The present invention relates to a multi-hop communication method and a multi-hop communication system.
Background of the Invention
Conventionally, there is known a multi-hop communication that enables communication by using other communication terminals in relaying communication if communication cannot be directly performed between communication terminals when communicating between the communication terminals existing on a communication network. The multi-hop communication is used particularly in a wireless network, one of communication networks.
In this communication network, it is sometimes the case that the communication terminals, previously capable of communicating, become unable to communicate due to the connection/disconnection of the communication terminals or the change in communication environments; and as a result, the network topology of the communication network undergoes a change. In order to perform good communication between the communication terminals, it is necessary to build a communication route between the communication terminals in the event that the network topology of the communication

network is changed (see, e.g., JP2006-67557A and JP2008-244679A).
As a communication route building method, it is possible to build a communication route between the communication networks by, e.g., exchanging route information between the communication terminals, searching for usable communication routes, and selecting one route having a good communication quality from the usable communication routes.
In recent years, there has been available a communication system, such as an automatic remote meter reading system, in which a plurality of parent terminals is installed on a region-by-region basis so that each of the parent terminals can communicate with a plurality of child terminals existing around each of the parent terminals. In the communication system of this kind, it has been proposed to build a communication network in which the aforementioned multi-hop communication is used in the communication between the parent terminals and the child terminals.
In the communication network using the parent terminals and the child terminals, each of the parent terminals arranged on a region-by-region basis acquires specified information directly from the respective child terminals existing around each of the parent terminals or indirectly from the respective child terminals using other child terminals as relay terminals. In other words, a

communication cell for multi-hop communication is formed by one parent terminal and a plurality of child terminals from which the parent terminal acquires specified information. A plurality of communication networks is built by providing a plurality of communication cells.
In the communication network using the parent terminals and the child terminals, wireless packets are sent and received by selecting one wireless channel from a plurality of wireless channels provided within a specified frequency band. If the adjoining communication networks intercommunicate through the use of the same wireless channel in case where there is provided a plurality of communication cells, it is likely that the wireless packets interfere with each other, thereby reducing the communication quality. Thus, to inhibit the interference between the communication cells, there has been proposed a method of allotting wireless channels to the respective communication cells (see, e.g., JP2007-158485A, JP2004-96148A, JP2001-128224A and JP10-313476A).
For example, it is plausible to use a method in which the parent terminals are connected to an upper network made up of an optical fiber network and in which the wireless channels used by the parent terminals are intensively managed by a server of the upper network pursuant to the positional information acquired from GPSs provided in the respective parent terminals. In this case, different

wireless channels are allotted to the respective communication cells if the distance between the parent terminals is short.
Also available is a method of making the adjoining parent terminals intercommunicate through the use of different wireless channels.
The dynamic allotment of wireless channels is a matter of multiple coloring in a graph theory and also a matter of NP-completeness. It is therefore difficult to derive a complete solution. It is only necessary to find a certain good solution. The algorithm for finding the good solution has been widely studied in the field of mathematical graphs.
In the communication network using the aforementioned multi-hop communication, the respective child terminals perform communication by relaying the wireless packets of other child terminals. Therefore, as compared with a case where the child terminals directly communicate with the parent terminals, the physical spatial distance formed by the communication network grows larger. For that reason, even if the parent terminals are distant from one another, it is often the case that the child terminals connected to the respective parent terminals are positioned close to one another. Relying only on the positional information of the parent terminals, it is impossible to sufficiently reduce the possibility of interference of the wireless packets. If the parent terminals are distant from one another, it is

impossible for the parent terminals to intercommunicate. This poses a problem in that it is impossible to sufficiently reduce the possibility of interference of the wireless packets.
Even when different wireless channels are used in the adjoining communication cells, if the frequency distance between the wireless channels is short, it is likely that the wireless packets interfere with each other due to the electric power leaked from the adjoining wireless channels.
Summary of the Invention
In view of the above, the present invention provides a multi-hop communication method and a multi-hop communication system, which are capable of making unnecessary intercommunications between parent terminals and intensive management for a server or the like, and inhibiting interference of wireless channels caused by the electric power leaked from other communication cells.
In accordance with one aspect of the present is provided a multi-hop communication system, including: a plurality of communication cells in each of which one parent terminal makes multi-hop communication with one or more child terminals by using one wireless channel selected from a plurality of wireless channels as an in-cell channel, wherein each of the child terminals detects the in-cell

channel of each communication cell adjacent to the communication cell to which the corresponding child terminal belongs, and notifies the in-cell channel to the parent terminal of the communication cell to which the corresponding child terminal belongs, and if a degree of interference, when the parent terminal communicates with another communication cell using the same in-cell channel as the communication cell to which the corresponding parent terminal belongs, is larger than a specified threshold value, the parent terminal selects, among one or more wireless channels not used as the in-cell channel in the communication cell to which the corresponding parent terminal belongs and in the adjacent communication cells, the wireless channel having the smallest frequency-distance-based interference level between the in-cell channel of the communication cell to which the corresponding parent terminal belongs and the in-cell channels of the adjacent communication cells to which the adjacent cells belong, and the parent terminal is configured to set the selected wireless channel as the in-cell channel of the communication cell to which the corresponding parent terminal belongs
In accordance with another aspect of the present is provide a multi-hop communication system, including: a plurality of communication cells in each of which one parent terminal makes multi-hop communication with one or more child terminals by using one wireless channel selected from

a plurality of wireless channels as an in-cell channel,
wherein the parent terminal and each of the child terminals
are configured to send, through the in-cell channel, a hello
packet including communication cell information for
identifying a communication cell to which the corresponding
parent terminal and the corresponding child terminal belong
and notifying the existence of the corresponding parent
terminal and the corresponding child terminal, if a
communication cell indicated by the communication cell information of the hello packet received through the in-cell channel differs from a first communication cell to which each of the child terminals belongs, each of the child terminals is configured to send a detection information, that includes the communication cell information of the received hello packet and notifies a detection of a second communication cell employing the same in-cell channel, to the parent terminal belonging to the first communication cell; if each of the child terminals receives the hello packet by sequentially changing over the wireless channels, each of the child terminals sends channel use information, that includes the communication cell information of the received hello packet and channel information on a wireless channel receiving the hello packet and notifies the in-cell channel employed by a third communication cell existing around the first communication cell, to the parent terminal belonging to the first communication cell; the parent

terminal belonging to the first communication cell is configured to derive a degree of interference in the communication of the first communication cell and the second communication cell based on the received detection information; and if the degree of interference is larger than a specified threshold value, the parent terminal belonging to the first communication cell is configured to select, among unused channels as wireless channels not used as the in-cell channels by the first and third communication cells, an unused channel remaining the smallest in a frequency-distance-based interference level between the unused channel and the in-cell channel of the first and third communication cells as a used channel based on the channel use information, the parent terminal belonging to the first communication cell is configured to set the selected unused channel as the in-cell channel of the first communication cell.
The parent terminal may be configured to, based on the channel use information, give a weight to the interference level depending on the number of the communication cells employing the used channel.
The parent terminal may be configured to exclude wireless channels adjacent to the used channels from the unused channels.
The parent terminal and each of the child terminals may be configured to send, through the in-cell channel, the

hello packet including the channel use information prepared by the communication cell to which the parent corresponding terminal and the corresponding child terminal belong; each of the child terminals belonging to the first communication cell is configured, to send the channel use information included in the hello packet received from the third communication cell to the parent terminal belonging to the first communication cell while sequentially changing over the wireless channels; and the parent terminal belonging to the first communication cell is configured to, based on the channel use information included in the hello packet, detect the in-cell channel employed by a fourth communication cell existing around the third communication cell and to exclude the in-cell channel employed by the fourth communication cell from the unused channels.
If there is no unused channel, the. parent terminal . belonging to the first communication cell may be configured to derive an evaluation function N+ᵦM with respect to each of the wireless channels, where N is the number of the third communication cells employing the wireless channels, M is the number of the fourth communication cells employing the wireless channels, and (3 is a weighting coefficient of from 0 to 1, and set the wireless channel having the smallest evaluation function as the in-cell channel.
The parent terminal belonging to the first communication cell may be configured to employ, as the used

channel, the in-cell channels of the first, third, and fourth communication cells, the interference level of the in-cell channel of the fourth communication cell with respect to the unused channels being given a weight lighter than a weight given to the interference level of the in-cell channel of the third communication cell with respect to the unused channels.
Each of the parent terminals may be given a priority and may be configured to, based on the priority, determine whether to set the unused channel as the in-cell channel if the degree of interference is larger than the specified threshold value.
Each of the wireless channels may be given a probability according to a specified probability density function, and the parent terminal may be configured to, based on the probability of the unused channel, determine whether to set the unused channel as the in-cell channel.
The parent terminal may be configured to change the probability of the unused channel, in which the degree of interference in the communication of the first communication cell and the second communication cell has previously exceeded a specified threshold value, so that the unused channel is not set as the in-cell channel.
In accordance with still another aspect of the present is provided A multi-hop communication system, including: a plurality of communication cells in each of which one parent

terminal and child terminals make multi-hop communication with each other through a same wireless channel, wherein each of the child terminals is configured to detect another communication cells employing the wireless channel used for communication by the corresponding child terminal and to notify said another communication cells to the parent terminal, and the parent terminal is configured to derive, on a cell-by-cell basis, a degree of interference in the communication between said another communication cells and the communication cell to which the parent terminal belongs, and to make communication through another wireless channel if the sum of the degrees of interference is larger than a specified threshold value.
In accordance with still another aspect of the present is providedA multi-hop communication system, including: a plurality of communication cells in each of which one parent terminal and child terminals make multi-hop communication with each other through a same wireless channel, wherein each of the terminals is configured to set one wireless channel selected from a plurality of wireless channels differing in frequency from one another as a communication channel used for communication and to send a hello packet including communication cell information for identification of the communication cell to which the corresponding terminal belongs and notifying the existence of the corresponding terminal; each of the child terminal is

configured such that, if the communication cell indicated by the communication cell information of the hello packet received through the communication channel differs from the communication cell to which the corresponding child terminal belongs, the corresponding child terminal sends detection information including the communication cell information of the received hello packet and notifying the detection of another communication cell employing the communication channel, to the parent terminal belonging to the communication cell to which the corresponding child terminal belongs; and the parent terminal is configured to derive, based on the received detection information, interference levels with respect to the respective communication cells indicated by the communication cell information of the detection information, and if the sum of the interference levels is larger than a specified threshold value, the parent terminal is configured to select one wireless channel other than the communication channel from the plurality of wireless channels and to set the selected wireless channel as a new communication channel used in the communication cell to which the parent terminal belongs.
Each of the communication cells may have individually given identification information, and the parent terminal may be configured to set the new communication channel only if the identification information of the communication cell indicated by the communication cell information of the

