GB2479378A

GB2479378A - Using SDMA to transmit from a base station to a first UE directly and from the base station to a second UE via a relay station

Info

Publication number: GB2479378A
Application number: GB1005794A
Authority: GB
Inventors: Yong Sun; Ngoc-Dung Dao; Filippo Tosato
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-04-07
Filing date: 2010-04-07
Publication date: 2011-10-12
Anticipated expiration: 2030-04-07
Also published as: JP2011239369A; GB2479378B; US8599739B2; US20110249613A1; GB201005794D0

Abstract

The present application concerns the use of space division multiple access (SDMA) to simultaneously transmit information from a base station NB directly to a first mobile user terminal UE, and from the base station to a second mobile user terminal RUE via a relay station RNB. The invention aims to minimize the interference which the first user terminal experiences due to relay transmissions, and also the interference which the second user terminal experiences due to transmissions to the first user terminal. In a first beamforming embodiment this is achieved by precoding base station transmissions so as to provide spatial orthogonality. In a second embodiment a spatial component transmitted to the relay node contains not only signal information for the second user terminal RUE but also signal information which is also sent directly to the first user terminal UE in another spatial component.

Description

REMOTELY WEIGHTED SDMA TRANSMISSION

The present invention concerns improvements to existing methods of establishing wireless communication.

Available for wireless communication relles upon access to a portion of the electromagnetic spectrum suitable for use for carriage of signals for reception by passive antennas. This portion of the spectrum is commonly known as the radio spectrum.

The radio spectrum provides a finite resource for use by a variety of operators. The use of the radio spectrum is very often Government regulated, and is considered a public asset of a strategic nature. This, and the physical bound of the radio spectrum, imposes a limit on the design of cellular radio systems.

Rather than allocating parts of the radio spectrum for exclusive use by particular operators, various forms of sharing of bands have been established. A variety of communications protocols allow operators to organise the manner in which they will share a particular band of frequency for intermittent, occasional or constant use.

In practice, all sharing methods involve some form of interference, which is proportional to the power of transmitted signals. For voice services, transmitter power control has been shown to improve performance by balancing signals and with respect to interference.

As the demand for transmission of data by wireless means increases, the allocation of resource to satisfy diverse quality of service (QoS) requirements is of interest. Since the characteristics of data services differ from those of voice services, a different approach to transmission needs to be investigated.

In a conventional mobile transmission system, communication is established by means of a base station which can establish communication with mobile stations within its range. This range is often known as a cell. Communication from between different mobile terminals with respective base stations (with associated cells) can cause interference between the cells. This is known as intercell interference. For a conventional deployment of base stations (with planned design of cells) the size of cells is sufficiently large, and radio spectrum can be designed and pre-allocated to avoid interference. At the edges of cells, mobility management has been developed to handle inter-cell interference by way of various handover mechanisms. Also, because of the relatively large size of such cells, the level of interference is generally low and can fairly easily be controlled. This can be by way of power control and handover can be affected reasonably quickly.

However, existing concepts have generally been developed with the intent to transmit voice data in a wireless communication system comprising a cellular plan. In contrast, data transmission, and particularly services such as Service on Demand (SoD) gives rise to a need to provide a system which can accommodate a range of different needs, such as low quality voice data to high quality real time video transmission.

Conventional interference illumination might not accommodate such new applications, as relatively high power high speed transmission might impact very negatively on a low power service.

Furthermore, there is no reason to assume that the existing concept of acellular coverage" need be retained for ever more. A more flexible approach needs to be accommodated particularly with the introduction of "small cell coverage" applications, such as multihop relays, femto-cells, and so on. With or without such implementations, the QoS of experience as a cell edge under future implementations could be much higher than the conventional definitions, anticipate.

With the presence of small cells, overlying a macrocell, power leakage becomes a significant issue, such as when high Q0S services are required by a mobile station.

Then, the power at the cell edge of a small cell could be sufficiently high that interference to and from adjacent cells can arise. This phenomenon is illustrated in Figure 1, which shows a base station (NB) defining a macrocell, the practical bounds of which are defined by the large solid ellipse. The NB is able to communicate directly with user equipment (UE), and additionally with a multihop relay (RNB). The RNB can enhance the communication possibility within a smaller zone (defined by a broken elliptical line). Further, femto-cells, which might be home installations, (HNB) define further small cells of usable transmission range, for use in communication with UEs.

