GB2453171A

GB2453171A - Conditioning OFDM signals for transmission by multiple antenna transmitters

Info

Publication number: GB2453171A
Application number: GB0719012A
Authority: GB
Inventors: Cheran Malsri Vithanage; Steve Carl Jamieson Parker; Magnus Stig Torsten Sandell
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2007-09-28
Filing date: 2007-09-28
Publication date: 2009-04-01
Anticipated expiration: 2027-09-28
Also published as: GB2453171B; GB0719012D0

Abstract

A method of conditioning Orthogonal Frequency Division Multiplexed (OFDM) signals for transmission by multiple antenna (6a,b) transmitters or transceivers includes determining antenna-sub-carrier allocations (9) and then phase pre-coding transmitted signals (11a,b) to produce an effective channel at the receiver which meets one or more criteria in terms of its frequency response. The one or more criteria may correspond to a transmission from fewer transmitters as expected by legacy receivers. A transmitter and transceiver are also described.

Description

A WIRELESS TRANSMISSION DEVICE AN)) METHOD The present invention relates to conditioning signals for multiple antenna wireless transmission systems configured to transmit signals having multiple subcarrier components. In particular, the invention relates to conditioning Orthogonal Frequency Division Multiplexing (OFDM) signals for transmission. Also in particular, the present invention relates to transmission of signals on Multiple-Input Multiple-Output (MIMO) systems and the transmission of signals on Multiple-Input Single-Output (MISO) systems to achieve legacy compatibility with a single antenna system.

The benefits of transmission by multiple antenna systems are well known. For example, these systems increase capacity and therefore, in practical implementations, it is possible to increase peak data rate and mean throughput.

Also well known are the advantages of using OFDM signals which have multiple subcarriers at frequencies selected to be orthogonal to each other.

Conditioning OFDM signals for transmission on multiple antenna systems can involve allocating subcarriers to individual antennas on the basis of known characteristics of the prevailing radio channel and the communications system.

The paper, An Adaptive Antenna Selection Scheme for Transmit Diversity in OFDM Systems' by Hui Shu et al published in VTC 2001 Fall, IEEE VTS 54th, describes allocation of antennas to individual subcarrier components. This is done on the basis of transfer functions of gain or, more accurately, path loss observed between given transmission antennas and a given receiver.

Ultra wideband (UWB) technology, using OFDM signals, has been adopted by the ECMA 368/369 standard and the WiMedia I specification for Wireless Personal Area Networks (WPANs).

Peak data rates in excess of 5 Gb/s are potentially achievable using UWB OFDM signals that exploit the FCC regulated UWB spectrum and maximise capacity through MIMO or MISO systems.

However, in the low signal-to-noise ratio (SNR) regime, which is the usual regime for UWB systems, channel capacity declines linearly with decreasing signal SNR at the receiver. Consequently, the range of these UWB transmissions is limited and performance suffers if propagation paths are obstructed by objects which attenuate radio waves.

One desired application of UWB is for wireless streaming of high definition (HD) video. However, this application requires a low Packet Error Rate (PER), even in the low SNR regime.

The use of MIMO and MISO systems with OFDM signals will contribute to achieving these low PERs.

One problem experienced when attempting to apply MIMO and MISO technology is backwards compatibility with legacy hardware which may have, for example, only a single receive antenna. Although it is necessary to adhere to the same communications protocol, it is also important to ensure that system performance is not likely to be compromised once implementation aspects are considered. The challenge is therefore to generate a multiple antenna transmission that can appear to a legacy receiver to have emanated from a single antenna system, albeit with an associated channel that suffers much less fading than normal.

