GB2426420A - Reducing peak to average power ratio (PAPR) in an orthogonal frequency division multiplexing transmitter - Google Patents

Abstract

To mitigate the problem of high peak to average power ratios in orthogonal frequency division multiplexing (OFDM) transmitters, an OFDM transmitter comprises division means (230) arranged in operation to divide a plurality of N sub-carriers into at least two blocks, and multicarrier modulation means operable to generate separate signals from said blocks of sub-carriers, for subsequent processing and transmission from respective antennas (291-294).

Description

ORTHOGONAL FREOUENCy DIVISION MULTIPLEXING TRANSMITTER The present

invention relates to orthogonal frequency division multiplexing (OFDM) transmitters, and in particular, mitigating the effects of peak to average power ratios in such transmitters.

In modern telecommunications systems, a range of transmission schemes exist to allow multiple users access to a common bandwidth resource. Such schemes include Time Division Multiplexing (TDM), wherein each user is allocated a short timeslot within the channel in which to transmit; Code Division Multiplexing (CDM), wherein each user is allocated a unique code by which to distribute their data over time and frequency; and Frequency Division Multiplexing (FDM), wherein each user is allocated a proportion of the frequency channel in which to communicate.

In FDM, sub-carriers on different frequency channels are spaced apart by a predetermined spacing to minimise inter-carrier interference, and this wastes potentially useful bandwidth. To improve this situation, an enhanced form of FDM known as orthogonal frequency division multiplexing, or OFDM, has been implemented.

In OFDM, the available channel bandwidth W is subdivided into N= W/4f subchannels, each having a bandwidth zlf The bandwidth 4f is chosen to be small enough so that the frequency response is constant over this interval (flat fading).

A sub-carrier x(t) is associated with each sub-channel n, wherein x, (1) = sin 2zJt for n=O, 1, ... N-i, and wheref is the centre frequency of the th sub-channel.

Figure 1 illustrates adjacent sub-channel frequency characteristics, with frequency on the x-axis and a y-axis representative of relative amplitude. In Figure 1, each sub-carrier has side lobes. The side lobes are a consequence of windowing the carrier signals in the time domain, causing a sinc function in the frequency domain. However, by selecting a symbol rate 1/Ton each of the sub-carriers to be equal to the separation zlf of adjacent sub-carriers, the sub-carriers become orthogonal as the centre frequency of each sub- channel coincides with the zero points of adjacent sub-carrier side lobes in frequency space. Figure 1 shows the orthogonality effect for sub- carriers 131 - 137 as labelled.

Consequently a relatively large number of sub-carriers can coexist adjacent each other, significantly improving spectral efficiency when compared with FDM.

It should be noted that, because 1/T=zl/, for an OFDM system with N subcarriers the symbol rate on each sub-carrier is N times slower than on a single carrier system employing the full bandwidth W. This provides a consequent improvement in channel robustness, over the comparable single carrier system.

A slower symbol rate renders OFDM advantageous for high data-rate applications that are affected by multi-path time dispersion, or channel spread, such as mobile telecommunications. Channel spread is caused by the reflection of signals in the propagation enviromnent causing multiple copies of the signal to arrive at the receiver at different times, with different amplitudes and phases. These copies can result in deep fades, where certain frequencies destructively interfere and result in the loss of information; and inter symbol interference (1ST), where delayed copies of the signal cause symbols to overlap.

Splitting the data over N sub-carriers, within a given bandwidth W results in symbol intervals N times longer than for a single channel with the same data rate, as noted above. When N is sufficiently large, the symbol period T becomes larger than the duration of channel spread, and the effect is to significantly reduce 151. In general terms, larger symbol intervals mean that, all else being equal, any 151 is spread over fewer symbols. This simplifies equalisation to correct for 151.

Splitting the data over N sub-carriers also provides the scope to distribute redundant coding such as forward error correction over the subcarriers, making the symbol stream more robust to fading at any given frequency.

Thus, OFDM has the potential to provide much greater channel spread resilience for the same data throughput than a single equivalent rate channel.

However, these properties of OFDM are subject to a number of conditions.

One condition is that the receiver and transmitter are perfectly synchronised in terms of clock frequency and timing, to ensure representative sampling of the signal. To address this, it is well known in the art for a data packet to comprise a pre-amble of known composition, which can be used to synchronise reception (the preamble also enables estimation of the channel transfer function, which is used during equalisation).

Similarly, one or more of the sub-carriers can be used as pilot channels, carrying known signal patterns to allow the tracking of any drift in frequency of the receiver relative to the transmitter.