As shown in Figure 1, UEs might have a desired signal to receive, such as a local HNB if in range, or the NB if the UE is not in range of a smaller cell. However, despite not being within the formal range of a small cell, such a device might be in receipt of interference from broadcasts of those HNBs and RNBs.

Figure 2 illustrates in a simpler form the signals involved in establishing communication between an NB and a user equipment. The user equipment in Figure 2 is designated RUE and UE respectively, RUE being that associated with a multihop relay RNB and the user equipment UE not being so associated. in order to communicate with user equipment UE, the base station NB sends a signal Si thereto. A signal S2 is sent to the multihop relay RNB, which then passes a consequent signal S2' on to the user equipment RUE. Corresponding interference signals from the NB and the RNB, Ii and 12 respectively, are encountered by the RUE and the UE respectively.

This phenomenon can be also explained further by reference to Figure 3. This figure shows a frame structure in which, in a first time period characterised by spatial operation, signals are beamformed to the RNB and the UE respectively, while a second time period is associated with interference between signals emanating from the RNB and the NB, both directed to the UE.

An aspect of the present invention provides a mechanism to pre-eliminate the potential interference of a transmission for use in wireless communications such as the presently standardised network MIMO approach and its further developments.

Further aspects of the invention can be determined by inspection of the appended claims. In particular, while specific implementation of the invention, given the following detailed description, will be evident to the skilled reader, the reader is appraised that the invention can be implemented by means of a computer program product, which might be delivered on computer readable storage means or by way of a signal. The product may provide a full implementation of the invention or may rely on general purpose functional elements, which may be hardware or software implemented, in order to configure a computer to perform in accordance with the invention.

The invention will now be further described with reference to specific embodiments thereof, with reference to the accompanyin drawings, in which: Figure 1 is a schematic illustration of a typical cellular communication network including a multi-hop relay and a femto cell deployment; Figure 2 is a schematic vector diagram of communication pathways in a typical

example;

Figure 3 is a transmission signal diagram demonstrating possible instances of interference in an SDMA communication; Figure 4 is a schematic vector diagram of communication pathways in accordance with an embodiment of the invention; Figure 5 is a schematic communication process diagram for a method of communicating in accordance with an embodiment of the invention; Figure 6 is a diagram illustrating 3D beamforming as used in an embodiment of the invention; Figure 7 is a schematic diagram illustrating a mode of implementation of the described embodiments of the invention; and Figure 8 is a vector diagram illustrating a method of formulating 3D beamforming in connection with an embodiment of the invention.

In the following description, the presented inventive concept will be demonstrated with the aid of specific embodiments thereof, and their operation, together with detailed

mathematical description as appropriate.

As shown in figure 2, 1 is the signal from NB to UE in macro-cell, S2 is the signal from NB to RNB. For only S1 and S2 communication can be conventionally supported with a simple consideration of MU-MIMO implementation. However, if the simple MU-MIMO concept is applied to this scenario, it will potentially cause interference, , as indicated in the figure, which is from NB to RUE. Furthermore, without proper control, interference will potentially arise when the RNB starts transmitting, the 2, from the RNB to UE, as also shown in figure 2.

It is noted that both of the interferences and 2 will only arise during a relaying transmission, as indicated in figure 3 as the "interference zone".

Two embodiments of the invention provide approaches to accommodate and avoid the impact of this interference.

In a first embodiment of the invention, focus is placed on interference avoidance and elimination. As shown in figure 2 and described previously, there are conceptually three users on the NB during the whole packet transmission, namely UE, RNB and RUE. The MU-MIMO concept can be applied to transmit virtually to the three users, for which precoding is applied at the NB to support the three users by virtually transmitting to the three users simultaneously to avoid interference. This is despite there not being any physical transmission to the RUE from the NB.

To achieve this, the NB must have the capability to provide sufficient spatial freedom, that is the capacity for support of at least three users in this case. Under current technical specifications for such equipment, this normally relies on antenna configurations, which means that the NB needs at least three antenna sets.

