Field of the Invention
The present invention relates to a wireless
communications system that performs wireless communications
by using a plurality of wireless channels.
Background of the Invention
Recently, there is known a wireless communications
system which exchanges control signals to initiate, change
or terminate functions of devices installed at a remote
place or exchanges measurement results from measuring
instruments installed at a remote place, by using a wireless
communications station such as a low-power wireless
communications station. Such a wireless communications
system is applied to various types of systems such as
illumination control systems, security systems, door phone
systems, fire information systems, and automatic remote
meter-reading systems. For example, there is disclosed a
fire information system in Patent document 1.
In the fire information system disclosed in Patent
document 1, a plurality of fire detectors (wireless units)
is installed at multiple portions and each of the fire
detectors has a wireless communications function. In the
fire information system, when any one of the fire detectors
detects a fire, the fire detector sounds the alarm while
continuously transmitting a wireless signal including fire
detection information to other fire detectors. In the
meantime, each of the other fire detectors receiving the
wireless signal from the fire detector activates a fire
alarm based on the fire detection information included in
the wireless signal.
Herein, the wireless signal transmitted or received by
the fire detectors includes a synchronous bit (preamble), a
frame synchronization (sync) pattern (unique word), a
destination address, a source address, a message, and an
error detection code. The preamble is a bit pattern for bit
synchronization of about 8 bits length in which 0 and 1 are
alternately arranged, for example. Further, the unique word
is a bit stream having a bit pattern differing from that of
the preamble, and uses a bit pattern that is hard to be
included in a pattern for a message or the like.
The detector detects the preamble and the unique word
included in the received wireless signal and determines that
the received wireless signal is a wireless signal used in
the system to which it belongs. Then, the detector can
obtain information by receiving the destination address or
the message following the unique word. Further, the error
detection code is formed of, e.g., a Cyclic Redundancy Check
(CRC) or the like, and is used in checking the integrity of
the received wireless signal and detecting a cause why the
bit inversion or the like has occurred.
In the above-described wireless communications system,
there is provided a plurality of wireless channels for
wireless signals within a specific frequency band and the
wireless signals are transmitted between multiple wireless
communications stations (transmitters and receiver). In
this case, the transmitter alternatively selects one among
the wireless channels and transmits a wireless signal
through the wireless channel. The receiver scans all
wireless channels at a predetermined time period interval,
for example, in a time division manner.
Further, the receiver measures signal strength of each
wireless channel based on a Received Signal Strength
Indication (RSSI) signal and, when the measured signal
strength is greater than a predetermined threshold, detects
a preamble and receives the wireless signal. In this way,
the receiver can receive the wireless signal through the
wireless channel arbitrarily selected by each transmitter,
and the wireless signal can be transferred between multiple
wireless communications stations.
[Patent document 1] Japanese Patent Application
Publication No. 2009-177340
However, when the transmitter selects a wireless
channel to transmit a wireless signal, a power of the
wireless signal may be leaked into wireless channels other
than the wireless channel selected to transmit the wireless
signal. Further, the receiver and the transmitter may be
installed close to each other, and there may be the case
where the signal strengths of other wireless channels exceed
a predetermined threshold. In this case, there is a
possibility that the receiver erroneously selects as a
reception target another wrong wireless channel and starts
reception of a wireless signal therefrom. For that reason,
it is concerned that the receiver may miss the wireless
signal transmitted from the transmitter via the correct
wireless channel.
On the other hand, in such a wireless communications
system, a wireless reception station may be shared by a
plurality of electronic devices. For example, a single
receiver can receive a detection signal from a human body
detection sensor of a door phone device, and receive a
dimming signal for manipulating a dimming level from an
illumination control device to operate a dimming level of a
lighting equipment. Further, a plurality of transmitters
can be connected to each of the door phone system and the
illumination control system.
In the conventional wireless communications system,
the bit length of the preamble is set to a minimum number of
bits required to perform the bit synchronization (8 bits in
the above-described fire information system). For that
reason, as the number of transmitters increases, the
probability becomes higher that the receiver scans another
wrong wireless channel when a certain transmitter transmits
a wireless signal. Accordingly, it is highly possible for
the receiver to miss the preamble of the target wireless
signal.
