COMMUNICATION SYSTEMS
Field of the Invention
Currently there exists significant interest in the use of multihop techniques in packet
based radio and other communication systems, where it is purported that such
techniques will enable both extension in coverage range and increase in system
capacity (throughput).
Background of the Invention
In a multi-hop communication system, communication signals are sent in a
communication direction along a communication path from a source apparatus to a
destination apparatus via one or more intermediate apparatuses. Figure 1 illustrates a
single-cell two-hop wireless communication system comprising a base station BS
(known in the context of 3G communication systems as "node-B" NB), a relay node RN
(also known as a relay station RS), and an item of user equipment UE (also known as
a mobile station MS or subscriber station SS; below, the term MS/SS is used to denote
either of these types of UE). In the case where signals are being transmitted on the
downlink (DL) from a base station to a destination user equipment (UE) via the relay
node (RN), the base station comprises the source station (S) and the user equipment
comprises the destination station (D). In the case where communication signals are
being transmitted on the uplink (UL) from the user equipment (UE), via the relay node,
to the base station, the user equipment comprises the source station and the base
station comprises the destination station. The latter form of communication includes
signals transmitted by the user equipment to identify itself to the base station (and

hence to the network) as part of a network entry procedure. This is of particular
relevance to the present invention as will be explained below.
The relay node is an example of an intermediate apparatus and comprises: a receiver,
operable to receive data from the source apparatus; and a transmitter, operable to
transmit this data, or a derivative thereof, to the destination apparatus.
Simple analogue repeaters or digital repeaters have been used as relays to improve or
provide coverage in dead spots. They can either operate in a different transmission
frequency band from the source station to prevent interference between the source
transmission and the repeater transmission, or they can operate at a time when there
is no transmission from the source station.
Figure 2 illustrates a number of applications for relay stations. For fixed infrastructure,
the coverage provided by a relay station may be "in-fill" to allow access to the
communication network for mobile stations which may otherwise be in the shadow of
other objects or otherwise unable to receive a signal of sufficient strength from the
base station despite being within the normal range of the base station. "Range
extension" is also shown, in which a relay station allows access when a mobile station
is outside the normal data transmission range of a base station. One example of in-fill
shown at the top right of Figure 2 is positioning of a nomadic relay station to allow
penetration of coverage within a building that could be above, at, or below ground level.

Other applications are nomadic relay stations which are brought into effect for
temporary cover, providing access during events or emergencies/disasters. A final
application shown in the bottom right of Figure 2 provides access to a network using a
relay positioned on a vehicle.
Relays may also be used in conjunction with advanced transmission techniques to
enhance gain of the communications system as explained below.
It is known that the occurrence of propagation loss, or "pathloss", due to the scattering
or absorption of a radio communication as it travels through space, causes the strength
of a signal to diminish. Factors which influence the pathloss between a transmitter and
a receiver include: transmitter antenna height, receiver antenna height, carrier
frequency, clutter type (urban, sub-urban, rural), details of morphology such as height,
density, separation, terrain type (hilly, flat). The pathloss L (dB) between a transmitter
and a receiver can be modelled by:

Where d (metres) is the transmitter-receiver separation, b(db) and n are the pathloss
parameters and the absolute pathloss is given by / = 10(L/10)
The sum of the absolute path losses experienced over the indirect link SI + ID may be
less than the pathloss experienced over the direct link SD. In other words it is possible
for:


Splitting a single transmission link into two (or more) shorter transmission segments
therefore exploits the non-linear relationship between pathloss verses distance. From
a simple theoretical analysis of the pathloss using equation (A), it can be appreciated
that a reduction in the overall pathloss (and therefore an improvement, or gain, in
signal strength and thus data throughput) can be achieved if a signal is sent from a
source apparatus to a destination apparatus via an intermediate apparatus (e.g. relay
node), rather than being sent directly from the source apparatus to the destination
apparatus. If implemented appropriately, multi-hop communication systems can allow
for a reduction in the transmit power of transmitters which facilitate wireless
transmissions, leading to a reduction in interference levels as well as decreasing
exposure to electromagnetic emissions. Alternatively, the reduction in overall pathloss
can be exploited to improve the received signal quality at the receiver without an
increase in the overall radiated transmission power required to convey the signal.
Multi-hop systems are suitable for use with multi-carrier transmission. In a multi-carrier
transmission system, such as FDM (frequency division multiplex), OFDM (orthogonal
frequency division multiplex) or DMT (discrete multi-tone), a single data stream is
modulated onto N parallel sub-carriers, each sub-carrier signal having its own
frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in
a given time interval) to be divided over a plurality of sub-carriers thereby increasing
the duration of each data symbol. Since each sub-carrier has a lower information rate,
multi-carrier systems benefit from enhanced immunity to channel induced distortion