Here, it is assumed that the NB is equipped with antenna sets intending to support two users simultaneously, as specified, RNB and UE. Consequently, it can also be capable to support UE and RUE, as the two different users.

Therefore in this embodiment of the invention, initially precoding is applied at the NB to support the UE and the RUE, rendering transmissions to the UE and RUE spatially orthogonal. Next, precoding is applied at the NB to support the UE and the RNB but the precoding at the UE should be maintained as for the first stage.

With this procedure, interference potentially arising from the NB to the RUE can be safely avoided. This operation does not require extra degrees of freedom on NB, for instance the provision of any extra antenna set(s).

As the RNB only needs to support one user (RUE), conventional beamforming techniques could be employed to perform directional transmission to the RUE.

However, due to the potential interference from RUE to UE as indicated, the beamforming has to be performed with due attention given to the interference. Thus, the RNB should treat both RUE and UE as its users (but UE as a "virtual user) to perform precoding, but actually providing only support for the RUE by using one precoding matrix specifically on the RUE. With this operation, there is no potential interference to the UE, so the NB and the RNB can perform transmission simultaneously, and effectively NB transmits to UE and RNB transmits to RUE without interfering with each other.

A second embodiment of the invention focuses on provision of a coordinated multiple-point transmitter. In the first embodiment, the problem to be addressed was treated with a conventional approach. However, in the second embodiment, the NB and the RNB are considered in combination as one joint network node to provide the service, The objective in this embodiment is to support the RUE and the UE by this composite node without the various consequent communications disturbing each other.

Without any limitations on the ability to consider NB and RNB as two independently functioning entities in the network, they can easily be operated in a coordinated manner in line with the concept of network MIMO.

However, there are several limitations on the deployment of a composite NB and RNB node. In practical applications, NB and RNB may well have fundamentally different backhaul support, signalling and controlling capacities, resource and power allocations, and so on.

It is reasonable to assume that RNB has no backhaul support and it has no capacity to control user access. The RNB may well have much lower transmit power than the NB.

All of these present many challenges to the coordinated operation of the RNB and the NB. In this embodiment of the invention, despite these challenges, the coordinated multiple-point transceiver through combination of the effect of the NB and the RNB is realised.

As described previously, the NB is able to support two users, UE and RUE, thus it is capable of supporting MU-MIMO as it will have two or more antennas. In practical systems, it is now commonplace to provide multiple antennas at the NB to support multi-antenna and/or multi-user transmission. This might not be the case at an RNB; however, in any relaying system, the critical problem is spectrum efficiency.

In this embodiment, the NB performs both a relaying transmission to the relayed UE (RUE) and a normal transmission to its own UE, by sharing the same radio resource.

In this embodiment, it is reasonably assumes that the NB has higher-level configurations than the RNB, and full control of the transmission. To achieve this, a remotely weighted transmission mechanism is used. The objective of this is to manage all of the potential interference and to convert the so-called interference into a desired signal. The mechanism and procedure are described as follows: * The NB initially divides the transmission to its target UE into two parts: o A first part of the transmission is synchronised with the transmission from NB to RNB; o A second part of the transmission is synchronised with the transmission from RNB to RUE; * The NB performs SDMA to transmit simultaneously to UE and RNB; * In contrast with conventional SDMA, in this first stage of the transmission, the NB also transmits to the RNB the signal/data of the second part of its transmission intended for its UE; * The RNB receives signal/data from both transmissions of NB to UE (2nd part) and NB to RNB; * After the first stage transmission, a second stage of the transmission comprises the NB and the RNB jointly performing coordinative transmission to the UE and the RUE; * In the coordinative transmission, the NB and RNB are jointly weighted to support both UE and RUE; * The weighting matrix on RNB is remotely derived by the NB and transmitted to the RNB by weighting the transmission from the NB and the RNB at the first stage of the transmission; * The remotely weighted approach is not only applied to the desired signal/data, but also applied to any remaining interference form the system inside; * For the remaining interference, or, potentially remaining interference, the NB initially produces a nulling space for the interference; * The NB weights antenna sets on both the NB and the RNB to steer the interference towards the nulling space, which eventually moves the interference away from the UE and the RUE.

This approach enables interference-free transmission within such radio-resource sharing configurations.