In this case, the transmitter transmits the same
wireless signal again. However, if a length of a message to
be transmitted is long, the time needed for the wireless
signal to be retransmitted after the preamble has been
missed is very long. As a result, there arises a problem
that the time taken in the transmission and reception of the
wireless signal becomes prolonged.
Summary of the Invention
In view of the above, the present invention provides a
wireless communications system in which a receiver can
reliably receive a wireless signal when the receiver is
installed close to a transmitter.
Further, the present invention provides a wireless
communications system which a preamble of a wireless signal
transmitted by a transmitter can be reliably received and
the transmission and reception of the wireless signal can be
performed in a short period of time.
In accordance with a first aspect of the present
invention, there is provided a wireless communications
system including: a transmitter which selects one wireless
channel among a plurality of wireless channels each having a
different frequency from each other and transmitting a
wireless signal through the selected wireless channel; and a
receiver which receives the wireless signal transmitted from
the transmitter, wherein the receiver includes: a signal
strength detection unit which scans each of target wireless
channels for a predetermined scan period by sequentially
switching the target wireless channels and measures a signal
strength thereof, a storage unit for storing the signal
strengths of the target wireless channels measured by the
signal strength detection unit; and a signal reception unit
which preferentially receives a wireless signal on the
wireless channel having a higher priority determined based
on the order of the signal strength stored in the storage
unit, among the target wireless channels.
With such configuration, the wireless signal is
detected on the wireless channel with the greatest signal
strength. Accordingly, when the transmitter which has
transmitted the wireless signal is positioned close to the
receiver, the receiver can reliably receive the wireless
signal through the wireless channel of the transmitter
positioned close to itself other than the wireless channel
into which the power of the wireless signal is leaked. As a
result, it is possible to reduce the probability that the
receiver misses the wireless signal from the transmitter
even if the transmitter and the receiver are positioned
close to each other.
In the above system, the wireless signal may include a
destination address indicative of a receiver of destination,
and the receiver may sequentially switch the target wireless
channels and receives the wireless signal on each of the
wireless channels in the order of the signal strength until
the destination address of the received wireless signal
matches its own address.
In accordance with a second aspect of the present
invention, there is provided a wireless communications
system including: one or more transmitters each of which
selects one wireless channel among a plurality of wireless
channels each having a different frequency from each other
and transmits a wireless signal through the selected
wireless channel; and a receiver which receives the wireless
signal transmitted from the transmitter, wherein the
wireless signal includes a preamble in which a predetermined
bit sync pattern is repeated, and data having an address for
identifying a transmitter and a transmission information,
the receiver includes a synchronous bit detection unit which
scans by sequentially switching the wireless channels in a
predetermined scan period in a time division manner, and
detects the bit sync pattern from each wireless channel, and
a data reception unit which receives the data following the
preamble when the bit sync pattern is detected, the preamble
has at least a total bit length of a bit length
corresponding to one scan period and a bit length required
to detect the bit sync pattern.
With such configuration, it is possible to increase
the probability that the receiver detects the bit sync
pattern of the wireless signal transmitted by the
transmitter. As a result/ the probability of missing the
preamble of the wireless signal transmitted by the
transmitter is decreased, and the wireless signal can be
transmitted and received in a short period of time.
The receiver may include a channel setting unit for
setting the target wireless channels, and the transmitter
may change a length of the preamble in accordance with the
number of wireless channels set by the channel setting unit.
Thus, since the length of the preamble of the
transmitter can be increased or decreased in accordance with
the number of scans of the wireless channels set by the
receiver, a more suitable preamble length can be selected.
As a result, the wireless signal can be transmitted and
received in a short period of time while decreasing the
probability of missing the preamble of the wireless signal
transmitted by the transmitter.
Further, the receiver may include a priority setting
unit for setting priorities for the target wireless
channels, and the synchronous bit detection unit may set the
number of scans of the respective wireless channels during
each scan period based on the priorities set for the
respective wireless channels.