received detection information has a specified relationship with the identification information of the communication cell to which the parent terminal belongs.
Each of the child terminals may be configured to receive the hello packet by sequentially changing over the wireless channels, and if the communication cell indicated by the communication cell information of the received hello packet differs from the communication cell to which the corresponding child terminal belongs, the corresponding child terminal sends channel use information including the communication cell information of the received hello packet and channel information indicating the wireless channel which has received the hello packet, to the parent terminal; and the parent terminal is configured to, based on the communication cell information and the channel information included in the channel use information, find the number of the communication cells employing the wireless channels with respect to the respective wireless channels, and to select a new communication channel by preferentially employing the wireless channel which is used by the least number of communication cells.
Each of the child terminals may be configured to send the detection information including a hop count in a communication route leading from the corresponding child terminal to the parent terminal of the communication cell to which the corresponding child terminal belongs, and the

parent terminal is configured to derive the interference levels such that the interference levels become higher as the hop count included in the detection information grows smaller.
The parent terminal may be configured to set the interference levels higher as the number of the child terminals belonging to the communication cell to which the parent terminal sending the detection information belongs grows larger.
Each of the terminals may be configured to send the hello packet that includes terminal information indicating whether the corresponding terminal is the parent terminal or the child terminal and indicating, if the corresponding terminal is the child terminals, a hop count in a communication route leading from the corresponding terminal to the parent terminal of the communication cell to which the corresponding terminal belongs, each of the child terminals is configured to send the detection information including the terminal information included in the received hello packet, and the parent terminal is configured to derive the interference levels pursuant to the terminal information included in the detection information.
In accordance with still anothr aspect of the present is providedA parent terminal for use in the multi-hop communication system of any one of above described aspects of the present invention.

In accordance with still another aspect of the present is provided a child terminal for use in the multi-hop communication system of any one of above described aspects of the present invention.
With the embodiments of the present invention, there is provided an effect of making it unnecessary for parent terminals intercommunicate, unnecessary for a server or the like to perform intensive management, and inhibits interference of wireless channels caused by the electric power leaked from other communication cells.
Brief Description of the Drawings
The objects and features of the present invention will become apparent from the following description of preferred embodiments made in conjunction with the accompanying drawings.
Fig. 1 is a configuration diagram schematically showing a wireless network made up of a multi-hop communication system according to a first embodiment of the present invention.
Fig. 2 is a block diagram showing the configuration of a communication terminal employed in wireless network.
Fig. 3 is a table diagram illustrating the configuration of an adjacent terminal management table employed in the wireless network.

Figs. 4A and 4B are table diagrams illustrating the configurations of communication route tables employed in the wireless network.
Fig. 5 is a table diagram illustrating the configuration of a detection information management table employed in the wireless network.
Figs. 6A, 6B and 6C are diagrams showing the formats of communication packets employed in the wireless network.
Fig. 7 is a sequence diagram showing a communication sequence performed by the wireless network.
Fig. 8 is a flowchart illustrating an in-cell channel changing process performed by the wireless network.
Fig. 9 is a diagram showing the format of a communication packet employed in the wireless network.
Fig. 10 is a table diagram illustrating the configuration of a channel information management table employed in the wireless network.
Fig. 11 is a channel configuration diagram illustrating the situation of use of wireless channels in the wireless network.
Fig. 12 is a table diagram illustrating the correspondence between frequency distances and separation indices.
Fig. 13 is a channel configuration diagram illustrating the situation of use of wireless channels in a second embodiment of the present invention.

Fig. 14 is a channel configuration diagram illustrating the situation of use of wireless channels in a fourth embodiment of the present invention.
Fig. 15 is a table diagram illustrating a detection information management table of a wireless network made up of a multi-hop communication system according to a fifth embodiment of the present invention.
Fig. 16 is a diagram illustrating the format of a communication packet of a wireless network made up of a multi-hop communication system according to a sixth embodiment of the present invention.
Fig. 17 is a table diagram illustrating a detection information management table of the wireless network made up of the multi-hop communication system according to the sixth embodiment of the present invention.
Detailed Description of the Preferred Embodiments
Certain embodiments of the present invention will now be described in detail with reference to the accompanying drawings which form a part of the subject specification. Throughout the drawings, identical or similar components will be designated by like reference symbols and will not be described repeatedly.
(First Embodiment)
Fig. 1 is a schematic diagram of a wireless network

made up of a multi-hop communication system according to the present embodiment. The wireless network is used in a dwelling unit group which includes a plurality of dwelling units K. Parent communication terminals 1 are installed within the dwelling unit group on a region-by-region basis (e.g., every 500 m in radius). Child communication terminals 2 are installed in the respective dwelling units K. In the following description, the parent communication terminals 1 will be referred to as "parent terminals 1" and the child communication terminals 2 will be referred to as "child terminals 2". If there is a need to individually identify the parent terminals 1, reference symbols 1-1, 1-2, 1-3, 1-4, etc. will be used to designate the parent terminals 1. If there is a need to individually identify the child terminals 2, reference symbols 2-1, 2-2, 2-3, etc. will be used to designate the child terminals 2.
The child terminals 2 serve to wirelessly send the specified information on the respective dwelling units K to one of the parent terminals 1. The parent terminals 1 serve to wirelessly acquire the specified information on the respective dwelling units K from the child terminals 2 and to send the acquired specified information to an upper management device (not shown) through the use of optical fiber links or the like. For example, the parent terminals 1 may acquire meter reading information, such as an electric power consumption amount, a gas consumption amount, and a

tap water consumption amount in the respective dwelling units K, from the child terminals 2. This makes it possible to form a remote meter reading system. In addition, the parent terminals 1 may send and receive pre-set specified information to and from the child terminals 2. This makes it possible to form a remote monitoring system for monitoring the conditions of devices arranged within the respective dwelling units K or a remote control system for controlling the conditions of devices arranged within the respective dwelling units K.
In the present wireless network, the parent terminals 1 and the child terminals 2 send and receive wireless signals to and from each other by virtue of multi-hop communication. More specifically, in the present wireless network, communication is directly or indirectly performed between the parent terminals 1 and the child terminals 2. The child terminals 2, incapable of directly communicating with the parent terminals 1, perform communication with the parent terminals 1 by using other child terminals 2, existing within a communication distance, to sequentially relay the communication packets.
In the multi-hop communication system, a communication cell as a small-size wireless network is formed of one parent terminal 1 and one or more child terminals 2 for directly or indirectly sending specified information to the parent terminal 1. In the present embodiment, a plurality

of communication cells C1, C2, C3, C4, etc. is arranged side by side. The parent terminals 1 of the respective communication cells select one communication channel for use in sending and receiving communication packets, from a plurality of wireless channels differing in frequency from one another. Each of the child terminals 2 sends and receives communication packets using the communication channel selected by the parent terminal 1 to which the subject child terminal 2 belongs. In other words, the parent terminal 1 and the child terminals 2, belonging to the same communication cell, send and receive wireless packets using the wireless channel having the same frequency. The single wireless channel used for communication in the parent terminal 1 and the child terminals 2 of each of the communication cells will be referred to as "in-cell channel". In the present embodiment, wireless channels Ch1, Ch2, etc. differing in frequency from one another are set as the wireless channels that can be used in the respective communication terminals A.
Fig. 2 is a block diagram showing one of the communication terminals A. In the present embodiment, the same communication terminals A are used as the parent terminals 1 and the child terminals 2. For example, the communication terminals A serve as the parent terminals 1 if they are set to become a parent by a setting means such as a****************

jumper switch or a changeover switch, and serve as the child terminals 2 if they are set to become a child. In the following description, the parent terminals 1 and the child terminals 2 will be referred to as "communication terminals A" if there is no need to distinguish the parent terminals 1 and the child terminals 2 from each other.
Each of the communication terminals A includes a memory unit 10, a control unit 20, and a wireless communication interface unit 30.
The memory unit 10 is formed of a nonvolatile memory such as a ROM or the like, a rewritable nonvolatile memory such as an EEPROM or the like, or a volatile memory such as a RAM or the like. The memory unit 10 includes a table storage unit 101 for storing link information on a communication route and adjacent terminals capable of making communication (the parent terminals 1 or the child terminals 2 capable of making direct communication). The memory unit 10 also stores various kinds of programs, such as a control program for operating the communication terminals A or other programs and various kinds of information required in executing the programs.
In the present embodiment, unique terminal IDs are allotted to the parent terminals 1 and the child terminals 2. The ID allotted to the subject terminal A is also stored in the memory unit 10 of each of the communication terminals A. In the present embodiment, terminal IDs "Ml", "M2",

