Introduction
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 (throughout).
Field of the Invention
In a multi-hop communication system, communication signals are sent in a
communication direction along a communication path (C) from a source apparatus
to a destination apparatus via one or more intermediate apparatuses. Figure 5
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 a user equipment UE (also
known as mobile station MS). 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 a 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 relay node is an
example of an intermediate apparatus (I) 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 6 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 6 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 6 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:
L = b + I0n log d (A)
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:
L(SI) + L(ID) < L(SD) (B)
Splitting a single transmission link into two 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 of 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 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 A/ is the sub-carrier separation in Hz, Ts = 1/A/ 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 e Cn, c = (c0, 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.
As an example, Figure 7 illustrates the single hop TDD frame structure used in the
OFDMA physical layer mode of the IEEE802.16 standard (WiMAX).
Each frame is divided into DL and UL subframes, each being a discrete
transmission interval. They are separated by Transmit/Receive and
Receive/Transmit Transition Guard interval (TTG and RTG respectively). Each DL
subframe starts with a preamble followed by the Frame Control Header (FCH), the
DL-MAP, and the UL-MAP.
The FCH contains the DL Frame Prefix (DLFP) to specify the burst profile and the
length of the DL-MAP. The DLFP is a data structure transmitted at the beginning of
each frame and contains information regarding the current frame; it is mapped to the
FCH.
Simultaneous DL allocations can be broadcast, multicast and unicast and they can
also include an allocation for another BS rather than a serving BS. Simultaneous
ULs can be data allocations and ranging or bandwidth requests.

Background of the Invention/Prior Art
This patent application is one of a set of ten UK patent applications filed on the
same date by the same applicant with agent reference numbers P1O6752GB00,
P106753GB00, P106754GB00, P106772GB00, P106773GB00, P106795GB00,
P106796GB00, P106797GB00, P106798GB00, and P106799GB00, describing
interrelated inventions proposed by the present inventors relating to communication
techniques. The entire contents of each of the other nine applications is
incorporated herein by way of reference thereto and copies of each of the other nine
applications are filed herewith.
In legacy single hop systems (e.g. 802.16-2004 and 802.16e-2005), standard
network entry procedures already exist for an MS entering a network. However, as
there is no concept of an RS in these systems, no suitable network entry procedure
is defined. Embodiments of the invention are suitable as a standard network entry
algorithm in the case that it is an RS entering the network.
Summary of Invention
Invention embodiments provide a method of assessing a potential communication
link in a wireless communication system, the system comprising communication
apparatuses comprising a base station (BS), a user equipment (UE) and at least
one intermediate apparatus (RS), said base station (BS) and said user equipment (UE) being operable to communicate using a communication path by transmitting information either directly along a single communication link or indirectly along the
communication path via the or each intermediate apparatus (RS), and the or each
intermediate apparatus (RS) being operable to receive information from a previous
communication apparatus along said path and to transmit the received information
to a subsequent apparatus along said path, the method comprising:
for a potential communication link between a particular said intermediate apparatus
(RS) and the base station (BS) of the communication system, establishing whether
the base station (BS) is of a first, relay-enabled type supporting intermediate
apparatuses or of a second type, which is not relay-enabled;

determining whether said link is suitable for communication in a first, relaying mode
or in a second mode in dependence upon the established type of said base station
(BS); and
if it is determined that said potential link is suitable for communication in said first,
relaying mode, concluding a link initiation process in order to enable communication
in said first, relaying mode along that link.
The invention is defined in the independent claims, to which reference should now
be made. Advantageous embodiments are set out in the sub claims.
Brief Description of the accompanying Drawings
Preferred features of the present invention will now be described, purely by way of
example, with reference to the accompanying drawings, in which:-
Figure 1 shows Standard MS network entry procedure;
Figure 2 shows Modification for capability negotiation;
Figure 3 shows Modification for obtaining RS uplink parameters;
Figure 4 shows Modification for switch uplink parameter usage;
Figure 5 shows a single-cell two-hop wireless communication system;
Figure 6 shows applications of relay stations; and
Figure 7 shows a single hop TDD frame structure used in the OFDMA physical layer
mode of the IEEE 802.16 standard.
Detailed Description of the Invention
RS Network Entry Procedure
The first stage is for the RS to follow the standard MS network entry procedure in
order to establish a connection with the BS. An example of the network entry
procedure for the case of the 802.16 system is given in Section 6.3.9 of the
standard. Figure 1 summarises these procedures that are detailed further in the
standard.