By doing so, for, e.g., a single scan period, a
wireless signal on the wireless channel having higher
priority is detected a plurality of times. Therefore, the
probability of detecting a wireless signal from a
transmitter having the higher priority is further increased.
Furthermore, the receiver may include a priority
setting unit for setting priorities for the target wireless
channels, and the synchronous bit detection unit may set the
time for which the respective wireless channels are scanned
during each scan period based on the priorities set for
respective wireless channels.
With such configuration, for, e.g., a single scan
period, a period time for which a wireless signal on a
wireless channel having higher priority is detected is
extended. As a result, the probability of detecting a
wireless signal from the transmitter having higher priority
is further increased.
The wireless signal may include a frame sync pattern
having a bit pattern differing from that of the bit sync
patterns which follows the bit sync pattern, the receiver
further may include a signal strength detection unit for
detecting signal strengths of the target wireless channels
and a frame detection unit for detecting the frame sync
pattern, and the wireless channels may be scanned again when
the frame sync pattern is not detected until a predetermined
period of time is elapsed after detecting a signal strength
of a predetermined strength or more by the signal strength
detection unit.
In this case, when a frame sync pattern is not
detected, detection of the wireless signal is performed
again. Accordingly, it is possible to prevent reception of
a normal wireless signal from being interrupted by an
abnormal wireless signal.
Further, the receiver may include a signal strength
detection unit for detecting signal strengths of the target
wireless channels, and, if a signal strength of the
receiving wireless channel, detected by the signal strength
detection unit while the data reception unit is receiving
the data, is less than a predetermined strength, scan of the
target wireless channels may be restarted while reception of
the wireless signal is stopped.
With this configuration, when the signal strength is
weakened during the reception of the wireless signal, the
reception is stopped and the detection of a wireless signal
is performed again. Accordingly, there can be avoided the
case in which a normal wireless signal is not received due
to another abnormal wireless signal.
Brief Description of the Drawings
The objects and features of the present invention will
be apparent from the following description of preferred
embodiments which is given in conjunction with the
accompanying drawings, as follows:
Fig. 1 is a schematic block diagram showing a
wireless communications system in accordance with a first
embodiment of the present invention;
Fig. 2A is a schematic diagram showing a format of a
wireless signal in the wireless communications system of the
first embodiment, Fig. 2B is a schematic diagram for
explaining a time division method by which a wireless signal
is scanned, in the wireless communications system of the
first embodiment, and Fig. 2C is a graph showing an example
of measurement results of wireless signal strengths, in the
wireless communications system of the first embodiment;
Fig. 3 is a schematic block diagram illustrating a
configuration of a wireless communications system in
accordance with a second embodiment;
Fig. 4A is a schematic diagram depicting a format of a
wireless signal in the wireless communications system of the
second embodiment, and 4B and 4C are schematic diagrams
showing the time for which the wireless signals are scanned,
in the wireless communications system of the second
embodiment; and
Fig. 5 is a table showing settings set by a receiver
in the wireless communications system of the second
embodiment.
Detailed Description of the Preferred Embodiments
Embodiments of the present invention will be described
in detail below with reference to the accompanying drawings
which form a part hereof. The same reference numerals will
be given to the same or similar components throughout the
drawings, and redundant descriptions thereof will be
omitted.

Hereinafter, a first embodiment in accordance with the
present invention will be described with reference to Figs.
1 and 2.
As shown in Fig. 1, a wireless communications system
of the present embodiment includes a single receiver 10 and
a plurality of transmitters 20. Further, in the present
embodiment, the transmitters 20 including seven transmitters
20a to 20d and 20k to 20z are described, but the present
invention is not limited thereto.
The transmitters 2 0a, ¦••, 2 0d together with the
receiver 10 may be included in a door phone system. Each of
the transmitters 20a, ¦•*, 2 0d includes a door phone outdoor
unit which transmits a call signal in response to
manipulation of a user, e.g., a human body detection sensor
implemented as a camera or the like to transmit a human body
detection signal, or a locking device attached to a door at
a porch or the like and configured to transmit a key
lock/unlock s ignal.