"M3", etc. are previously allotted to the parent terminals 1-1, 1-2, 1-3, etc. If the below-mentioned communication route is built, terminal IDs "Tl", "T2", "T3", etc. are allotted to the child terminals 2-1, 2-2, 2-3, etc. by the parent terminals 1.
A device ID such as a serial number (production number) or a MAC address is previously allotted to each of the communication terminals A. The device ID is previously stored in the memory unit 10 of each of the communication terminals A. By adding the device ID, communication control is performed with respect to the communication packets sent and received by the communication terminals A.
A unique communication cell ID is allotted to each of the communication cells. Each of the communication terminals A sends a communication packet containing the communication cell ID of the communication cell to which the subject terminals A belong. In the present embodiment, the terminal ID of the parent terminal 1 belonging to the communication cell is used as the communication cell ID. "Ml", "M2", "M3", etc. are used as the communication cell IDs of the communication cells CI, C2, C3, etc. The communication cell IDs correspond to the communication cell information of the present invention.
An adjacent terminal management table TB11 shown in Fig. 3 is stored in the memory unit 10 of each of the parent terminals 1 and the child terminals 2. A detection

information management table TB12 shown in Fig. 5 is stored in the table storage unit 101 of each of the parent terminals 1. Moreover, A communication route table TB21 shown in Fig. 4A is stored in the table storage unit 101 of each of the parent terminals 1. A communication route table TB22 shown in Fig. 4B is stored in the table storage unit 101 of each of the child terminals 2.
As shown in Fig. 3, the adjacent terminal management table TB11 stores, in a tabular form, the information (adjacent terminal information) on the communication terminals A (hereinafter referred to as "adjacent terminals A") capable of making direct communication with the relevant communication terminals A without being relayed by other communication terminals A. More specifically, the adjacent terminal management table TB11 is provided with individual fields for the adjacent terminal IDs, the terminal kinds, the receiving link communication quality, the sending link communication quality and the link communication quality.
In the adjacent terminal management table TBll, the adjacent terminal IDs are terminal IDs allotted to the communication terminals A capable of making direct communication with a subject communication terminal A. The terminal kinds indicate the kinds of the adjacent terminals A (namely, the parent terminal 1 as a "parent" or the child terminal 2 as a "child"). The receiving link communication quality indicates the communication quality of a

communication link from the adjacent terminal A to a subject communication terminal. The sending link communication quality indicates the communication quality of a communication link from a subject communication terminal to the adjacent terminal A. The link communication quality indicates the communication quality in a communication link between the adjacent terminal A and a subject communication terminal.
For example, a communication quality value SQ is used to indicate the link communication quality in a communication link between two communication terminals A and A capable of making direct communication with each other. The communication quality value SQ is indicated by an integer value of ten steps or twenty steps which grows smaller as the received signal intensity between two communication terminals A and A capable of making direct communication with each other becomes larger. In other words, the attenuation of communication packets becomes smaller and the communication state becomes better as the communication quality value SQ, i.e., the integer value, grows smaller.
If noise levels or interference levels are different in the communication packet receiving places, the bidirectional communication qualities in the communication link differ from one another. The bidirectional communication qualities in the communication link include

the receiving link communication quality and the sending link communication quality. The receiving link communication quality refers to the received signal intensity in the link between two communication terminals A and A capable of making direct communication with each other, when a subject communication terminal A receives communication packets from other communication terminals A. The sending link communication quality refers to the received signal intensity in the link between two communication terminals A and A capable of making direct communication with each other, when a subject communication terminal A sends communication packets to other communication terminals A and when other communication terminals A receive the communication packets.
With a view to assure communication certainty (reliability) when communication is performed between the communication terminals A and A, the receiving link communication quality or the sending link communication quality, whichever is poor in the communication state (whichever is larger in the value of the link communication quality), is employed as the link communication quality between two communication terminals A and A.
The sum of the link communication qualities of the respective communication links making up the communication routes between the parent terminals 1 and the child terminals 2 is employed as the below-mentioned route

communication quality which is the evaluation of the communication quality of the communication routes between the parent terminals 1 and the child terminals 2.
While the aforementioned communication quality value SQ is associated with the received signal intensity, it may be possible to calculate the communication quality value SQ in association with other factors such as a signal-noise ratio, an error vector magnitude, a bit error rate, and a packet error rate, in place of the received signal intensity.
The detection information management table TB12 stores, in a tabular form, the information (detection information) indicating the communication cells (interfering cells) which are detected by the respective child terminals 2 belonging to the same communication cell as the parent terminal 1 and which make use of the same communication channel. More specifically, the detection information ' management table TB12 is provided with individual fields for the terminal IDs (child terminal IDs) of the child terminals detecting the interfering cells, the communication cell IDs (interfering cell IDs) of the detected interfering cells, and the detection time. The interfering cells correspond to the second communication cells of the present invention. The detection information management table TB12 is set such that the detection information is periodically deleted when a specified time elapses from the detection time thereof.

The communication routes between the parent terminals 1 and the child terminals 2 are formed of one or more communication links. The communication route table TB21 (see Fig. 4A) held by each of the parent terminals 1 stores, in a tabular form, the information (communication route information) on the communication routes between the parent terminals 1 and the child terminals 2 remaining under the control of the parent terminals 1. More specifically, the communication route table TB21 is provided with individual fields for the terminal IDs, the route communication quality, the hop count, and the hop destination.
In the communication route table TB21 held by each of the parent terminals 1, the terminal IDs are the terminal IDs allotted to the subordinate child terminals 2 by which the communication routes are built. The route communication quality indicates the communication quality in the communication routes leading to the child terminals 2 registered in the terminal ID field. The hop count indicates the hop count in the communication routes leading to the child terminals 2 registered in the terminal ID field. The hop destination indicates the communication terminals A of the destinations of the respective hops in the communication routes leading to the child terminals 2 registered in the terminal ID field.
The communication route table TB22 (see Fig. 4B) held by each of the child terminals 2 stores, in a tabular form,

the information (communication route information) on the communication routes between the child terminals 2 and the parent terminals 1 capable of making communication with the child terminals 2. More specifically, the communication route table TB22 is provided with individual fields for the terminal IDs, the route communication quality, the hop count and the hop destination.
In the communication route table TB22 held by each of the child terminals 2, the terminal IDs are the terminal IDs allotted to the parent terminals 1 capable of communicating with the child terminals 2. The route communication quality indicates the communication quality in the communication routes leading to the parent terminals 1 registered in the terminal ID field. The hop count indicates the hop count in the communication routes leading to the parent terminals 1 registered in the terminal ID field. The hop destination indicates the communication terminals A of the destinations of the respective hops in the communication routes leading to the parent terminals 1 registered in the terminal ID field.
In the communication route tables TB21 and TB22, the hop count is the number of the communication terminals A in the communication route extending from a subject communication terminal to a destination terminal. For example, the hop count is two in case of the communication route where the child terminal 2-2 communicates with the

parent terminal 1 through the child terminal 2-1. In the hop destination, the terminal IDs of the communication terminals A arranged between a subject communication terminal and a destination terminal are registered in the arrangement order. The terminal IDs of the communication terminals A registered in the terminal ID field are finally registered in the hop destination.
The wireless communication interface unit 30 is a communication interface circuit for making communication with other communication terminals A using a wireless signal. The wireless communication interface unit 30 is used in sending and receiving the communication packet within the communication cell to which a subject communication terminal belongs, at which time one wireless channel selected from a plurality of wireless channels differing in frequency from one another is used as an in-cell channel.
The control unit 20 is a device that controls the respective parts of the communication terminals A to thereby control the overall operations of the communication terminals A. The control unit 20 is formed of, e.g., a microprocessor and peripheral circuits thereof. The control unit 20 includes a table management unit 201, a communication processing unit 202, and a sending timer unit 203. The control unit 20 performs a communication route building process for directly or indirectly building

communication routes between the child terminals 2 and the parent terminals 1.
The control unit 20 further includes a channel changeover processing unit 204, an interference detecting unit 205 and a changing timer unit 206. The control unit 20 performs an in-cell channel deciding process for deciding and changing an in-cell channel used in sending and receiving the communication packets between the child terminals 2 and the parent terminals 1.
The table management unit 201 manages the registration contents of the respective tables stored in the table storage unit 101 of the memory unit 10. The communication processing unit 2 02 sends and receives the communication packets to and from other communication terminals A through the use of the wireless communication interface unit 30. By performing the below-mentioned operation, the communication processing unit 202 performs a communication route building process for building communication routes between the parent terminals 1 and the child terminals 2. The sending timer unit 203 is a clock means for measuring a specified time lapse. The sending timer unit 203 notifies the communication processing unit 202 of arrival of the sending timing of various kinds of communication packets at a specified time interval.
The channel changeover processing unit 204 selects one in-cell channel from a plurality of wireless channels, based

on the situation of communication interference between the communication cells detected by the interference detecting unit 205, and sets the selected in-cell channel in the wireless communication interface unit 30. The interference detecting unit 205 detects the state of interference between the communication cells from the communication packets sent from other communication terminals A and received by the communication processing unit 202. The interference detecting unit 205 outputs the state of interference to the channel changeover processing unit 204. The changing timer unit 206 is a clock means for detecting a specified time. The changing timer unit 206 notifies the channel changeover processing unit 204 of arrival of the communication channel changing timing.
Next, description will be made on how to build a communication route in the present wireless network.
If the communication terminals A are started up, the sending timer unit 203 of the control unit 20 of each of the communication terminals A begins to measure the time in order to send a hello packet (hereinafter referred to as "H packet") . Upon the time-up, the sending timer unit 203 notifies the communication processing unit 202 to the effect that it is now the sending timing of the H packet. Responsive to this notification, the communication processing unit 202 sends the H packet to the wireless network through the use of broadcast communication.

The H packet is a communication packet by which each of the communication terminals A notifies other communication terminals A of existence of the subject communication terminal A. Fig. 6A illustrates the format of the H packet which includes a source terminal ID section, a destination terminal ID section, a cell ID section, an operation code section, a terminal kind section and a communication route section.
The source terminal ID section of the H packet holds the terminal ID of the communication terminal A which has sent the H packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes the destination of the H packet. In the case of the H packet, a broadcast communication code "BC" such as broad cast or the like is held in the destination terminal ID section. The cell ID section holds the communication cell ID allotted to the communication cell to which the communication terminal A sending the H packet belongs. In the present embodiment, each of the parent terminals 1 holds the terminal ID of the subject terminal 1. Each of the child terminal 2 holds the terminal ID of the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The operation code section holds the code of the H packet. The terminal kind section holds the information for identifying which of the parent terminal 1 and the child terminal 2 has sent the H packet.

