GB2459832A

GB2459832A - Beamforming in a radiated power controlled multi-antenna transmission

Info

Publication number: GB2459832A
Application number: GB0804792A
Authority: GB
Inventors: Cheran Malsri Vithanage
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2008-03-14
Filing date: 2008-03-14
Publication date: 2009-11-11
Anticipated expiration: 2028-03-14
Also published as: GB0804792D0; GB2459832B

Abstract

Beamforming is described for a two antenna transmitter. The approach involves determining a beamforming relationship between the two antennas, which antennas are separated by a distance which is less tan half a wavelength of operating wireless signals, the beamforming relationship being determined on the basis of an equivalent isotropic radiated power restriction (EIPR) and wherein the beamformer is operable to determine said beamforming relationship on the basis of signal to noise ratio at a receiver with which said apparatus seeks to communicate. Another approach to the invention comprises beamforming on the basis of a first test to determine if antenna selection is acceptable in comparison with antenna mixing. If antenna mixing is the better approach, then the mixing is determined on the basis of an optimisation with respect to signal to noise ratio at the receiver. In one embodiment, a codebook approach is used, such that only a selection of possible mixings are tested and the best is selected. The invention is particularly suited to ultra wideband (UWB).

Description

S

Wireless Communications Apparatus The present invention concerns wireless communications apparatus, and particularly transmit beamforming for use in arrangements wherein there is an equivalent isotropic radiated power (EIRP) restriction. It is particularly suited to applications involving ultra wideband (UWB) but is not restricted thereto.

Apparatus such as UWB apparatus is, in many regulatory environments, restricted by an EIRP restriction. This means that transmitted power over the whole angular range of an antenna should not exceed a particular value. In general, transmitted power should not exceed a particular level in any particular direction.

Consumer devices in the field of UWB are, in common with other wireless communications apparatus, desired to be as small as possible. It is generally undesirable for bulky and therefore space consuming apparatus to be placed in such environments. This is generally known as providing a small footprint. In the context of multiple antenna configurations, such a device can, in some circumstances, be configured such that there are only two transmit antennas and, because of the restrictions placed on the design due to the physical size of the device, that these are spaced apart at less than half a wavelength. Examples of such small devices include wireless universal serial bus (USB) flash drives, and the smaller types of mobile telephone handsets.

Multiple antenna configurations are of potentially significant use in the delivery of multiple input multiple output (MIMO) technology. This has the potential to deliver high data rate and/or robust communication, by exploiting the additional degrees of freedom and diversity afforded by the spatial domain, in addition to the frequency and/or time domains.

It will be appreciated that many problems arise when data is transmitted from multiple antennas simultaneously. For example, a signal received at a corresponding receiver comprises a superposition of the transmitted signals. This results from the nature of transmission over a wireless medium. The superposed signals must be separated by a MIMO detector of the receiver. Some MIMO apparatus aim to use knowledge of the wireless channel at the transmitter to precondition the transmitted message so as to facilitate detection at the receiver. This conditioning is known as beamforming or precoding. In order to be effective, this generally requires a degree of knowledge at the transmitting device of the characteristics of the wireless channel between the transmitting device and the receiving device. This channel knowledge can be ascertained either from a feedback channel dedicated to the transmission from the receiver to the transmitter of such channel knowledge, or by using channel reciprocity, particularly if the communication arrangement between the transmitter and receiver uses time division duplexing.

Whereas optimal precoding algorithms are known, these need to be placed in the context of other performance constraints imposed on MIMO apparatus. In particular, systems such as UWB are restricted by EIRP constraints. This imposes greater restrictions on performance than would a conventional total transmit power constraint.

Any beamforming scheme applied at the transmitter for such systems would need to be compliant with regulatory EIRP restrictions.

One particularly useful and commonplace type of beamforming is known as antenna selection. This is investigated in "Performance analysis of combined transmit-SC/receive-MRC," (S. Theon, L. V. Perre, B. Gyselinckx and M. Engels, IEEE Transactions on Communications, vol. 49(1), January 2001).