Furthermore, the operations are all performed at the NB but take advantage of the RNB as an extra spatial freedom for both generation of a desired signal and avoidance or management of potential/remaining interference. A further advantage of this is that it achieves the relaying transmission in a conventional manner without requiring any additional function from the RNB.

For this embodiment of the invention, the basic configuration comprises an NB with a minimum of two antennas, and an RNB with one or more antennas. The UE and RUE also each have at least one antenna.

It will be appreciated that any extensions of this configuration and its various combinations are clearly possible with the same principles.

For both multiuser transmission with M transmitter antennas and N users, and, multi-antenna system with M transmitter antennas and N receiver antennas, the system can be modelled as y=Hx+n (1) where X [x1,x2,. ..,XM] and -[y,y2,.. .,YMI are the transmitted and received sequence, respectively, with the noise vector.,nN] and the N X M matrix H. For simplicity without loss of generality in precoding, the symbols prior to precoding are defined as v = [vl,v2,...,vN]T to be transmitted. V can be processed linearly or non-linearly. The processed V is defined as U. For linear precoding, V does in factor not need to be processed, thus, U = V. However, for non-linear precoding, it can be processed as (2) where is a constant related to the constellation size and is a complex vector.

A generic precoding matrix is defined as W The precoded sequence, S, can be expressed as s=Wu (3) Prior to transmission, the precoded symbols need to be scaled, which is required in order to ensure no increase in transmit power due to the precoding operation, such that (4) where the scaling factor HI, and Now, equation (1) can be extended to y=pffWu+n (5) It should be noted here that the precoding matrix, W, applies to any precoding algorithm, including both linear and non-linear precoding.

Focusing specifically on MU-MIMO precoding, based on equation ( 5), the received signal on each user can be expressed as Yk =HkpIW,ul+nk (6) Now with reference to figure 2, the related transmission channel is represented in figure 4. With the NB performing MU-MIMO to the UE and the RNB, the received signal at the UE and the RNB can be expressed as YUE = PUEHS1WUE1UE + PRNBHSWRNBURNR + UE YRNB = PRWBHS2WRNBURNB + PIJEHSZWI/EUUE + (7) For optimised precoding, Ik, WB = 0 IISWUE=0 (8) In that case, there is no interference within the MU-MIMO transmission. However, in a practical transmission, the interference needs to be considered. For this purpose, the interference terms are always retained in equations here and later.

As shown in figure 3, the transmission has two parts; the first part is the transmission from NB to UE and NB to RNB, and the second part is the transmission from NB to UE and RNB to RUE.

Due to the potential interference as described previously, the following transmission mechanism and procedures are used in accordance with this embodiment of the invention.

The transmission sequence is separated into two parts: UUE = [uUE 11UE,,2] (9) The is the transmission part synchronised with NB -RNB and the is the transmission part synchronised with RNB -RUE.

The NB transmits the UUE2 to the RNB while NB performs transmission to UE and RNB. Then,

YUE PUEPIHSWUEPIUUEP, + +PRNBHSWRNBUUE + UE YB PRNBHS2WRNBURNB +pJ?NBHSW/NBU(/j ( 10) +pJEHcWJEuhJE + RNB As indicated in equation (10), this is specific to the first part of transmission. The ULJE2 transmitted to RNB is in preparation for the second part of the transmission, in which the NB and RNB coordinate.

The second, coordinative transmission includes two signals: URUE -which is the signal transmitted from RNB to RUN; and UUE2 -which is the second part of signal transmitted from NB to UE.

UROE is in factor the same signal as the Upjqj which is the relayed signal from NB to RUE. The two required signals are available to both NB and RNB, which enable the requirement of the coordinative transmission. For the coordinative transmission from both NB and RNB, the channel conditions are changed as well. For simplicity, the joint channel can be expressed as On UE, HUE_c [Hs,H,]T HRJE [H *,H1 OnRUE, -L S2 With this transmission, the original interference channels are now transferred to the signal channels for the coordinative operations.

YUE PUE2HUE_C'/E2hJUE2 +pRUEHiE CWRUEURI/E + UL (11) YRUE PRUEHRUE_CWRUEUR(/E +puEHRUEWJE 11UE2 + RUE The precoding weights, WJE2 and WR/E are the coordinative weights on both NB and RNB antenna sets. The weights on the RNB are calculated on NB and delivered to the RNB together with the signals transmitted to the RNB.

