WO2010115463A1

WO2010115463A1 - A method and apparatus for load balance in a relay-based multi-hop wireless network

Info

Publication number: WO2010115463A1
Application number: PCT/EP2009/054267
Authority: WO
Inventors: Haifeng Wang; Jiang Wang; Jing Xu
Original assignee: Nokia Siemens Networks Oy
Priority date: 2009-04-08
Filing date: 2009-04-08
Publication date: 2010-10-14

Abstract

A method comprising determining a load imbalance between a plurality of access points in the form of relay nodes, a user equipment having been allocated to one access node; and causing said user equipment to be reallocated to another access point.

Description

METHOD AND APPARATUS FOR LOAD BALANCE IN A RELAY-BASED MULTI-HOP WIRELESS NETWORK

FIELD OF THE INVENTION

The present invention relates to a method and apparatus and in particular but not exclusively for use in an arrangement using relays.

BACKGROUND

A communication system can be seen as a facility that enables communication sessions between two or more entities such as mobile communication devices and/or other stations associated with the communication system. A communication system and a compatible communication device typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standard or specification may define if a communication device is provided with a circuit switched carrier service or a packet switched carrier service or both. Communication protocols and/or parameters which snail be used for the connection are also typically defined. For example, the manner how the communication device can access the communication system and how communication shall be implemented between communicating devices, the elements of the communication network and/or other communication devices is typically based on predefined communication protocols.

In a wireless communication system at least a part of the communication between at least two stations occurs over a wireless link. Examples of wireless systems include public land mobile networks (PLMN)₁ satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). The wireless systems can be divided into cells, and are therefore often referred to as cellular systems.

A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. Typically a communication device is used for enabling the users thereof to receive and transmit communications such as speech and data. In wireless systems a communication devices provides a transceiver station that can communicate with e.g. a base station of an access network servicing at least one cell and/or another communications device. Depending on the context, a communication device or user equipment may also be considered as being a part of a communication system. In certain applications, for example in ad-hoc networks, the communication system can be based on use of a plurality of user equipment capable of communicating with each other.

The communication may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-iimiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. The user may also be provided broadcast or multicast content. Non-limiting examples of the content include downloads, television and radio programs, videos, advertisements, various alerts and other information.

3^rd Generation Partnership Project (3GPP) is standardizing an architecture that is known as the iong-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The aim is to achieve, inter alia, reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. A further development of the LTE is referred to herein as LTE-Advanced. The LTE-Advanced aims to provide further enhanced services by means of even higher data rates and lower latency with reduced cost. The various development stages of the 3GPP LTE specifications are referred to as releases.

Since the new spectrum bands for international mobile telecommunications (IMT) contain higher frequency bands and LTE-Advanced is aiming at a higher data rate, coverage of one Node B (base station) can be limited due to the high propagation loss and limited energy per bit. Relaying has been proposed as a possibility to enlarge the coverage. Apart from this goal of coverage extension, introducing relay concepts may also help in the provision of high-bit-rate coverage in a high shadowing environment, reducing average radio-transmission power at the User Equipment (UE). This may lead to long battery life, enhanced cell capacity and effective throughput, e.g., increasing cell- edge capacity, balancing cell load, enhancing overall performance, and reducing deployment costs of radio access networks (RAN). The relaying would be provided by entities referred to as Relay stations (RSs) or Relay Nodes (RNs). The relay nodes can be fixed or mobile, for example mounted to a high-speed train. In some systems the relay stations may be opportunistically available user equipment / mobile terminals that are not owned by the network itself.

Thus it can be contemplated that for future LTEs standard releases, releases 9 and upwards, relay nodes (RNs) are provided.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method comprising determining a load imbalance between a plurality of access points in the form of relay nodes, a user equipment having been allocated to one access node; and causing said user equipment to be reallocated to another access point

According to another aspect of the present invention, there is provided an apparatus comprising means for determining a load imbalance between a plurality of access points in the form of relay nodes, a user equipment having been allocated to an access node; and means for causing the reallocating of said user equipment to another access point.

