GB2434064A

GB2434064A - Assigning time-frequency codes in MB-OFDM on the basis of channel information and data rate

Info

Publication number: GB2434064A
Application number: GB0600157A
Authority: GB
Inventors: Jiun Siew; Justin Coon; Steve Carl Jamieson Parker
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2006-01-05
Filing date: 2006-01-05
Publication date: 2007-07-11
Anticipated expiration: 2026-01-05
Also published as: GB0600157D0; GB2434064B

Abstract

In Multi-band OFDM an UWB spectrum is divided into several band groups (32, Fig. 3a), and each band group into several bands (34, Fig. 3a). For each band group a time-frequency code (TFC) defines the time sequence of bands used for successive symbols, i.e. it defines how frequency hopping should be performed (see Fig. 3b). Each piconet can be assigned a different TFC. The present invention sets out a method of assigning TFCs based on channel information and data rate. Preferably the channel information is the length of the channel's excess delay, L, and this is compared with the length of a symbol's cyclic extension, Q, or the length of zero padding. When L is less than Q a TFC which uses the same band in consecutive slots may be employed without interference. However when L is greater than Q such a TFC may only be used if the data rate exceeds a threshold rate.

Description

M&C Folio: GBP93469 Document: 1135817

A METHOD OF ASSIGNING CODES IN OFDM SYSTEMS

Field of the Invention

This invention relates to apparatus and methods for assigning codes in OFDM (Orthogonal Frequency Division Multiplexed) communication systems. More particularly, it relates to assigning codes in multi-band OFDM (MB-OFDM) systems.

Background of the Invention

OFDM is a well-known technique for transmitting high bit rate digital data signals.

Rather than modulate a single carrier with the high speed data, the data is divided into a number of lower data rate channels each of which is transmitted on a separate subcarriers. In this way the effect of multipath fading is mitigated. In an OFDM signal the separate subcarriers are spaced so that they overlap, as shown for subcarriers 12 in spectrum 10 of Figure 1. The subcarrier frequencies are chosen so that the subcarriers are mutually orthogonal, so that the separate signals modulated onto the subcarriers can be recovered at the receiver. One OFDM symbol is defined by a set of symbols, one modulated onto each subcarrier (and therefore corresponds to a plurality of data bits).

The subcarriers are orthogonal if they are spaced apart in frequency by an interval of 1/T, where T is the OFDM symbol period.

An OFDM symbol can be obtained by performing an inverse Fourier transform, preferably an Inverse Fast Fourier Transform (IFFT), on a set of input symbols. The input symbols can be recovered by performing a Fourier transform, preferably a fast Fourier transform (FFT), on the OFDM symbol. The FFT effectively multiplies the OFDM symbol by each subcarrier and integrates over the symbol period T. It can be seen that for a given subcarrier only one subcarrier from the OFDM symbol is extracted by this procedure, as the overlap with the other subcarriers of the OFDM symbol will average to zero over the integration period T. It should be noted that, because 1/T=z4f for an OFDM system with N subcarriers the symbol rate on each subcarrier 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.

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 ISI. In general terms, larger symbol intervals mean that, all else being equal, any 1ST is spread over fewer symbols. This simplifies equalisation to correct for 1ST.

Splitting the data over N sub-carriers also provides the scope to distribute redundant coding such as forward error correction over the sub-carriers, 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 subcarriers 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.

The second condition is that there is minimal transmission distortion of the signal that might affect recovery of the N subcarriers. However, prior to transmission, the process of converting the N subcarriers 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 intermodulation 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.

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 subcarriers. 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.lla). 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.