Throughout it is assumed that the network could consist of some legacy BS and
some relaying enabled BS. 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.
The first modification to the sequence above is that during the negotiation of basic
capabilities the RS will identify itself as an RS to the BS using a new signalling entity
(referred to as a TLV) that indicates that the device registering has the capability to
act as a relay. Amongst other parameters the relay shall identify its capability to act
as a relay on DL and/or UL traffic. It shall also declare the type of relaying
supported (i.e. transparent or not). The required processes that need to be included
into the procedure shown in Figure 1 are shown in Figure 2 in underlined text.
I As a result, the BS will now know that the connecting device is an RS, if it completes
i this stage. If the BS is a legacy BS then it will not complete this stage as it will not
acknowledge the use of the extended relay related capabilities. However the RS
may continue the network entry procedure as it may be able to operate in an
alternative mode that does not require the BS to have knowledge that it is a RS and
not an MS.

If the RS is to perform uplink relaying (as identified above) then the second
modification is that at some point between the RS becoming successfully registered
with the BS and the RS becoming operational it will require the BS to inform it of the
RS specific uplink parameters. In particular, this is required as during the normal
ranging region, the RS will have to be receiving signals from MS or other RS and
hence cannot be transmitting to the BS.
It is assumed that if the BS is not already advertising these parameters through an
appropriate message, it will at least start once it is aware that an RS is entering the
network as determined during the RS capability negotiation stage. Therefore if the
RS cannot determine the RS specific uplink parameters because they are not being
advertised by the BS (usually after a timeout period of waiting for the parameters to
be broadcast) it will assume that the BS does not support RSs (i.e. it is a legacy BS)
and will mark the downlink channel associated with this BS as unusuable and restart
the network entry procedure scanning for other potential downlink channels.
The required processes that need to be included into the procedure shown in Figure
1 are shown in Figure 3 in underlined text.
Once the RS uplink parameters are identified the RS then switches to using these
new parameters on the uplink prior to becoming operational. This is required before
the RS is operational and is the final amendment required to the procedure shown in
Figure 1, as shown in Figure 4 in underlined text.

The RS completes the network entry procedure and now becomes operational,
receiving the preamble to maintain synchronisation and the DL and UL-MAP
messages to understand the allocation of resources within the frame for
communication with the MS and BS.
Extension for the case of RS transmitting a preamble
If the RS is required to provide transmission of broadcast control information (i.e. the
MS cannot receive this information directly from the BS or RS to which the RS is
connecting) then prior to becoming operational one final step is required. In this
case, the BS or RS will have identified to the RS during the capability negotiating
phase that the RS should operate in such a mode. The RS will then stop listening to
the normal preamble and MAP messages, so that it can transmit its own. Instead, it
will ascertain from the BS or RS to which it is connecting the location of the relay
amble, or other RS specific information signal that can be used to identify the
transmitter and train the various distortion correction units within the receiver in the
absence of the preamble knowledge.
At this point the RS can then begin to broadcast the normal preamble and as and
when required, the MAP messages.
During operation the RS continually monitors the RS uplink parameters and other
RS specific information signals on the downlink (i.e. Relay Amble and control
information) as the BS or RS may change these based on the dynamically changing
operational environment. For example, as more uplink channels are required to
report HARQ related ACK/NACKs, channel quality reports or increase the ranging
region.

Benefits
In summary the benefits of invention embodiments are:
o Defines a simple modification to an existing procedure that supports the entry
of both MS and RS into a communication network.
o Minimises the impact on existing BS designs as the number of modifications
required are minimal.
o Enables the RS to closely mimic the procedure already developed and in use
in the MS, thus enabling reuse of existing software developed to support
network entry procedures in the MS.
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.

WE CLAIM:
1. A method of assessing a potential communication link in a wireless
communication system, the system comprising communication apparatuses
comprising a base station (BS), a user equipment (UE) and at least one
intermediate apparatus (RS), said base station (BS) and said user
equipment (UE) being operable to communicate using a communication path
by transmitting information either directly along a single communication link
or indirectly along the communication path via the or each intermediate
apparatus (RS), and the or each intermediate apparatus (RS) being operable
to receive information from a previous communication apparatus along said
path and to transmit the received information to a subsequent apparatus
along said path, the method comprising:
for a potential communication link between a particular said intermediate
apparatus (RS) and the base station (BS) of the communication system,
establishing whether the base station (BS) is of a first, relay-enabled type
supporting intermediate apparatuses or of a second type, which is not relay-
enabled;
determining whether said link is suitable for communication in a first, relaying
mode or in a second mode in dependence upon the established type of said
base station (BS); and
if it is determined that said potential link is suitable for communication in said
first, relaying mode, concluding a link initiation process in order to enable
communication in said first, relaying mode along that link.
2. The method as claimed in claim 1, wherein communication in said first,
relaying mode involves use of a set of capabilities of the particular
intermediate apparatus (RS), and wherein communication in said second
mode involves use of a subset of said set of capabilities of the particular
intermediate apparatus (RS).

3. The method as claimed in claim 2, comprising:
if it is determined that said link is suitable for communication in said second
mode, concluding a link initiation process in order to enable communication
in said second mode along that potential link.
4. The method as claimed in claim 1, wherein communication in said first,
relaying mode involves use of some or all of a set of capabilities of the
particular intermediate apparatus (RS), and wherein communication in said
second mode does not allow conclusion of a link initiation process for the
potential link, so that if it is determined that said potential link is suitable for
communication in said second mode, the potential link is preferably marked
as unusable.
5. The method as claimed in any of the preceding claims, comprising carrying
out said establishing in said base station (BS).
6. The method as claimed in any of the preceding claims, comprising carrying
out said establishing in the particular intermediate apparatus (RS).
7. The method as claimed in claim 6, comprising carrying out said establishing
based upon information received from said base station (BS).
8. The method as claimed in claim 6 or 7, comprising carrying out said
establishing based upon information stored within the particular intermediate
apparatus (RS).
9. The method as claimed in any of the preceding claims, comprising carrying
out said determination in the particular intermediate apparatus (RS).
10. The method as claimed in any of the preceding claims, comprising
configuring a mode of operation of said particular intermediate apparatus
(RS) based upon the established type of said base station (BS).

11. The method as claimed in any of the preceding claims 5 comprising
configuring a communication format for use in communication between the
particular intermediate apparatus (RS) and said base station (BS) based
upon the established type of said base station (BS).
12. The method as claimed in any of the preceding claims, wherein said base
station (BS) is a previous said apparatus along said path relative to the
particular intermediate apparatus (RS), the method comprising configuring a
communication format for use in communication between the particular
intermediate apparatus (RS) and a subsequent apparatus along said path
relative to the particular intermediate apparatus (RS) based upon the
established type of said other apparatus.
13. The method as claimed in any of the preceding claims, wherein the system is
an OFDM or OFDMA communication system.
14. A wireless communication system, comprising:
communication apparatuses comprising a base station (BS), a user
equipment (UE) and at least one intermediate apparatus (RS), said base
station (BS) and user equipment (UE) being operable to communicate using
a communication path by transmitting information either directly along a
single communication link or indirectly along the communication path via the
or each intermediate apparatus (RS), and the or each intermediate
apparatus (RS) being operable to receive information from a previous
communication apparatus along said path and to transmit the received
information to a subsequent apparatus along said path;
establishing means operable, for a potential communication link between a
particular said intermediate apparatus (RS) and the base station (BS) of the
communication system, to establish whether said base station (BS) is of a
first, relay-enabled type supporting intermediate apparatuses or of a second
type, which is not relay-enabled;
determining means operable to determine whether said link is suitable for
communication in a first, relaying mode or in a second mode in dependence
upon the established type of said base station (BS); and

concluding means operable, if it is determined that said potential link is
suitable for communication in said first, relaying mode, to conclude a link
initiation process in order to enable communication in said first, relaying mode
along that link.
15. An intermediate apparatus (RS) for use in a wireless communication system,
the system comprising communication apparatuses comprising:
a base station (BS) and a user equipment (UE), said base station (BS) and
user equipment (UE) being operable to communicate using a communication
path by transmitting information either directly along a single communication
link or indirectly along the communication path via the intermediate apparatus
(RS), and the intermediate apparatus (RS) being operable to receive
information from a previous communication apparatus along said path and to
transmit the received information to a subsequent apparatus along said path,
the intermediate apparatus (RS) comprising:
establishing means operable, for a potential communication link between the
intermediate apparatus (RS) and the base station (BS) of the communication
system, to establish whether said base station (BS) is of a first, relay-enabled
type supporting intermediate apparatuses or of a second type, which is not
relay-enabled;
determining means operable to determine whether said link is suitable for
communication in a first, relaying mode or in a second mode in dependence
upon the established type of said base station (BS); and
concluding means operable, if it is determined that said potential link is
suitable for communication in said first, relaying mode, to conclude a link
initiation process in order to enable communication in said first, relaying mode
along that link.

ABSTRACT

TITLE "COMMUNICATION SYSTEM"
The invention relates to a method of assessing a potential communication link in
a wireless communication system, the system comprising a source apparatus, a
destination apparatus and at least one intermediate apparatus, said source
apparatus being operable to transmit information in a communication direction
towards the destination apparatus either directly along a single communication
link or indirectly along a communication path via the or each intermediate
apparatus, and the or each intermediate apparatus being operable to receive
information from a previous communication apparatus along said path in said
communication direction and to transmit the received information to a
subsequent apparatus along said path in said communication direction, the
method comprising: for a potential communication link between a particular said
intermediate apparatus and another apparatus of the communication system,
establishing whether said other apparatus is of a first type or of a second type
different from said first type; determining whether said link is suitable for
communication in a first mode or in a second mode in dependence upon the
established type of said other apparatus; and if it is determined that said
potential link is suitable for communication in said first mode, concluding a link
initiation process in order to enable communication in said first mode along that
link.