DESCRIPTION OF DRAWINGS

For a better understanding of some embodiments of the invention, reference will be made by way of example only to the accompanying drawings in which:

Figure 1 shows an arrangement with three relay nodes in a sector: Figure 2 shows a flowchart of a first scheme for resource partition:

Figure 3 shows a second arrangement with three relay nodes per sector:

Figure 4 shows a second flowchart of a second scheme for resource partition;

Figure 5 shows a cumulative distribution function CDF of spectrum efficiency gain Figure 6 shows a cumulative distribution function of balance between different RNs;

Figure 7 shows a communication device; Figure 8 shows a base station; and Figure 9 shows a relay node.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

Due to the higher frequency carrier for the IMT-advanced systems, relay technology is utilized for coverage extension and spectrum efficiency improvement. In relay enhanced cellular systems (REC systems), how to share the frequency resource between one-hop users (local UEs) and two-hop users (remote UEs) is an interesting issue. In the previous research D6.13.7 WINNER Il Test scenarios and calibration cases issue 2- ,{see Table 0.1 and figure 2.2 of D6.13.7 WINNER II) only one relay node (RN) per sector is considered and FDMA is assumed between local UEs and remote UEs. Resource partition between them is implemented based on traffic load and UEs' affiliations are based on spectrum efficiency [ see D3.5.1 v1.0 IST-4-027756 WINNER Il Relaying concepts and supporting actions in the context of CGs, (see annex B of D3.5.1 v1.0 IST-4-027756).

However, in embodiments of the there might be more than one relay node (RN) per sector or cell to provide full coverage for REC systems. For different RNs in one sector, SDMA (Spatial Division Multiplexing Access) for the first hop and reuse one for the second hop may be efficient frequency utilization. Since the traffic load distribution is not uniform in the real environment, load imbalance between RNs may exist. In some embodiments of the invention, a resource partition scheme based on load balance among relays and BS (base stations) for REC systems is proposed to increase the spectrum efficiency.

REC systems with three RNs for each sector are considered, as shown in Figure 1. It should be appreciated that embodiments of the invention which have balancing between relay nodes alternatively may have only two RNs per sector or more than three. FDMA (frequency division multiple access) is used with local UEs and remote UEs. Reuse of frequencies between different RNs is assumed. In the arrangement of Figure 1 , a base station 4 is shown which serves three sectors 2a, 2b and 2c. By way of example, the first sector 2a is shown as having three relay nodes 3a, 3b, 3c. It should be appreciated, that other of the sectors may include no relay nodes or at least one relay node. It may be such that each sector has the same number of relay nodes, or in alternative embodiments, the sectors may have differing numbers of relay nodes The relay nodes 3a, 3b and 3c are shown located towards the edge of the sector 2a. The coverage area associated with each relay node 3a to c is illustrated and referenced 6a to 6c respectively. The coverage area of each relay node overlaps the coverage area of at least one relay node. In the case of the second relay node 3b, this coverage area 6b of this relay node overlaps two coverage areas. The coverage areas may extend beyond the edge of the sector defined by the base station 4.

In the arrangement of Figure 1 , a number of user equipment UE1-UE12 are shown in the cell sector and other are shown in the coverage area provided by a respective relay node which lies outside the cell sector.

A UE may be served by the base station or one of the relay nodes.

In the arrangement shown, the coverage area provided by the relay nodes may be arranged so that they do not extend significantly beyond the cell sector.

Reference is made to Figure 2 which shows the flowchart of a scheme. This scheme can be implemented by the following steps.

In step S1 , the UEs are in the cell sector.

In step S2, the spectrum efficiency of each UE is calculated for both one-hop and two- hop transmissions since UE's affiliation is unknown at the initial stage. A UE affiliation is which base station or which relay node the UE is arranged to communicate with.

In step S3, the UE's affiliation is determined based on the maximum spectrum efficiency determined in step S2. If the spectrum efficiency of one-hop transmission is larger than that of any two-hop transmission, then a given UE should be a local UE and communicate with the base station directiy. Otherwise, this UE should be considered a remote UE and thus communicate with the base station via a relay node. Based on the maximum spectrum efficiency, it is determine if a UE belong to BS or to a particular RN.

In step S4, resource partitioning is done based on the spectrum requirements for the one-hop transmissions and the two-hop transmissions. In particular, the resource partition is based on the maximum required bandwidth with reuse one for the second hop. Reuse one means all RNs use the same frequency resource. The resource partition for the relay link (to a relay node) and the access link {to the base station) is according to the maximum traffic load of the relay node. The requirement of the two-hop transmission is achieved by selecting the maximum spectrum requirement of different

RNs'.

S5 is the end of the flow.

A second scheme will now be discussed which takes into account the imbalance of traffic loads of RNs to improve spectrum efficiency

For the real network environment, the spectrum requirement of the RNs in one sector may be very different because of non-uniform toad distribution. Furthermore, if the UEs' affiliation is based only on maximum spectrum efficiency, this may result in a significant load imbalance between RNs. Since reuse one between the second hop of RNs is assumed, imbalance between the RNs will decrease the spectrum efficiency.