The Multi-band OFDM Alliance (MBOA) proposal (see A. Batra, et al., "Multiband OFDM physical layer proposal for IEEE 802.15 task group 3a", IEEE P802.15- 04/0493r1, September 2004) is fundamentally an OFDM system. Each OFDM symbol has 128 subcarriers, where 10 subcarriers are assigned to guard carriers (the values of these tones are simply replicas of other tones' values), and 12 subcarriers are reserved for pilot symbols. The subcarrier spacing is 4.125 MHz, and 32 zero samples are added to each OFDM symbol in order to mitigate the effects of multipath. Zero-padding is used in this proposal rather than the more common cyclic prefix. Due to the available ultra-wide spectrum, each OFDM symbol occupies a bandwidth of 528 MHz and the entire UWB spectrum is divided into 14 bands. Each of these bands 34 is assigned to a specific band group 32 as shown in Table 30 of Figure 3a. To facilitate band switching, an additional 5 zero samples are added to every OFDM symbol. The total duration of an OFDM symbol is 312.5ns.

For each band group 32, the time-frequency code (TFC) defines the time sequence of bands used for successive OFDM symbols. The TFC is defined over 6 symbol periods, after which, the sequence is repeated. Each piconet is assigned a different TFC in order to minimise collisions from occurring. As an example, the sequence of bands 42 used by TFC 1 is 1, 2, 3, 1, 2, 3 as is depicted in Figure 4.

The MBOA proposal defines four different TFCs for Band Groups 1-4, and two TFCs for Band Group 5. The TFCs are listed in Table 36 in Figure 3b. Note that the band numbers 38 listed in Table 36 refers to the relative band number within each band group. Hence, if a piconet was operating in Band Group 2, the first, second, and third band used will correspond to Band Number 4, 5, and 6 as listed in Table 30 of Figure 3a. It is also worth highlighting that with these TFC definitions, a maximum of four piconets can operate simultaneously within a band group.

Another feature of the MBOA proposal is the use of conjugate symmetry and time-spreading. With conjugate symmetry, data is replicated and conjugated about the centre frequency within each OFDM symbol. Hence, the output of the IFFT 220 (Figure 2a) at the OFDM transmission system 200 is real, thereby simplifying the hardware of low data rate devices. With time-spreading, each OFDM symbol is transmitted twice, thus allowing the receiver to combine both transmitted symbols to obtain an improvement in error rate performance. When both conjugate symmetry and time-spreading are used, the overall spreading factor is said to be four. When only time-spreading is used, the overall spreading factor is said to be two, and when none of them are used, the overall spreading factor is said to be one. Obviously, the higher the spreading factor, the lower the transmission rate, but the better the error rate performance for a given SNR. As shown in Figure 5a, Table 50 lists the data rates specified in the proposal and whether or not conjugate symmetry and time-spreading are used.

Referring to Figure 6, the purpose of adding a cyclic extension 62 to each OFDM symbol 66 is to mitigate the multipath effects of the channel. In the ideal case where the cyclic extension is greater than the length of the channel 64, the overlap or leakage' of one OFDM symbol into the subsequent OFDM symbol will only affect the samples of the cyclic extension. Since these samples are discarded, the channel does not affect the information bearing samples of the following OFDM symbol.

However, when the channel's excess delay 74 exceeds the cyclic extension 72, the energy of one OFDM symbol effects the information bearing samples of the subsequent OFDM symbol 78, as shown in Figure 7. The leaking' of energy beyond the interval of the cyclic extension causes a degradation in performance and can be distinguished by two forms of interference: (i) inter-block interference (IBI)-this occurs as a direct result of the energy leaked from one OFDM symbol on the information samples of the following OFDM symbol, and (ii) inter-carrier interference (ICI)-this occurs when the removal of the cyclic extension insufficiently characterises the cyclic nature of the symbol.

With the use of TFCs, not only can collisions be minimised, but IBI can also be alleviated if TFCs with non-consecutive bands are used. Note however, that ICI cannot be mitigated. Even so, this still constitutes a performance gain as compared to the presence of both IBI and ICI, if a TFC uses consecutive bands for transmitting symbols.

Hence, when the channel's excess delay exceeds the cyclic prefix or zero padding length, TFC 1 should outperform TFC 4. From Figure 8, results for the MB-OFDM system operating in a 70-tap i.i.d channel shows that this is indeed the case.

The current specification does not take into account the user's channel characteristics in assigning TFCs to piconets and thus, potentially degrades performance unnecessarily.