compared with single carrier systems. This is made possible by ensuring that the
transmission rate and hence bandwidth of each subcarrier is less than the coherence
bandwidth of the channel. As a result, the channel distortion experienced on a signal
subcarrier is frequency independent and can hence be corrected by a simple phase
and amplitude correction factor. Thus the channel distortion correction entity within a
multicarrier receiver can be of significantly lower complexity than its counterpart within
a single carrier receiver when the system bandwidth is in excess of the coherence
bandwidth of the channel.
Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is
based on FDM. An OFDM system uses a plurality of sub-carrier frequencies which are
orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap
without interference due to the fact they are mutually independent. The orthogonality
of OFDM systems removes the need for guard band frequencies and thereby increases
the spectral efficiency of the system. OFDM has been proposed and adopted for many
wireless systems. It is currently used in Asymmetric Digital Subscriber Line (ADSL)
connections, in some wireless LAN applications (such as WiFi devices based on the
IEEE802.11a/g standard), and (of particular relevance to the present invention) in
wireless MAN applications such as WiMAX (based on the IEEE 802.16 standard).
OFDM is often used in conjunction with channel coding, an error correction technique,
to create coded orthogonal FDM or COFDM. COFDM is now widely used in digital
telecommunications systems to improve the performance of an OFDM based system in
a multipath environment where variations in the channel distortion can be seen across
both subcarriers in the frequency domain and symbols in the time domain. The system
has found use in video and audio broadcasting, such as DVB and DAB, as well as
certain types of computer networking technology.

compared with single carrier systems. This is made possible by ensuring that the
transmission rate and hence bandwidth of each subcarrier is less than the coherence
bandwidth of the channel. As a result, the channel distortion experienced on a signal
subcarrier is frequency independent and can hence be corrected by a simple phase
and amplitude correction factor. Thus the channel distortion correction entity within a
multicarrier receiver can be of significantly lower complexity than its counterpart within
a single carrier receiver when the system bandwidth is in excess of the coherence
bandwidth of the channel.
Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is
based on FDM. An OFDM system uses a plurality of sub-carrier frequencies which are
orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap
without interference due to the fact they are mutually independent. The orthogonality
of OFDM systems removes the need for guard band frequencies and thereby increases
the spectral efficiency of the system. OFDM has been proposed and adopted for many
wireless systems. It is currently used in Asymmetric Digital Subscriber Line (ADSL)
connections, in some wireless LAN applications (such as WiFi devices based on the
IEEE802.11a/g standard), and (of particular relevance to the present invention) in
wireless MAN applications such as WiMAX (based on the IEEE 802.16 standard).
OFDM is often used in conjunction with channel coding, an error correction technique,
to create coded orthogonal FDM or COFDM. COFDM is now widely used in digital
telecommunications systems to improve the performance of an OFDM based system in
a multipath environment where variations in the channel distortion can be seen across
both subcarriers in the frequency domain and symbols in the time domain. The system
has found use in video and audio broadcasting, such as DVB and DAB, as well as
certain types of computer networking technology.

In an OFDM system, a block of N modulated parallel data source signals is mapped to
N orthogonal parallel sub-carriers by using an Inverse Discrete or Fast Fourier
Transform algorithm (IDFT/IFFT) to form a signal known as an "OFDM symbol" in the
time domain at the transmitter. Thus, an "OFDM symbol" is the composite signal of all
N sub-carrier signals. An OFDM symbol can be represented mathematically as:

where ∆f is the sub-carrier separation in Hz, Ts = 1/∆f is symbol time interval in
seconds, and cn are the modulated source signals. The sub-carrier vector in (1) onto
which each of the source signals is modulated c Cn, c = (co, C1..CN-1) is a vector of N
constellation symbols from a finite constellation. At the receiver, the received time-
domain signal is transformed back to frequency domain by applying Discrete Fourier
Transform (DFT) or Fast Fourier Transform (FFT) algorithm.
OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access variant
of OFDM. It works by assigning a subset of sub-carriers, to an individual user. This
allows simultaneous transmission from several users leading to better spectral
efficiency. However, there is still the issue of allowing bi-directional communication,
that is, in the uplink and download directions, without interference.