Reference is made to Figure 3 which shows a system in which embodiments of the invention may be provided is shown. Those elements which are the same as shown in Figure 1 will be referenced by the same reference numbers.

As with Figure 1 , a base station 4 is shown as serving three cell sectors 2a-2c. Each cell sector is shown as having three relay nodes 3a- 3c. In this example, the coverage area 6a-c associated with the respective relay nodes 3a-c are contained within the cell sector served by the base station. However, as shown in Figure 1 , the coverage areas may extend beyond the edge of the cell sector. Again, more or less than three relay nodes may be provided in alternative embodiments of the invention. It should be appreciated that in some embodiments of the invention, different numbers of relay nodes may be provided in the cell sectors.

Each coverage area of a relay node has an area 10 which is considered to provide an area of overlap with the base station. In this area, a UE can be served by the base station or the relay node. Each coverage area of a relay node has one or two areas 12 where there is overlap with the coverage area of a neighboring relay node. A UE in this area of overlap could be served by either of the relay nodes in question.

Thus for the overlapped zones shown as in Figure 3, a UE's spectrum efficiency to the neighboring access points might be very close.

With the scheme illustrated in Figures 3 and 4, the imbalance between RNs is alleviated by re-affiliating the overlapped UEs. The load balance is used for resource partitioning between one-hop transmissions and two-hop transmissions. Furthermore, the proposed intra-cell load balance may not complicate the handover process since the BS controls both one-hop and two-hop transmissions in one ceil. The scheme may provide increased spectrum efficiency and fairness for REC systems.

The flowchart of 4 will now be described. The first three steps T1 , T2 and T3 are similar to steps S1 to S3 of Figure 2. Each UE should calculate spectrum efficiencies for both one-hop and two-hop transmissions firstly (T2) and then achieve initial affiliation based on the maximum spectrum efficiency T3. The spectrum efficiency threshold, denoted as T_h, may be defined as the ratio of the selected spectrum efficiency to the maximum spectrum efficiency. T_h may be set as required. In step T7, the UE should also determine the virtual affiliation set based on spectrum efficiency threshold. The spectrum efficiency threshold is defined as the ratio of the selected spectrum efficiency to the maximum spectrum efficiency. Step T7 takes place after step T2 and can take place in parallel with step T3, before step T3 or after step T3.

Virtual affiliation means that one UE can be belonged to more than one access nodes (base station or relay node) if that UE's spectrum efficiencies for these access nodes are similar. For example, for one local UE, its spectrum efficiency of one-hop transmission is highest. If spectrum efficiency of its two-hop transmission is higher than the maximum spectrum efficiency multiplied by the spectrum efficiency threshold, it can be determined that this UE exists in an overlapped zone between the BS and the respective RN. For this local UE, besides the initial affiliation based on the maximum spectrum efficiency, it also has one or more virtual affiliation and such virtual affiliation can be used for load balance. This method can also be applied between RNs. Step T7 thus determines if a UE could be associated another node, if necessary.

The fourth step T4 is same as step S4. Step T5 is to judge if the load between RNs is balanced or not. If yes, the procedure ends at step T6. Otherwise, a resource repartition procedure starts in step T8 based on the UE affiliation set information determined in step T7. In step T8, load balance between RNs and between BS and RNs are considered jointly. The balance factor = load of the lightest RN/ load of the heaviest RN. This parameter may be used to denote the balance level, that is to say, the larger balance factor, the higher the balance level. The balance level may be considered to be high enough if the balance factor is larger than the balance threshold. The balance threshold may be set as desired.

In one embodiment, load balance between BS and RNs is achieved by reducing the heaviest RN's load and increasing the lightest RN's load. If there are more than one UE that can be used for load balance, one UE is selected at random for simplification. More complex criteria can be used to select a UE for load balance. Once one UE has changed its affiliation, a balance judgment is made in step T9 to determine if the different RNs are balanced. If so, the scheme is ended -step T10. If not, a determination is made in step T11 , if there are any further UEs in an overlap zone. If not the procedure is ended in step T12. If there is one or more further UE in an overlap zone, the process reverts to step T8 and the process continues until the balance between RNs is achieved or there is no more UEs at the overlapped zones for balancing.