Further, the transmitters 2 Ox, ..., 20z together with
the receiver 10 may be included in an illumination control
system. In this case, each of the transmitters 20x, ..., 20z
may be a remote controller for transmitting a dimming signal
in accordance with manipulation of a user, a lighting
equipment for outputting a lighting signal indicative of a
lighting state of a light source, or an illuminance sensor
attached to a ceiling surface or the like and configured to
output an illuminance detection signal indicative of
brightness in a room.
Each transmitter 2 0 is provided with a transmission
unit 21 serving as, e.g., a low-power wireless station. The
transmitter 20 is configured to transmit to the receiver 10
a wireless signal including information about its own type
and the type of system to which it belongs. The
transmission unit 21 selects a wireless channel among a
plurality of wireless channels provided within a specific
frequency band, and then transmits the wireless signal to
the receiver 10 through the selected wireless channel.
Further, in the present embodiment, the wireless channels
used by the transmitters 20a, ..., 20d and 20x, ..., 20z,
include wireless channels ch1, ch2, ..., ch7.
The receiver 10 serves as a main control part common
to both the door phone system and the illumination control
system, and includes a control unit 11 such as a
microcomputer or the like, a reception unit 12 such as,
e.g., a low-power wireless station, and a storage unit 13
such as non-volatile semiconductor memory.
The reception unit 12 is operated under the control of
the control unit 11. Specifically, when a wireless channel
to be scanned is designated by the control unit 11, the
reception unit 12 measures the signal strength of the
designated wireless channel by using a signal strength
detection unit 12a based on, e.g., RSSI, and outputs the
measurement result to the control unit 11. Further, the
reception unit 12 receives a wireless signal flowing through
the designated wireless channel when the wireless channel is
designated by the control unit 11, and outputs various types
of control information or the like included in the received
wireless signal to the control unit 11.
The information is stored in the storage unit 13 by
the control unit 11. In the storage unit, signal strengths
measured by the signal strength detection unit 12a of the
reception unit 12 are stored in association with respective
wireless channels.
The control unit 11 activates the reception unit 12 at
every scan period interval using a built-in timer (not
shown) to measure signal strengths of target wireless
channels. Further, the control unit 11 controls the
reception unit 12 to receive a wireless signal transmitted
from one of the transmitters 20 based on the signal strength
of each wireless channel. In the present embodiment, the
signal strength is measured, e.g., for 2ms per a wireless
channel, and the time corresponding to one scan period
interval required to measure the signal strengths of seven
wireless channels Chi to Ch7 is set to 14 ms (See Fig. 2B)-
The wireless signal transmitted from the transmitter
20 to the receiver 10 has a format shown in Fig. 2A, and
includes a preamble PA, a unique word UW, a destination
address DA, a source address SA, a message MG, and an error
detection code CR. The preamble PA is a bit stream formed
of, e.g., a bit synchronization (sync) pattern in which 0
and 1 are alternately repeated, and in which, for example,
the first bit is set to 0 and the last bit is set to 1, or
vice versa. The receiver can obtain frequency pulling-in
range while receiving the bit stream.
The unique word UW includes bits differing from those
of the preamble PA, and is set to have a length of, e.g., 32
bits in the present embodiment. The destination address DA
is a bit stream required to identify the receiver 10 as a
destination for receiving the wireless signal. The source
address SA is a bit stream required to uniquely identify
each transmitter 20 which transmits the wireless signal.
Further, the message MG is a bit stream including
transmission information that is transmitted by the
transmitter 20, and may have a variable bit length depending
on contents of the transmission information. The error
detection code CR is a bit stream such as, e.g. , a Cyclic
Redundancy Check (CRC), and is set such that an abnormality
such as bit inversion occurring at least in the bit stream
of the message MG can be detected upon transmission and
reception.
Hereinafter, operation of the receiver 10 receiving a
wireless signal is described. First, the control unit 11 of
the receiver 10 activates the reception unit 12 at every
scan period interval, and measures signal strengths of all
wireless channels to be scanned. The results of
measurements are sequentially stored in the storage unit 13
by the control unit 11. The signal strengths of the
respective wireless channels are given as shown in, e.g.,
Fig. 2C.