The communication route section of the H packet holds the communication route information indicating the communication routes from the child terminals 2 to the parent terminal 1 and the route quality information indicating the communication qualities of the communication routes. The communication route information is indicated by sequentially enumerating the terminal IDs of the communication terminals A arranged in the communication route extending from the child terminals 2 to the parent terminal 1. The route quality information is indicated by the sum of the link communication qualities within the communication routes. The sum of the link communication qualities will be called a route communication quality.
For example, it is assumed that the child terminal 2-2 builds a communication route between the child terminal 2-2 and the parent terminal 1 via the child terminal 2-1 and further that the route communication quality is equal to 17. In this case, the communication route section of the H packet sent by the child terminal 2-2 holds "T2->T->M1:17". The hop count of the communication route is equal to "2". The communication route section of the H packet sent by the parent terminal 1 remains "null" (or data-free) , in which case the hop count is "0" and the route communication quality is "0".
In order to reduce the communication traffic of the wireless network, the H packet is sent from each of the

communication terminals A in a case where the communication terminal A is set to the parent terminal 1 and in a case where the communication terminals A are the child terminals 2 building a communication route leading to the parent terminal 1. The H packet sending process is performed at a specified time interval after the startup.
The child terminals 2 not building any communication route leading to the parent terminal 1 receives the H • packet, finds the communication cost between the child terminals 2 and the parent terminal 1 based on the route communication quality information of the H packet, and builds a communication route leading to the parent terminal 1 which is smallest in the communication cost. The method of building communication routes between the parent terminal 1 and the child terminals 2 is well-known and, therefore, will not be described in detail.
Next, a method of deciding a communication channel in the present wireless network will be described with reference to the sequence shown in Fig. 7.
If the parent terminal 1 is started up, the channel changeover processing unit 204 sets a specified wireless channel as an tentative in-cell channel in the communication interface unit 30 (step SI). In this regard, the tentative in-cell channel is set because it is unclear which of the wireless channels is used as an in-cell channel in the communication cells and because it is assumed that the in-

cell channel is changed by performing following operations. When deciding the tentative in-cell channel, it may be possible to measure the wireless signal intensity for every available wireless channel and to select a wireless channel which is lowest in the wireless signal intensity. Alternatively, it may be possible to randomly select a wireless channel from'the available wireless channels.
At this time, each of the child terminals 2 arranged around the parent terminal 1 receives the H packet sent via the tentative in-cell channel. Each of the child terminals 2 build a communication route between the subject terminal 2 and the parent terminal 1 if the communication quality is higher than that of the communication cell to which the subject terminal 2 belongs. Consequently, the child terminals 2 arranged around the parent terminal 1 make direct or indirect communication with the parent terminal through the use of the same in-cell channel, thereby forming a communication cell that includes the parent terminal 1 and the child terminals 2. The terminal ID allotted to the parent terminal 1 of the communication cell to which the child terminals 2 belong is stored as a communication cell ID in the memory units 10 of the child terminals 2.
In the present embodiment, as shown in Fig. 1, the communication cell C1 includes a parent terminal 1-1 and child terminals 2-1, 2-2 and 2-3. The communication cell C2 includes a parent terminal 1-2 and child terminals 2-4 and

2-5. The communication cell C3 includes a parent terminal 1-3 and a child terminal 2-6. The child terminal 2-2 makes indirect communication with the parent terminal 1-1 through the child terminal 2-1. The child terminals 2-1 and 2-3 make direct communication with the parent terminal 1-1. The child terminal 2-5 makes indirect communication with the parent terminal 1-2 through the child terminal 2-4. The child terminal 2-4 makes direct communication with the parent terminal 1-2. The child terminal 2-6 and the parent terminal 1-3 make direct communication with each other.
In each of the parent terminals 1-1, 1-2 and 1-3, channel Ch1 is set as the in-cell channel used in sending and receiving the communication packet. The child terminals 2-1 and 2-5 exist in the range where direct communication can be made with the child terminal 2-2. In other words, while the child terminals 2-2 and 2-5 are included in different communication cells, they make wireless packet communication in the respective communication cells through the use of the same in-cell channel. The parent terminal 1-1 and the child terminal 2-6 exist in the range where direct communication can be made with the child terminal 2-3. In other words, while the child terminals 2-3 and 2-6 are included in different communication cells, they make wireless packet communication in the respective communication cells through the use of the same in-cell channel.

The respective child terminals 2 periodically receive the H packet as stated above and receive the H packet sent from the surrounding communication terminals A. In the present embodiment, the respective child terminals 2 receive the H packet sent through the use of the in-cell channel of the communication cell to which the subject terminals 2 belong.
Upon receiving the H packet, each of the child terminals 2 compares the communication cell ID included in the H packet with the communication cell ID of the communication cell (first communication cell) to which the subject terminal 2 belongs. In the present embodiment, the terminal ID of the parent terminal 1 is used as the communication cell ID. Therefore, in reality, each of the child terminals 2 compares the communication cell ID included in the H packet with the terminal ID of the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. If the result of comparison reveals that the communication cell IDs differ from each other, the channel detection information packet (hereinafter referred to as "D packet") indicating the detection of an interfering cell (second communication cell), i.e., another cell using the same in-cell channel as the communication cell to which the subject terminal 2 belongs, is directly or indirectly sent to the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The D packet

corresponds to the detection information of the present invention.
Fig. 6B illustrates the format of the D packet which includes a source terminal ID section, a destination terminal ID section, a cell ID section, an operation code section and a detection information section.
The source terminal ID section of the D packet holds the terminal ID of the communication terminal A which has sent the D packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes the destination of the D packet. In the case of the D packet, the terminal ID of the parent terminal 1 of the communication cell to which the communication terminal A belongs is held in the destination terminal ID section. The cell ID section holds the communication cell ID allotted to the communication cell to which the communication terminal A sending the D packet belongs. In the present embodiment, each of the parent terminals 1 holds the terminal ID of the subject terminal 1. Each of the child terminals 2 holds the terminal ID of the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The operation code section holds the code of the D packet. The detection information section holds, e.g., an interfering cell ID which has the same value as that of the cell ID section included in the received H packet and which indicates the communication cell ID of the detected interfering cell.

the D packet sent from the child terminal 2-2, the parent terminal 1-1 stores the terminal ID of the child terminal 2-2, the interfering cell ID of the D packet and the detection time in the detection information management table TB12 using the current time as the detection time (step S5).
Next, the channel changeover processing unit 204 of the parent terminal 1-1 determines whether there is a need to change the in-cell channel to another wireless channel, based on the information included in the detection information management table TB12. If such a need exists, the channel changeover processing unit 204 changes the in-cell channel. In the present embodiment, the in-cell channel changing process is started using the reception of the D packet as a trigger. Alternatively, the in-cell channel changing process may be started in a specified time period.
By referring to the respective records of the detection information management table TB12, the interference detecting unit 205 calculates the degree of interference in the in-cell channel Ch1 on a cell-by-cell basis. In the present embodiment, the degree of interference of the interfering cell whose ID is stored in the detection information management table TB12 is always set equal to "1" by the interference detecting unit 205.
The parent terminal 1-1 compares the sum of the interference degrees calculated on a cell-by-cell basis

(namely, the number of the interfering cells using the in-cell channel Ch1 of the parent terminal 1-1) with a specified threshold value. If the number of the interfering cells is larger than the threshold value, the parent terminal 1-1 performs the in-cell channel changing process to be described later. In the present embodiment, a threshold value "1" is previously stored in the memory unit 10.
More specifically, as shown in Fig. 7, the parent terminal 1-1 stores various kinds of information in the detection information management table TB12 based on the D packet sent from the child terminal 2-2 via the child terminal 2-1. Thereafter, the interference detecting unit 205 finds the number of the interfering cells. At this time, only the information sent from the child terminal 2-2 is stored in the detection information management table TB12. The number of the interfering cells is equal to "1". Therefore, the in-cell channel changing process is not carried out.
Next, if the child terminal 2-6 belonging to the communication cell C3 sends the H packet (step S6) , the child terminal 2-3 receives the H packet. The child terminal 2-3 receiving the H packet belongs to the communication cell C1. The communication cell ID included in the H packet differs from the communication cell ID of the communication cell to which the subject terminal 2-3

scanning each of the wireless channels is set longer than the sending interval of the H packet so that the H packet can be reliably received if there is another communication terminal A that makes use of the wireless channel to be scanned. Upon receiving the H packet from each of the wireless channels thus scanned, the child terminal 2 sends a channel use situation notifying packet indicating the use situation of the wireless channels to the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The channel use situation notifying packet corresponds to the channel use information of the present invention.
Fig. 9 illustrates the format of the channel use situation notifying packet. The channel use situation notifying packet includes a source terminal ID section, a destination terminal ID section, a cell ID section, an operation code section and a channel use information section.
The source terminal ID section of the channel use situation notifying packet holds the terminal ID of the communication terminal A which has sent the channel use situation notifying packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes the destination of the channel use situation notifying packet. In the case of the channel use situation notifying packet, the terminal ID of the parent terminal 1

of the communication cell to which the communication terminal A belongs is held in the destination terminal ID section. The cell ID section holds the communication cell ID allotted to the communication cell to which the communication terminal A sending the channel use situation notifying packet belongs. In the present embodiment, each of the parent terminals 1 holds the terminal ID of the subject terminal 1. Each of the child terminals 2 holds the terminal ID of the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The operation code section holds the code of the channel use situation notifying packet. The channel use information section holds, in pair, the channel information indicating the wireless channel through which the H packet is received and the communication cell ID (the communication cell information) which is based on the cell ID section of the H packet received through the use of the wireless channel indicated by the channel information. In other words, the channel use situation notifying packet notifies the communication cell ID of the adjacent cell (third communication cell), i.e., another communication cell existing around the communication cell (first communication cell) to which the parent terminal 1 receiving the channel use situation notifying packet belongs, and also notifies the in-cell channel used by the adjacent cell. In the present embodiment, the channel information includes the

channel number which is "1" in the case of the wireless channel Ch1 and "2" in the case of the wireless channel Ch2. The channel number corresponds to the identification number of the wireless channel of the present invention.
The channel information management table TB13 shown in Fig. 10 is further stored in the table storage unit 101 of the parent terminal 1. The channel information management table TB13 stores, in a tabular form, the information (channel use information) indicating the use situation of the in-cell channel in the adjacent cell, which information is detected by the respective child terminals 2 of the communication cell to which the parent terminal 1 belongs. More specifically, the channel information management table TB13 is provided with individual fields for the child terminal ID, the communication cell ID of the adjacent cell (the adjacent cell ID), the detection time, and the in-cell channel (used channel) of the adjacent cell.
Upon receiving the channel use situation notifying packet sent from the child terminal 2 of the communication cell to which the subject terminal 1 belongs, the parent terminal 1 causes the information included in the channel use situation notifying packet to be stored in the channel information management table TB13.
In the parent terminal 1 configured as above, the interference detecting unit 205 calculates the interference level of the changed wireless channel by referring to the

respective records of the channel information management table TB13.
For example, the interference detecting unit 205 of the parent terminal 1-1 derives used channels by referring to the used channel field of the channel information management table TB13. Fig. 11 illustrates a derivation example of the used channels in a case where there are wireless channels Ch1 to Chl0. The used channels are Ch1, Ch6, Ch7 and ChlO (see the horizontal line regions in Fig. 11) . The used channels are formed of the in-cell channel Ch1 of the parent terminal 1-1 and the in-cell channel of the adjacent cell of the parent terminal 1-1.
Next, the interference detecting unit 205 of the parent terminal 1-1 determines whether unused channels other than the used channels exist in the wireless channels (step S101) . In Fig. 11, the unused channels are Ch2 to Ch5, Ch8 and Ch9 (see the white regions in Fig. 11).
Then, the interference detecting unit 205 of the parent terminal 1-1 narrows down candidates for the in-cell channel from the unused channels based on the separation indices of the unused channels (step S102).
First, the interference detecting unit 205 of the parent terminal 1-1 finds the interference level of the unused channels Ch2 to Ch5, Ch8 and Ch9. The interference level is derived pursuant to the frequency distance between the unused channels and the used channels. More