Summary of the invention

The present invention aims to minimise collisions between each OFDM symbol.

In a first aspect of the present invention, there is provided a method of assigning codes to an OFDM signal, the method comprising the steps of determining the channel information of a communication channel, comparing said determined information with predetermined information, determining the data rate of an OFDM signal on said channel, and assigning codes to said signal based on said compared information and said data rate.

The channel information may be the length of channel's excess delay of said communication channel.

Preferably, said predetermined information is any one of: a. length of the cyclic prefix of said OFDM signal; b. length of zero padding of said OFDM signal.

Preferably, said codes are time frequency codes (TFCs) defining a time sequence of frequency bands for said signal.

Preferably, said TFCs are allocated into one of two categories, herein defined as a first category and a second category, wherein said first category includes designation of non-consecutive TFC bands and said second category includes designation of consecutive TFC bands.

Preferably, said first category is assigned to said OFDM signal if said length of channel's excess delay is greater than said length of predetermined information.

Preferably, said second category is assigned to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is greater than 320 Mbps.

Preferably, said first category is assigned to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is lower than 320 Mbps.

In another embodiment of the present invention, there is provided an OFDM transmitter comprising: estimation means operable to determine the channel information of a communication channel, comparison means operable to compare said determined information with pre-determined information, data rate determining means operable to determine the data rate of a signal on said channel, codes assigning means operable to assign codes to said signal based on said compared information and said data rate, and storage means operable to store said pre-determined information and said codes.

Preferably, said estimation means is operable to determine the length of channel's excess delay of said communication channel.

Preferably, said assigning means is operable to assign non-consecutive TFC bands and consecutive TFC band.

Preferably, said assigning means is operable to assign said non-consecutive TFC bands to said OFDM signal if said length of channel's excess length is greater than said length of predetermined information.

Preferably, said assigning means is operable to assign said consecutive TFC bands to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is greater than 320 Mbps.

Preferably, said assigning means is operable to assign said non-consecutive TFC bands to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is lower than 320 Mbps.

Brief description of the Drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 shows the subcarriers of an OFDM signal spectrum, with frequency on the x-axis, and an arbitrary scale on the y-axis; Figure 2 is a schematic diagram of an example communication device; Figure 2a shows an example of a conventional MB-OFDM transmission system, as known in the art; Figure 3a shows the band group definition, band number, and centre frequency for each band as defined in the MBOA proposal; Figure 3b shows the time frequency codes for MB-OFDM system as defined in the MBOA proposal; Figure 4 shows an example ofTFCI as defined in the MBOA proposal; Figure 5a shows the data rates and their corresponding code rate, conjugate symmetry use, time-spreading use, and overall spreading factor as defined in the MBOA proposal; Figure 5b shows the channel model characteristics as detailed in J. Foerster, "Channel Modelling Sub-committee Report Final", IEEE P802.1 5-02/368r5-SG3a, December 2002; Figure 6 shows the function of cyclic extension added to the OFDM symbols, as known in the art; Figure 7 shows an example of interference when the channel's excess delay is greater than the length of the cyclic extension, as known in the art; Figure 8 shows the performance of an MB-OFDM system in a 70-tap i.i.d channel, as known in the art; Figure 9 shows the performance of an MB-OFDM system in a 30-tap i.i.d channel, as known in the art; Figure 10 shows an OFDM transmission system in accordance with an embodiment of the present invention; Figure 11 shows an algorithm for assigning TFCs based on channel length in accordance with an embodiment of the present invention; Figure 12 shows the performance of MB-OFDM system in CM 4 at 320 Mbps with a cyclic extension of either 16 or 32 zeros.

Detailed Description

The present invention will be described in further detail on the basis of the attached diagram.