In that approach, the transmitter consists of multiple antennas, and knowledge of the prevailing condition of the wireless channel is used to determine from which antenna a message should be transmitted. Antenna selection can be applied in wideband systems by using orthogonal frequency division multiplexing (OFDM). In an OFDM system, antenna selection can be performed on the basis of selecting per subcarrier or per groups of subcarriers. Consequently, on any given subcarrier, a particular antenna may be chosen for transmission, whereas another antenna may be chosen for transmission on a different subcarrier. In that way, transmission may be optimised across the bandwidth according to some specified cost (or utility) function. Examples of such functions include instantaneous receive signal-to-noise ratio (SNR), capacity, and uncoded bit error rate (BER). In EIRP constrained systems, such as UWB, it transpires that per subearrier antenna selection can maximise system capacity in many practical cases.

In many portable wireless communication devices, the position of the antennas can present problems. In particular, it is desirable, in theory, to separate transmit antennas of a device intended to be used for multi antenna transmissions, by at least a half of a wavelength. This may not be possible when taking into account the mechanical design of the device, and the constraint that the device should have a small footprint. Per subcarrier antenna selection schemes may no longer be optimal, even under the specific EIRP constraints as discussed above. Consequently, it may be beneficial to find beamforming solutions for such cases, which improve performance as measured against a particular criterion.

One aspect of the invention comprises an EIRP constrained wireless communications apparatus, including two transmit antennas, governed by the received SNR as the performance criterion.

Another aspect of the invention provides a wireless communications device including two transmit antennas, wherein beamforming is governed by a beamforming solution selected from a codebook of possible beamforming solutions. A subset of these beamforming solutions may include per subcarrier antenna selection.

Another aspect of the invention comprises a wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, wherein the antennas are separated by a distance which is less than half a wavelength of operating wireless signals, the apparatus further comprising a beamformer for imposing beamforming on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas on the basis of an equivalent isotropic radiated power restriction and wherein the beamformer is operable to determine said beamforming relationship on the basis of signal to noise ratio at a receiver with which said apparatus seeks to communicate.

Another aspect of the invention comprises wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, the apparatus further comprising a beamformer for imposing beamforining on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then being operable to optimise said beamforming relationship, and said beamformer being responsive to said first test determining that antenna selection is the better approach to select on the basis of a second test one or other of the antennas to be used.

Another aspect of the invention comprises a method of establishing a beamforming relationship for a wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, wherein the antennas are separated by a distance which is less than half a wavelength of operating wireless signals, the method comprising determining a beamforming relationship between the two antennas on the basis of an equivalent isotropic radiated power restriction and determine said beamforming relationship on the basis of signal to noise ratio at a receiver with which said apparatus seeks to communicate.

Another aspect of the invention comprises a method of determining a beamforming relationship between two antennas of a wireless communications apparatus, comprising determining a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then optimising said beamforming relationship, and responsive to said first test determining that antenna selection is the better approach then selecting on the basis of a second test one or other of the antennas to be used.

Another aspect of the invention comprises a wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, the apparatus further comprising a beamformer for imposing beamforming on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then being operable to test each of a plurality of prestored beamforming relationships and to select the best performing of said beamforming relationships for use, and said beamformer being responsive to said first test determining that antenna selection is the better approach to select on the basis of a second test one or other of the antennas to be used.

Another aspect of the invention comprises a method of determining a beamforming relationship between two antennas of a wireless communications apparatus, comprising determining a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then testing each of a plurality of prestored beamforming relationships and selecting the best performing of said beamforming relationships for use, and responsive to said first test determining that antenna selection is the better approach then selecting on the basis of a second test one or other of the antennas to be used.