Counter-intuitively, it is also a condition that there is minimal channel spread distortion of the signal. Channel spread causes intersymbol interference when echoes of the previous symbol (signal block) reach the receiver at the start of the next symbol, causing signal distortion that might affect FFT decoding of the received signal for recovery of the N sub-carriers. Whilst the increased length of symbol interval T reduces the proportion of echo overlap, it does not eliminate it. Thus, although OFDM reduces the degree of overlap between symbols, it is very sensitive to any overlap that remains.

The reflection of signals in the propagation environment is commonplace. To eliminate this problem, it is similarly well known in the art to add a guard interval to the transmitted signal equal to an estimate of the maximum multi-path delay spread. This adds an appreciable overhead to the data transmission rate, which is proportional to the ratio of the delay spread to the symbol period T (e.g., 20% for IEEE 802.1 la). The interval is referred to as a cyclic prefix, where a portion of the signal tail is prepended to the signal itself to occupy the interval.

As noted previously, redundancy within the symbol in the form of forward error correction enables recovery of the information in the symbol, but again at the cost of an overhead.

A third condition is that there is minimal transmission distortion of the signal that might affect recovery of the N sub-carriers. However, prior to transmission, the process of converting the N sub-carriers into a waveform via inverse FFT can result in a large peak to average power ratio (PAPR), when signals modulating the OFDM sub-carriers add constructively in phase. This in turn can lead to signal distortion when the transmitter contains a non-linear component such as a power amplifier.

The resulting non-linear effects cause intra-band interference due to intennodulation and warping of the signal constellation, and inter-band interference in the form of adjacent channel interference through spectral spreading. Both types of interference increase the bit error rate (BER) at the receiver.

One possible method by which to alleviate transmission distortion is to reduce the power amplifier gain by an amount approximately equal to the PAPR, so avoiding distortion in the first place. However this significantly reduces the efficiency of the amplifier, which is undesirable, particularly in portable wireless devices where power consumption is an important consideration.

Consequently, a number of techniques are known in the art for reducing the PAPR directly.

The first technique is redundant coding. In an OFDM system with N subcarriers, for example using quadrature phase shift keying (QPSK), there are 22N possible symbol combinations. Redundant coding relies on selecting as valid, for the transmission scheme, only those symbols that combine to generate relatively low peak powers. This approach has the advantage that it does not apply any additional distortion or processing to the signal. However, it does necessarily reduce throughput as there are fewer symbol combinations available. In Shepherd S., et. al. Asymptotic limits in peak envelope power reduction by redundant coding in orthogonal frequency-division multiplex modulation' (IEEE Trans. Comms., Vol 46 No. 1, Jan 1998), it is shown that with one redundant bit, the PAPR can be reduced from 15 to 4 for 15 sub-carriers. However, as the number of sub-carriers N increases, Shepherd et. a!. also show that the amount of redundancy needed to achieve a PAPR of below 3 converges toward a rate code.

Additionally, as the number of sub-carriers increases, the computational task of combination selection becomes onerous, placing power and processing costs on the host device (for typical values of N, it is impractical to store so many combinations in advance).

A second technique is peak clipping, whose performance is independent of the number of sub-carriers N. However, clipping causes spectral leakage into adjacent sub-carriers.

To reduce the resulting adjacent channel interference, the signal must be filtered, although this filtration typically causes 4 to 5dB re-growth of the original peak. The overall effect is some reduction in PAPR at the cost of some inter-band interference. In addition there is some intraband interference, as the clipped peak corresponds to lost information and so blurs the FFT decoding. In Mestdagh D., et. al., Analysis of clipping effects in DMT-based ADSL systems' (IEEE 1994), it was shown that the effective intra-band noise of clipped information can be compensated for by increased fidelity in the A to D and D to A converters; additional quantisation bits can be used such that the reduced quantisation noise compensates for the increased clipping noise.

However, this approach assumes that clipping is sufficiently frequent within a given symbol to approximate a Gaussian noise distribution; if it does not, then there is little benefit. Moreover, it adds cost in the form of higher fidelity converters.