In order to enable bi-directional communication between two nodes, two well known
different approaches exist for duplexing the two (forward or download and reverse or
uplink) communication links to overcome the physical limitation that a device cannot
simultaneously transmit and receive on the same resource medium. The first,
frequency division duplexing (FDD), involves operating the two links simultaneously but
on different frequency bands by subdividing the transmission medium into two distinct
bands, one for forward link and the other for reverse link communications. The second,
time division duplexing (TDD), involves operating the two links on the same frequency
band, but subdividing the access to the medium in time so that only the forward or the
reverse link will be utilizing the medium at any one point in time. Both approaches
(TDD & FDD) have their relative merits and are both well used techniques for single
hop wired and wireless communication systems. For example the IEEE802.16
standard incorporates both an FDD and TDD mode. IEEE Std 802.16-2004 "Air
Interface for Fixed Broadband Wireless Access Systems" is hereby incorporated by
reference in its entirety.
In a single-hop communication system in which communication takes place directly
between an MS/SS and a BS, a network entry procedure is followed by the MS/SS in
conjunction with the BS. However, the known network entry procedure is not sufficient
for a multi-hop system in which communication between the BS and MS/SS takes
place via one or more relay stations RS. There is consequently a need for an improved
network entry procedure applicable in such a case.

Summary of the Invention
The invention is defined in the independent claims, to which reference should now be
made. Advantageous embodiments are set out in the sub claims.
Embodiments of the invention provide a communication method, communication
system, intermediate apparatus (e.g., a relay station RS) and a base station (BS)
employing a novel protocol adopted as a network entry procedure followed by the BS
and RS, to enable entry of a legacy MS or SS into a relaying enabled communication
network. The protocol allows centralised control of the overall process. The protocol
may be implemented as an adaptation of the current network entry procedure followed
in the IEEE802.16 standard and is primarily designed for the case of the transparent
style of relaying (i.e. a relay that does not broadcast control signals such as the
preamble or MAP). The present invention also embraces computer software for
executing the novel protocol on a BS or RS.
Brief Description of the Drawings
Preferred features of the present invention will now be described, by way of example,
with reference to the accompanying drawings, in which:-
Figure 1 shows a single-cell two-hop wireless communication system;
Figure 2 shows applications of relay stations;

Figure 3 shows standard MS network entry procedure; and
Figure 4 shows a BS ranging code detection procedure in a relay enabled network,
embodying the invention.
Detailed Description
In legacy single hop systems (e.g. 802.16-2004 and 802.16e-2005), standard network
entry procedures already exist to support entry of an MS or SS into a communication
network. However, when the network is modified to support relaying functionality, of
which a legacy MS or SS has no knowledge, a modified network entry procedure is
required from the network point of view to facilitate fast and efficient support of MS/SS
network entry.
This invention relates to a protocol that is intended to be adopted as the modified
network entry procedure from the network point of view, i.e. adopted in the RS and BS.
In particular it is designed with application to the IEEE802.16 standard in mind and
requires no changes to the procedure from the MS or SS point of view. It is also
designed for the case of transparent relaying where it is assumed that control of
network entry will be predominately performed in a centralised manner (i.e. in the BS,
with some limited assistance from the RS).

Figure 3 illustrates the network entry procedure described in the IEEE802.16 standard
which supports network entry of an MS or SS into a single-hop communication system.
Here, it is assumed that any RS with which the MS is communicating during the
network entry procedure is already known to the network (incidentally, in this
specification, the terms "network" and "system" are used interchangeably). For
example, the RS may have already completed entry into the network following a
separate procedure, such as the one described in the applicant's UK application no.
0616475.0, the disclosure of which is hereby incorporated by reference. It is also
assumed that, as the network is required to support legacy users, the MS or SS still
follows the same network entry procedure from its point of view, as illustrated in Figure
3. However, the procedure followed by the RS is defined here and the one followed by
the BS is modified from that followed for the case of a single hop network. For ease of
explanation, a two-hop configuration as in Fig. 1 will be considered although the
present invention is not limited to this.
Referring to Figure 3, the following operations take place during the identified stages:
Scan for Downlink Channel
During this stage the MS/SS scans for BS preamble transmissions (note RS will not
transmit preamble in this case). Once all potential preambles are detected, the MS will
select which channel it wishes to use from the available set of channels, in line with the
standard procedure. It will then synchronise its receiver with the transmitter.