In the following description, it is assumed that the
signal strength reaches a maximum at Ch4 and gradually
decreases in the order of Ch2, Chi, Ch5, Ch3, Ch6, and Ch7.
Further, the signal strengths at the channels Chi to Ch5 are
greater than a threshold th, which is determined to be a
valid signal strength for the reception unit 12, and those
at the channels Ch6 and Ch7 are smaller than the threshold
th.
Next, the control unit 11 sets as a target wireless
channel of the reception unit 12 the wireless channel (Ch4
in this case) having the strongest signal strength, and
controls the reception unit 12 to start receiving the
wireless signal. The reception unit 12 fixes the target
wireless channel, and detects bit patterns corresponding to
the preamble PA, the unique word UW, the destination address
DA, the source address SA, the message MG, and the error
detection code CR.
Further, if the respective bit patterns are correctly
detected, there is no abnormality in the signal information
based on the error detection code, and the address of the
reception unit 12 itself is included in the destination
address DA, the reception unit 12 determines the wireless
signal to be a valid wireless signal, and outputs
information stored in the message MG to the control unit 11.
Accordingly, the wireless signal flowing through the
wireless channel having the strongest signal strength can be
correctly received by the receiver 10.
On the other hand, when the respective bit patterns
are not normally detected from the target wireless channel,
or when the address of the reception unit 12 itself is not
included in the destination address DA, the control unit 11
stops the reception unit 12 from receiving a wireless signal
flowing through the wireless channel. Then, the control
unit 11 sets a wireless channel (Ch2 in this case) having
the second-highest signal strength among the signal
strengths of respective wireless channels stored in the
storage unit 13, as a target wireless channel, and controls
the reception unit 12 to start receiving a wireless signal
through the wireless channel.
The control unit 11 repeats the above-described
receiving operation on wireless channels having the third
signal strength (CHI) , the fourth signal strength (CH 5) , ...,
and the like, in the order of signal strength until the
reception unit 12 receives a valid wireless signal and
obtains the message MG included in the received wireless
signal. However, when the signal strength of the target
wireless channel is less than the threshold th, the control
unit 11 starts measurement of signal strengths again at
every predetermined scan period interval without performing
reception through subsequent wireless channels.
That is, in the case of the signal strengths shown in
Fig. 2C, the reception of wireless signals can be attempted
in the order of Ch4, Ch2, Ch1, Ch5, and Ch3 until a valid
wireless signal is received. Further, when a valid wireless
signal has not been received even in the reception of
wireless signals through the wireless channel Ch3, the
measurement of signal strengths is repeated at every scan
period interval.
In this way, since the reception of the wireless
signal is sequentially performed through all wireless
channels in the order of signal strength, it is possible to
reduce the occurrence of the case where the reception starts
through another wrong wireless channel having the signal
strength which is greater than the threshold th due to the
leakage of power, and a wireless signal is missed on the
target wireless channel through which the transmitter
transmitted the wireless signal.
Further, reception of a wireless signal is performed
sequentially from wireless channels in the order of signal
strength until the wireless signal is. received. If the
corresponding reception unit is not included in the
destination of a wireless signal having a higher signal
strength, the reception unit can receive wireless signals
flowing through other channels. Accordingly, even when
wireless communications systems are arranged close to each
other, the occurrence of the case where the reception unit
misses wireless signals which include the reception unit as
a destination, through the transmitted and received wireless
signal, can be reduced, thereby improving communication
efficiency.
Further, when the reception unit 12 cannot normally
detect each of the bit patterns, or when the reception unit
12 is not included in the destination address DA, the signal
strength may be measured again and may set a wireless
channel having the next-highest signal strength to a
reception target. By doing this, it is possible to always
receive a wireless signal based on the latest signal
strengths, and reduce the probability that a newly
transmitted wireless signal is missed.

Hereinafter, a second embodiment of the present
invention will be described with reference to Figs. 3 to 5.
As shown in Fig. 3, in a wireless communications
system of the present embodiment, a receiver 10 further
includes a manipulation input unit 14 such as a liquid
crystal panel, a switch, or the like. Below, a description
will be made while focusing on a configuration different
from that of the first embodiment.
A reception unit 12 is operated under the control of a
control unit 11, and a signal strength detection unit 12a
measures a signal strength of a wireless channel based on,
e.g., RSSI, when a wireless channel to be scanned is
designated. When the measured signal strength is greater
than a predetermined threshold, the reception unit 12
receives a wireless signal flowing through the wireless
channel and outputs the wireless signal to the control unit
11. When the signal strength is less than the predetermined
threshold, the reception unit 12 does not receive the
wireless signal and waits for a new designation of a
wireless channel by the control unit 11.
A storage unit 13 stores information recorded by the
control unit 11, so that in the present embodiment, pieces
of information such as the priorities, identification
addresses and wireless channel numbers of respective
transmitters 20 are stored. In the present embodiment, the
manipulation input unit 14 to be described later includes a
channel setting unit 14a and a priority setting unit 14b.
Further, setting information as shown in, e.g., Fig. 5, is
stored in accordance with a manipulation input from the
manipulation input unit 14. For example, for a transmitter
20a, an identification address of 001, a wireless channel of
chl, and a priority of r2j are set, and an identification
address, a wireless channel, and a priority is set in the
same manner for transmitters 20b, ..., 2 0d, 20x, ..., 20z. As
for the priorities, it is assumed that the priority is
higher as the number is smaller, that is, rlj is the
highest priority.
The manipulation input unit 14 outputs a manipulation
input signal to the control unit 11 in response to the
manipulation input by a user. The user manipulates a switch
or the like of the manipulation input unit 14 based on the
character or the figure displayed on the liquid crystal
panel of the manipulation input unit 14, and may modify
settings by, e.g., adding, changing or deleting the
identification addresses, priorities, and wireless channel
numbers of respective transmitters 2 0 and the total number
of transmitters 2 0. Further, the setting information
performed by the user using the manipulation input unit 14
is stored in the storage unit 13 by the control unit 11, and
the control unit 11 performs operations based on the
information recorded in the storage unit 13.
In this case, the wireless signal transmitted from the
transmitter 2 0 to the receiver 10 nas the format as shown in
Fig. 4A, and includes a preamble PA, a unique word UW, data
DA, and an error detection code CR. The preamble PA is a
bit stream such as a bit sync pattern in which 0 and 1 are
alternately repeated and the first and the last bit are set
to 0 and 1, respectively. In the present embodiment, it is
determined that the bit stream is the preamble PA only when
its length is 64 or more bits.
The unique word UW (frame sync pattern) includes bits
differing from those of the preamble PA, and is set to have
a length of 32 bits in the present embodiment. The data DA
is a bit stream including the identification address of a
transmitter 2 0 that transmits the wireless signal and
transmission information transmitted by the transmitter 2 0.
Further, the data DA may have a variable bit length
depending on the contents of the transmission information to
be transmitted. The error detection code CR is a bit stream
such as CRC, and is set such that an abnormality such as a
bit inversion occurring at least in the bit stream of the
data DA can be detected upon transmission and reception.
The control unit 11 activates the reception unit 12 at
every predetermined time interval (scan period) by using a
built-in timer (not shown) , sequentially scans preset
wireless channels, and receives a wireless signal
transmitted from the transmitter 20. In the present
embodiment, as shown in Fig. 4B, each wireless channel is
scanned for 2 ms based on the setting information stored in
the storage unit 13. In this case, the priorities of the
wireless channels ch5 to ch7 allotted to the transmitters
2 Ox to 20z are set to rlj , so that the wireless channels
ch5 to ch7 are set to be scanned twice for one scan period.
The priorities of the transmitters 20a to 20d are set to
1 2j , so that the corresponding channels are set to be
scanned once for one period. Accordingly, the time interval
corresponding to one scan period required to scan all
wireless channels is 20ms.
Next, an operation of scanning each of the wireless
channels is described. The control unit 11 controls the
reception unit 12 to detect a signal strength of a target
wireless channel by using the signal strength detection unit
12a. Then, if the detected signal strength is sufficient,
the reception unit 12 inputs the wireless signal to the
control unit 11.