A third technique is to add artificial signals to the present combination of sub-carriers in the OFDM symbol, in order to destructively interfere with the peaks. If a proportion of the sub-carriers are sacrificed in each OFDM symbol, they may be amplitude and phase weighted such that, in combination within the OFDM symbol, a lower overall PAPR is generated. In Yang Jun, et. al., Reduction of peak to average power ratio of the multicarrier signal via artificial signals', (IEEE 2000), a desired maximum value of the sample envelope is equated with the envelope of the combined symbol plus one or two artificial signals. The amplitudes, phases and frequencies of these signals can then be found if the equation has valid roots. Yang Jun, et. a!. show that the PAPR can be reduced by 6 dB for 16 sub-carriers when using two such artificial signals. An improved solution utilising all empty carriers is also possible using convex optimisation.

However, it will be appreciated that these techniques improve PAPR by increasing the number of sub-chaimel transmissions, which increases power demands and so can be disadvantageous for portable devices. Also, the computational overhead for determining two or more artificial signals for every transmitted symbol is not trivial, and increases with the number of sub-carriers N. Thus, whilst there are a number of techniques available to reduce PAPR, none are ideal in all circumstances. In addition, the two methods that do not distort the symbol, namely coding redundancy and artificial signals, become less effective as the number of sub- carriers N increases. However, current trends in telecommunications suggest that such an increase is likely. For example, in the current IEEE 802.11 a and single 20MHz band 802.1 ig telecommunications standards, OFDM is used to efficiently transmit data by modulating 48 out of 64 sub-carriers with data and using cyclic guard intervals, whilst 4 sub-carriers provide pilot tones, and 12 sub-carriers are set to zero (null) for suppression of out-of band interference.

However, 128 sub-carriers are desirable for OFDM over a 40MHz wide band rather than the 20MHz band currently used by IEEE 802.11 a and IEEE 802. llg.

Similarly, whilst the IEEE 802.1 in standard also uses 64 sub-carriers per 20MHz, there has been debate over the possible use of 128 sub- carriers in order to reduce the proportional overhead of the cyclic guard interval. Increasing the number of sub-carriers lengthens the OFDM symbol duration T (as it is proportional to 1/Af), whilst the guard interval is a function of channel spread and thus does not change with N. The result is that the cyclic guard overhead is proportional to 1/N. This effect tends to encourage an increase in the number of sub-carriers provided in OFDM systems.

Therefore, it can be expected that the number of sub-carriers in ODFM systems is set to increase in the future. Consequently, it is desirable to find a technique complementary to those proposed above that will reduce PAPR for OFDM systems needing larger numbers of sub- carriers.

In a first aspect of the present invention, an OFDM transmitter comprises means to divide the total N sub-carriers into a plurality of groups. Following additional processing, generated signals corresponding with said groups are passed to respective power amplifiers.

In one, preferred configuration of the above aspect, the respective power amplifiers are each part of a separate transmit chain comprising a multicarrier modulator, cyclic guard means and serial to parallel converter means, a D/A converter and an up-converter.

In another, alternative configuration of the above aspect, division of the N sub-carriers is achieved for at least part of a transmission chain by the use of components operating sequentially upon each group of subcarriers, by switching between them.

In a second aspect of the invention, an OFDM transmitter operable to transmit on at least some of a plurality of sub-carriers, and comprising sub-carrier allocation means operable to allocate the sub-carriers into a plurality of sub-carrier groups, and respective data processing means for each sub-carrier group, each data processing means operable to prepare said sub-carriers of said sub-carrier group for OFDM transmission.

In a configuration of the above aspect, the data processing means may include a power amplifier.

In another configuration of the above aspect, the data processing means may include a multi-carrier modulator.

In another configuration of the above aspect, the data processing means may include a cyclic prefixer and serial to parallel converter.

In another configuration of the above aspect, the data processing means may include a digital to analog converter.

In another configuration of the above aspect, the data processing means may include an up converter.

In another configuration of the above aspect, the data processing means may include an antenna.

In another aspect of the present invention, a transceiver comprises the OFDM transmitter described herein.

Similarly, in another aspect of the present invention, a communications device comprises the OFDM transmitter described herein.

In a further aspect of the present invention, a transmission of two or more coordinated signals bears data modulated onto sub-carriers that are arranged as respective groups of sub-carriers, said groups of sub-carriers generated by partitioning a plurality of sub- carriers into two or more respective blocks of sub-carriers.

In a yet further aspect of the present invention, a method of OFDM transmission comprises the step of allocating a plurality of sub-carriers into two or more groups.

Signals generated from these groups of sub-carriers are then sent via respective power amplifiers prior to transmission.