Note that no new operations are required on the network side.
Obtain Uplink Parameters
During this stage the MS/SS obtains uplink parameters which includes location of the
uplink control information transmission region that will be used by the MS/SS in the
next stage. Note that according to the frame structure for this mode of operation, the
uplink parameters advertised by the BS must be common for the MS to RS uplink.
Note that no new operations are required on the network side.
Ranging & Automatic Adjustments
The MS/SS will transmit a ranging code or ranging message, as defined in the
IEEE802.16 standard, as a form of identification information to identify itself to the
network. (Incidentally, the term "ranging message" is more correct when OFDM is
being used, and "ranging code" more appropriate to OFDMA, but in the following
description "ranging code" is used for both). It is possible that a number of receivers in
the multi-hop network receive this transmission.
The BS attempts to detect the transmission of a ranging code during this stage.
However, if the transmit power used by the MS/SS was too low, detection may not
occur. Further, if the BS detects the code but the received signal power is too low, it

may ignore or ask the MS/SS to continue ranging such that it retransmits using a
higher transmission power or applies some other adjustment to its transmission to
make detection more reliable. In the standard procedure, once the BS successfully
detects the code and is satisfied with the transmission parameter setting
(synchronisation, received signal power, etc), it will inform the MS/SS of completion of
the ranging process. The MS and the BS then continue the remainder of the network
entry procedure in the known manner.
Referring now to Figure 4, in a relay enabled system, some modification is required to
the operations on the network side, as described in the preceding paragraph. As the
BS knows that an RS exists, not only will it check for direct ranging code reception from
an MS, it will also check for ranging code detection at the RS, before deciding on
whether or not to transmit a ranging code related response to the MS. This is a first
feature of the invention.
Any of the following three different mechanisms may be employed to inform the BS of
the reception of a ranging code at the RS. These mechanisms form additional features
of the invention:
(a) The RS simply receives and retransmits the ranging code on to the BS. In doing
so, it is assumed that the RS ensures that the transmission power at the RS is
reasonable. For example, the received carrier-to-interference plus noise ratio - CINR -
on the ranging code at the BS should be similar to the received CINR on the ranging
code at the RS. Such a situation will automatically occur if the invention in the

applicant's EP application no. 05253783.4, the disclosure of which is hereby
incorporated by reference, is applied. If this is not ensured then the detection
probability will not correctly represent the conditions at the RS receiver. If this situation
(i.e. lack of CINR balance) is known at the BS, which could be the case as described in
the just-mentioned [UK] application, it is possible that the BS can then correct for this
knowledge following detection by adjusting the observed CINR appropriately.
Received signal strength - RSSI - may be used as an alternative to CINR.
(b) The RS detects the code and rather than forwarding the code, instead it forwards
the detection information on to the BS. The detection information could include, but is
not limited to, the code index used by the transmitter and the received CINR at the RS.
It could also include information about the timing or frequency accuracy of the received
signal from the MS.
(c) Alternatively, the BS informs the RS of a ranging acceptance threshold (i.e. the
level of CINR that must be observed) and then the RS simply informs the BS when it is
has detected a user.
Once the BS has the appropriate information from the RS via one of the mechanisms
detailed above, the BS then combines the relayed information regarding code detection
with that of any information relating to direct code detection at the BS during the normal
uplink ranging transmission interval. Note that it is possible that it receives relayed
detection information from a number of relays so it may actually have more than two

sets of information to arbitrate. The relays may be from multiple RS receiving the same
ranging code in parallel from the MS/SS.
Alternatively, in a multi-hop configuration, multiple RS may be interposed in the
communication path between the MS/SS and BS. In such a case, the above procedure
is modified to include one RS receiving, and/or relaying, a ranging code or detection
information from/to another RS.
The procedure in the BS for managing the process is illustrated in Figure 4.
Once ranging is complete the remainder of the existing network entry procedure is
followed by the BS and MS with the flow of data taking place through the selected
route. The transmission route may vary between the uplink and the downlink; in
particular, there may be no need for information on the downlink to be relayed via the
RS, so that the response from the BS can be transmitted directly to the MS.
Alternatively a plurality of RS may be include in the uplink with fewer or no RS involved
in the downlink.
In summary, the present invention defines an initial ranging procedure that enables a
network to support entry of a legacy MS or SS into a relaying enabled communication
network. Only a minimal number of modifications are required in the BS to the legacy
network entry procedure. Embodiments of the present invention provide three different
approaches for relaying MS detection information at the RS to the BS, such that it is