Aspects of the invention may comprise a computer program product comprising computer executable instructions operable to cause a computer to become configured to perform a method in accordance with any of the above identified aspects of the invention. The computer program product can be in the form of an optical disc or other computer readable storage medium, a mass storage device such as a flash memory, or a read only memory device such as ROM. The method may be embodied in an application specific device such as an ASIC, or in a suitably configured device such as a DSP or an FPGA. A computer program product could, alternatively, be in the form of a signal, such as a wireless signal or a physical network signal.

Specific embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a wireless communications apparatus incorporating a communications unit in accordance with a specific embodiment of the invention; Figure 2 is a schematic diagram of a communications unit of the specific embodiment of the invention; Figure 3 is a schematic diagram of a communications unit of the specific embodiment, further detailing a multiple subcarrier implementation thereof; Figure 4 is a flow diagram illustrating a method of computing beamformer attributes for use by the apparatus in Figures 1, 2 and 3; and Figure 5 is a graph illustrating performance of experimental implementations of the specific embodiment of the invention in comparison with prior art arrangements.

The wireless communication device 100 illustrated in Figure 1 is generally capable of being used in a MIMO context, to establish a MIMO communications channel with one or more other devices and, in accordance with a specific embodiment of the invention, to take account of channel information so as to derive a pre-coding (or otherwise described as beamforming) scheme appropriate to the quality of the channel. The reader will appreciate that the actual implementation of the wireless communication device is non-specific, in that it could be a base station or a user terminal.

Figure 1 illustrates schematically hardware operably configured (by means of software or application specific hardware components) as a wireless communication device 100.

The wireless communication device 100 comprises a processor 120 operable to execute machine code instructions stored in a working memory 124 and/or retrievable from a mass storage device 122. By means of a general purpose bus 130, user operable input devices 136 are capable of communication with the processor 120. The user operable input devices 136 can comprise, in this example, a keyboard and a mouse though it will be appreciated that any other input devices could also or alternatively be provided, such as another type of pointing device, a writing tablet, speech recognition means, or any other means by which a user input action can be interpreted and converted into data signals.

Audio/video output hardware devices 138 are further connected to the general purpose bus 130, for the output of information to a user. Audio/video output hardware devices 138 can include a visual display unit, a speaker or any other device capable of presenting information to a user.

A communications unit 132, connected to the general purpose bus 130, is connected to a plurality of antennas 134. In the illustrated embodiment in Figure 1, the working memory 124 stores user applications 126 which, when executed by the processor 120, cause the establishment of a user interface to enable communication of data to and from a user. The applications in this embodiment establish general purpose or specific computer implemented utilities that might habitually be used by a user.

Communications facilities 128 in accordance with the specific embodiment are also stored in the working memory 124, for establishing a communications protocol to enable data generated in the execution of one of the applications 126 to be processed and then passed to the communications unit 132 for transmission and communication with another communications device. It will be understood that the software defining the applications 126 and the communications facilities 128 may be partly stored in the working memory 124 and the mass storage device 122, for convenience. A memory manager could optionally be provided to enable this to be managed effectively, to take account of the possible different speeds of access to data stored in the working memory 124 and the mass storage device 122.

On execution by the processor 120 of processor executable instructions corresponding with the communications unit 132, the processor 120 is operable to establish communication with another device in accordance with a recognised communications protocol.

The communications unit 132 will now be described in further detail. As illustrated in figures 2 and 3, single carrier and multiple subcarrier versions of the communications unit 132 are exemplified.

In figure 2, data (x) to be transmitted is input to a beamformer 202, which is configured by a beamformer vector computation unit 204. The beamformer vector computation unit 204 is itself governed by channel state information (H) which is derived from whatever available source. In many circumstances, a channel estimate will be available from an assumption of channel reciprocity, as the unit will itself be operable as a receiver as well as being a transmitter, or the receiver at the other end of the channel might transmit, for instance on another lower capacity channel, channel information.

Such channel information could be transmitted in full, or in a compressed format.