At this point, the control unit 11 detects the above-
described bit sync pattern from the wireless signal inputted
from the reception unit 12 by using a synchronous bit
detection unit 11a. When the bit sync pattern is not
detected in the time of 2 ms for every wireless channel, the
control unit 11 switches a wireless channel to be scanned to
the next wireless channel and detects a bit sync pattern
therefrom again. On the other hand, when the bit sync
pattern is detected in 2 ms, the control unit 11 fixes the
wireless channel, continues to scan for a predetermined
period of time, and detects a unique word UW by using a
frame detection unit lib.
In the present embodiment, the period of time required
to perform the detection is set to 40 ms from the start of
the detection of the bit sync pattern, and this time period
is obtained by summing up the time of 20 ms corresponding to
one scan period required to scan all wireless channels, the
time of 13.3 ms corresponding to 64 bits required to
determine that the bit stream is the preamble PA, and the
time of 6.7 ms corresponding to 32 bits of the unique word
UW.
Next, the receiver 10 stops receiving the wireless
signal through the wireless channel when the unique word UW
is not detected, and starts to scan each wireless channel
again. When the unique word UW is detected, the data DA
subsequent to the unique word UW is received by the data
reception unit 12b, and it is determined whether signal
information is normal or abnormal by using an error
correction code (ECC) CR. Further, information included in
the wireless signal is obtained if the signal information is
normal.
In the meantime, the reception unit 12 periodically
detects the signal strength of the wireless signal even
while receiving the data DA. If the signal strength becomes
lower than a predetermined threshold, the reception unit 12
outputs a signal indicative of a decrease in the signal
strength to the control unit 11. Further, the control unit
11 controls the reception unit 12 to terminate the reception
of the wireless signal through the wireless channel, and to
start scan of a wireless signal flowing through a subsequent
wireless channel.
Thus, the receiver 10 receives wireless signals, each
including a preamble PA, through respective wireless
channels at every predetermined scan period interval (20
ins) .
Here, the bit length of the preamble PA is set to 160
bits in which 96 bits correspond to the scan period (20 ms)
of the receiver 10 and 64 bits correspond to the period
required to detect the preamble PA. Accordingly, the
probability that the receiver 10 detects the bit sync
pattern for an interval of 9 6 bits from the start bit of the
preamble PA is further increased.
Further, since the bit sync pattern of 64 bits
required to detect at least the preamble PA follows the
interval, the receiver 10 can reliably detect the preamble
PA and then receive a wireless signal. Consequently, it is
possible to increase the probability that the wireless
signal transmitted from the transmitter 20 is received by
the receiver 10, while reducing a need for the transmitter
20 to retransmit the same wireless signal, thus reducing the
time required for transmission and reception.
Further, as shown in Fig. 4C, the receiver 10 may scan
for a longer time period for the wireless channels ch5 to
ch7 having higher priorities than the wireless channels chl
to ch4 having lower priorities. For example, the wireless
channels chl to ch4 may be scanned for 2 ms and the wireless
channels ch5 to ch7 for 4 ms.
Furthermore, in the present embodiment, the bit length
of the preamble PA of the wireless signal transmitted from
each of the transmitters 20 is set to a fixed length of 160
bits, but it may be possible to provide a setting unit in
each of the transmitters 2 0 and to change the bit length of
the preamble in accordance with the number of wireless
channels set in the receiver 10. Alternatively, it is
possible to change the bit length of the preamble in
accordance with the length of one period required to scan
each of the wireless channels.
As for changing the bit length, when the user changes
settings for the receiver 10 via the manipulation input unit
14, the receiver 10 may send a signal indicative of the
length of the preamble to the transmitter 20, and the
transmitter 20 may change a preamble length in a wireless
signal to be transmitted in response to the received signal.
