COMMUNICATION SYSTEMS Introduction Currently there exists significant interest m the use of muhihop techniques in packet based radio and other communication systems, whew ii is purported that such techniques will enable both extension in coverage range and increase in syslem capacity (throughput). 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 intermediaic apparatuses. Figure 5 illustrates a single-cell twn-hop wireless communication system comprising a base station BS (known in the context oHG communication systems as "node-B" NH) a relay node RN (also known as a relay station RS) and a user equipment UE (also known as mobiie station MS). In the case where signals are being transmitted on the downlink (l>L) from a base station lo 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 signate are being transmitted on the uplink (UL) from a user equipment (UE), via the relay node, to the base slation, the user equipment comprises the source station and the base station comprises the destination station. The relay node h an example of an intenncdiate apparatus (I) and comprises: a receiver, operable to receive data from the source apparatus; and a Iransmitter, operable to transmit this data, or a derivative thereof, to the destination apparatus. Simple analu£uc repeaters or digital repealers have been used as relays lo 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 ai a time when there is no transmission from the source station. Figure 6 illustrates a number of appliealions 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 stati&nx which may otherwise be in the shadow of olhor objects or otherwise unable to receive a signal of sufficient strength from the base station despite being within the normal range of ihc base station. "Range ex-icnsion" is also shown, in which a relay slation allows access when a mobile station is outside the normal dala Iransmmion range of abase slaliun. One citamplc of in-fill shown al the lop right of Figure G is positioning of a nomadic relay station to allow penetration of coverage within a building thai could be above, at, or below ground level Other applications arc nomadic relay stations which arc brought into effect for icmporary cover, providing access during events or emergencies/disasters. A final application shown in Ihe bellum righl of Figure 6 provides access to a network using a relay positioned on s 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 ihe occurrence of propagation loss, or "pathloss", due to the scattering or absorption of a radio communication as il travels Ihrougb space, causes ihe strength of a signal to diminish. Factors which influence the pathldss between a transmitter and a receiver include: transmitter antenna height, receiver antenna height, carrier frequency, duller 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 palhloss parameters and Ihe absolute pathloss is given by f = 10'; '. The sum of absolute path losses experienced over indirect link SI + ID may be less than the pathloss experienced over the direct link SD, In other words il is possible for: l(SI) + L(ID)r use with multi-carrier transmission. In a multi-carrier transmission system, such as FDM (frequency division multiplex), OFDM (orthogunal frequency division multiplex) Form a signal known as an "OKDM symbol" in the time domain at the transmitter, Ihus, an "OFDM symbol" is the composite signal of all N sub-carrier signals. An OFDM symbol can be represented mathematically as: where Lf is the sub-carrier separation in Hz, Is - l/A/ is symbol time interval in seconds, and cn are the modulated source signals. The sub-carrier vector in (I) onto which each of the source signals is modulated c e C, c - (c P is a data structure transmitted at the beginning of each frame and contains information regarding Ihc current frame; it is mapped to the FC1L Simultaneous 111. 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. This patent application is one of a set often UK patent applications filed on the same date by Ihe same applicant with Application No. GB0616477.G'. GB0616481.S, GB06164S2.il, GB0606478.4, GB0616479.2, GB061647L9, GB0616472.7, GB06I6474.3, GB0616475.O, and GB0616476.S describing interrelated inventions proposed by the present inventors relating to communication techniques. The entire contents of each of the other nine applications are incorporated herein by reference. In WiMAX and other networks, a mobile station (MS) may face many possible paths to exchange information with base station (BS). Approaches should be designed for MS to select an optimal path. Especially, in mobile environment, the radio channel condition will be dynamically changing, thus a MS should dynamically seleel a proper relay station (RS), or BS for handover. Preferred features of ihc present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows Scenario 1: BS can cover (he MS, An MS will have many possible paths to communication with BS in WiMAX relay systems. The system has to choose an optimal path for the communication belween MS and BS; Figure 2 shows Scenario 2: HSs cannot cover MS. The system has to choose either RS lft or RS 2tt to relay the information between MS and RSs; Figure 3 shows an example of calculating PoR values; Figure 4 shows an example of the signaling related to PoR table broadcast; 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 OFDM A physical layer mode of the IEEE 802.16 standard. Figures 1 and 2 illustrate scenarios of WiMAX relay systems, where the Mobile Station (MS) could connect to BS directly, or ask RS 1# or RS 21/ fur relay, Obviously, in Figure 1, following paths are possihle for the communication between MS And BS: a. MS->DS b. MS->RSli¥->BS c. MS->RSI#->RS2->BS d. MS->RS2#->BS e. MS->RS2#->RS1->BS The system shall decide an optimal path for the handover of MS. This decision should be based on the link quality, QoS demand, and so on. Especially in high mobilily case, the MShs status will he dynamically changing, thus KK/HS selection approach will be very important to maintain an acceptable QoS level in MS. An MS will have many possible paths to communication with BS in WiMAX relay systems. The system has to choose an optimal path for the communication between MS and BS. The sysLcm has to choose cither RS 1# or RS 2# to relay the information between MS and HSs. The previous method for MS to decide the handover is using link quality, such as ClNR, which could be measured from preamble or o; ,h > 0; Xm> 0; 7m > 0) where tjn, n = ], 2, 3..., is a negative factor, kn is the corresponding