specifically, as shown in Fig. 11, the frequency distance X1 between the nearest used and unused channels at the low frequency side of the unused channels and the frequency distance X2 between the nearest used and unused channels at the high frequency side of the unused channels are found with respect to each of the unused channels. The low-frequency-side frequency distance X1 and the high-frequency-side frequency distance X2 are indicated by the difference in the channels numbers of the wireless channels.
For example, in the case of the unused channel Ch2 and the used channels Ch1 and Ch6, the low-frequency-side frequency distance X1 is equal to 1 and the high-frequency-side frequency distance X2 is equal to 4. In the case of the unused channel Ch3 and the used channels Ch1 and Ch6, the low-frequency-side frequency distance X1 is equal to 2 and the high-frequency-side frequency distance X2 is equal to 3. In the case of the unused channel Ch4 and the used channels Ch1 and Ch6, the low-frequency-side frequency distance X1 is equal to 3 and the high-frequency-side frequency distance X2 is equal to 2. In the case of the unused channel Ch5 and the used channels Ch1 and Ch6, the low-frequency-side frequency distance X1 is equal to 4 and the high-frequency-side frequency distance X2 is equal to 1. In the case of the unused channel Ch8 and the used channels Ch7 and ChlO, the low-frequency-side frequency distance X1 is equal to 1 and the high-frequency-side frequency distance

X2 is equal to 2. In the case of the unused channel Ch9 and the used channels Ch7 and Chl0, the low-frequency-side frequency distance X1 is equal to 2 and the high-frequency-side frequency distance X2 is equal to 1.
As shown in Fig. 12, the isolation indices corresponding to the frequency distances are set in advance. The frequency distances X1 and X2 are converted to the isolation indices. In this regard, the frequency distance "1" corresponds to the isolation index "10". The frequency distance "2" corresponds to the isolation index "3". The frequency distance "3 or more" corresponds to the isolation index "1". The isolation index grows smaller as the frequency distance becomes larger. For example, if the low-frequency-side frequency distance X1 is equal to 1 and if the high-frequency-side frequency distance X2 is equal to 4, the low-frequency-side isolation index corresponding to the low-frequency-side frequency distance X1 becomes equal to "10" and the high-frequency-side isolation index corresponding to the high-frequency-side frequency distance X2 becomes equal to "1".
The sum of the respective isolation indices corresponding to the frequency distances X1 and X2 is derived as an interference level. In other words, the interference level is equal to the low-frequency-side isolation index plus the high-frequency-side isolation index.

For example, if the respective isolation indices corresponding to the low-frequency-side frequency distance X1 and the high-frequency-side frequency distance X2 are "10" and "1", the interference level is equal to 11 (=10+1). This interference level denotes the interference level of the unused channel and the used channel. As the interference level becomes lower, it becomes more difficult for the unused channels and the used channels to interfere with each other.
Therefore, in Fig. 11, the interference level of the unused channel Ch2 is equal to 11 (=10+1). The interference level of the unused channel Ch3 is equal to 4 (=3 + 1) . The interference level of the unused channel Ch4 is equal to 4 (=1+3). In addition, the interference level of the unused channel Ch5 is equal to 11 (=1+10). The interference level of the unused channel Ch8 is equal to 13 ( = 10+3) . The interference level of the unused channel Ch9 is equal to 13 ( = 3+10) .
In other words, the interference levels of the unused channels Ch2, Ch3, Ch4, Ch5, Ch8 and Ch9 are respectively equal to "11", "4", "4", "11", "13" and "13". In this case, the unused channels Ch3 and Ch4 having a minimum interference level "4" become the candidates for the in-cell channel.
The interference detecting unit 205 of the parent terminal 1-1 refers to the interfering cell IDs of the

detection information management table TB12. The interfering cell IDs are the communication IDs of the parent terminals 1 belonging to the interfering cells that make use of the same in-cell channel as the subject terminal. Depending on whether the communication IDs are large or small, a priority n is given to each of the parent terminals 1 belonging to the interfering cells that make use of the same in-cell channel as the subject terminal. In this regard, the interference detecting unit 205 of the parent terminal 1 applies the priority n of each of the subject terminals 1 to a function f(n) and obtains a calculation result of the function f (n) (step S103) . The interference detecting unit 205 of each of the parent terminals 1 includes a random number generating means. Depending on the magnitude relationship between the calculation result of the function f(n) and the generation result of random numbers, the interference detecting unit 205 of each of the parent terminals 1 determines whether the in-cell channel is changed in the subject terminal 1 (step S104). For example, if "a" denotes an integer, the function f(n) is expressed by f(n)=exp{-a(n-1}} . The priority n is set higher as the communication IDs of .the parent terminals 1 become smaller. If so, the in-cell channel changing probability grows higher as the communication IDs of the parent terminals 1 become smaller.
If the in-cell channels are simultaneously changed in

the communication cell to which a subject terminal belongs and the interfering cell, it is highly likely that re-interference is generated after the change of the in-cell channels. However, as stated above, the in-cell channel changing probability is set based on the priority of a subject terminal and the priority of the parent terminal 1 belonging to the interfering cells. It is therefore possible to reduce the likelihood of generation of re-interference after the change of the in-cell channels.
If it is determined that the in-cell channel has to be changed, the interference detecting unit 205 of each of the parent terminals 1 calculates a probability density function p(j) using a channel number j as a parameter, with respect to each of the unused channels Ch3 and Ch4 which are the candidates for the in-cell channel (step S105). For example, if "b" and "c" are integers and if "Rj" is the interference level of a wireless channel Chj, the probability density function p(j) is expressed by p{j)=b/(Rj+cj). As the channel number j becomes smaller, the probability density function p(j) grows higher. The integer b is set such that Ʃp(j) becomes equal to 1. On the other hand, if it is determined in step S104 that the in-cell channel is not changed, the present process comes to an end.
The interference detecting unit 205 of each of the .parent terminals 1 selects, as the changed in-cell channel,

the unused channel Ch3 whose probability density function p(j) is higher than that of the unused channel Ch4. The channel changeover processing unit 204 of the parent terminal 1-1 sets the unused channel Ch3 as a new in-cell channel of the communication cell C1 (step S106).
For example, if a new in-cell channel is randomly selected from the unused channels Ch3 and Ch4 as the candidates for the in-cell channel, the number of the in-cell channels used in the system as a whole is increased after the in-cell channel re-setting process in the system comes to an end. However, as stated above, the wireless channel having a smaller channel number j is preferentially selected a new in-cell channel. This makes it possible to operate the system with a reduced number of wireless channels.
It may also be possible to use a probability density function p(j, n)=b/(Rj+c | j-n | ) . In this case, it is highly probable that the unused channel having a channel number j is selected as a new in-cell channel for the parent terminal 1 of priority n. Accordingly, it is less probable that the same in-cell channel is selected with respect to a plurality of communication cells. This makes it possible to inhibit re-interference after the change of the in-cell channel.
It is assumed that the interference detecting unit 205 of the parent terminal 1-1 has a history in which the wireless channel of channel number jl used as the in-cell

channel in the past is changed to another wireless channel due to the interference with an interfering cell. In this case, if the wireless channel of channel number jl exists in the unused channels as the candidates for the in-cell channel, the probability of selection of the channel number jl may be reduced by changing the probability density function p(j) or p(j, n) in such a direction in which the probability density function of channel number jl becomes lower. In this case, it is possible to inhibit repetition of an operation by which the wireless channel having a past interference history is set as an in-cell channel and then the in-cell channel is returned to the original wireless channel.
If it is determined in step S101 that there is no unused channel, the interference detecting unit 205 of the parent terminal 1-1 determines whether the wireless channel having a smallest number of interfering cells is the current in-cell channel Ch1 (step S107). More specifically, the interference detecting unit 205 of the parent terminal 1-1 calculates the number of the interfering cells using the same wireless channel by referring to the channel information management table TB13 stored in the table storage unit 101. Then, the interference detecting unit 205 of the parent terminal 1-1 compares the number of interfering cells in the current in-cell channel Ch1 with the number of interfering cells in other wireless channels.

If the number of interfering cells is smallest in the current in-cell channel Ch1 among all the wireless channels, the present process is terminated and the communication cell C1 keeps the in-cell channel Ch1 unchanged. On the other hand, if the number of interfering cells is not smallest in the current in-cell channel Ch1, one or more other wireless channels having a smallest number of interfering cells are selected as the candidates for the in-cell channel (step S108) . Then, the processes of steps S103 to S106 are performed with respect to the in-cell channel candidates having a smallest number of interfering cells. One of the candidates is selected as the changed in-cell channel and is set as a new in-cell channel of the communication cell C1.
Unlike the present embodiment, the in-cell channel changing process may be performed is such a way that one wireless channel differing from the in-cell channel currently set in the parent terminal 1-1 is selected from the available wireless channels, thereby deciding a new communication channel.
In other words, one wireless channel is arbitrarily selected from the unused channels of the channel information management table TB13 and is used as a new in-cell channel. At this time, if no unused channel exists in the channel information management table TB13, the number of interfering cells using the available wireless channels is found on a channel-by-channel basis from the respective records stored

in the channel information management table TB13. The wireless channel having a smallest number of interfering cells is used as a new in-cell channel.
Next, the parent terminal 1-1 finds the changing time for changing the in-cell channel from the current time. By virtue of broadcast communication, the parent terminal 1-1 sends a channel change notifying packet including the changing time and the new in-cell channel to all the child terminals 2 belonging to the communication cell C1 to which the parent terminal 1-1 belongs. In this regard, the changing time may be, e.g., the time five minutes later than the current time, and may be set such that all the child terminals 2 belonging to the communication cell C1 to which the parent terminal 1-1 belongs can normally receive the channel change notifying packet.
The format of the channel change notifying packet is illustrated in Fig. 6C. The channel change notifying packet includes a source terminal ID section, a destination terminal ID section, a cell ID section, an operation code section, a changing time section, and a communication channel section.
The source terminal ID section of the channel change notifying packet holds the terminal ID of the communication terminal A which has sent the channel change notifying packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes

the destination of the channel change notifying packet. In the case of the channel change notifying packet, the code "BC" of broadcast communication such as broadcast or the like is held in the destination terminal ID section. The cell ID section holds the communication cell ID allotted to the communication cell to which the communication terminal A sending the channel change notifying packet belongs. In the present embodiment, each of the parent terminals 1 holds the terminal ID of the subject terminal 1. Each of the child terminals 2 holds the terminal ID of the parent terminal 1 of the communication cell to which the subject terminal 2 belongs. The operation code section holds the code of the channel change notifying packet. The changing time section holds the changing time at which the change of the communication channel is started. The communication channel section holds a value that enables identification of the communication channel changed at the changing time.
More specifically, as shown in Fig. 7, the parent terminal 1-1 uses channel Ch3 as a new communication channel and sets the time five minutes later than the current time as the communication channel changing time (step S9) . The parent terminal 1-1 sends the channel change notifying packet by virtue of broadcast communication (step S10).
The child terminal 2 of the communication cell C1 receiving the channel change notifying packet starts up the changing timer unit 206 thereof in order to change the

communication cell at the changing time stored in the changing time section of the channel change notifying packet (step S12). At this time, the child terminal 2-2 receives the channel change notifying packet from the parent terminal 1-1 via the child terminal 2-1 relaying the channel change notifying packet (step Sll) . The communication cell to which the subject terminal 2 belongs does not process the channel change notifying packet received from other communication cells.
Then, if the changing time arrives after the lapse of a specified time, the channel changeover processing unit 204 of each of the parent terminal 1-1 and the respective child terminals 2 receives a notification from the changing timer unit 206 and changes the communication channel to channel Ch3 (step S13) . In this manner, the communication channel used in the communication cell Cl including the parent terminal 1-1 and the respective child terminals 2 is changed to channel Ch3, a new communication channel.
Accordingly, the child terminals 2-2 and 2-5 and the child terminals 2-3 and 2-6 send and receive communication packets within the respective communication cells through the use of different communication channels. This makes it possible to reduce the interference of communication packets.
The communication channel changing process performed pursuant to the sum of interference levels is carried out

under the same conditions in the parent terminals 1-2 and 1-3. In other words, when other child terminals 2 for detecting other communication cells exist within the communication cells C2 and C3 to which the parent terminals 1-2 and 1-3 belong, each of the parent terminals 1 (1-2 and 1-3) performs a communication channel changing process if the sum of interference levels (the number of interfering communication cells) exceeds a specified threshold value. For that reason, if the parent terminals 1-1 and 1-2 jointly set the new communication channel to channel Ch2 when the sum of interference levels calculated by the interference detecting unit 205 of the parent terminal 1-2 exceeds the threshold value, it is sometimes the case that the interference remains unsolved despite the change of the communication channel.
In view of this, the channel changeover processing unit 204 of each of the parent terminals 1 may compare the communication cell ID of the communication cell to which the subject terminal 1 belongs, with the communication cell ID included in the received D packet. The change of the communication channel may be performed only when the communication cell ID of the communication cell to which the subject terminal 1 belongs is larger than the communication cell ID included in the received D packet. More specifically, it is only necessary to compare the numerals of the communication cell ID "Ml" of the communication cell

to which the parent terminal 1-1 belongs and the communication cell ID "M2" of the communication cell to which the parent terminal 1-2 belongs. In this case, only the parent terminal 1-2 performs the change of the communication channel.
In addition, various kinds of information such as the serial number, the IP address and the MAC address of the parent terminal 1 may be included in the communication packet. Determination as to whether to perform the communication channel changing process may be made by comparing these values. Comparison-purpose numerical values applied to each of the communication cells may be stored in the memory unit 10. The numerical values applied to the communication cells may be compared by referring to the communication cell ID of the communication packet and the memory unit 10.
In the present embodiment, the interference detecting unit 205 of each of the parent terminals 1 sets the interference level of the communication cell whose detection cell ID is stored in the detection information management table TB12, equal to "1" at all times. However, the value corresponding to the total number of the child terminal IDs stored in the detection information management table TB12 may be set as the interference level. More specifically, in the parent terminal 1-1, not only the child terminal 2-2 but also the child terminal 2-1 receives the H packet from the

child terminal 2-5 and sends the D packet to the parent terminal 1-1. Two records corresponding to the communication cell C2 exist in the detection information management table TB12. At this time, the interference level of the communication cell C2 calculated by the interference detecting unit 205 of the parent terminal 1-1 is equal to "2".
This makes it possible to accurately change the communication channel even if the number of the child terminals 2 sending and receiving the communication packet through the use of the same communication channel is large and even if errors are likely to occur in sending and receiving the wireless packet.
In the present system, the communication between the parent terminals 1 or the intensive management of the server becomes unnecessary. It is also possible to reduce the wireless packet interference between the communication cells. Since a wireless channel less interfered by the electric power leaked from the used channels can be selected from the unused channels, it is possible to reduce the wireless packet interference caused by the electric power leaked from other communication cells, thereby reducing the interference which is seen in the frequency axis.
(Second Embodiment)
The multi-hop communication system of the present embodiment has the same configurations as those of the first

embodiment but differs in the unused channel setting method from the first embodiment. The same configurations will be designated by like reference symbols and will not be described in detail.
Fig. 13 illustrates a derivation example of the unused channels in the parent terminal 1-1 in a case where there are wireless channels Ch1 to Chl0.
The used channels are Ch1, Ch6 and ChlO (see the horizontal line regions in Fig. 13). The used channels include not only the wireless channels (the in-cell channels used by the adjacent cells) stored in the channel information management table TB13 but also the in-cell channel Ch1 of the parent terminal 1-1.
Next, the interference detecting unit 205 of the parent terminal 1-1 derives the wireless channels (adjacent channels) adjoining the high frequency side and the low frequency side of the used channels Ch1, Ch6 and ChlO. In Fig. 13, the adjacent channels are Ch2, Ch5, Ch7 and Ch9 (see the dot regions in Fig. 13).
Next, the interference detecting unit 205 of the parent terminal 1-1 derives, as the unused channels, the wireless channels other than the used channels and the adjacent channels from the wireless channels Ch1 to ChlO. In Fig. 13, the unused channels are Ch3, Ch4 and Ch8 (see the white regions in Fig. 13).
Then, the interference detecting unit 205 of the

parent terminal 1-1 performs an in-cell channel changing process in the same manner as in the first embodiment through the use of the unused channels Ch3, Ch4 and Ch8.
By not setting the adjacent channels adjoining the used channels as new in-cell channels, it is possible to reduce the interference caused by the spurious of the adjacent channels or the like.
(Third Embodiment)
The multi-hop communication system of the present embodiment differs from the first embodiment in that, when finding the interference levels in step S102 of Fig. 8, weights are given to the interference levels depending on the number of the communication cells that make use of the respective used channels. The same configurations will be designated by like reference symbols and will not be described in detail.
In Fig. 11, the low-frequency-side isolation index and the high-frequency-side isolation index for each of the unused channels are found in the same manner as in the first embodiment. Then, the interference detecting unit 205 of the present embodiment calculates the number of the communication cells (the adjacent cells or the terminals of the communication cells) employing the used channels by referring to the channel information management table TB13. . In the present embodiment, the number of the communication cells employing the used channel Ch1 is equal to "2". The

number of the communication cells employing the used channel Ch6 is equal to "4". The number of the communication cells employing the used channel Ch7 is equal to "1". The number of the communication cells employing the used channel Chl0 is equal to "1".
When finding the interference level of each of the unused channels Ch2 to Ch5, Ch8 and Ch9, the interference detecting unit 205 multiplies the low-frequency-side isolation index by the number of the communication cells employing the low-frequency-side used channels. Moreover, the interference detecting unit 205 multiplies the high-frequency-side isolation index by the number of the communication cells employing the high-frequency-side used channels. Then, the sum of the high-frequency-side multiplication result and the low-frequency-side multiplication result is used as the interference level.
In Fig. 11, the interference level of the unused channel Ch2 is equal to 24 (=10x2+1x4). The interference level of the unused channel Ch3 is equal to 10 .(=3x2 + 1ᵡ4) . The interference level of the unused channel Ch4 is equal to 14 (=1x2+3x4). The interference level of the unused channel Ch5 is equal to 42 (=1x2 + 10x4) . The interference level of the unused channel Ch8 is equal to 13 ( = 10x1+3x1) . The interference level of the unused channel Ch9 is equal to 13 (=3x1+10x1).
In other words, the interference levels of the

respective unused channels Ch2, Ch3, Ch4, Ch5, Ch8 and Ch9 are "24", "10", "14", "42", "13" and "13". In this case, the unused channel Ch3 having a minimum interference level "10" becomes the candidate for the in-cell channel.
By using the unused channel Ch3 as the candidate for the in-cell channel, the interference detecting unit 205 of the parent terminal 1-1 performs an in-cell channel changing process in the same manner as in the first embodiment.
The traffic becomes larger and the time-dependent interference probability grows higher in a case where the wireless channels are commonly used by a plurality of communication cells. However, if the weights are given to the interference levels depending on the number of the communication cells employing the respective used channels, it is possible to reduce the interference in terms of the frequency base and the time base.
(Fourth Embodiment)
The communication terminal A of the multi-hop communication system of the present embodiment periodically sends the H packet including the channel information management table TB13 prepared by the parent terminal 1 of the communication cell to which the subject terminal A belongs, through the use of the in-cell channel of the communication cell to which the subject terminal A belongs. In the present embodiment, each of the child terminals 2 can send the H packet including the channel information

management table TB13 by periodically acquiring, from the parent terminal 1, the channel information management table TB13 prepared by the parent terminal 1 of the communication cell to which the subject terminal 2 belongs.
For example, the child terminal 2 belonging to the communication cell C1 (first communication cell) sequentially changes over all the wireless channels and receives the H packet from the adjacent cell (third communication cell) which is another communication cell existing around the communication cell C1. Then, the child terminal 2 belonging to the communication cell C1 derives the channel information management table TB13 from the received H packet and sends the packet including the channel information management table TB13 to the parent terminal 1-1 of the communication cell C1.
In this regard, the communication cell existing around the adjacent cell is called a quasi-adjacent cell (fourth communication cell). The channel information management table TB13 included in the H packet sent from the adjacent cell is the information that indicates the use situation of the in-cell channel in the quasi-adjacent cell.
Based on the channel information management table TB13, the interference detecting unit 205 of the parent terminal 1-1 can detect the in-cell channel used by the quasi-adjacent cell (the used channel from the viewpoint of the adjacent cell),