The weighting factors for the two signals transmitted to the RNB as described previously could depend upon the precoding weights to RUE or UE in the second part of the transmission.

NB and RNB coordinatively steer interference to nulling space. This is achieved as follows. The transmission includes two required signals as indicated in equation (11), which are: LlUE2 -the second part of transmission from NB to UE UR -the relayed signal transmitted from RNB to RUE As described in equation (10), three signals are transmitted during the first part of transmission. With an optimal SDMA, only U1/E,,2 and URUE should be seen during the second transmission as described in equation (11).

However, in a practical application, it is rare to have an optimal deployment, such that the SDMA operation during the first transmission might not have an ideal precoding to guarantee the orthogonal transmission from the NB to UE and RNB.

As indicated in equation (10), there might be three potential sources of interference during the first transmission.

PRNBHSIWRNBURNB. an interference to UE from the signal transmitted from NB to RNB; pJq8H1WpJ/Bh1uEp2; an interference to UE from the second part of the signal transmitted to the UE; and PIJE H WUE UI/F p1 2 p1 ": an interference to RNB from the first part of the signal transmitted to the UE.

During the second transmission, on'y the third interference could remain as it is the interference to RNB, which could be retransmitted through the RNB.

The interference can be predicted after the precoding performed at the NB during the first transmission. Hence, just at the beginning of the second transmission, the interference might be precoded through the antenna sets of NB and RNB. For this, the NB has to produce a virtual user channel for the interference, H,, to meet the requirement of f H, JHE c° --(12) IH jT v I''RUE c in which case the virtual interference channel is uncorrelated to all the channels of the desired signals.

In other words, the H,1 is the null space for H(JEC and "RUE_C Therefore, the precoded interference can be expressed as YIUE,, H,JW,JUUh 7,JWV,NBI (13) = H, 71W,, _NB_2 U(JJ,.

PuE, H WUE WV, _RNB 7vj is the power control on NB to limit the UhF impact on the second transmission, even though "- Also, the W,,,1NB might have an impact on the desired signal.

Ideally, the WV,_RNB needs to be optimised together with the weights on the desired signal of the second transmission.

To simplify the problem, the precoding matrix might be only applied at the NB based on the existing weighting factor at the RNB: _NB_1 YI_uE, = 1v! r_1w, _NB_2 ( 14) With regard to equations (13) and (14), there are two special cases that should be considered: Case I: known channel state information The key issue when channel state information is known, is the accuracy thereof that can be obtained by the eNB and, more particularly, that the might need to be.

Case-Il: unknown channel state information In this case, the eNB might be able to be retain information of its experience of the system operation, when the H52 is not fully detected. The H52 then becomes unknown or, more precisely, becomes uncertain.

Case I relies heavily on the channel estimation engine and feedback mechanism(s) operated in the system. In this case, the interference can be easily steered away on the same plane of the desired signals.

In case II, there might not be complete lack of knowledge of channel information but it might be not accurate, so the third weighting factor, P/FHSWJJ: ,becomes uncertain.

However, this is coherent interference that needs to be taken into account.

For both cases, one solution is to apply 3D beamforming effectively to steer the interference out of the plane of the desired signal to avoid any uncertainties. 3D beamforming is described in US6602194. It is well-understood that multi-dimensional beamforming can be decomposed into two linear step processes, on either a planar phased array beamformer or a linear array beamformer. For a currently practical 3D beamformer, it is complicated to steer in different directions in the 3D domain, and an example illustrating this is set out below. In the present embodiment, the desired signals are transmitted on one plane and the 3D beamforming applied here is only to steer the interference to the orthogonal plane as demonstrated in figure 6, which highly simplifies the beamforming processing.

Targeting on drawbacks of those conventional relaying systems, especially on low efficiency of transmission and/or potential interference with radio resource sharing, the presently described approach uses a remotely weighted SDMA mechanism to allowing a base station to support relaying transmission and its normal macro-cell transmission simultaneously with shared radio resources.