It should be appreciated that load balance between two-hop transmissions and load balance between one-hop transmission and two-hop transmission are considered jointly in this embodiment. For load balance between one-hop transmission and two-hop transmission, it is achieved by both reducing the heaviest RN's load and increasing the lightest RN's load. It should be appreciated that in alternative embodiments of the invention, the load balance between two hop transmissions and load balance between one hop and two hop transmissions can be dealt with separately. In alternative embodiments of the invention, only one of the load balance between two hop transmissions and load balance between one hop and two hop transmissions may be considered.

In the above example, an iterative procedure is described where one UE is re-affiliated at a time. In alternative embodiments of the invention, a procedure may be used where more than one UE is re-affiliated at a time. This procedure may or may not be iterative.

In a further example, the iterative procedure may be replaced by a single step which re- affiliates the UEs based on for example the size of the load imbalance.

In one embodiment, step T7 may only be performed if it is determined that there is an imbalance.

A simulation for an embodiment of the invention will now be described.

Assumptions: - FDMA between BS and RNs

- Uniform UE distribution

- Reuse of one between RNs for the second hop

- Both LUE (local UE₁ one-hop transmission) and RUE (remote UE, two-hop transmission)have the same data rate requirement for simplification - Resource for RUE with load balance is restricted so that it is not larger than the original maximum traffic load of RN's without balancing

- Resource partition for relay link and access link according to the updated maximum traffic load of RN's after balancing:

The related parameters for simulation are presented as Table 1.1 : Table 1.1 simulation parameters

The spectrum efficiency gain of this simulation is shown in Figure 5 and Table 1.2. Figure 5 shows CDF against spectrum efficiency gain. Table 1.2 shows the relationship between T_h with average spectrum efficiency gain and the probability when the gain larger than 5%.

Table 1.2 T_h vs. spectrum efficiency gain

From Figure 5 and Table 1.2, it can be sent that the spectrum efficiency gain of an embodiment of the invention may be significant and the gain increases when the threshold T_h decreases. The reason for this is that there are more UEs in the overlapped zone and thus more flexibility to ioad balance for the lower threshold.

This simulation is based on the assumption of a uniform distribution of UEs. if UEs distribution between RNs is non-uniform, such as might occur when one of the RNs is placed at a hotspot, the gain of some embodiments of the invention can be further increased. Figure 6 presents a graph of CDF against different balance factors for different schemes. Figure 6 shows that balance between RNs of embodiments of the invention may be improved for different efficiency threshoids.

Some embodiments of the invention may have one or more advantages:

- Increases spectrum efficiency significantly, especially for lower spectrum efficiency threshold;

- A single UE's spectrum efficiency and the balance level between RNs are both considered in some embodiments of the invention and the resource partition flexibility is increased through UE virtual affiliation;

- Embodiments of the invention may alleviate the imbalance between RNs and increases the UE fairness between RNs.

A communication device or UE can be used for accessing various services and/or applications provided via a communication system. In wireless or mobile communication systems the access is provided via an access interface between mobile communication devices (UE) 101 and an appropriate wireless access system. A mobile communication device 101 can typically access wirelessly a communication system via at least one base station or similar wireless transmitter and/or receiver node of the access system. The communication devices can access the communication system based on various access techniques, such as code division multiple access (CDMA), or wideband CDMA

(WCDMA), the latter technique being used by communication systems based on the third Generation Partnership Project (3GPP) specifications. Other examples include time division multiple access (TDIvIA), frequency division multiple access (FDMA), space division multiple access (SDMA) and so on. In a wireless system a network entity such as a base station provides an access node for communication devices.

Non-limiting examples of appropriate access nodes are a base station of a cellular system, a base station of a wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access), a relay node or the like. In certain systems the base station is referred to as Node B, or enhanced Node B (e-NB). Each mobile device may have one or more radio channels open at the same time and may receive signals from more than one base station and/or other communication device. A transceiver may need to be able to send and receive at the same time. This can be addressed by applying a multiplexing scheme to communications to and from a transceiver. Time-Division Duplex (TDD) is an application of time-division multiplexing where outward and return signals are separated based on time. It emulates full duplex communication over a half duplex communication link, in Frequency-Division Duplex (FDD) the transmitter and receiver operate at different carrier frequencies, and the uplink and downlink sub-bands are separated by the "frequency offset". Both of these techniques may be used by a transceiver.

A non-limiting example of mobile architectures where the herein described principles may be applied is known as the Evolved Universal Terrestrial Radio Access Network (E- UTRAN). The eNBs may provide E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical layer protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the user devices.