The beamformer produces two streams, one for each antenna 134. Each stream is passed to a digital to analogue converter 208, a frequency upeonverter 210 and a power amplifier 212. The output of each power amplifier 212 is suitable to be passed to a respective antenna 134.

With regard to figure 3, many of the same principles apply. A multi-subcarrier signal is input to the communications unit 132, which comprises as a first stage a serial to parallel converter 301. The output of the serial to output converter 301 (a plurality of intended subcarrier signals) is passed, again, to a train of units per antenna. In each train, the subcarrier signals are subjected to a per subcarrier beamformer 302. Each per subcarrier beamformer 302 is subject to a beamforming vector determined by a beamforming vector computation unit 304. This, again, is influenced by channel state information acquired as discussed above.

The output of each beamformer 302 is sent to a multicarrier modulator (implemented by an Inverse Fast Fourier Transform) 308 which then passes the resultant parallel signals to a cyclic prefixer and parallel to serial converter 310. The consequent serial signal is then converted to the analogue domain by a DAC 312, and prepared for transmission on the corresponding antenna by way of a frequency upconverter 314 and a power amplifier 316.

There now follows an explanation of the function of the beamforming vector computation unit, particularly that illustrated in figure 3. A generic model (for the purpose of illustration of the invention) of a baseband communication system can be described as follows. With R denoting the number of receive antennas, the communication system employs two transmit antennas. H, which is an x 2 matrix, denotes the equivalent channel between the transmitter and the receiver. To demonstrate use of the system, it is supposed that, at a particular time instant, the transmitter intends to transmit the scalar symbol x, pre-multiplied by a 2 x 1 beamforming vector v. Using the R x 1 vectors n and y to denote the additive noise manifesting at the receiver and the resultant total signal at the receiver, respectively, y =Hvx+n.

It will be noted that the elements of the vectors and the matrix above are complex numbers for a baseband representation.

Design of the beamforming vector v is addressed herein. Such design can be made according to various criteria such as maximising the received SNR, maximising the resultant system capacity and minimising the decoded error rates at the receiver.

Furthermore, the design of v can be subjected to various constraints. Traditional designs, such as that in "Maximal Ratio Transmission" (T. K. Y. Lo, IEEE Transactions on Communications, October 1999), were subject to limitations on the total transmitted power. The constraint of having limitations on the equivalent isotropic radiated power (EIRP) was considered in "Performance of multiple-receive multiple-transmit beamforming in WLAN-type systems under power or EIRP constraints with delayed channel estimates" (P. Zetterberg, M. Bengtsson, D. McNamara, P. Karisson and M. A. Beach, Proceedings of the IEEE Vehicular Technology Conference, 2002.), in which the authors presented iterative optimisation methods to find the optimal beamforming vector.

The described specific embodiment of the invention therefore concerns the provision of a beamforming vector which improves the received SNR for systems subject to EIRP constraints, when there are only two antennas at the transmitter. Being a closed form solution, this approach is different from that used in Zetterberg et al. The practical importance of a closed form solution, compared with a numerical iterative optimisation scheme, stems from the computation time being reduced as well as fixed. Furthermore, the solution reverts to the well known antenna selection scheme, when the transmit antenna spacing is at least half a wavelength. For a smaller spacing, which can be necessary for compact devices, the produced beamforming vector is non trivial.

As shown in figure 2, each data symbol x is multiplied by the beamforming vector v before being fed to the signal paths leading to the two transmit antennas. It will be noted that the product vx is a two element vector: the first element is fed to the signal path leading to the first antenna, and the second element is fed to the signal path leading to the second antenna.

The beamformirig vector is computed using knowledge of the channel matrix H, as will be described later in the flow chart of Figure 4. The beamforming vector computation unit (204), which performs this computation, is the main focus of the present

description.

Systems such as ultra wideband (UWB) systems conforming to the WiMedia specifications (WiMedia Alliance, "Multiband OFDM physical layer specification", January 14, 2005), are subject to EIRP restrictions per subcarrier. For such systems, this beamforming is applied per subcarrier as illustrated in Figure 3.

An example of an algorithm for computing the beamforming vector of the specific embodiment will now be described. The flow chart of Figure 4 illustrates the computation of this beam forming vector v, for a given channel matrix H. The two columns of the R x 2 matrix H are denoted h1 and h2, respectively. For a vector h, h denotes its conjugate transpose. Again, for a vector h, the Euclidean norm is given by hO = Iii. For a complex number C, �J1(c) denotes its real part.

The wavelength of the radio waves at transmission is denoted by 2, and d denotes the spacing between the two transmit antennas. The algorithm requires the input of the parameters T, and p, which can be hard coded for a particular system. Here, T is a large real positive constant and is a constant, which is dependent on factors such as losses in the transmit chain and the EIRP restriction. The purpose of is to scale the product vx such that the EIRP restriction is not exceeded. Computation of p is as given in Figure 4.

The description of the algorithm executed by the apparatus is as follows. Step 31 begins the algorithm. The algorithm is assumed to have been fed the parameters: T, x and p. In step 32, the new channel matrix H is input to the algorithm.

By way of background, apart from the degenerate cases where transmit antenna selection is optimal, the optimal beamforming vector is assumed to be of the form v = with t being a non-negative real number and /3 being a real number.

Extensive simulations have indicated that, to maximise the received SNIR, /3 = r is potentially optimal. Thus, the beamforming vector would, using this assumption, take the form v=xr('j.

It will be apparent to the reader that the task of maximising the received SNR for this scenario is thus reduced to finding the stationary points of a quadratic equation in t. In step 33, the parameters A, B and C are computed. These are the coefficients of the quadratic equation.

Parameter t is computed in step 34. t corresponds to the maximising solution to the quadratic equation mentioned above. Step 35 checks whether t is in the real interval (0, T] The preceding notation indicates that the interval does not include zero, but does include T. If t is in the interval, this indicates that transmit antenna selection is sub-optimal and the optimal beamforming vector is of the assumed form v = zi{1j.

If t were zero, negative, or greater than or equal to T, (selected to be a large number, such as 10,000, to represent the case whereby t tends to infinity) this would be indicative that there is sufficient imbalance between the usefulness of the two antennas in presenting a high SNR solution that antenna selection would be appropriate.

In all other cases, however, the test indicates that both antennas may have a contribution to make in presenting a suitable beamforming solution which enhances SNR to the receiver.

Thus, if the test of step 35 indicates that t is in the interval, the algorithm then moves to step 36; otherwise it will move to step 37.

In step 36, the parameter y is computed. In step 40, the beamforming vector v is computed as shown and fed to step 41.

The alternative path, through step 37 (which implements antenna selection) will now be described. Step 37 checks whether the Euclidean norm of the first column of H is greater than the Euclidean norm of the second column of H. If the answer is "yes", the algorithm moves to step 38, otherwise it moves to step 39. Both steps 38 and 39 compute the beamforming vector v as shown and feed it to step 41. Steps 38 and 39 represent antenna selection: step 38 sets antenna 1 to be active and antenna 2 to be off' and, for step 39, the reverse applies.

In step 41 the computed beamforming vector v is output and, in step 42, the algorithm ends.

As can be seen from steps 40, 38 and 39 of the algorithm, the beamforming vector v always takes one of the following forms: z[J or with t being a real number. Thus, another approach, contemplated as an embodiment of the invention, is to select a codebook of possible beamforming vectors such that its element vectors are of the form above. One of the element vectors of this codebook can be selected based on a suitable optimality criterion, as the beamforming vector v. An illustration of a corresponding four element codebook will be described below.

This arrangement renders up the beamforming vector which improves link performance subject to EIRP restrictions at the transmitter, when there are only two transmit antennas. Previous work of Zetterberg et al. had given an iterative numerical solution to obtain this vector. The algorithm of this invention gives a closed form solution, and thereby computes the beamforming vector with a much reduced, and fixed, computational burden.

The conventional method of per subcarrier antenna selection is optimal, provided that the two transmitting antennas are spaced apart by at least half a wavelength. The described algorithm also reverts to such antenna selection, for sufficient spacing between the transmit antennas. More importantly, for more closely spaced transmit antennas, the described embodiment improves the link performance compared to conventional schemes such as per subcarrier antenna selection.

In addition, the use of codebooks helps to reduce the feed back burden required for a receiver to inform a transmitter as to the most suitable beamforming vector to be used.

Practical communication systems, such as UWB systems, can take compact forms and can be restricted by EIRP limitations. In the case of having two closely spaced transmit antennas, the proposed algorithm can give improved link performance compared to alternative schemes such as performing transmit antenna selection.

In practice, the receiver can inform the transmitter of the most appropriate beamforming vector to use. A codebook scheme is attractive for such scenarios, since the maximum bit rate required for this feed back can be controlled by the size of the codebook. The present disclosure gives a structure for codebooks. In simulations, the codebook scheme gives good performance, while utilising just a four element codebook.

Figure 5 gives an illustration of the packet error rate (PER) performance due to the use of the proposed beamforming vector. Simulations were made in conformity to the OFDM based WiMedia specifications for UWB systems. Since the EIRP restrictions apply per subcarrier for such systems, the beamforming was also applied per subcarrier.

For the simulations, the number of receive antennas, R was set to be 1. The two transmit antennas were spaced apart by 1cm, which implies a spacing of 0.1 to 0.12 wavelengths at transmission. It should be noted that, for a UWB specific comparison, the various schemes investigated were ensured to have the same EIRP at transmission and not necessarily the same transmit power. The SNR shown in the x-axis is that due to the use of the antenna selection scheme for the particular EIRP concerned.

The PER performance can be compared with a system which uses per subcarrier antenna selection (as in UK patent application 0719012.7) and also against a reference legacy WiMedia system which utilises only a single transmit antenna.

The PER performance of a codebook based scheme is also given. The codebook used for this simulation had only four vector elements as given below. Here, the scaling factors, and ensures that the EIRP restrictions are satisfied.

Illustrative J {x(J, [1J [_2)} four element codebook As can be seen from the graph, the per-subcarrier antenna selection scheme with the use of two transmit antennas performs close to 2dB better at a PER of 0.001, compared to the legacy scheme which only has one transmit antenna. A further 1 dB improvement is possible with the use of the proposed algorithm, which is highly attractive given the low SNRs of operation. It should also be noted that the performance of the proposed scheme was identical to the performance of the iterative numerical solution of Zetterberg et al. for this simulation, which suggests that the desired closed form solution may in fact be received SNR optimal.

The four element codebook gives performance very similar to the algorithm described above. The advantage of using the codebook is in an implementation where the receiver has to inform the proper beamforming vector to the transmitter. When using the four element codebook, two bits per channel matrix are sufficient to inform the transmitter of the best beamforming vector to use.

Although the above described embodiments of the invention are intended to inform the reader as to the possibilities for implementation of the invention, the invention is not limited to such embodiments. Indeed, the reader will appreciate that many alternative embodiments, and modification, replacement or omission of individual features of the illustrated embodiments are possible within the scope of the invention. The invention should instead be read as being defined by the appended claims, which can be read in conjunction with, but should not be considered limited by, the present description and accompanying drawings.

Claims

CLAIMS: 1. A wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, wherein the antennas are separated by a distance which is less than half a wavelength of operating wireless signals, the apparatus further comprising a beamformer for imposing beamforming on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas on the basis of an equivalent isotropic radiated power restriction and wherein the beamformer is operable to determine said beamforrning relationship on the basis of signal to noise ratio at a receiver with which said apparatus seeks to communicate.
2. Apparatus in accordance with claim 1 wherein said beamformer is operable to determine a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then being operable to optimise said beamfonning relationship, and said beamformer being responsive to said first test determining that antenna selection is the better approach to select on the basis of a second test one or other of the antennas to be used.
3. A wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, the apparatus further comprising a beamformer for imposing beamforming on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then being operable to optimise said beamforming relationship, and said beamformer being responsive to said first test determining that antenna selection is the better approach to select on the basis of a second test one or other of the antennas to be used.
4. Apparatus in accordance with claim 2 or claim 3 wherein said beamformer is operable to perform said first test on the basis of an optimisation variable, wherein said first test determines that antenna selection is the better approach if said optimisation variable lies outside a predetermined range.
5. Apparatus in accordance with claim 4 wherein said beamformer is operable to determine a beamforming relationship proportional to [1) wherein t is said optimisation variable.
6. Apparatus in accordance with claim 5 wherein t is a real number.
7. Apparatus in accordance with claim 6 wherein said predetermined range is defined between zero and a predetermined upper limit.
8. Apparatus in accordance with any one of claims 2 to 7 wherein said beamformer is operable to determine a scaling factor on the basis of a restriction on equivalent isotropic radiated power and to impose said scaling factor on said beamforming relationship.
9. Apparatus in accordance with any one of claims 2 to 8 wherein said beamformer is operable, in applying said second test, to compare magnitudes of columns of a channel matrix for a channel between the apparatus and an intended receiver, to determine which of the antennas, to which said respective columns relate, will be more effective.
10. A method of establishing a beamforming relationship for a wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, wherein the antennas are separated by a distance which is less than half a wavelength of operating wireless signals, the method comprising determining a beamforming relationship between the two antennas on the basis of an equivalent isotropic radiated power restriction and determine said beamforming relationship on the basis of signal to noise ratio at a receiver with which said apparatus seeks to communicate.
11. A method in accordance with claim 10 wherein said determining comprises performing a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then optimising said beamforming relationship, responsive to said first test determining that antenna selection is the better approach then selecting on the basis of a second test one or other of the antennas to be used.
12. A method of determining a beamforming relationship between two antennas of a wireless communications apparatus, comprising determining a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then optimising said beamforming relationship, and responsive to said first test determining that antenna selection is the better approach then selecting on the basis of a second test one or other of the antennas to be used.
13. A method in accordance with claim 11 or claim 12 wherein said determining comprises performing said first test on the basis of an optimisation variable, wherein said first test comprises determining that antenna selection is the better approach if said optimisation variable lies outside a predetermined range.
14. A method in accordance with claim 13 wherein said beamforming relationship is proportional to wherein t is said optimisation variable.
15. A method in accordance with claim 14 wherein t is a real number.
16. A method in accordance with claim 15 wherein said predetermined range is defined between zero and a predetermined upper limit.
17. A wireless communications apparatus comprising two antennas, each suitable for emitting a wireless signal comprising one or more of a plurality of subcarriers, the apparatus further comprising a beamformer for imposing beamforming on a signal to be transmitted from said antennas, the beamformer being operable to determine a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then being operable to test each of a plurality of prestored beamforming relationships and to select the best performing of said beamforming relationships for use, and said beamformer being responsive to said first test determining that antenna selection is the better approach to select on the basis of a second test one or other of the antennas to be used.
18. A method of determining a beamforming relationship between two antennas of a wireless communications apparatus, comprising determining a beamforming relationship between the two antennas by virtue of a first test to determine if antenna selection or optimisation is the better approach, responsive to said first test determining that optimisation is the better approach then testing each of a plurality of prestored beamforming relationships and selecting the best performing of said beamforming relationships for use, and responsive to said first test determining that antenna selection is the better approach then selecting on the basis of a second test one or other of the antennas to be used.
19. A computer program product comprising computer executable instructions operable, when executed on a computer, to cause said computer to perform a method in accordance with any one of claims 10 to 16 or claim 18.
20. A computer program product in accordance with claim 19 wherein said product comprises a computer readable carrier medium.