To avoid any potential interference caused inside the system, two methods have been described above. The first method involves interference avoidance and elimination, which applies formed beam(s) on interference. The second method is characterised by coordinated multiple-point transmission.

The coordinative transmission is proposed to co-ordinatively support a macro-cell mobile terminal and a relaying mobile terminal on both a macro base station and a relay station.

In a practical cellular system, the two methods can be flexibly operated according to deployment conditions and environments. For an isolated relaying transmission coverage, where any relayed mobile terminal is not able to communicate with the macro base station, and the relay station has no impact on any macro mobile terminal, the coordinative transmission may not be required.

An operational procedure is depicted in figure 5, as an example to show the key steps of the coordinative transmission. However, this process might be only a part of the practical operation. Since the operation is fully controlled by the macro base station, the base station is able to determine the best mode of operation based on the practical deployment and environment.

Fundamentally, the base station will request all feedback from the relay station (RNB), the relayed mobile user (RUE) and the macro mobile user (UE). It is possible for the macro base station to collect all the channel state information for the channels between the mobile terminals and the relay station and macro base station. Consequently, it is possible for the base station to determine the potential interference in the system. It is also reasonable to assume that the base station is fully aware of the antenna configurations of the relay station and mobile terminals.

If the relay station covers a fairly isolated area as mentioned previously, the macro base station can employ SDMA to the relay station and the macro mobile terminal and the relay station can directly transmit to the relayed mobile terminal, without any need for interference avoidance.

In the event that the mobile terminals can see both the macro base station and the relay station, the second embodiment involving coordinated multiple-point transmitter can be employed. The key operation of the coordinated multiple-point transmitter is on both the macro base station and the relay station. Besides the precoding on the desired signal, it also performs precoding on interference to steer the interference away from the desired signal.

Providing multiple antennas on a relay station might create more spatial freedom for beamforming and/or interference elimination.

The major advantage of the proposed remotely weighted SDMA transmission is that it is operated at the macro base station without any required extra functionality at a relay station or at mobile terminals. Even though it has been described in the context of multihop relay transmission, it can be extended to femto-cell transmission. The proposed mechanism provides high efficiency with radio resource sharing but interference-free relaying transmission.

Relay technologies have been actively studied and developed in the standardization process of next-generation mobile broadband communication systems such as 3GPP LTE-Advanced, IEEE 802.16j, and IEEE 802.16m. One example is on 3GPP LTE.

Two types of Relay Stations have been defined in 3GPP LTE-Advanced: Type-I and Type-Il as set out in 3GPP TR 36.814 V1.2.1, "Further Advancements for EUTRA: Physical Layer Aspects," Tech. Spec.n Group Radio Access Network Rel. 9, June 2009, and as shown in the upper portion of figure 7.

Specifically, a Type-I Relay Station can assist a remote UE unit, which is located far away from an eNB (or a BS), to access the eNB. On the other hand, a Type-Il Relay Station can help a local UE unit, which is located within the coverage of an eNB (or a BS) and has a direct communication link with the eNB, to improve its service quality and link capacity.

The remotely weighted SDMA mechanism of the described embodiment can be deployed perfectly in the Type-Il Relay Station scenario as shown in figure 7. With this transmission, the potential capacity can be double that of the current specification in the standard. The interface between eNB and RNB has been well defined between eNB and RNB for data transmission, which is suitable for the application of the proposal.

For the 3D beamforming to steer away interference as described above, an example is now provided. A formulation for 3D beamforming is shown in figure 8. The decomposition of the 3D beamforming into two linear steps is expressed as follows: B(f,,a,/3,çb,R) [M_l 1 (15) (JfiR).[ x (f).s11, (faR)j With the two separated steering vectors expressed as Sm(f,a,R) = exp{ j2 [2 + x -2xmRcosa -R]} ____________ (16) sfl(ffiR)_exP{i2f[ -2yRcosfl _R]} Following the same concept, further steering vectors can be obtained along the z-axis.

k (f,�,R) = eXP{J2f [z: -2:kR cos -R]} (17) Then, while a -r / 2, /3 -r 12, 0 -o.

Consequently, in 3D beamforming, while the steering vector is pointing along the z-axis, there will be no energy along either x-axis or y-axis.