Then, the interference detecting unit 205 of the parent terminal 1-1 excludes the in-cell channel used by the quasi-adjacent cell from the unused channels. Using the unused channels, the interference detecting unit 205 of the parent terminal 1-1 performs an in-cell channel changing process in the same manner as in one of the first to third embodiments.
For example, if the in-cell channel changing process is performed in the communication cell C1, the interference level gets increased in the adjacent cell employing the changed in-cell channel of the communication cell C1. Thus, an in-cell channel changing process may be possibly performed in the adjacent cell. It is likely that a prolonged time may be required in stabilizing the overall system.
In the present embodiment, however, the wireless channels used by the quasi-adjacent cells as well as the wireless channels (used channels) used by the adjacent cells are excluded from the candidates for the in-cell channel. Accordingly, even if the change of the in-cell channel is performed in one communication cell, it is less likely that the in-cell channel is changed in the adjacent cell thereof. This makes it possible to shorten the processing time in the overall system.
By taking the adjacent cells and the quasi-adjacent cells into account, the interference detecting unit 205 of

the parent terminal 1-1 narrows down, in step S102 of Fig. 8, the candidates for the in-cell channel from the unused channels in the following manner.
Fig. 14 illustrates a derivation example of the unused channels in the parent terminal 1-1 in a case where there are wireless channels Ch1 to Chl0.
The used channels in the parent terminal 1-1 are Ch1, Ch6 and Ch7 (see the horizontal line regions in Fig. 14). The used channels are formed of the in-cell channel Ch1 of the parent terminal 1-1 and the in-cell channel of the adjacent cell of the parent terminal 1-1.
The wireless channels used by the quasi-adjacent cells are quasi-used channels Ch4 and ChlO (see the wave line regions in Fig. 14).
In this case, the unused channels Ch2, Ch3, Ch5, Ch8 and Ch9 exist in Fig. 14 (see the white regions in Fig. 14).
In step S102 of Fig. 8, the interference detecting unit 2 05 of the parent terminal 1-1 narrows down the candidates for the in-cell channel from the unused channels based on the isolation indices of the respective unused channels.
First, the interference detecting unit 205 finds the interference levels of the unused channels Ch2, Ch3, Ch5, Ch8 and Ch9. The interference levels are derived based on the frequency distance between the unused channels, the used channels and the quasi-used channels. More specifically, in

the case of the unused channel Ch2, the used channel Ch1 and the quasi-used channel Ch4 shown in Fig. 14, the low-frequency-side frequency distance X1 is equal to 1 and the high-frequency-side frequency distance X2 is equal to 2. In the case of the unused channel Ch3, the used channel Ch1 and the quasi-used channel Ch4, the low-frequency-side frequency distance X1 is equal to 2 and the high-frequency-side frequency distance X2 is equal to 1. In the case of the unused channel Ch5, the quasi-used channel Ch4 and the used channel Ch6, the low-frequency-side frequency distance X1 is equal to 1 and the high-frequency-side frequency distance X2 is equal to 1. In the case of the unused channel Ch8, the used channel Ch7 and the quasi-used channel Chl0, the low-frequency-side frequency distance XI is equal to 1 and the high-frequency-side frequency distance X2 is equal to 2. In the case of the unused channel Ch9, the used channel Ch7 and the quasi-used channel ChlO, the low-frequency-side frequency distance X1 is equal to 2 and the high-frequency-side frequency distance X2 is equal to 1.
As in the first embodiment, the frequency distances X1 and X2 are converted to isolation indices (see Fig. 12).
Next, the sum of the respective isolation indices corresponding to the frequency distances X1 and X2 is derived as an interference level. At this time, a weight is given to the interference level by taking the adjacent cell and the quasi-adjacent cell into account.

More specifically, the interference level is equal to α ᵡ low-frequency-side isolation index + β ᵡ high-frequency-side isolation index. If the low-frequency-side wireless channel is the used channel, α is set equal to 1. If the low-frequency-side wireless channel is the quasi-used channel, a is set equal to 0.3. Moreover, if the high-frequency-side wireless channel is the used channel, β is set equal to 1. If the high-frequency-side wireless channel is the quasi-used channel, β is set equal to 0.3. In other words, when finding the interference level with the communication cell to which a subject terminal belongs, a weight is given such that the interference level with the quasi-adjacent cell is lighter than the interference level with the adjacent cell. Accordingly, it is possible to derive an interference level by which the influence of the interference of the quasi-adjacent cell on the subject terminal is made smaller than the influence of the interference of the adjacent cell on the subject terminal.
In Fig. 14, the interference level of the unused channel Ch2 is equal to 10.9 (=10>ˣl+3x0.3) and the interference level of the unused channel Ch3 is equal to 6 (=3x1+10x0.3). Moreover, the interference level of the unused channel Ch5 is equal to 13 (=10x0.3+10x1). The interference level of the unused channel Ch8 is equal to 10.9 (=10x1+3x0.3). The interference level of the unused channel Ch9 is equal to 6 (=3x1+10x0.3).

In other words, the interference levels of the unused channels Ch2, Ch3, Ch5, ch8 and Ch9 shown in Fig. 11 are "10.9", "6", "13", "10.9" and "6". In this case, the unused channels Ch3 and Ch9 having a minimum interference level "6" become the candidates for the in-cell channel.
Using the unused channels Ch3 and Ch9, the interference detecting unit 205 of the parent terminal 1-1 performs an in-cell channel changing process in the same manner as in the first embodiment.
Just like the third embodiment, the low-frequency-side isolation index may be multiplied by the number of the communication cells employing the low-frequency-side used channels. The high-frequency-side isolation index may be multiplied by the number of the communication cells employing the high-frequency-side used channels. If the weights are given to the interference levels depending on the number of the communication cells employing the respective used channels, it is possible to reduce the interference in terms of the frequency axis and the time axis.
In the present embodiment, the interference detecting unit 205 of the parent terminal 1-1 performs the following operations if no unused channel exists in step S101 of Fig. 8.
First, the number of the adjacent cells employing the respective wireless channels is assumed to be N. The number

of the quasi-adjacent cells employing the respective wireless channels is assumed to be M. It is assumed that a weighting coefficient p is equal to or larger than 0 and equal to or smaller than 1. If there is no unused channel, the interference detecting unit 2 05 of the parent terminal 1-1 derives an evaluation function N+PM with respect to each of the wireless channels. From the viewpoint of the communication cell C1 to which the parent terminal 1-1 belongs, the interference with the quasi-adjacent cell is smaller than the interference with the adjacent cell. For that reason, the weight given to the interference with the quasi-adjacent cell may be light. One or more wireless channels remaining the smallest in the evaluation function are used as the candidates for the in-cell channel of the communication cell C1. If 3 is equal to 1, the same weight can be given to the interference with the adjacent cell and the interference with the quasi-adjacent cell.
As stated above, even if there is no unused channel when finding the interference with the communication cell to which a subject terminal belongs, the weight can be given such that the interference with the quasi-adjacent cell becomes lighter than the interference with the adjacent cell. Accordingly, if the change of the in-cell channel is performed in one communication cell when there is no unused channel, it is possible to reduce the possibility of the in-cell channel being changed in the adjacent cell, while

reducing the interference with the subject terminal.
(Fifth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols and will not be described in detail.
In place of the detection information management table TB12 of the first embodiment, the detection information management table TBI 4 shown in Fig. 15 is stored in the table storage unit 101 of the parent terminal 1. The detection information management table TB14 stores, in a tabular form, the detection information detected by the respective child terminals. More specifically, the detection information management table TB14 is provided with individual fields for the child terminal ID, the detection cell ID, the detection time, and the hop count (the child terminal hop count) from the child terminal 2 detecting another communication cell to the parent terminal 1.
In this regard, the hop count from the child terminal 2 to the parent terminal 1 is equal to the hop count stored in the communication route table TB21 of the parent terminal 1. In the present embodiment, reference is made to the child terminal hop count field of the detection information management table TB14. However, it may be possible to refer to the hop count field of the communication route table

TB21.
Next, the interference detecting unit 205 of the parent terminal 1 derives the interference level such that the interference level becomes larger as the child terminal hop count of the records of the detection information management table TBI4 grows smaller. For example, the interference levels of the respective communication cells are found by a numerical formula, [the interference level of each of the communication cells] = 1/(a x [the hop count to the child terminal 2 detecting the communication cell]) where a is a numerical value larger than 0. The interference detecting unit 2 05 of the parent terminal 1 compares the sum of the interference levels of the respective communication cells with a threshold value.
Thus, the change of the communication channel is easy to perform if the child terminal 2 positioned near the parent terminal 1 interferes with another communication cell and if the degree of interference between the communication cell, to which a subject terminal belongs, and another communication cell becomes larger. Accordingly, if there is a need to change the communication channel, it is possible to accurately perform the change of the communication channel and to reduce the interference of the wireless packets.
(Sixth Embodiment)
The configurations of the multi-hop communication

system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols and will not be described in detail.
In the present embodiment, upon receiving the H packet sent from another communication cell, each of the child terminals 2 sends a detection information region including the kind of the communication terminal A sending the H packet and the hop count to the parent terminal 1 of the communication cell to which the communication terminal A sending the H packet belongs.
Fig. 16 illustrates the details of the detection information region of the D packet. The detection information region of the D packet includes a detection number section, a detected cell ID section, a detected terminal kind section, and a detected hop count section.
The detection number section holds the number of the detected cell ID section, the detected terminal kind section, and the detected hop count section, all of which come after the detection number section. The detected cell ID section holds the communication cell IDs held in the H packets sent from other communication cells. The detected terminal kind section holds the kinds of the communication terminals A belonging to other communication cell which has sent the H packets. The value indicating the parent terminal 1 or the child terminal 2 is held in the detected

terminal kind section. If the communication terminal A belonging to another communication cell which has sent the H packet is the child terminal 2, the detected hop count section holds the hop count between the parent terminal 1, to which the communication terminal A belongs, and the communication terminal A. If the communication terminal A is the parent terminal 1, the detected hop count section holds "null". The detected terminal kind section may not be provided. In this case, the communication terminal A can be regarded as the child terminal 2 if the value of the detected hop count section is equal to or larger than 1. The communication terminal A can be regarded as the parent terminal 1 if the value of the detected hop count section is equal to 0.
Instead of the detection information management table TB12 of the first embodiment, the detection information management table TB15 shown in Fig. 17 is stored in the table storage unit 101 of the parent terminal 1. The detection information management table TB15 stores, in a tabular form, the detection information detected by the respective child terminals. More specifically, the detection information management table TB15 is provided with individual fields for the child terminal ID, the detected cell ID, the detection time, the terminal kind (detected terminal kind) of the detected communication terminals A, and the hop count (detected hop count) to the parent