Further, in the description of the above embodiments,
an example was given in which the wireless communications
system is applied to the door phone system and the
illumination control system, but, it is not limited thereto,
and the wireless communications system of the present
invention can be applied to various systems such as remote
meter-reading systems. For example, the present invention
may be applied to remote meter-reading systems in such a way
that a measurement is automatically performed by using
measuring instruments such as power meters, gas meters and
water meters installed at remote places, and measurement
results are transmitted as remote meter-reading information.
Further, the present invention may also be applied to fire
information systems.
Although the preferred embodiments of the present
invention have been described above, the present invention
is not limited to those specific embodiments. Various
modifications and variations can be realized without
departing the scope of the following claims, and fall within
the scope of the present invention.
WE CLAIM:
1. A wireless communications system comprising:
a transmitter which selects one wireless channel among
a plurality of wireless channels each having a different
frequency from each other and transmitting a wireless signal
through the selected wireless channel; and
a receiver which receives the wireless signal
transmitted from the transmitter,
wherein the receiver comprises:
a signal strength detection unit which scans each of
target wireless channels for a predetermined scan period by
sequentially switching the target wireless channels and
measures a signal strength thereof,
a storage unit for storing the signal strengths of the
target wireless channels measured by the signal strength
detection unit; and
a signal reception unit which preferentially receives
a wireless signal on the wireless channel having a higher
priority determined based on the order of the signal
strength stored in the storage unit, among the target
wireless channels.
2. The system of claim 1, wherein:
the wireless signal includes a destination address
indicative of a receiver of destination, and
the receiver sequentially switches the target wireless
channels and receives the wireless signal on each of the
wireless channels in the order of the signal strength until
the destination address of the received wireless signal
matches its own address.
3. A wireless communications system comprising:
one or more transmitters each of which selects one
wireless channel among a plurality of wireless channels each
having a different frequency from each other and transmits a
wireless signal through the selected wireless channel; and
a receiver which receives the wireless signal
transmitted from the transmitter,
wherein the wireless signal includes a preamble in
which a predetermined bit sync pattern is repeated, and data
having an address for identifying a transmitter and a
transmission information,
the receiver includes a synchronous bit detection unit
which scans by sequentially switching the wireless channels
in a predetermined scan period in a time division manner,
and detects the bit sync pattern from each wireless channel,
and a data reception unit which receives the data following
the preamble when the bit sync pattern is detected,
the preamble has at least a total bit length of a bit
length corresponding to one scan period and a bit length
required to detect the bit sync pattern.
4. The system of claim 3, wherein:
the receiver includes a channel setting unit for
setting the target wireless channels, and
the transmitter changes a length of the preamble in
accordance with the number of wireless channels set by the
channel setting unit.
5. The system of claim 3 or 4, wherein:
the receiver further includes a priority setting unit
for setting priorities for the target wireless channels, and
the synchronous bit detection unit sets the number of
scans of the respective wireless channels during each scan
period based on the priorities set for the respective
wireless channels.
6. The system of claim 3 or 4, wherein:
the receiver further includes a priority setting unit
for setting priorities for the target wireless channels, and
the synchronous bit detection unit sets the time for
which the respective wireless channels are scanned during
each scan period based on the priorities set for respective
wireless channels.
7. The system of any one of claims 3 to 6, wherein:
the wireless signal includes a frame sync pattern
having a bit pattern differing from that of the bit sync
patterns which follows the bit sync pattern,
the receiver further includes a signal strength
detection unit for detecting signal strengths of the target
wireless channels and a frame detection unit for detecting
the frame sync pattern, and
the wireless channels is scanned again when the frame
sync pattern is not detected until a predetermined period of
time is elapsed after detecting a signal strength of a
predetermined strength or more by the signal strength
detection unit.
8. The system of any one of claims 3 to 6, wherein:
the receiver further includes a signal strength
detection unit for detecting signal strengths of the target
wireless channels, and
if a signal strength of the receiving wireless
channel, detected by the signal strength detection unit
while the data reception unit is receiving the data, is less
than a predetermined strength, scan of the target wireless
channels is restarted while reception of the wireless signal
is stopped.
9. The system of any one of claims 1 to 8, wherein the
wireless communications system is a remote meter-reading
system and the wireless signal is a signal including remote
meter reading information.