Claims

CLAIMS: 1. A wireless communications station for establishing communication with one or more mobile stations, either directly or via a relay station, the station comprising a communications means operable to emit a space division multiple access wireless communication signal, the wireless communication signal comprising a first spatial component directed towards a first mobile station and a second spatial component directed towards a relay station for relaying to a second mobile station, the station comprising precoding means operable to render the first and second spatial components substantially spatially orthogonal with respect to said first and second mobile stations.
2. A wireless communications station in accordance with claim 1 wherein said precoding means is further operable to form said spatial components.
3. A wireless communications station for establishing communication with one or more mobile stations, either directly or via a relay station, the station comprising communication means operable to emit a first space division multiple access, SDMA, wireless communication signal, the first signal comprising first and second spatial components, the first spatial component being directed towards a first user station and bearing a first portion of signal information intended for said first user station, the second spatial component being directed towards a relay station and comprising a second portion of signal information intended for said first user station and a third portion of signal information intended for a second user station, the communication means being further operable to emit a second SDMA wireless signal synchronised with re-emission of said second spatial component by said relay station, said second SDMA wireless signal and said re-emission in combination comprising third and fourth spatial components directed, respectively, to said first and second mobile stations and bearing, respectively, said second portion and said third portion of signal information.
4. A wireless communications station in accordance with claim 3 wherein said communication means is operable to transmit to said relay station beamforming information to enable said relay station to generate a third SDMA signal which, in combination with said second SDMA signal causes formation of said third and fourth spatial components.
5. A wireless communications station in accordance with claim 3 or claim 4 and further operable to apply beaniforming to said first and second SDMA signals, to direct interference away from said mobile stations.
6. A wireless communications station in accordance with any one of claims 3 to 5 wherein the communication means is operable to emit the second spatial component such that it is suitable for re-emission by a relay station without further processing thereby.
7. A wireless communications station in accordance with claim 6 and operable to generate said second SDMA wireless signal with regard to an assumption that the relay re-emits said second spatial component without further processing thereof.
8. A wireless communications station in accordance with claim 7 and operable to generate said second SDMA wireless signal with regard to an assumption that the relay re-emits said second spatial component without spatial diversity.
9. A method of establishing communication with one or more mobile stations, either directly or via a relay station, comprising emitting a space division multiple access, SDMA, wireless communication signal, the wireless communication signal comprising a first spatial component directed towards a first mobile station and a second spatial component directed towards a relay station for relaying to a second mobile station, the first and second spatial components being substantially spatially orthogonal with respect to said first and second mobile stations.
10. A method in accordance with claim 7 wherein said precoding forms said spatial components.
11. A method of establishing communication with one or more mobile stations, either directly or via a relay station, comprising emitting a first space division multiple access, SDMA, wireless communication signal, the first signal comprising first and second spatial components, the first spatial component being directed towards a first user station and bearing a first portion of signal information intended for said first user station, the second spatial component being directed towards a relay station and comprising a second portion of signal information intended for said first user station and a third portion of signal information intended for a second user station, and emitting a second SDMA wireless signal synchronised with re-emission of said second spatial component by said relay station, said second SDMA wireless signal and said re-emission in combination comprising third and fourth spatial components directed, respectively, to said first and second mobile stations and bearing, respectively, said second portion and said third portion of signal information.
12. A method in accordance with claim 11 including transmitting to said relay station beamforming information to enable said relay station to generate a third SDMA signal which, in combination with said second SDMA signal causes formation of said third and fourth spatial components.
13. A method in accordance with claim 11 or claim 12 and further comprising beamforming said first and second SDMA signals, to direct interference away from said mobile stations.
14. A method in accordance with any one of claims 11 to 13 wherein the second spatial component is emitted in a form suitable for re-emission by a relay station without further processing thereby.
15. A method in accordance with claim 14 wherein said second SOMA wireless signal is generated with regard to an assumption that the relay re-emits said second spatial component without further processing thereof.
16. A method in accordance with claim 15 wherein said second SDMA wireless signal is generated with regard to an assumption that the relay re-emits said second spatial component without spatial diversity.
17. A computer program product comprising computer executable instructions which, when executed by a computer, cause said computer to perform a method in accordance with any one of claims 9 to 16.