Figure 7 shows a schematic, partially sectioned view of a UE 101 that can be used for communication with at least one access node such as a base station or a relay node of an access system. An appropriate mobile communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS), a portable computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A wireless mobile communication device is often referred to as a user equipment (UE).

A mobile communication device may be used for voice and video calls, for accessing service applications provided via a data network. The mobile device 101 may receive signals via an appropriate radio receiver of the mobile device. In Figure 7 the receiver is designated schematically by block 107. The receiver may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device. The receiver arrangement of the mobile device may be configured for enabling tuning to different carrier frequencies. A mobile device is also typically provided with at least one data processing entity 103, at ieast one memory 104 and other possible components 109 for use in tasks it is designed to perform. The data processing, storage and other entities can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 106. The user may control the operation of the mobile device by means of a suitable user interface such as key pad 102, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 105, a speaker and a microphone are also typically provided. Furthermore, a mobile device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

Reference is made to Figure 8 which schematically shows a base station 4 embodying the invention. The base station has an antenna 48 which is arranged to transmit signals to user equipment. The antenna 48 may optionally be used to transmit signals to one or more relay nodes. The signals to the relay node may alternatively be transmitted via a different connection {not shown) which may be wired.

The antenna is also arranged to receive signals from the user equipment. Optionally signals may be received from one or more relay nodes. Alternatively signals from a relay node may be received via the different connection.

A radio frequency/baseband circuit 50 is connected to process baseband signals up to the radio frequency for transmitted signals and to down convert radio signals down to the baseband frequency for received signals. Separate paths and/or circuits may be provided for the transmitted and received signals.

The circuit 50 is connected to processing block 52. The processing block comprises a first block 52a which is arranged to provide initial allocation information. The first block 52a may comprise memory which stores this information which is received from the UE. Alternatively or additionally, the first block may be arranged to determine initial allocation based on received information, in this situation, the first block may additionally or alternatively comprise a processor.

The second block is a load imbalance block 52b which is arranged to determine an imbalance in the loading of two access points, for example, two relay nodes. The second block may comprise a processor and optionally a memory. The second block is connected to a third block 52c which provides stored or determined information on the resource partition. The third block 52c comprises a memory and/or a processor. The resource partition information may be determined from information which the third block has received or may be received, for example from other access points and/or UE.

The load imbalance block 52b comprises a processor which is arranged to determine an imbalance between the loading of two nodes, based on one or more of the information from the first block 52a and the third block 52c. The load imbalance block 52b is connected to a reallocation block 52e.

Responsive to imbalance, the reallocation block 52e is arranged to cause the reallocation of a UE. The reallocation block 52e is connected to a fourth block 52d which is arranged to store information or to determine information relating to UE in a region of overlap between two access points. The overlap information may be determined from information which the fourth block has received or may be received, for example from other access points and/or UE.

The reallocation block 52e is also connected to the first block 52a and to a sixth block 52f. The fifth block provides virtual affiliation information. This information may be stored in a memory, having been received or may be calculated by a processor in this block, based on received information.

The reallocation block 52e causes reallocation based on the information from one or more of the blocks 52a, 52b, 52d and 52f.

Reference will now be made to Figure 9 which shows a relay node embodying the invention. This relay station has a simple function. The relay station is arranged to receive signals via receiver 58 from the base station, amplify the signals using amplification circuitry 60 comprising one or more amplifiers and transmit the signals to the UEs via transmitter 62. The receiver and transmitter are connected to an antenna 64. This arrangement can also be used to receive signals from the UEs and transmit those signals to the base station. In the alternative a parallel arrangement is provided for dealing with the signals from the UEs. The relay node may in alternative embodiments of the invention, be arranged to process the received signals before retransmitting those signals. The processing may include down converting and up converting the signais, filtering and/or the like.

The relay node may do some or all of the processing instead of the base station. The relay node may therefore include at least some of the blocks of the processing block 52 .The relay node may for example be arranged to determine which UE are affiliated therewith and transmit the information to the base station.

It should be appreciated that in those embodiments where there is a wired connection between the base station and the relay node, the communication between the base station and the relay node will be via the wired connection.

At least some of the processing of processing block may be carried out by one or more processors in conjunction with one or more memories.

Processing block may be provided by an integrated circuit or a chip set.

At least some of the processing block may alternatively or additionally be provided by a controller of the access points, for example a radio network controller or the like.