In another aspect of the present invention, a data carrier comprises processor implementable instructions that, when loaded into a computer, cause the computer to operate as an OFDM transmitter in which a plurality of sub-carriers are allocated into two or more groups of sub-carriers, wherein said groups of sub-carriers are subsequently processed and transmitted via parallel transmit chains.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates an orthogonal set of sin(x)/x spectral distributions for the introductory example set out above.

Figure 2 is a schematic diagram of an example communications device.

Figure 3 is a schematic illustration of an example OFDM transmission system.

Figure 4 is a schematic diagram of a communications device in accordance with an embodiment of the present invention.

Figure 5 is a schematic illustration of an OFDM transmission system in accordance with an embodiment of the present invention.

An OFDM transmitter and a method of transmission are disclosed. In the following description, a number of specific details are presented in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, to a person skilled in the art that these specific details need not be employed to practise the present invention.

Figure 2 illustrates schematically a laptop computer device 20 providing an example of background to the invention. The laptop 20 comprises a processor 24 operable to execute machine code instructions stored in a working memory 26 and/or retrievable from a mass storage device 22. By means of a general-purpose bus 25, user operable input devices 30 are in communication with the processor 24. The user operable input devices 30 comprise, in this example, a keyboard and a touchpad, but could include a mouse or other pointing device, a contact sensitive surface on a display unit of the device, a writing tablet, speech recognition means, haptic input means, or any other means by which a user input action can be interpreted and converted into data signals.

Audio/video output devices 32 are further connected to the generalpurpose bus 25, for the output of information to a user. Audio/video output devices 32 include a visual display unit, and a speaker, but can also include any other device capable of presenting information to a user.

A communications unit 100 is connected to the general-purpose bus 25, and further connected to an antenna 190. By means of the communications unit 100 and the antenna 190, the laptop computer 20 is capable of establishing wireless communication with another device. The communications unit 100 is operable to convert data passed thereto on the bus 25 to an RF signal carrier in accordance with a communications protocol previously established for use by a system in which the laptop computer 20 is appropriate for use.

In the device 20 of Figure 2, the working memory 26 stores user applications 28 which, when executed by the processor 24, cause the establishment of a user interface to enable communication of data to and from a user. The applications 28 thus establish general purpose or specific computer implemented utilities and facilities that might habitually be used by a user.

Referring also now to Figure 3, the communications device 100 comprises an OFDM transmitter 101 as known in the art. Connection with the generalpurpose bus 25 provides a data source 110. Data from the data source 110 is passed to a serial to parallel buffer 120, which assigns data to N subcarriers 130. A multicarrier modulator modulates the sub-carriers 130 with the data, and converts the result to a discrete time series. A cyclic prefix is added and subsequent prefixed time series are then concatenated by serial to parallel converter 150 to produce a continuous series. A digital to analog converter 160 uses this series to generate an analog signal, before an upconverter 170 generates a corresponding RF signal. Finally, this signal is amplified by a power amplifier 180 before being transmitted by an antenna 190. It will be appreciated that the transmitter 101 is a simplified illustration, omitting elements such as filters for the purposes of clarity.

Specifically, in typical operation the serial to parallel buffer 120 takes Bf bits from the data source 110, and allocates them to N groups of b bits such that = Bf. The N groups are each assigned to one of N sub- carriers 130.

The multicarrier modulator 140 may then be viewed as generating N independent QAM sub-channels, where the symbol rate for each sub-channel is J/T. The number of signal points in the th sub-channel is therefore M=2b.

Denoting the complex values of the signals in each sub-channel byX, n = 0, 1, ... N-i, then the symbols X,, can be considered as the values of a discrete Fourier transform (DFT) of an OFDM transmission signal x(t).

Therefore, to obtain x(t) for transmission, an inverse Fourier transform of X must be performed. Where the protocol requires conjugate symmetry (for example, as proposed in IEEE 802.1 5.3a), K=2N symbols are typically generated by defining XK = X,, and all K are inverse transformed, resulting in x(t) being real-valued. Other protocols, such as IEEE 802.11 a, g or as proposed in 11 n use just the original N symbols and demodulate both I and Q components. For the purposes of clarity, and without loss of generalisation, a real-valued x(t) is discussed below.

The inverse DFT (typically in the form of an inverse fast Fourier transform, IFFT) thus generates the real-valued sequence 1 K Vv j2,dcn/K k= 1 Xk- fL n,... - V1\ n=O where 1 /\JK is a scaling only. xk represents a sequence of samples, or a sample block, of the desired OFDM signal x(t), output by the multicarrier modulator 140.