The present disclosure considers the use of per-subcarrier antenna selection to reduce the impact on performance of frequency selective fading. However, if this technique is used in isolation then there is a risk that the effective' channel seen' by a legacy device will have different characteristics from those which would normally be expected by the receiver's hardware as dictated by an implementer. For example, the phase of the measured channel frequency response on adjacent subcarriers is liable to jump discontinuously if these particular subcarriers have been assigned to different transmit antennas. However, a receiver may assume that the coherence bandwidth of the channel is far in excess of the subcarrier spacing and so a smooth and small change in the phase of the frequency response will be anticipated. Filters implemented in the receiver may assume a smoothly varying phase of the frequency response. Therefore, per-subcarrier antenna selection may cause aliasing in a receiver and subsequently poor PER performance. In the time domain, the discontinuous phase between signals transmitted on adjacent subcarriers manifests itself as a spreading of the channel's impulse response, leading to channel lengths longer than expected. The applicant has observed that it may be advantageous to provide a transmission system where signals transmitted from multiple antennas are conditioned to make the signals appear to have emanated from a single antenna. The applicant has also observed that, in practice, this may be achieved in the frequency domain, for example, by linearising the phase profile.

Figure 1 depicts an example of allocation of subcarriers to antennas for an OFDM signal. Figure 1 also shows a set of channel transfer functions having attenuation in dB on a vertical axis and frequency in megahertz relative to a carrier on the horizontal axis.

Each transfer function is for a channel between a particular transmit antenna and the receive antenna. It is apparent from Figure 1 that an ideal antenna allocation is likely to correspond to allocating signals carried on subcarriers to antennas which experience low attenuation, or high gain, when propagating through the channel at that particular frequency.

SUMMARY OF INVENTION

An aspect of the present invention provides a method of conditioning signals with multiple subcarrier components for multiple antenna systems by rotating the phase of signals transmitted in the frequency domain to make the transmissions appear to a receiver to have been produced by a device with fewer antennas, while advantageously reducing the frequency selectivity of the new effective channel. For example, the phase of the signals may be rotated to linearise the phase of the frequency response and make the transmission appear to have originated from a single antenna with an associated channel that is favourable to the anticipated receivers.

In one aspect the invention is a method of conditioning a signal with multiple subcarrier components for multiple antenna transmission systems, the method including: measuring the amplitude and phase of the channel frequency response from the transmitter to the receiver, either by measuring the received channel in the reverse direction and assuming channel reciprocity (with calibration used to correct for any potential non-reciprocity), or by the use of an explicit feedback channel; allocating subcarriers to antennas based on some metric of performance, such as the amplitude of the channel's frequency response; and rotating the phase of the signals transmitted on the subcarriers of the multiple antennas by an amount calculated to make transmissions appear to have originated from a transmitter with a fewer number of antennas, with an associated channel response that appears to suffer less fading than before.

As used herein, the term phase rotation' refers to the relative phase advancement or retardation imparted to each signal transmitted on the subcarriers relative to those transmitted on other subcarrier signals during propagation between a transmitter and a receiver.

As used herein the term direction' is defmed solely in tenns of receiver to transmitter.

The phase response of the channel combined with the phase rotation of the signals on the subcarriers may include a weighting factor m representing the slope of a linear profile of phase signals transmitted over subcarrier frequency.

The substantially linear phase profile of the channel combined with the phase rotation of the signals on the subcarriers may correspond to that expected or observed for transmission from a single antenna.

The weighting factor may correspond to a predetermined position for a peak in the resulting channel impulse response viewed in the time-domain for a predefined receiver.

In another aspect, the invention provides a transmitter having multiple transmit antennas, being adapted for transmission of signals on multiple subcarrier frequencies and having available channel state information relating to the transmission of signals, the transmitter including: a subcarrier allocator operable to allocate subcarriers of the signal to antennas according to a metric of performance determined for subcarrier frequencies from the channel state information; a phase adjustor operable to rotate the phase of the signals transmitted on the subcarriers so as to make the effective channel measured by a receiver meet one or more predetermined criteria.

As used herein the term rotate' used in reference to the phase of signals refers to advancing or retarding the phase of a signal to be transmitted on a subcarrier. It will be understood by those skilled in the art that the rotation is subject to modulo 2ir phase wrapping.

As used herein the term channel state information' is intended to include any information pertaining to the state which allows one or more predetermined criteria to be evaluated for transmission of signals.

After subcarrier allocation, the phase of signals may change discontinuously or by a unexpectedly large amount between adjacent subearriers. The phase rotation is preferably applied to individual signals to be transmitted on different subcarriers to adjust each signal to meet a predetermined criteria defined for the set of signals transmitted on all subcarriers. Therefore, the signals to be transmitted may be adjusted arbitrarily so that the signals collectively appear to the receiver to have originated from a transmitter with fewer antennas.