The required data processing apparatus and functions of a relay node and a base station apparatus as well as an appropriate communication device may be provided by means of one or more data processors. The above described functions may be provided by separate processors or by an integrated processor. The data processing may be distributed across several data processing modules. A data processor may be provided by means of, for example, at least one chip. Appropriate memory capacity can also be provided in the relevant nodes. An appropriately adapted computer program code product or products may be used for implementing the embodiments, when loaded on an appropriate data processing apparatus, for example in a processor apparatus associated with the base station, processing apparatus associated with relay node and/or a data processing apparatus associated with a UE. The program code product for providing the operation may be stored on, provided and embodied by means of an appropriate carrier medium. An appropriate computer program can be embodied on a computer readable record medium. A possibility is to download the program code product via a data network.

It is noted that whilst embodiments have been described in relation to LTE, similar principles can be applied to any other communication system where relaying is employed. Therefore, although certain embodiments were described above by way of example with reference to certain exemplifying architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein.

It should be noted that although embodiments of the invention have been described in the context of relative high carrier frequencies, embodiments of the invention can be used with lower frequency carriers.

It is also noted herein that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention.

Claims

1. A method comprising: determining a load imbalance between a plurality of access points in the form of relay nodes, a user equipment having been allocated to one access node; and causing said user equipment to be reallocated to another access point.

2 A method as claimed in claim 1 , wherein said causing of the reallocating of said user equipment is in dependence on on virtual affiliation.

3. A method as claimed in claim 1 or 2, comprising initially allocating said user equipment based on a determined spectrum efficiency.

4. A method as claimed in claim 3, comprising calculating a spectrum efficiency for said user equipment for an access point in the form of a base station and for at least one relay node and initially allocating said user equipment based on a better one of said calculated spectrum efficiencies.

5. A method as claimed in any preceding claim, wherein said causing of said reallocating comprises causing reallocation from an access point having a higher loading to an access point having a lower loading.

6. A method as claimed in any preceding claim, comprising determining if said user equipment is allocatable to a further one, other than said one, of said access points.

7. A method as claimed in any preceding claim, wherein said access points comprise a base station and a plurality of relay nodes serving a cell. .

8. A method as claimed in any preceding claim, causing of said reallocating of said user equipment is responsive to said user equipment being in a region of overlap between two access nodes.

9. A method as claimed in any preceding claim, wherein said determining and causing reallocation are repeated until no imbalance is determined.

10. A method as claimed in any preceding claim, wherein said determining and causing reallocating are repeated until no further user equipment is reallocatable.

11. A method as claimed in any preceding claim, said determining and optionally causing said reallocating are repeated after each user equipment is reallocated.

12. A computer program, comprising program code means adapted to perform any of the steps of claims 1 to 11 , when the program is run on a processor.

13. Apparatus comprising: means for determining a load imbalance between a plurality of access points in the form of relay nodes, a user equipment having been allocated to an access node; and means for causing reallocation of said user equipment to another access point.

14 Apparatus as claimed in claim 13, wherein said means for causing realiocation of said user equipment is arranged to be responsive to virtual affiliation.

15. Apparatus as claimed in claim 13 or 14, comprising means for initially allocating, said means being responsive to a determined spectrum efficiency.

16. Apparatus as claimed in claim 15, comprising means for calculating a spectrum efficiency for said user equipment for an access point in the form of a base station and for at least one relay node and initially allocating said user equipment based on a better one of said calculated spectrum efficiencies.

17. Apparatus as claimed in any of claims 13 to 16, wherein said means for causing reallocation is arranged to reallocate a user equipment from an access point having a higher loading to an access point having a tower loading.

18. Apparatus as claimed in any of claims 13 to 17, wherein said means for causing reallocation is arranged to determine if said user equipment is ailocatable to a further one, other than said one, of said access points.

19. Apparatus as claimed in any of claims 13 to 18, said means for causing reallocation is arranged to cause reallocation of said user equipment when said user equipment is in a region of overlap between two access nodes.

20. Apparatus as claimed in any of claims 13 to 19, wherein said means for causing reallocation is arranged to repeatedly cause reallocation of user equipment until no imbalance is determined.

21. Apparatus as claimed in any of claims 13 to 20, wherein said reallocation means is arranged to repeatedly reallocate user equipment until no further user equipment is realloca table.

22. Apparatus as claimed in any of claims 13 to 21 , wherein said determining means and means for causing reallocation are repeatedly operated after each user equipment is reallocated.

23. An access point comprising apparatus as claimed in any of claims 13 to 22.

24. An access point controller comprising apparatus as claimed in any of claims 13 to 22.

25. An integrated circuit or set of integrated circuits comprising an apparatus as claimed in any of claims 13 to 22.