A method and apparatus for assigning codes to an OFDM signal is 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 practice 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 22 operable to execute machine code instructions stored in a working memory 23 andlor retrievable from a mass storage device 21. By means of a general-purpose bus 25, user operable input devices 26 are in communication with the processor 22. The user operable input devices 26 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 27 are further connected to the general-purpose bus 25, for the output of information to a user. Audio/video output devices 27 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 is connected to the general-purpose bus 25, and further connected to an antenna 255. By means of the communications unit 200 and the antenna 255, the laptop computer 20 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 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 23 stores user applications 24 which, when executed by the processor 22, cause the establishment of a user interface to enable communication of data to and from a user. The applications 24 thus establish general purpose or specific computer implemented utilities and facilities that might habitually be used by a user.

Referring to Figure 2a, the communications device 200 comprises an OFDM transmitter 201 as known in the art. Connection with the general-purpose bus 25 provides a data source 205. Data from the data source 205 is passed to a serial to parallel buffer 210, which assigns data to N subcarriers 215. A multicarrier modulator 220 modulates the sub-carriers 215 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 parallel to serial converter 225 to produce a continuous series. A time frequency encoder 230 then assigns time frequency codes (TFCs) to this series. A digital to analog converter 235 uses this series to generate an analog signal, before an upconverter 240 generates a corresponding RF signal. Finally, this signal is amplified by a power amplifier 250 before being transmitted by an antenna 255. It will be appreciated that the transmitter 201 is a simplified illustration, omitting elements such as filters for the purposes of clarity.

Specifically, in typical operation the serial to parallel buffer 210 takes B1 bits from the data source 205, and allocates them to N groups of b bits such that = B. The N groups are each assigned to one of N sub-carriers 250.

The multicarrier modulator 220 may then be viewed as generating N independent QAM sub-channels, where the symbol rate for each sub- channel is 1/T. The number of signal points in the th sub-channel is therefore M=2b1.

Denoting the complex values of the signals in each sub-channel by X,, 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 5.3 a), 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 802.1 Ia, g or as proposed in un 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 j2kn/K fl 1 K-Xk r,...

JJ\ n=O where l/'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 220.

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 225.

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(1) + 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 1/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, XKm+/, ... XK..I 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-I, produces the received signal rk. Since by definition any intersymbol interference only affects the first m samples of this block, discarding these will recover the desired block rk, 0 k K-i without any intersymbol interference present.

Parallel to serial conversion simply concatenates sample blocks prior to the time frequency encoder. The time frequency encoder then assigns TFCs defining the time sequence of bands for successive sample blocks. The output is then passed to the D to A converter 235. The output of the converter is the analog OFDM signal x(t).

The analog signal x(1) is then upconverted to the transmission frequency by upconverter 240. Finally this RF signal is amplified by a power amplifier 250 and is then transmitted by the antenna 255.

In order to incorporate channel information into TFC designation, it is important to consider whether the channel's excess delay is significantly greater than the length of the cyclic prefix or zero padding. If this is the case, i.e. the excess delay is greater than the guard interval, and if a TFC designation belonging to the non-consecutive' group is used, then IBI cannot arise. This is because, when using a non-consecutive TFC designation, any multipath energy from the transmission of an OFDM symbol does not affect the subsequent symbol since the two symbols are inevitably transmitted on different parts (non-adjacent) of the allocated spectrum.

While the method described above uses an estimate of the length of the channel's excess delay, it will be appreciated that other metrics can be coupled with the channel length information in determining how TFCs are assigned, for example, quality of service requirements, traffic types, and range. This enables account to be taken of other criteria as well in arriving at an appropriate TFC designation. For instance, quality of service requirements may be sufficiently relaxed that a best' selection of TFC need not be provided, and a theoretically less desirable TFC designation will be sufficient. Even in such a case, account is taken of channel length, in ensuring appropriate assignment of TFC.

Figure 8 illustrates the performance comparison of a MB-OFDM system in a 70-tap i.i.d channel when non-consecutive band TFC (e.g. TFC 1) and consecutive band TFC (TFC 4) are used, as known in the art. The results in Figure 8 show that the error floor is reduced by as much as an order of magnitude when TFC I is used when the channel's excess delay exceeds the cyclic prefix or zero padding length. However, in figure 9, the performance comparison of a MB-OFDM system in a 30-tap i.i.d channel shows that it is preferable to assign TFCs with consecutive bands for higher data rates with spreading factor of 1. The reason the systems with TFC 4 outperform systems with TFC I when excess delay is shorter than the cyclic extension is due to the design of the interleaver.

Hence it is important to consider the length of the channel's excess delay and the data rate when assigning TFCs in a MB-OFDM system.

Referring now to Figure 10, in accordance with an embodiment of the present invention, the communication unit 200 comprises of an OFDM transmission system 301.

Connection with the general-purpose bus 25 provides a data source 305. Data from the data source 305 is passed to a serial to parallel buffer 310, which assigns data to N subcarriers 315. A multicarrier modulator 320 modulates the sub-carriers 315 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 parallel to serial converter 325 to produce a continuous series. Infonnation on the length of the cyclic prefix is passed to the comparator 340. The channel delay estimator estimates the length of the channel's excess delay and passed its output to the comparator. The output of the comparator is passed to a time frequency encoder 330 where it assigns the TFCs to the series based on the output of the comparator. A digital to analog converter 335 uses this series to generate an analog signal, before an upconverter 340 generates a corresponding RF signal. Finally, this signal is amplified by a power amplifier 350 before being transmitted by an antenna 355. It will be appreciated that the transmitter 201 is a simplified illustration, omitting elements such as filters for the purposes of clarity.

The operation of assigning TFCs to the series blocks in accordance with an embodiment fl of the present invention is shown in the flow chart of Figure jO, where L is the length of the channel's excess delay, and Q is the length of the cyclic extension.

The operation of assigning the TFCs works as follows: Step 1: Estimate L. Step 2: If L> Q, assign TFC 1 or 2.

Step 3: Else, if L Q, and if data rate is ? 320 Mbps, assign TFC 3 or 4.

Step 4: Else if L Q, and if data rate is 320 Mbps, assign TFC I or 2.

For the step where assigning of TFCs is to be done, TFCs can be assigned on a first-come-first-served basis.

It will be appreciated that one measure of the length of the cyclic extension Q is m, the number of samples provided in the cyclic extension. in such a case, L would be expressed in terms of samples in the same way.

It should also be stressed that this method is not only applicable to the MBOA proposal but can be used more generally with any band-hopping MB-OFDM system.

This algorithm provides a simple, intelligent method of designating TFCs to piconets such that performance gains can be achieved at minimaladditional complexity. The performance gains of assigning TFCs based on channel conditions can improve performance, as shown in Figure 8. Furthermore, if such channel conditions do not exist, then the system performs no worse than randomly assigning TFCs, and in some cases can perform better as shown in Figure 9.

This simple algorithm enables the MB-OFDM system to operate more robustly in channels with large excess delays. For example, many audio-video applications require bit error rates in the order of I 0, for instance as described in "Steps for Delivering Multimedia Over 5GHz WLANs" (James Crawford and David Critchiow, published on the CommsDesign website www.commsdesign.com/designcorner, 26 February 2004).

From Figure 8, the highest data rate that can achieve this bit error rate is the 320 Mbps mode. However, with TFC 1, the SNR required to achieve this bit error rate is approximately 12 dBs. In contrast, with TFC 4, an SNR of 20 dBs or greater is needed.

Although the channel used to generate the results in Figure 8 may not be often encountered in practical scenarios, this algorithm does provide a significant advantage if, in future standards, the cyclic extension were to be shortened.

Since the cyclic extension carries no additional data, it is inefficient to transmit an excessively long cyclic extension if it is not needed. Hence, by shortening the cyclic extension, the transmissions become more efficient, as the percentage overhead is reduced. This method has already been adopted the IEEE 802.16 standard as a method of decreasing overhead.

Using the proposed algorithm enables the system to reduce the length of the cyclic extension without substantial performance loss. The performance difference can be seen in Figure 12, where only results for an MB-OFDM system with 32 and 16 zero padded samples are shown. For brevity, only results for operation at 320 Mbps in CM 4 are shown.

The channel model CM 4 is one of the channel models proposed by the IEEE 802.15.3a task group and was designed to have a RMS delay spread of 25ns (See, J. Foerster, "Channel Modelling Sub-committee Report Final", lEE P802.1 5-02/368r5-SG3a, December 2002). The channel is a modified version of the Saleh-Valenzuela channel model. Table 55 in Figure Sb lists some of the parameters of all four channel models specified in (see, J. Foerster, "Channel Modelling Sub-committee Report Final", lEE P802.1 S-02/368r5-SG3a, December 2002).

Note from Figurel2 that the performance of the system operating with TFC 4 and 32 zero padded samples is identical to the system operating with TFC 1 and 16 zero padded samples for SNRs less than 14 dB. Hence, assigning the TFC selection intelligently, it is possible to operate at 10% higher spectral efficiency for CM 4. For higher SNRs an irreducible error floor occurs due to ICI.

The advantages of this method are easily illustrated by a simple example. If a bit error rate of 1 0 is needed and an SNR at the receiver of 10-15 dB is obtainable, then the most efficient choice would be to operate with TFC I and reduce the length of the zero padding samples to 16. Doing so will provide a similar performance to the system operating with TFC 4 and 32 zero padding samples but with a 10% improvement in throughput. If the bit error rate requirement is more demanding, or if the SNR were to drop, the system could simply increase its zero padding length to 32 and operate with TFC 1.

Claims

CLAIMS: A method of assigning codes to an OFDM signal, the method

comprising the steps of: determining the channel information of a communication channel; comparing said determined information with predetermined information; determining the data rate of an OFDM signal; and assigning codes to said OFDM signal based on said compared information and said data rate.

2. A method of assigning codes to an OFDM signal according to claim 1, wherein said channel information is the length of channel's excess delay of said communication channel.

3. A method of assigning codes to an OFDM signal according to claim 1, wherein said predetermined information is any one of: a. length of the cyclic prefix of said OFDM signal; b. length of zero padding of said OFDM signal.

4. A method of assigning codes to an OFDM signal according to claim 1, wherein said codes are time frequency codes (TFCs) defining a time sequence of frequency bands for said signal.

5. A method of assigning codes to an OFDM signal according to claim 5, wherein said TFCs are allocated into one of two categories, herein defined as a first category and a second category, wherein said first category includes designation of non-consecutive TFC bands and said second category includes designation of consecutive TFC bands.

6. A method of assigning codes to an OFDM signal according to any preceding claim, wherein said first category is assigned to said OFDM signal if said length of channel's excess delay is greater than said length of predetermined information.

7. A method of assigning codes to an OFDM signal according to any preceding claim, wherein said second category is assigned to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is greater than 320 Mbps.

8. A method of assigning codes to an OFDM signal according to any preceding claim, wherein said first category is assigned to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is lower than 320 Mbps.

9. An OFDM transmitter, comprising: estimation means operable to determine channel information of a communication channel; comparison means operable to compare said determined information with pre-determined information; data rate determining means operable to determine the data rate of an OFDM signal; codes assigning means operable to assign codes to said OFDM signal based on said compared information and said data rate; and storage means operable to store said pre-determined information and said codes.

10. An OFDM transmitter in accordance with claim 9, wherein said estimation means is operable to determine the length of said channel's excess delay of said communication channel.

11. An OFDM transmitter in accordance with claim 9 or claim 10, wherein said assigning means is operable to assign non-consecutive TFC bands and consecutive TFC band.

12. An OFDM transmitter in accordance with any one of claims 9 to 11, wherein said assigning means is operable to assign said non-consecutive TFC bands to said OFDM signal if said length of channel's excess length is greater than said length of predetermined information.

13. An OFDM transmitter in accordance with any one of claims 9 to 12, wherein said assigning means is operable to assign said consecutive TFC bands to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is greater than 320 Mbps.

14. An OFDM transmitter in accordance with any one of claims 9 to 13, wherein said assigning means is operable to assign said non-consecutive TFC bands to said OFDM signal if said length of channel's excess delay is lesser than said length of predetermined information and said data rate is lower than 320 Mbps.

15. A receiver operable to receive a signal in accordance with any one of claims 1 to 8.