However, prior to obtaining x(t), a cyclic prefix should be added to the sample block xk, k= 0, 1, ...K-1 by the cyclic prefix and parallel to serial converter 150.

The required length of the cyclic prefix may be derived by considering the received signal; this may be modelled as r(t) = x(t) *c(t) + n(t), where c(t) is the impulse response of the channel and * is convolution and n(t) is noise. Recalling that the symbol period T oc]/zlf is large compared to the channel impulse response (channel spread duration); however, to completely remove intersymbol interference one may insert a guard interval of duration mT/K between successive sample sequences, where m+1 samples would match or exceed the impulse response duration.

The guard interval can be a cyclic prefix prepended to each block of K samples, using samples XKm, XK.m+J, ... x to create a augmented block of K+m samples duration.

Then, if sample values of the channel response are Ck, 0 k m, the convolution of Ck with xk, -m k K-], produces the received signal rk. Since by definition any intersymbol interference only affects the first m+ i samples of this block, discarding these will recover the desired block rk, 0 k K-i without any intersymbol interference present.

Serial to parallel conversion simply concatenates sample blocks prior to D to A conversion by the D to A converter 160. The output of the converter is the analog OFDM signal x(t).

The analog signal x(t) is then upconverted to the transmission frequency by upconverter 170. Finally this RF signal is amplified by a power amplifier 180 and is then transmitted by the antenna 190.

However as noted previously, x(t) can have a high peak to average power ratio (PAPR) if sub-carrier signals in Xk, k = 0, 1, .. . K-i constructively interfere, and this can cause undesirable non-linear distortion in the power amplifier.

In contrast to the solutions for PAPR reduction discussed previously, the inventors of the present invention have appreciated that the PAPR can be shown to be proportional to N, and seek to exploit this property.

The peak to average power ratio of a time domain OFDM symbol x, where all sub- carriers share the same constellation and a cyclic prefix is added, is given by: max X PAPR{x[n/L]}=N 2) (1)

EXI I

where n is a time index, L is the number of samples in the symbol, N is the number of sub-carriers, X, is the complex constellation value on the flt sub-carrier and E{.} is the expectation function.

To take a specific example, define the signal for N sub-carriers as = a cos wt + jb sinat, where a and b provide in-phase and quadrature modulations. If each sub-carrier has amplitude A, then the maximum PAPR is (NA)2/ -2N /N(A2/2) - In essence, the more sub-carriers that exist, then the more scope there is for in-phase addition during the IFFT (Note however that the maximum possible PAPR itself is reached less often as the signal approaches a Normal distribution for large numbers of channels).

The inventors of the present invention have appreciated that therefore an alternative approach to those previously discussed for reducing PAPR is possible.

Referring now to Figure 4, this figure illustrates schematically a laptop computer device 220 in accordance with an embodiment of the present invention. Components similar to the laptop of Figure 2 are numbered similarly for clarity. The laptop 220 comprises, as previously, a processor 24 operable to execute machine code instructions stored in a working memory 26 and/or retrievable from a mass storage device 22. By means of a general-purpose bus 25, user operable input devices 30 are in communication with the processor 24. The user operable input devices 30 comprise, in this example, a keyboard and a touchpad, but could include a mouse or other pointing device, a contact sensitive surface on a display unit of the device, a writing tablet, speech recognition means, haptic input means, or any other means by which a user input action can be interpreted and converted into data signals.

Audio/video output devices 32 are further connected to the generalpurpose bus 25, for the output of information to a user. Audio/video output devices 32 include a visual display unit, and a speaker, but can also include any other device capable of presenting information to a user.

A communications unit 200 in accordance with an embodiment of the present invention is connected to the general-purpose bus 25, and further connected to a plurality of antennas 290. By means of the communications unit 200 and the antennas 290, the laptop computer 220 is capable of establishing wireless communication with another device. The communications unit 200 is operable to convert data passed thereto on the bus 25 to a plurality of RF signal carriers in accordance with a communications protocol previously established for use by a system in which the laptop computer 220 is appropriate for use.

Referring now to Figure 5, in an embodiment of the present invention, the communications unit 200 comprises an OFDM transmission system 201. Notably, the OFDM transmission system 201 divides the sub-channels between a plurality of transmit antennas, collectively numbered 290. Thus in this example, the OFDM system comprising 256 sub-carriers employs 4 antennas 291, 292, 293, 294, with 64 sub- carriers assigned to each.

It will be appreciated that the embodiment described herein is illustrative, and other configurations may provide other than 256 subcarriers, or comprise other than 4 antennas.

As shown in Figure 5, each antenna 290 has a corresponding transmission chain similar to that seen in the illustrative OFDM system of Figure 3. Advantageously, however, in this example the power amplifier in each chain is now only exposed to a PAPR proportional to M= N/4, namely 64 subcarriers.

Specifically, serial to parallel buffer 220 generates N=256 inputs as before, but now also comprises means to separate these into, in this example, four supra sets s = 1, 2, 3, 4 each comprising 64 inputs.

These 64 inputs (typically each a constellation point representing a number of bits b) in each set s are then each assigned to 64 corresponding sub-carriers, producing in this example four blocks of 64 sub-carriers 231-234, corresponding with input sets s = 1, 2, 3,4.

These four blocks of 64 sub-carriers then feed into four parallel transmit chains. Each chain comprises a multicarrier modulator (24 1-244, collectively 240), a cyclic prefixer and parallel to serial converter(251-254, collectively 250), a D to A converter (261-264, collectively 260), an up-converter (271-274, collectively 270), a power amplifier (28 1- 284, collectively 280) and an antenna (29 1-294, collectively 290). Each transmit chain operates substantially as described for the OFDM transmission system described with reference to Figure 3, with the notable difference that each transmit chain now only serves 64 sub- carriers, instead of a single transmit chain serving 256.

Consequently, power amplification occurs for each of four separate signals x(t), s 1, 2, 3, 4 where, as noted above for this example, the PAPR compared to the undivided signal x(t) seen for the OFDM system of Figure 3 is reduced by a factor of 4.

In consequence, distortion for each signal x(t) will be reduced or eliminated within its respective power amplifier, all else being equal.

In an alternative embodiment, the number of sets s may be determined by the number of antennas available, or by the number of antennas chosen from those available to provide a suitable number of sets for the desired number of sub-carriers.

Advantageously, parallel transmit chains as described above can be used to provide OFDM systems with a number of sub-carriers other than an exact power of 2, as is currently required to use a fast Fourier transform. For example, an OFDM with 192 sub- carriers can now be constructed from three transmit chains each servicing a 64 sub- carriers block.

Dividing the sub-carriers over a number of antennas also results in several other advantages.

Firstly, whilst there are more elements in the embodiment of Figure 5, it should be noted that the multicarrier modulator 240 in each transmit chain is considerably simpler than that in the system of Figure 1, as the relative complexity of the individual inverse Fourier transforms drop from N log2 N to M log2 M, where M< <N. Similarly, the parallel to serial converter 250 will require a smaller buffer.

Secondly, the reduced PAPR allows each separate power amplifier to operate closer to saturation, where they are substantially more efficient. This provides a considerable saving in power.

Thirdly, each separate antenna and power amplifier can be designed for the specific, smaller bandwidth covered by the N/4 sub-carriers in their respective block. A comparatively narrow-band antenna can be made correspondingly smaller physically, and therefore integration with the RF/baseband electronics is more feasible. Such an antenna would need to be dynamically tunable to operate in other channels so as to avoid other WLANs.

Fourthly, the complementary methods of artificial signal insertion and redundant coding discussed previously become simpler to implement when there are fewer sub-carriers in each transmit chain.

Finally, embodiments of the present invention may facilitate Alamouti space-frequency coding. The Alamouti space-time code is a popular and simple method for improving system robustness by transmitting a second, coded copy of the data from a different antenna. However, this introduces a duplication/decoding latency, and requires channel constancy over two symbol periods (or more for other space-time block codes). The frequency variant approach is to transmit the redundant information on two subcarriers instead of over two time slots. However, this is only as good as space-time coding if the channel transfer function is identical for both sub-carriers. Currently, the problems described above mean there is a practical limit to how many sub-carriers can be used in a given bandwidth, and so usually the spacing between them is too large to ensure that the channel response is identical. Embodiments of the present invention allow more sub-carriers across the band due to removal of the PAPR N constraint. Consequently closer spacing is possible and therefore so are more consistent channel transfer functions between neighbouring sub-carriers. This may allow the Alamouti space- frequency variant to succeed in applications where performance would normally have been poor.

In addition, reducing the PAPR increases the number of sub-carriers N that can be defined for an equivalent distortion level. Because, as noted previously, an increase in the number of sub-carriers leads to an increase in symbol period T, this allows a reduction in the proportional overhead of the cyclic guard interval, all else being equal.

In the described embodiments, signals x5(t) are transmitted from their respective antennas together as an OFDM transmission. Consequently, at the receiver, assuming that the transmissions for each antenna are synchronised, then a normal cooperating OFDM receiver will see a superposed signal comprising all N sub-carriers, which can be decoded as per normal.

Preferably, each signal x5(t) comprises information from at least one pilot channel.

In an embodiment of the present invention, the preamble is assembled piece-wise across the plurality of transmission streams, with components of the pre-amble being transmitted in a complementary manner from each antenna.

It will be appreciated that by replacing, for example, a 256-channel IFFT with four 64 channel IFFTs, the relative frequency information of channels between each of the four blocks is lost. That is to say, the frequencies of channels n=1, n'=65, n=129 and n=193 for example will appear identical for each 64 channel IFFT, as they now each occupy the same position in one of the smaller blocks passed to each IFFT.

As a result, a simultaneous transmission would result in superposition of four signals occupying the same W/4 sub-bandwidth.

Consequently, in an embodiment of the present invention, each signal x(t), s = 1, 2, 3, 4, is transmitted using a shifted RF carrier. Preferably the shift is equal to the base bandwidth Wdivided by the number of blocks s, so for example for s 1, 2, 3, 4, each signal is upconverted to an RF carrier further shifted by W/4. The net result is that the signals are restored to their relative positions in the RF band.

In an alternative embodiment of the present invention, the relative frequency of channels in each block s is preserved from the outset by performing a full N-sub-carrier IFFT in each transmit chain, but with all carriers other than those of the respective block zeroed to maintain the reduction in PAPR due to limited constructive interference. This avoids the need to shift the RF carrier for each signal s, but does lose the advantage of a smaller IFFT (and thus simpler circuitry) for each transmit chain.

It will be clear to a person skilled in the art that the N sub-carriers may preferably be divided such that the size of each block equates with a power-of-two. However, it will also be clear that there is no need for each block to be the same size, although this could require a flexible parallel/serial buffering system, as the input/output sequences for different size blocks would be of different lengths.

It will similarly be clear to a person skilled in the art that, while preferable, the division of N sub-carriers need not result in blocks of adjacent channels. For example, four interleaved blocks could be generated by selecting every fourth sub-carriers starting from sub- carriers 0, 1, 2 and 3. This reduces the scope for tailored antenna design, and use of the Alamouti frequency variant, but may in some circumstances improve overall robustness by spreading the frequency range used by each set s. Similarly, dynamic allocation of symbols among the sub-carriers may be employed to reduce PAPR.

It will be clear to a person skilled in the art that the invention described herein is applicable to any field where OFDM is used, for example wireless LAN, digital audio and digital video broadcasting, and ADSL or SDSL (DSL).

In the case of DSL, it will therefore be clear to a person skilled in the art that the use of an antenna is not essential for wire line DSL, and that the apparatus is equally applicable to wire line or fibre-optic transmissions, provided that the signals are combined in a lossless manner.

It will similarly be clear to a person skilled in the art that the above apparatus is applicable to variants of OFDM (here all classed under the term OFDM), such as coded OFDM.

It will also be clear to a person skilled in the art that the above means to arrange the apparatus is applicable to mitigate the effect of any nonlinear component in the transmit chain, and is not limited to power amplifiers.

It will similarly be clear to a person skilled in the art that the power amplifiers and other components of the transmit chain or chains may comprise a single entity or a plurality of entities (for example for each transmit chain), and furthennore may form part of a host device such as a laptop, PDA, mobile phone or entertainment device, or may form part of a peripheral such as a PCMCIA card. Similarly, it may form part of a communications network such as a router, access point, repeater, or piconet controller.

It will similarly be clear to a person skilled in the art that the host device or components thereof may be adapted to take advantage of embodiments of the present invention, for example being adapted to increase data rate, or encode Alamouti space-frequency coding. In particular the processor 24, working memory 26, mass storage device 22 andlor general-purpose bus 25 may be adapted as appropriate.

Thus the present invention may be implemented in any suitable manner to provide suitable apparatus or operation; in particular, an OFDM transmitter may consist of a single discrete entity, a single discrete entity such as a PCMCIA card added to a conventional host device such as a laptop, multiple entities added to a conventional host device, or may be formed by adapting existing parts of a conventional host device, such as by software reconfiguration, e.g. using a software plug-in. Alternatively, a combination of additional and adapted entities may be envisaged. For example, switchable multicarrier modulation may be performed by a laptop, whilst cyclic prefixing and parallel to serial conversion is performed on a PCMCIA card. Thus adapting existing parts of a conventional host device may comprise for example reprogramming of one or more processors therein. As such the required adaptation may be implemented in the form of a computer program product comprising processor- implementable instructions stored on a storage medium, such as a floppy disk, hard disk, PROM, RAM or any combination of these or other storage media or signals.