terminal 1 of the communication cell to which the detected child terminal 2 belongs.
Next, the interference detecting unit 205 of the parent terminal 1 finds an interference level on a record-by-record basis by referring to the respective records of the detection information management table TB15. In this regard, the interference level is derived such that if the detected terminal kind is the parent terminal, the interference level becomes larger and such that, if the detected terminal kind is the child terminal, the interference level becomes larger as the detected hop count of the records grows smaller.
For example, the interference levels can be found by a formula, [interference level] = 1/α x (([detected hop count]) + 1). In a case where the detected terminal kind is the parent terminal, the interference levels are found by using "0" as the detected hop count.
Then, the interference detecting unit 205 sets the largest value of the interference levels as the interference level of each of the communication cells with respect to each of the detected cell IDs included in the detection information management table TB15. If the sum of the interference levels is larger than the threshold value, a communication channel changing process is started.
Thus, the change of the communication channel is easy to perform if the communication terminal A interfering with

the communication cell to which a subject terminal belongs is the parent terminal 1 or is the child terminal 2 positioned near the parent terminal 1 and if the degree of interference becomes larger. Accordingly, if there is a need to change the communication channel, it is possible to accurately perform the change of the communication channel and to reduce the interference of the wireless packets.
While certain preferred embodiments of the present invention have been described above, the present invention is not limited to these specific embodiments but may be changed or modified in many different forms without departing from the scope of the invention defined in the appended claims. Such changes or modifications shall be construed to fall within the scope of the invention.

1. A multi-hop communication system, comprising:
a plurality of communication cells in each of which one parent terminal makes multi-hop communication with one or more child terminals by using one wireless channel selected from a plurality of wireless channels as an in-cell channel, wherein each of the child terminals detects the in-cell channel of each communication cell adjacent to the communication cell to which the corresponding child terminal belongs, and notifies the in-cell channel to the parent terminal of the communication cell to which the corresponding child terminal belongs, and
if a degree of interference, when the parent terminal communicates with another communication cell using the same in-cell channel as the communication cell to which the corresponding parent terminal belongs, is larger than a specified threshold value, the parent terminal selects, among one or more wireless channels not used as the in-cell channel in the communication cell to which the corresponding parent terminal belongs and in the adjacent communication cells, the wireless channel having the smallest frequency-distance-based interference level between the in-cell channel of the communication cell to which the corresponding parent terminal belongs and the in-cell channels of the adjacent communication cells to which the adjacent cells

belong, and the parent terminal is configured to set the selected wireless channel as the in-cell channel of the communication cell to which the corresponding parent terminal belongs.
2. A multi-hop communication system, comprising:
a plurality of communication cells in each of which one parent terminal makes multi-hop communication with one or more child terminals by using one wireless channel selected from a plurality of wireless channels as an in-cell channel, wherein the parent terminal and each of the child terminals are configured to send, through the in-cell channel, a hello packet including communication cell information for identifying a communication cell to which the corresponding parent terminal and the corresponding child terminal belong and notifying the existence of the corresponding parent terminal and the corresponding child terminal,
if a communication cell indicated by the communication cell information of the hello packet received through the in-cell channel differs from a first communication cell to which each of the child terminals belongs, each of the child terminals is configured to send a detection information, that includes the communication cell information of the received hello packet and notifies a detection of a second communication cell employing the same in-cell channel, to

the parent terminal belonging to the first communication cell;
if each of the child terminals receives the hello packet by sequentially changing over the wireless channels, each of the child terminals sends channel use information, that includes the communication cell information of the received hello packet and channel information on a wireless channel receiving the hello packet and notifies the in-cell channel employed by a third communication cell existing around the first communication cell, to the parent terminal belonging to the first communication cell;
the parent terminal belonging to the first communication cell is configured to derive a degree of interference in the communication of the first communication cell and the second communication cell based on the received detection information; and
if the degree of interference is larger than a specified threshold value, the parent terminal belonging to the first communication cell is configured to select, among unused channels as wireless channels not used as the in-cell channels by the first and third communication cells, an unused channel remaining the smallest in a frequency-distance-based interference level between the unused channel and the in-cell channel of the first and third communication cells as a used channel based on the channel use information,

the parent terminal belonging to the first communication cell is configured to set the selected unused channel as the in-cell channel of the first communication cell.
3. The system of claim 2, wherein the parent terminal is configured to, based on the channel use information, give a weight to the interference level depending on the number of the communication cells employing the used channel.
4. The system of claim 2 or 3, wherein the parent terminal is configured to exclude wireless channels adjacent to the used channels from the unused channels.
5. The system of any one of claims 2 to 4, wherein the parent terminal and each of the child terminals are configured to send, through the in-cell channel, the hello packet including the channel use information prepared by the communication cell to which the parent corresponding terminal and the corresponding child terminal belong;
each of the child terminals belonging to the first communication cell is configured, to send the channel use information included in the hello packet received from the third communication cell to the parent terminal belonging to the first communication cell while sequentially changing over the wireless channels; and
the parent terminal belonging to the first

communication cell is configured to, based on the channel use information included in the hello packet, detect the in-cell channel employed by a fourth communication cell existing around the third communication cell and to exclude the in-cell channel employed by the fourth communication cell from the unused channels.
6. The system of claim 5, wherein, if there is no unused channel, the parent terminal belonging to the first communication cell is configured to derive an evaluation function N+βM with respect to each of the wireless channels, where N is the number of the third communication cells employing the wireless channels, M is the number of the fourth communication cells employing the wireless channels, and (3 is a weighting coefficient of from 0 to 1, and set the wireless channel having the smallest evaluation function as the in-cell channel.
7. The system of claim 5 or 6, wherein the parent terminal belonging to the first communication cell is configured to employ, as the used channel, the in-cell channels of the first, third, and fourth communication cells,
the interference level of the in-cell channel of the fourth communication cell with respect to the unused channels being given a weight lighter than a weight given to the interference level of the in-cell channel of the third

communication cell with respect to the unused channels.
8. The system of any one of claims 2 to 7, wherein each of the parent terminals is given a priority and is configured to, based on the priority, determine whether to set the unused channel as the in-cell channel if the degree of interference is larger than the specified threshold value.
9. The system of any one of claims 1 to 8, wherein each of the wireless channels is given a probability according to a specified probability density function, and
the parent terminal is configured to, based on the probability of the unused channel, determine whether to set the unused channel as the in-cell channel.
10. The system of claim 9, wherein the parent terminal is configured to change the probability of the unused channel, in which the degree of interference in the communication of the first communication cell and the second communication cell has previously exceeded a specified threshold value, so that the unused channel is not set as the in-cell channel.
11. A multi-hop communication system, comprising:
a plurality of communication cells in each of which one parent terminal and child terminals make multi-hop communication with each other through a same wireless

channel,
wherein each of the child terminals is configured to detect another communication cells employing the wireless channel used for communication by the corresponding child terminal and to notify said another communication cells to the parent terminal, and
the parent terminal is configured to derive, on a cell-by-cell basis, a degree of interference in the communication between said another communication cells and the communication cell to which the parent terminal belongs, and to make communication through another wireless channel if the sum of the degrees of interference is larger than a specified threshold value.
12. A multi-hop communication system, comprising:
a plurality of communication cells in each of which one parent terminal and child terminals make multi-hop communication with each other through a same wireless channel,
wherein each of the terminals is configured to set one wireless channel selected from a plurality of wireless channels differing in frequency from one another as a communication channel used for communication and to send a hello packet including communication cell information for identification of the communication cell to which the corresponding terminal belongs and notifying the existence

of the corresponding terminal;
each of the child terminal is configured such that, if the communication cell indicated by the communication cell information of the hello packet received through the communication channel differs from the communication cell to which the corresponding child terminal belongs, the corresponding child terminal sends detection information including the communication cell information of the received hello packet and notifying the detection of another communication cell employing the communication channel, to the parent terminal belonging to the communication cell to which the corresponding child terminal belongs; and
the parent terminal is configured to derive, based on the received detection information, interference levels with respect to the respective communication cells indicated by the communication cell information of the detection information, and if the sum of the interference levels is larger than a specified threshold value, the parent terminal is configured to select one wireless channel other than the communication channel from the plurality of wireless ■ channels and to set the selected wireless channel as a new communication channel used in the communication cell to which the parent terminal belongs.
13. The system of claim 12, wherein each of the communication cells has individually given identification

information, and the parent terminal is configured to set the new communication channel only if the identification information of the communication cell indicated by the communication cell information of the received detection information has a specified relationship with the identification information of the communication cell to which the parent terminal belongs.
14. The system of claim 12 or 13, wherein each of the child terminals is configured to receive the hello packet by sequentially changing over the wireless channels, and if the communication cell indicated by the communication cell information of the received hello packet differs from the communication cell to which the corresponding child terminal belongs, the corresponding child terminal sends channel use information including the communication cell information of the received hello packet and channel information indicating the wireless channel which has received the hello packet, to the parent terminal; and
the parent terminal is configured to, based on the communication cell information and the channel information included in the channel use information, find the number of the communication cells employing the wireless channels with respect to the respective wireless channels, and to select a new communication channel by preferentially employing the wireless channel which is used by the least number of

communication cells.
15. The system of any one of claims 12 to 14, wherein each of the child terminals is configured to send the detection information including a hop count in a communication route leading from the corresponding child terminal to the parent terminal of the communication cell to which the corresponding child terminal belongs, and the parent terminal is configured to derive the interference levels such that the interference levels become higher as the hop count included in the detection information grows smaller.
16. The system of any one of claims 12 to 15, wherein the parent terminal is configured to set the interference levels higher as the number of the child terminals belonging to the communication cell to which the parent terminal sending the detection information belongs grows larger.
17. The system of any one of claims 12 to 16, wherein each of the terminals is configured to send the hello packet that includes terminal information indicating whether the corresponding terminal is the parent terminal or the child terminal and indicating, if the corresponding terminal is the child terminals, a hop count in a communication route leading from the corresponding terminal to the parent terminal of the communication cell to which the