possible to select the technique that is most appropriate for the system into which the
technique is to be employed (i.e. signalling overhead, RS complexity, BS complexity,
protocol reliability).
In the above description, it is assumed that the network could consist of some legacy
BS (i.e., base stations operating in compliance with existing protocols) and some
relaying enabled BS (i.e., base stations modified so as to be able to operate in
accordance with the present invention). It is also assumed that a relaying enabled BS
may be operating in a legacy mode until it receives a request from an RS for it to enter
the network. The reason the BS may operate in such a mode would be to preserve
transmission resources by not having to broadcast relay specific information when
there are no relays benefiting from the transmission.
Embodiments of the present invention may be implemented in hardware, or as
software modules running on one or more processors, or on a combination thereof.
That is, those skilled in the art will appreciate that a microprocessor or digital signal
processor (DSP) may be used in practice to implement some or all of the functionality
of a transmitter embodying the present invention. The invention may also be embodied
as one or more device or apparatus programs (e.g. computer programs and computer
program products) for carrying out part or all of any of the methods described herein.
Such programs embodying the present invention may be stored on computer-readable
media, or could, for example, be in the form of one or more signals. Such signals may
be data signals downloadable from an Internet website, or provided on a carrier signal,
or in any other form.

A program embodying the invention could also be used to add the functionality of the
RS as described above to a MS/SS having suitable hardware.

We Claim:
1. A multi-hop wireless communication system comprising:
a source apparatus configured to transmit a code;
an intermediate apparatus configured to receive said code and to transmit
notification to a destination apparatus in response to reception of said code; and
a destination apparatus configured to check not only whether said code is directly
received from source apparatus but also whether said code is received by said intermediate
apparatus for a communication path selection.
2. The multi-hop wireless communication system according to claim 1, wherein a
response to said code is transmitted to said source apparatus directly from said destination
apparatus.
3. The multi-hop wireless communication system according to claim 2, wherein said
response is not relayed by said intermediate apparatus.
4. The multi-hop wireless communication system according to claim 1, 2, or 3,
wherein a path selected by said communication path selection is used for subsequent
communication between said destination apparatus and said source apparatus.
5. The multi-hop wireless communication system according to any preceding claim,
wherein said intermediate apparatus informs said destination apparatus of reception of said
code when a criteria is satisfied with respect to said code.
6. The multi-hop wireless communication system according to claim 5, wherein said
criteria is notified from said destination apparatus.
7. The multi-hop wireless communication system according to any preceding claim,
wherein said notification includes information relating to code index, CINR relating to said
code, a receiving timing of said code or a receiving frequency of said code.
8. The multi-hop wireless communication system according to any preceding claim,
wherein a response to said code is transmitted to said source apparatus directly from said

destination apparatus, the response including adjustment information when a criteria is not
satisfied by said code.
9. A method for a multi-hop wireless communication system, said method comprising:
transmitting a code from a source apparatus;
receiving, by an intermediate apparatus, said code and transmitting notification to a
destination apparatus in response to reception of said code from said intermediate
apparatus; and
checking, by said destination apparatus, not only whether said code is directly
received from source apparatus but also whether said code is received by said intermediate
apparatus for a communication path selection.
10. A source apparatus used in a multi-hop wireless communication system, said
source apparatus comprising:
a transmitter to transmit a code;
a receiver to receive a response to said code directly from a destination apparatus
that receives said code directly from said source apparatus and receives notification from an
intermediate apparatus that transmits said notification in response to reception of said code
from said source apparatus;
wherein said source apparatus communicates with said destination apparatus
using a path that is selected by said destination apparatus based on said code received
directly and said notification.
11. An intermediate apparatus used in a multi-hop wireless communication system,
said intermediate apparatus comprising:
a receiver to receive a code transmitted from a source apparatus;
a transmitter to transmit detection information of said code instead of said code
itself to a destination apparatus,
wherein said detection information is used for a communication path selection by
said destination apparatus.
12. A destination apparatus used in a multi-hop wireless communication system, said
destination apparatus comprising:
a receiver to receive a code directly from a source apparatus and notification from
an intermediate apparatus that transmits said notification in response to reception of said
code from said source apparatus;

wherein said destination apparatus checks not only whether said code is directly
received from source apparatus but also whether said code is received by said intermediate
apparatus for a communication path selection.

A wireless communication system, in which transmissions between a mobile
station (MS/SS) and a base station (BS) are relayed via at least one relay station (RS).
The relay station determines whether the mobile station has issued a ranging code and
if so, notifies the base station of this. The base station detects any ranging code
received directly or notified from the relay station and uses all such detections to
decide how to respond to the mobile station. In this way, with minimal modifications to
an existing BS, legacy MS/SS can be supported within a multi-hop wireless
communication system. The system may in other respects conform to the IEEE802.16
standard for single-hop wireless communication.