weight for the n negative factor, ymt m-1, 2, 3 ... , is a positive factor, Am is the weight for the m"1 positive factor, and a is the weight for the link from apparatus A to apparatus 0; for at least the or said one of the multi-link paths, combining the link-suitability information for each link ^ the path concerned by multiplying the PoR for each of those links together so as to generate path-suitability information indicative of the suitability of the path concerned for transmission and/or reception; and selecting one of said paths for transmission and/or reception in dependence upon said path-suitability information. Main benefits Invention embodiments give an efficient solution for BS/Rs selection for MS's handover in WiMAX system. The benefits from this are; \. Through the associated improvement in performance (due to select an optimal HH or RS for handover to guarantee the QoS) to differentiate FUJITSU'S wireless/cable OFDMA (such as WiMAX) from that of our competitors; 2. Ihe proposed method gives an approach of selecting the optimal BS/RS for handover in non-relay system, and multi-hop relay system; 3. By dynamically selecting an optimal HS/RS, the MS can adapt to the dynamically changing radio environment and QoS demands; 4. A novel metric, named the potential of relay (PoR), is defined to combine various QoS, or link quality related metrics, to aid the BS/RS selection and decrease the signaling overhead. This melric gives a fusible way to calculate the quality of multi-hop or single-hop links; 5. A genetic signaling mechanism is designed for the pruned seJeelitwi meibud, wbidi can be compatible withiEEE802.l6e; 6. More flexible for distributed implementation. A distributed implementation can release the computation and signaling bad in BS; 1. The definition of RS region gives an feasihte method for RSs to hroadcastftransmit information to other RSs. and MSs. 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. Sueh 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 signal* downloadable from an Internet website, or provided on a carrier signal, or in any other form. We Claim: 1. A path selection method for use in a communication system, the system comprising at least three communication apparatuses, a particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either * single-link palh extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or heing a multi-link path extending from the particular apparatus lo said olher or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link along the path, and at least one said path being such a multi-link path, the method comprising: for each link along at least the nr one of the multi-link paths, obtaining link-suiiabiliiy information indicative of the suitability of the lir>k concerned for transmission and/or reception; for at least the or said ane of the multi-link path?, Gcimhmiiig the link-suitability infbrmalion Tor each link of the path concerned so as to generate path-suitability information indicative of the suitability of the path concerned tor transmission and/or reception; and selecting one of said paths Tor transmission and/or reception in dependence upon said path-suitability information. 2. The method according to claim 1, wherein said particular apparatus is a user terminal and/or a mobile terminal and/or a relay apparatus operable to receive a communication signal over such a communication link arid transmit a communication signal over another such communication link. 3. The method according to claim 2, wherein the or each intermediate apparatus is a relay apparatus operable to receive a communication signal over such a communication link and transmit a communication signal over another such communication link. 4. The method according to any preceding claim, ^herein the apparatus at the opposite end of each path to the particular apparatus is a base station apparatus, 5. The method according to any preceding claim, wherein at least two of said paths are such multi-link paths. 6. The method according to any preceding claim, further comprising: tor the or each link of each said pathT obtaining such link-suitnhility information, such link-suitability information being such path-suitability information in the case of a single-link path; for each said multi-link path, combining the fink-suitability information for each link of ihe path concerned so as to generate path-suitability informaiiun fur the path concerned; and carrying out said selection in dependence upon the path-suitability information for each said palh. 7. The method according to any preceding claim, wherein each obtained iink- suitability information comprises a numerical factor, and wherein said combining comprises mathematically combining the factors concerned, 3. The method according to any preceding claim, comprising canying ouE said flection in the particular apparatus. 9. Ihe method according to any preceding claim, uumprising carrying out said selection in accordance with predetermined selection criteria, preferably cilher by sniveling the path having a better suitability than that of the other said paths, or by selecting one of the paths whose path-suitabilily informaliun indicate* thai its suitability is above a predetermined suitability threshold. 10. The method according to any preceding claim, wherein the link-suitability information for at least one of said links comprises a combination factor being the result of a mathematical combination of a plurality of component factors for that link. 11. The method according to any preceding claim, wherein said system is an OFDM or OFDMA communication system. 12. A suite of computer program* which, when executed by computing devices of a communication system, causes the system to carry out a path selection method, the system comprising at least three communication apparatuses, a particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-link path extending from Ihe particular apparatus lo another said apparatus directly over a single coininunicatioii link therebetween, or being a multi-link path extending from the particular apparatus lo said olher or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-Jink along the path, and at least one said paih being such a multi-link path, and ihe method comprising: for each link along at least ihe or one of the multi-link paths, obtaining link-suitabiliiy information indicative of the suitability of ihe link concerned for transmission and/or reception; for at least the or said one of the multi-link paths, combining the link-suitabiliiy information for each link of the path concerned so as lo generate path-suitahility information todteaifoc of iht suitability of fre path concerned for transmission and/or reception; and sclenting one of said paths for transmission and/or reception in dependence upon said path-suitability information. 