Whilst the above discussions have referred to transmit antennas and transmit chains, it will be appreciated that many devices commonly comprise corresponding transmit and receiver chains, and such transceivers are envisioned within the scope of the present invention.

A method of OFDM transmission in accordance with the present invention comprises the step of dividing a plurality of N sub-carriers into two or more groups. Signals generated from these groups of sub-carriers are then sent via respective power amplifiers prior to transmission.

An OFDM signal in accordance with the present invention comprises data obtained by dividing a plurality of N sub-carriers in to two or more groups. These groups are processed in parallel transmit chains to generate two or more streams x(t), which are then upconverted, amplified and transmitted.

It will be understood that examples of the OFDM transmitter and method of transmission as described above may provide at least one or more of the following advantages: i. Complementary methods of PAPR reduction that are sensitive to the number of sub-carriers are simplified when applied to separate transmit chains; ii. An OFDM transmission may be provided wherein the total number of sub- carriers is not a power of 2, giving added flexibility; iii. An OFDM transmitter can be designed in response to narrower bandwidths for each transmit chain, making some or all of them more efficient; iv. Enabling an increase in N can improve the conditions required for Alamouti frequency encoding (for fixed bandwidth), and; v. Enabling an increase in N can reduce the proportional overhead of the cyclic guard interval.

vi. The sampling rate of individual components is reduced by dividing the overall workload among parallel sets of components, and so enabling a scaling of the technology to higher rates.

vii. The use of narrower band antennas means that they can be made smaller and may therefore be integrated with the basebandlRF electronics for simplification of packaging and scalability.

Claims (14)

1. CLAIMS: 1. An OFDM transmitter, comprising; division means arranged in

   operation to divide a plurality of sub-carriers into at least two blocks of sub- carriers, and multicarrier modulation means operable to generate separate outputs from said blocks of sub-carriers, for subsequent processing and transmission.
2. 2. An OFDM transmitter according to claim 1 further comprising respective transmit chains, wherein each transmit chain comprises a power amplifier, the OFDM transmitter being further arranged in operation to transmit said separate signals generated from said blocks of sub-carriers via respective transmit chains.
3. 3. An OFDM transmitter according to claim 2 wherein said respective transmit chains each further comprise any or all of: i. a digital to analogue converter, and; ii. an upconverter.
4. 4. An OFDM transmitter according to claim 3 wherein said respective transmit chains each further comprise any or all of: i. a multicarrier modulator; ii. a cyclic prefixer, and; iii. a parallel to serial converter.
5. 5. An OFDM transmitter according to any one of claims 2 to 4 wherein said respective transmit chains each further comprise an antenna adapted for the frequency range occupied by their respective block of sub-carriers.
6. 6. An OFDM transmitter according to any one of claims 2 to 5 wherein one or more respective power amplifiers are adapted for the frequency range occupied by their respective block of sub-carriers.
7. 7. An OFDM transmitter according to any one of the preceding claims wherein the total number of sub-carriers is not equal to a power of 2.
8. 8. An OFDM transmitter according to any one of the preceding claims wherein the amplifier, RF components and antenna are integrated on a single chip for each transmit chain.
9. 9. An OFDM transceiver comprising a transmitter in accordance with any one of the preceding claims.
10. 10. A communications device comprising a transmitter in accordance with any one of claims 1 to 7.
11. 11. A communications device according to claim 10 wherein the communications device is any one of: i. a laptop; ii. aPDA; iii. a mobile phone; iv. an entertainment device; v. a PCMCIA card; vi. a router; vii. an access point; viii. a repeater, and; ix. a piconet controller.
12. 12. An OFDM transmission comprising two or more substantially cotransmitted signals, each bearing information derived from a respective group of sub-carriers obtained by division of a plurality of sub-carriers into two or more respective blocks of sub-carriers.
13. 13. A method of OFDM transmission comprising the steps of: dividing a plurality of sub-carriers into two or more blocks of sub-carriers, and; multicarrier modulating each block to generate a separate output for subsequent processing and transmission.
14. 14. A data carrier comprising computer readable instructions that, when loaded into a computer, cause the computer to operate as an OFDM transmitter according to any one of claims ito 8.