The channel state information may include the phase of the channel frequency response.

It may also include the amplitude of the channel frequency response.

The one or more predetermined phase criteria may be that the phase of the signal precoding, combined with the phase imparted in propagation between the transmitter and a receiver, decreases with the frequency of the subcarrier on which they are to be transmitted, subject to modulo 2it phase wrapping.

The phase of signal precoding combined with the phase imparted in propagation between the transmitter and a receiver may decreases linearly with the frequency of the subcarrier on which they are to be transmitted. This linearity of the phase of the frequency response may have a slope chosen to suit a given receiver.

The transmitter may be operable to obtain channel state information by measuring a received channel and assuming reciprocity. This situation may arise naturally where the transmitter is included in a transceiver.

The transmitter may be operable to calibrate the transmitted signals to correct for potential non-reciprocity in the channel or the radio transmit and receive hardware.

Another aspect of the present invention provides a transceiver including the transmitter.

Alternatively, the transmitter may be operable to receive a feedback channel providing channel state information. The channel state information may comprise a matrix substantially mapping a vector characterising symbols transmitted to a vector characterising symbols received.

The multiple antennas of the transmitter may be arranged in a favourable configuration.

The favourable configuration may be a ring. Alternatively, the favourable configuration may be a linear array.

Another aspect of the present invention provides a method of conditioning a signal to be transmitted on subcarriers for multiple antenna transmission systems, the method including: measuring the channel frequency response between a transmitter and a receiver; allocating subcarriers to antennas based on a metric of performance relating to transmission of signals; rotating the phase of the signals transmitted on the subcarriers of the multiple antennas such that the phase of signals transmitted meet one or more predetermined phase criteria.

The one or more predetermined phase criteria may be chosen such that transmissions appear to have originated from a transmitter with a fewer number of antennas, or even a single antenna.

The one or more predetermined criteria may be chosen such that the transmissions appear to have originated from a transmitter with a single antenna.

This allows compatibility of a multiple antenna transceiver with legacy transceivers which may have a single antenna.

Measurements of the channel frequency response that will be experienced by the transmitted signals may involve measuring the received channel by a transceiver and assuming reciprocity. These measurements may be calibrated to account for any non-reciprocity effects. Alternatively the channel state information at the transmitter may be determined using an explicit feedback channel.

In another aspect, the present invention provides a computer readable medium containing computer readable instructions operable to cause a computer to become configured to carry out a method of conditioning signals for a multiple antenna transmission system configured to transmit signals with multiple subcarrier components, the method including: measuring the channel frequency response between a transmitter and a receiver; allocating subcarriers to antennas based on a metric of performance relating to the measured channel frequency response; rotating the phase of the signals transmitted on the subcarriers of the multiple antennas such that the phase of signals transmitted meet one or more predetermined phase criterion.

The method may include measuring the amplitude of the channel frequency response between a transmitter and a receiver. The phase, or phase and amplitude, of the channel frequency response may be measured by assuming a reciprocal channel. Alternatively, the phase, or phase and amplitude, may be communicated to the transmitter using an explicit feedback channel.

Another aspect of the invention provides a transmitter having multiple antennas and being adapted for transmission of signals having multiple components each transmitted on a different subcarrier, the transmitter including: a data store of measured phase data said data characterising phases of signal components relative to other signal components when transmitted to a receiver; a data store of predetermined phase data characterising predetermined phases of signal components relative to other signal components; a phase adjustor operable to compare the data store of measured phase data and the data store of predetermined phase data affect an adjustment of phase of the signal components according to the comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a multiple input antenna transmission system transmitting an OFDM signal where individual subcarriers are allocated to different antennas; Figure 2 depicts a schematic diagram depicting a multiple input antenna transmission system according to a preferred embodiment of the present invention; Figure 3 is a flow diagram of a method of conditioning a signal according to a preferred embodiment of the present invention; Figure 4 depicts an example of how the phase of the effective channel seen' by a receiver appears discontinuous due to the allocation of subcarriers onto multiple transmit antennas; Figure 5 depicts how the phase of the effective channel measured by a receiver may be made to appear to have originated from a single antenna by phase precoding the symbols transmitted from the multiple antennas; Figure 6 depicts a time domain effective channel impulse response seen' at a receiver once the signals have been phase pre-coded according to Figure 5; Figure 7 shows the ergodic capacity of a transmission link including a transmission system according to a preferred embodiment of the present invention; Figure 8 shows the packet error rate (PER) performance of a conventional single antenna system, a multiple antenna system with per subcarrier selection and a multiple antenna system with per subcarrier selection and phase pre-coding of the transmitted signals; Figure 9 schematically depicts a transceiver according to an embodiment of the invention.

Figure 2 is a schematic diagram of a transmission system 3 according to a specific embodiment of the present invention. This transmission system may be included in a transceiver. The transmission system 3 has a signal input 4 and two signal outputs 5a and Sb which supply signals for input antennas 6a and 6b respectively. It will be apparent to those skilled in the art that the preferred embodiment depicted in Figure 2 represents a very simple case where only two input antennas are used. However, it will also be apparent how the embodiment depicted can be scaled to alternative embodiments with an arbitrary number of input antennas. Alternative embodiments may have antennas arranged in a favourable geometry, such as a ring or a linear array.

Some embodiments may use combinations of directional and omni-directional antennas.

It will be apparent to those skilled in the art that optimal antenna arrangements may differ between applications such as Ultra Wide-Band (UWB) and Wireless Local Area Networks (WLAN).

The OFDM signal received at the input 4 passes to a serial-to-parallel buffer 7 and, from there, onto a subcarrier antenna allocator 9. The subcarrier antenna allocator 9, or simply antenna allocator 9, allocates each subcarrier of the OFDM signal 8 to an antenna 6a or 6b.

The subcarrier antenna allocator 9 determines antenna allocations for the subcarrier components based on a performance metric for the particular frequency. This metric may be any metric known to those skilled in the art suitable for determining optimal antenna selection. One suitable metric may be the gain of the channel's frequency response determined from channel estimate unit 17. Other metrics relating to performance may be used with various advantages and disadvantages known to those skilled in the art. Some other examples are channel capacity and anticipated bit error rate (BER) at the receiver. The antenna allocations are based on Channel State Information available at the Transmitter (CSIT). This knowledge characterises the transmission link at the RF frequencies of the subcarriers. The CSIT consists of the amplitude and the phase of the channel frequency response in the forward direction.

Suitable processes for measurement of CSIT will be apparent to those skilled in the art.

For example, CSIT may be estimated by measuring the channel response in the reverse direction, by the reception of a known preamble, with channel reciprocity assumed.

Alternatively, an explicit feedback channel may be provided to feed back channel state information.

The subcarrier antenna allocator will result in a composite transmission from multiple antennas. If a legacy receiver assumes that the transmission has come from a system with a lower number of antennas, and it makes assumptions about properties of the channel, such as its coherence bandwidth, then the channel estimates may be conditioned in an inappropriate manner leading to poor performance. This invention resolves this potential problem by phase pre-coding transmissions to make them appear to have originated from the expected number of antennas. Hence, the OFDM signals lOa and lOb, for which different antennas have been allocated for the subcarriers, pass onto phase adjustors ha and lib. The phase adjustors are capable of rotating the phase of the signals transmitted on each subcarrier relative to other subcarriers. Suitable phase adjustors I la and 1 lb will be apparent to those skilled in the art. In practice, the signal applied to each subcarrier is comprised of two components: an in phase component and a quadrature component.

The phase adjusted, antenna allocated OFDM signals then pass on to multicarrier modulators 12a and l2b, which perform an Inverse Fast Fourier Transform (IFFT).

Next, the signals pass onto a cyclic prefixer and parallel-to-serial converters I 3a and 13b, then digitaltoanalogUe converters 14a and 14b and up-converters 15a, 15b.

Finally, the signals pass through power amplifiers 1 5a and 1 5b and then the input antenna 6a and 6b.

The phase adjustors 11 a and 11 b communicate with a phase and amplitude comparator 19 to obtain the phase and amplitude adjustments to be applied. It will be apparent to those skilled in the art that the comparator 19 may be incorporated into the phase adjustors ha or lIb. The comparator may determine the amplitude and phase corrections based on the channel estimates stored at 17 and the optimum responses stored at 20 required to make transmissions appear to have originated from a single antenna. This data is stored in the data store 21.

In the preferred embodiment, the data store 21 contains the phase rotations and amplitude changes that must be applied to signals transmitted from the subcarriers on the chosen antennas. For a transmission system with 128 subcarriers and 2 antennas, the data store 21 will have 128 entries (one entry consists of both amplitude and phase coefficients).

The comparator 19 compares the phase of the measured (and calibrated) channel frequency response from 17 with the phase profile from 20 desired to make emissions appear to have originated from a system with a reduced number of antennas (e.g. one antenna). The data store 21 is then populated with the phase rotations (and optionally amplitude changes) that must be used to phase pre-code the transmitted signals to make them appear to have originated from the system with a reduced number of antennas. In the preferred embodiment, the phase rotations represent a phase adjustment over OFDM subcarrier frequencies that result in the receiver estimating a channel frequency response that is consistent with transmissions from a single antenna transmitter. The phase rotations applied on the signals transmitted on the subcarriers are made relative to one other and an arbitrary constant phase change may be applied without any effect.

The phase pre-coding is implemented by the phase adjustors ha and 1 lb to implement the calculated phase rotation. That is, the phase of the OFDM subearrier components relative to each other are rotated or adjusted so that after transmission, the OFDM signal transmitted from multiple antennas will appear to have been transmitted from only a single antenna. For example, this is achieved when the combination of the phase of the actual channel frequency response and the phase rotation imparted by the phase pre-coding results in a measured phase of the frequency response that decreases linearly with frequency. Anomalies in the phase of the frequency response expected to occur in transmissions of subearrier components from multiple antennas will therefore have been compensated. In practice, the amplitude of the frequency response may be similarly smoothed to achieve an optimum effective channel frequency response, although this may be precluded due to EIRP restrictions. Suitable methods for acquiring this knowledge of phase rotations for various antenna allocations will be known to those skilled in the art and may involve a calibration process to populate data store 21 with observed relative phase rotations.

The phase adjustments are made to the preamble of the signal as well as the rest of the signal. This ensures that the receiver automatically combines the signal pre-coding with the measured channel so that the actual channel estimated is a combination of both the real channel response and the phase pre-coding. This combination is referred to as the effective channel'. If the preamble is not pre-coded in the same way as the transmitted data then the OFDM symbols cannot be decoded properly. Also, pre-coding of the preamble ensures that any smoothing of the channel estimates at the receiver does not result in aliasing and a subsequence loss in performance.

Plot 52 in figure 6 depicts the effective channel impulse response of the system where the phase rotation has been adjusted by the phase adjustor 11 to yield an effective frequency response at the receiver that appears to have originated from a single antenna with significantly reduced frequency selective fading. The effective channel impulse response depicted by line 52 shows a much higher peak in which the power is concentrated within a much narrower interval. The delay between peaks 51 and 52 may be affected by adjusting the slope parameter m of the distribution 41 in Figure 5.

Figure 3 shows a process 60 carried out by a transmission system 3 according to the preferred embodiment of the present invention. The process 60 is shown as a sequential set of steps although it will be apparent to those skilled in the art that in alternative embodiments some of these steps may be performed in parallel and, even, that the order of some of the steps may be altered. The process is described with reference to the phase and amplitude comparator (19 in figure 2), although it will be apparent to those skilled in the art that the comparator may be incorporated into other parts of the transmission system such as the phase adjustor 11 for example. Suitable modifications to the process will be apparent to those skilled in the art in this case.

The process 60 begins at step 61. In step 62, the channel estimate is received from the transceiver. This estimate characterises how the phase of the channel frequency response jumps discontinuously between subcarrier frequencies when these subcarriers are from different transmit antennas. A comparator 63 is then used to determine how the phase and amplitude of the signals transmitted from the subcarriers must be pre-coded to make the receiver believe that the effective' channel is consistent with emissions from a single antenna. In some embodiments the amplitude change may be ignored to satisfy EIRP restrictions.

In step 64, the phase and amplitude changes needed to make transmissions appear to a receiver as having emanated from a single antenna with reduced frequency selective fading are determined from the comparator 63.

The required phase rotation is typically selected to make the combined phase change imparted by the channel and the phase pre-coding of the transmitted signals result in a linear decrease in the phase with frequency, with the slope selected to be optimal for a given legacy receiver adapted for SISO transmissions.

In step 65, the antenna allocator 9 allocates antennas to given subcarriers of the OFDM signal 8 to create the OFDM signals lOa and lOb allocated to the outputs 5a and 5b.

At step 66 the signals allocated to the respectively mapped subcarriers are adjusted in amplitude and phase as dictated by 64.

The process ends at step 67.

Figure 4 depicts the measured phase of the frequency response of the channel in the absence of phase precoding, with frequency in MHz on the horizontal axis and wrapped phase in radians, subject to modulo 2t phase wrapping, on the vertical axis. Line 31 depicts the phase of the channel frequency response. Arrows 32 to 37 depict OFDM subcarrier frequencies.

Figure 5 depicts the desired effective channel frequency response 41 after the measured phase of the frequency response of the channel has been combined with the phase precoding, with frequency in MHz shown on the horizontal axis and wrapped phase in radians shown on the vertical axis. Subcarrier frequencies of the OFDM signal are depicted by arrows 42 to 47. Typically, the phase rotations applied to signals transmitted on the subcarriers will be selected to optimise perfonnance of the transmission from the multiple antenna transmission system 3 to a legacy single antenna receiver. In this case, the optimal phase rotation distribution will result in a linear phase response, where the resulting effective phase is give n by = m v +c prior to phase wrapping, where m is the slope and is selected for a particular category of receiver.

The predetermined criteria may include a weighting factor m representing the slope of the desired effective phase response of the channel. This translates into a definable shift of the impulse response in the time domain. If a transmitter knows in advance the properties of the receiver, such as knowing it is a legacy device or one designed with knowledge of this new technology, then the weighting factor m can be chosen to inaximise performance in the target receiver.

Figure 6 shows the graph 50 with a time index shown on the horizontal axis and the magnitude of the channel impulse response shown on the vertical axis. The line 51 depicts a time domain impulse response for an effective channel that has had per-subcarrier antenna selection applied without any phase pre-coding of the transmitted signals. The phase of the subcarriers jumps discontinuouslY due to the use of spatially separated multiple antennas. Line 52 depicts the time domain impulse response of the effective channel once the transmitted signals have been phase pre-coded to eliminate abrupt changes in phase between adjacent subcarriers.

Figure 7 shows a graph 80 with distance in metres on the horizontal axis and ergodic capacity shown on the vertical axis in bits/s. Line 81 depicts the ergodic capacity of a multiple input antenna system communicating with a single antenna receiver according to the preferred embodiment of the present invention with four transmit antennas and 128 OFDM subcarriers.

Line 82 depicts an equivalent case where channel state information is not used at all at the transmitter and all the four transmit antennas are utilised optimally from an ergodic capacity perspective (spatial multiplexing). Line 83 depicts a reference single transmit antenna system for benchmarkiflg performance.

Figure 7 shows that, in terms of ergodic capacity, a multiple antenna transmitter shows only a limited advantage over a single antenna system when channel state information is not utilized at transmission. Figure 7 also shows that a multiple antenna transmitter utilizing channel state information to perform a subcarrier to antenna allocation provides a significant advantage over single antenna transmitters used with single antenna receivers.

Figure 8 shows a graph 90 with signal-to-noise ratio (SNR) in decibels (dB) on the horizontal axis and Packet Error Rate (PER) on the vertical axis. Line 91 depicts the result for the same four antenna 128 subcarrier system as line 81 of Figure 7, with antenna allocation and phase adjustment according to the present invention. Line 92 represents a case corresponding to a similar system but with no phase precoding applied according to the present invention. Line 93 depicts a result for a reference single transmit antenna system.

Figure 8 shows that, in terms of PER, multiple antenna transmitters with per subcarrier antenna allocation and no phase pre-coding provide only a limited advantage at low SNRs and even suffer a disadvantage at high SNRs. However, Figure 8 shows that multiple antenna transmitters with phase pre-coding according to the present invention show a significant improvement in PER over the wide range of SNRs.

Figure 9 schematically depicts a transceiver 100 according to another embodiment of the present invention. The transceiver 100 includes a transmitter 103, equivalent in functionality to the transmission system 3, with multiple antennas 105 and 106.

Alternative embodiments may have more antennas as appropriate for given applications.

The transceiver also includes a receiver 107 which is also in communication with the antennas 105 and 106 of the transmission system 104.

The reader will appreciate that the foregoing is but one example of implementation of the present invention, in that further aspects, variations and advantages may arise from using the invention in different embodiments. The scope of protection is intended to be provided by the claims appended hereto, which are to be interpreted in the light of the description with reference to the drawings and not to be limited to the above.

Claims

CLAIMS: I. A transmitter having multiple transmit antennas, being adapted for transmission of signals on multiple subcarrier frequencies and having available channel state information, the transmitter including: a subcarrier allocator operable to allocate subcarriers of the signal to antennas according to a metric of performance determined for subcarrier frequencies from the channel state information; and a phase adjustor operable to pre-code the phase of the signals according to a measured phase response of the channel to make transmissions fulfil one or more predetermined criteria.
2. A transmitter according to claim I, wherein said predetermined criterion that the phase of the signal precoding combined with the phase imparted in propagation between the transmitter and a receiver decreases with the frequency of the subcarrier on which they are to be transmitted, subject to modulo 2it phase wrapping.
3. A transmitter as claimed in claim 2, wherein the predetermined criteria is also that the phase of the signal precoding combined with the phase imparted in propagation between the transmitter and a receiver decreases linearly with the frequency of the subcarrier on which they are to be transmitted, subject to modulo 27t phase wrapping.
4. A transmitter as claimed in claim 3, wherein the predetermined phase pre-coding criteria includes a weighting factor which defines the rate of the linear decrease in phase.
5. A transmitter as claimed in any one of the preceding claims wherein the transmitter is operable to obtain said channel state information from a received channel assuming reciprocity.
6. A transmitter as claimed in claim 5, wherein the transmitter is operable to calibrate the channel state information to correct for potential non-reciprocity.
7. A transmitter as claimed in any one of claims I to 4, wherein the transmitter is operable to receive a feedback channel providing channel state information.
8. A transmitter as claimed in any one of the preceding claims, wherein said multiple antennas are arranged favourably such as a ring or linear array.
9. A transmitter as claimed in any one of the preceding claims, wherein said multiple antennas include both directional and omni-directional antennas.
10. A transceiver including a transmitter as claimed in any one of the preceding claims.
11. A transceiver as claimed in claim 10 including a receiver in communication with at least two of the multiple antennas of the transmitter.
12. A method of conditioning a signal with multiple subcarrier components for multiple antenna transmission systems, the method including: measuring the channel frequency response between a transmitter and a receiver; allocating subcarriers to antennas based on a metric of performance related to the measured channel frequency response; rotating the phase of the signals transmitted on the subcarriers of the multiple antennas such that the phase of signals transmitted meet one or more predetermined phase criteria.
13. The method of claim 12, wherein said one or more predetermined criteria is chosen such that transmissions have a phase corresponding to those would originate from a transmitter with a fewer number of antennas.
14. The method of any one of claims 12 or 13 including providing feedback channel to feed data relating to measurements of amplitude and phase of signals transmitted from the receiver to the transmitter.
15. The method of any one of claims 11 or 12, including receiving a transmission at the transmitter, measuring the amplitude and phase of the channel and assuming reciprocity.
16. The method of claim 15 including calibrating measurements of amplitude and phase to correct any potential non-reciprocity.
17. A computer readable medium containing computer readable instructions operable to cause a computer to become configured to carry out a method of conditioning signals for a multiple antenna transmission system configured to transmit signals with multiple subcarrier components, the method including: measuring the channel frequency response between a transmitter and a receiver; allocating subcarriers to antennas based on a metric of performance related to the measured channel frequency response; rotating the phase of the signals transmitted on the subcarriers of the multiple antennas such that the phase of signals transmitted meet one or more predetermined phase criteria.