13, A communication systemH comprising; at least three communication apparatuses, a particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-link path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from Ihe particular apparatus to said other or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consc unlive such links link-by-link along the path, and at least one said path being such a multi-link path; obtaining means operable, for each link along at least the or one of the multi-link paths, to obtain link-suitability information indicative of the suitability of the link concerned for transmission and/or reception; combining means operable for at least the or said one of the mulli-iink paths, to combine the link-suitability information for each link of the path concerned so as to generate path-suitability information indicative of the suitability of the path concerned for transmission and/or reception; and selecting means operable to select one of said paths for transmission and/or reception in dependence upon said path-suitability information. 14, A path selection method for use in a particular communication apparatus of a communication system, the system comprising at least three such communication apparatuses, the particular one of said apparatuses being operable lo transmit and/or receive a communication signal along at Icasi two different communication paths, each said path being either a single-link path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from the particular apparatus to said other or another said apparatus indirectJy via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link along the path, and at least one said path being such a multi-link path, the method comprising: for tadi link m for groups of links along aV \v&*\ me or one of the mufti-link paths, obtaining link-suitability information and/or link-group-suitabilily information, as the case may be, indicative of the suitability of the link or group of links concerned for transmission and/or reception; for at least the or said one of the multi-link paths, combining the link-suitability information and/or the I ink-group-suitability information for the path concerned so as to generate path-suitability information indicative of the suitability of the path concerned for transmission and/or reception; and selecting one of said paths for transmission and/or reception in dependence upon said path-suitability information. 15, A computer program which, when executed by a computing device of a particular communication apparatus of a communication system, causes the particular apparatus lo carry out a path selection method, the system comprising at least three such communication apparatuses, the particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-link path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from the particular apparatus to said other or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link along the path, and at least one said path being such a multi-link path, the method comprising: for each link or tor groups of links along at least the or one of the multi-link paths, obtaining link-suitability information and/or link-group-suitability information, as the case may be, indicative of the suitability of the link or group of links concerned for transmission and/or reception; for at least the or said one of ihe multi-link paths, combining the link-suitability information and/or the Iink-group-suitability information far the path concerned so as to generate path-s liability information indicative of the suitability of the path concerned for transmission and/or reception; and selecting one of staid paths for transmission and/or reception in dependence upon said path-suitability information. \6. A particular communication apparatus of a communication system, the system comprising al least three such communication apparatuses, the particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-link path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from ihe particular apparatus to said other or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link along the path, and at least one said path being such a multi-link path, the particular communication apparatus eompricing: obtaining mean*; operable, for each link or for groups of links along at least the or one of the multi-link paths, to obtain link-suitability information and/or link-group-suitability information, as the case may be, indicative of the suitability of the link or group of links concerned for transmission and/or reception: combining means operable, for al least the or said one ol the multi-link paths, to combine the link-suitability information and/or the link-group-suitability information for the path concerned so as to generate path-suitability information indicative of Ihe suitability of the path concerned for transmission and/or reception; and selecting means operable to select one of said paths for transmission and/or reception in dependence upon said path-suitability information. 17, The particular communication apparatus according to claim 16. being a user terminal and/or a mobile terminal and/or a relay apparatus. IS, A path selection method for use in a communication system, ihe system comprising at least three communication apparatuses, a particular one of said apparatuses being operable to transmit and/or receive a communication signal along at least two different communication paths, each said path being either a single-Jink path extending from the particular apparatus to another said apparatus directly over a single communication link therebetween, or being a multi-link path extending from the particular apparatus to said other or another said apparatus indirectly via one or more intermediate said apparatuses over a plurality of consecutive such links link-by-link alon^ the path, and a! least t>ne said path being such a multi-link path, the method comprising; for each link along at least the or one of the multi-link paths, amaining link-suitability information, PoR, indicative of the suitability of Ihe link concerned for transmission and/or reception, wherein the link suitability information, PoR, for each link is defined for a link between apparatuses A and B as: