WO2004073219A1

WO2004073219A1 - Interleaving method for ofdm communications

Info

Publication number: WO2004073219A1
Application number: PCT/CN2003/000133
Authority: WO
Inventors: Jaap Van De Beek; Branislav M. Popovic
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2003-02-14
Filing date: 2003-02-14
Publication date: 2004-08-26
Also published as: CN1742450A; CN100544239C; AU2003218598A1

Abstract

The present invention is related to a method for interleaving in an OFDM system, wherein the method exploits the two-dimensional time-frequency structure of an OFDM system. The method comprises the steps of: dividing the available frequency band into several sub-bands, each sub-band comprising essentially only adjacent frequencies; mapping a sequence of constellation symbols to a sequence of OFDM units, where one OFDM unit comprises a group of constellation symbols, after which step the constellation symbols are only processed groupwise or OFDM-unit-wise; and interleaving a sequence of OFDM units, where each OFDM unit may be mapped onto one of said sub-bands. Preferably the last step is performed separately for the real and imaginary parts, respectively, of OFDM units. The invention is further related to such a transmitter and system.

Description

INTERLEAVING METHOD FOR OFDM COMMUNICATIONS

Field of the invention

The present invention relates to the field of radio communication systems, and in particular to an improved interleaving method for OFDM (Orthogonal Frequency Division Multiplexing) communication.

Background of the invention

OFDM (Orthogonal Frequency Division Multiplexing) is a transmission technique that allows high data rates to be transmitted over very noisy channels, yet at a comparatively low complexity, and is used for digital audio broadcasting (DAB) and digital video broadcasting (DVB) . OFDM has several favourable properties like high spectral efficiency and robustness to channel dispersion, for which reasons it will most likely be used for future broadband applications such as digital mobile radio communication.

Briefly, in an OFDM system the data to be transmitted is spread over a large number of carriers, and the data rate to be transferred by each of these carriers is consequently reduced in proportion to the number of carriers. The carriers have an equal, precisely chosen frequency spacing, and the frequency bands of the sub-carriers are not separate but overlap. By using an IFFT (Inverse Fast Fourier Transform) as modulation, the spacing of the sub-carriers are chosen in such a way that at the frequency, where a received signal is evaluated, all other signals are zero. The choice of carrier spacing is made so that orthogonality is preserved, giving the method its name.

A properly designed OFDM system turns a dispersive, fading channel into a flat-fading channel making advanced time-domain equalizers obsolete. This provides an obvious implementation advantage in that no equalizers are needed and thus decreasing the transceiver complexity, but this also involves the risk of losing the frequency diversity offered by the channel. As opposed to a single-carrier system (for which a well-designed equalizer captures the available frequency-diversity in the channel), an OFDM system must invoke specific means to reach the higher-order diversity performance that the radio environment offers .

This is well recognized in the literature on OFDM, and one accepted means of capturing the channel's frequency diversity is to employ a code (for example a convolutional code or a turbo code), whose output bits are spread over all sub-carriers As is described in 'A comparison of a single-carrier system using a DFE and a coded OFDM system in a broadcast Rayleigh- fading channel', S.K. Wilson and J.M.Cioffi, International

Symposium on Information Theory (ISIT) , September 1995, this approach captures the available frequency diversity provided that the code's diversity order is larger than the channel's frequency diversity order.

Because OFDM effectively creates a set of flat fading channels, any technique, designed to improve the performance of communication over flat-fading channels can be applied to OFDM.

One way to improve the diversity performances of generally uncoded (i.e. where no redundancy is added to the information bit stream) communication signals in a flat-fading channel is to increase the diversity order of the modulation symbols, i.e. to increase the so-called modulation diversity. In "Signal Space Diversity: A Power- and Bandwidth-Efficient Diversity Technique for the Rayleigh Fading Channel", IEEE Trans. Information Theory, Vol. 44, No. 4, July 1998, Joseph Boutros and Emanuele Viterbo, this alternative diversity technique is analyzed. The modulation diversity is increased by applying a certain rotation to a classical signal constellation in such a way that any two constellation points obtain a maximum number of distinct components . The fading of a transmitted signal vector is thus spread out over many components and a better protection against the effects of noise is achieved, since the components are differently affected by a fade and no two points would collapse together. It is shown in this document that a multidimensional QAM (Quadrature Amplitude Modulation) constellation becomes insensitive to fading when the modulation diversity is large.

However, the above document assumes ideal interleaving and does not exploit any specific feature of OFDM systems, and in particular not any features of mul ti -user OFDM systems (in which a single user uses, during each OFDM symbol interval, just a part of the total available spectrum) . A drawback of modulation diversity is that in the presence of non-ideal interleaving the performance gain may be negligible.

Further related prior art is found in "On the Design of Multidimensional Signal Sets for OFDM Systems", IEEE Trans.

Communications, Vol. 50, No. 3, March 2002, Dennis L. Goeckel and Ganesh Ananthaswamy . In this document the concept of combining several sub-carriers, carrying symbols from one multidimensional constellation, is developed for a general OFDM transmission system. A coded modulation technique is proposed that draws symbols from a signal set in multiple complex dimensions, and each complex dimension of the selected symbol is then placed on a properly chosen sub-carrier of the OFDM system, and a diversity improvement is achieved. In this document scattered sub-carriers are used in each OFDM symbol interval for transmission of the components of the constellation symbols. There are several drawbacks of this method. Firstly, this document does not describe application to multi -user systems. Even if it could be applied to a multiuser system, channel estimation is difficult because of the scattered nature of the sub-carriers. Secondly, because of the particular, non-standard, non-rotated constellation structures, detection is more complex, and thirdly, the performance in a Gaussian channel is not optimal .

On a frequency-selective fading channel, a typical uncoded OFDM system achieves significantly less diversity than an uncoded single-carrier system employing a proper channel equalizer. One way to retrieve the frequency diversity for the OFDM system is to apply coding/interleaving over the sub-carriers, another is to employ high-diversity symbol-constellations (a finite set of real-valued vectors onto which the coded bits are mapped) with component-interleaving in a concept known as signal space or modulation diversity. The advantage of the first method is that the code redundancy yields an additional performance gain in the form of increased diversity, but at the cost of increased bandwidth. The advantage of the second method is that the frequency diversity is captured without any bandwidth increase (no redundancy) . However, both methods require that a proper interleaving scheme decorrelates the symbols and symbol - components, respectively.

Because one interleaving scheme may yield a significantly worse performance than another, it would be desirable to provide OFDM interleaving schemes that optimize the diversity performance in all coded OFDM systems, i.e. for OFDM systems utilizing redundancy codes (such as convolutional codes for example) as well as OFDM systems utilizing non-redundancy codes (such as modulation diversity) . Further, an additional aim is that the diversity gain, in case of multi-user OFDM, can be obtained for all users in the system.

Thus, what is needed is an OFDM interleaving method that optimizes the diversity performance of OFDM systems.

Summary of the invention

It is an object of the present invention to provide, in an OFDM system, an interleaving method, which optimizes the diversity performance of the diversity methods used to create frequency diversity.

It is further an object of the present invention to provide the diversity gain for all users of an OFDM system.

Another object of the present invention is to provide higher frequency diversity without the need to increase the bandwidth, as compared to known multi-user OFDM systems. Modulation diversity does not add any redundancy to the data stream, and a diversity performance gain can thus be obtained while maintaining the data rate.

These objects are achieved, according to a first aspect of the invention, by a method as defined in claim 1, a transmitter as claimed in claim 11 and system as claimed in claim 19.

In accordance with the present invention, a method for interleaving in an OFDM system is provided, wherein the method exploits the two-dimensional time-frequency structure of an OFDM system. The method comprises the steps of: dividing the available frequency band into several sub-bands, each sub-band comprising essentially only adjacent frequencies; mapping a sequence of constellation symbols to a sequence of OFDM units, where one OFDM unit is defined as a group of constellation symbols, after which step the constellation symbols are only processed group-wise or OFDM-unit-wise; and interleaving a sequence of OFDM units, where each OFDM unit may be mapped onto one of said sub-bands. Preferably said interleaving is performed separately for the real and imaginary parts, respectively, of OFDM units.

In accordance with one embodiment of the present invention the step of dividing the available OFDM frequency bandwidth into several sub-bands comprises dividing the frequency band into equally sized sub-bands. This feature provides a simple management of the available resources. For example, the allocation of channels to a number of users may be performed in an easy manner. The use of non-equal frequency sub-bands is also possible, and an operator of a mobile communication system is thus provided great design flexibility.

In accordance with another embodiment of the present invention the method is performed after a constellation mapping step. This is thus an additional interleaving, not performed in the prior art. This additional interleaving step, exploiting the time-frequency correlation structure of an OFDM system, provides an additional diversity.

In accordance with another embodiment of the present invention, the step of mapping the sequence of constellation symbols comprises the mapping of constellation symbols onto OFDM units after a rotation of the constellation symbols and the interleaving of the components of the constellation symbols. In accordance with this feature, an additional interleaving is performed after the interleaving of components of the constellation symbols, and provides thereby additional diversity. In accordance with another embodiment of the present invention, the step of interleaving is a time and frequency mapping of the sequence of OFDM units, where each OFDM unit is allocated to a unique time and f equency-band position within the available transmission time interval and available frequency spectrum. By means of this, the specific time-frequency correlation of an OFDM flat fading channel is accounted for, and an additional frequency diversity of a channel is captured.

In accordance with another embodiment of the present invention, the mapping pattern used is a randomised time-frequency mapping. This random mapping pattern can be shown to provide an excellent diversity performance, but other patterns may be used as well . It is for example possible to use mapping patterns only differing in time or frequency for the different users.

The invention further relates to a transmitter including means for performing the interleaving method. The invention further relates to a communication system comprising at least one of said transmitters .

The corresponding advantages, described above, are achieved by a transmitter and a system as claimed in claims 8 and 13, respectively.

When applying the present invention to a turbo-coded system (or some other method using soft-decision) , the soft bits fed to the first stage of the iterative decoder are improved. Therefore the decoded bits will have lower error probability than systems not capturing this diversity.

Further, when applied to a multi-user system, all the users in the system will obtain a similar diversity improvement by means of the present invention. The present invention further provides a method which can easily be coalesced with the existing standards for OFDM, thus making it a very attractive alternative.

Brief description of the drawings Fig. 1 shows a prior art method.

Fig. 2 shows the basics of an OFDM system.

Fig. 3 shows the final baseband stage of an OFDM transmitter, the definition of an OFDM unit and the grouping of the frequency band in accordance with the present invention.

Fig. 4 shows a typical downlink transmitter processing chain.

Fig. 5 shows final stages of an OFDM transmitter processing chain in accordance with the present invention.

Figs. 6a-d show exemplary time-frequency mapping patterns used in the present invention.

Figs. 7-8 show some experimental results.

Detailed description of preferred embodiments

As was explained in the introductory part, OFDM effectively creates a set of flat fading channels, and any technique, designed to improve the performance of communication over flat- fading channels can be applied to OFDM. However, the particular correlation structure of the OFDM flat fading channels offers specific opportunities for improvements, which are recognized by the inventors of the present invention. Specifically, a multiple-carrier system such as OFDM has a two-dimensional structure (time and frequency dimensions) offering great designer opportunities regarding frequency correlation and time correlation, respectively. In a multiuser OFDM system, a single user uses during each OFDM symbol interval just a part of the total available spectrum. It makes an opportunity for an additional time-frequency interleaving that, for example in combination with existing interleaving of data bits and constellation mapping, also can capture a potential frequency diversity gain of the channel .

As was described earlier, in the document "Signal Space Diversity: A Power- and Bandwidth-Efficient Diversity Technique for the Rayleigh Fading Channel", IEEE Trans. Information Theory, Vol. 44, No. 4, July 1998, Joseph Boutros and Emanuele

Viterbo, one method to achieve modulation diversity is provided. As was also explained earlier, the key point to increase the modulation diversity is to apply a certain rotation to a classical signal constellation in such a way that any two constellation points obtain as much as possible distinct components. Such a rotation operation will not change the performances on an AWGN (Additive White Gaussian Noise) channel, but will improve the performances on a flat, Rayleigh-fading channel. The basic prerequisite for such improved performances is that the real and imaginary parts of the transmitted signal are uncorrelated. To create this prerequisite a so-called component interleaver is introduced in one of the signal quadrature branches . The component interleaver destroys the correlation between the in-phase and quadrature signal components, so that a deep fade hits only one of the components of the transmitted constellation symbol.

The above described prior art method is illustrated in Fig. 1. The inventors of the present invention have realized that this method can be adjusted and implemented in OFDM systems, which is done in accordance with the present invention. Fig. 2 illustrates that the FFTs (Fast Fourier Transform) and the cyclic prefix in an OFDM system convert a dispersive channel into a flat, Rayleigh fading channel and the inventors of the present invention have realized that the concept of modulation diversity therefore is applicable to an OFDM system. Accordingly, the present invention concerns a method which boosts the diversity gain even further, compared to the modulation diversity gain described in the document referred to above, by appropriate time-frequency interleaving in multi-user OFDM, based on separation of OFDM units in time and frequency. The time-frequency interleaver of OFDM units has the task to separate every pair of OFDM units as much as possible in frequency and time domain, in a manner that de-correlates the received constellation symbols.

With reference now to Fig. 3, some basic features of an OFDM transmitter will be described, as well as a feature in accordance with the present invention. An OFDM unit is here defined as a group of constellation symbols to be mapped onto one of a number of equal sub-bands within the total available frequency spectrum. In accordance with the present invention the total frequency spectrum is divided into preferably equal sub-bands. Equal frequency sub-bands are preferred since the resource management is facilitated (it is easier to allocate the available resources for example) , but division into non- equal frequency sub-bands is also possible. The OFDM units are fed into an Inverse Fast Fourier Transform (IFFT) processor, where pulse forming and modulation are performed. In the receiver the reverse operations are performed, by means of a Fast Fourier Transform (FFT) , as is realized by a person skilled in the art. The signal is then fed into a parallel to serial unit (P/S) , where the N parallel data streams are combined into one data stream. Thereafter a cyclic prefix is added . With reference now to Fig. 4 a typical processing performed by a transmitter on the downlink of a mobile communication system is described. A block of bits is first extended with a number of redundancy check bits and then encoded using, for instance, a convolutional coding or a turbo coding. Then, the resulting block of bits is punctured and interleaved and mapped onto a constellation of complex numbers. After this stage there is a block of N_s complex-valued constellation symbols. Reference is made to the 3^rd generation partnership project, 3GPP TS 25.212 V5.2.0 (2002-09) (e.g. chapter 4.2) for details.

One embodiment of the invention is related to the final part of the transmitter processing chain: the mapping of the N_s constellation symbols onto the OFDM physical resources (characterized by sub-carrier indexes and OFDM symbol interval indexes), or physical channel mapping. The interleaving method in accordance with the present invention may however be performed elsewhere in the transmission chain. Ultimately this OFDM physical channel mapping associates each of the N₃ constellation symbols with an OFDM symbol interval and a part of the FFT (a sub-carrier frequency) . The present invention sub-divides the general structure of the OFDM physical channel mapping into three steps, as will be described with reference to Fig. 4.

In contrast to the prior art method referred to above ("On the Design of Multidimensional Signal Sets for OFDM Systems"), which uses scattered sub-carriers in each OFDM symbol interval for transmission of the components of the constellation symbols, the present invention collects groups of adjacent sub-carriers. Though most of the frequencies of a sub-band are adjacent to each other, some deviating frequencies may form part of the sub-band. These deviating frequencies would be used for non- user traffic, such as pilot signals. This gathering of sub- carriers simplifies channel estimation by providing better conditions for the channel estimation. OFDM channel estimation methods usually exploit strong correlation of channel attenuations on sub-carriers near each other. Thus, knowing the attenuation of one frequency this knowledge may be used to estimate the attenuation, or fading, of an adjacent frequency. The need to send pilot signals, i.e. a known signal sequence, thus decreases (there is no need to send one pilot signal for each frequency) , and the scarce resources may thereby be saved and the data rate of user data increased.

There are thus two conflicting wishes: on the one hand it would be desirable to spread the groups in order to achieve diversity; on the other hand it would be desirable to have the groups gathered, since this would facilitate the channel estimation.

The present invention accommodates both of these wishes in that the available frequency band is divided into several sub-bands (i.e. groups of essentially only adjacent sub-carriers are gathered) and by at the same time providing modulation diversity (by a proper time-frequency interleaving) .

As was explained earlier, the time-frequency interleaver unit of OFDM units has the task to separate every pair of OFDM units as much as possible in frequency and time domain, in a manner that de-correlates the received constellation symbols. Particularly, and with reference to Fig. 4, the OFDM time- frequency interleaver in accordance with the present invention consists of the following three basic steps:

1. Mapping of the sequence of constellation symbols to a sequence of OFDM units. After this stage constellation symbols are only processed group-wise, or OFDM-unit-wise. Specifically, the mapping of constellation symbols onto OFDM units is done after the rotation of the constellation symbols and the interleaving of the components of the constellation symbols.

2. OFDM-unit interleaving. The sequence of OFDM units is interleaved. This is an additional (when used in the example described) interleaving, not performed in the prior art, and is preferably performed separately for real and imaginary parts of OFDM units.

3. Time-frequency mapping of the sequence of OFDM units (i.e. a group of constellation symbols) , where each OFDM unit is allocated to a unique time and frequency-band position within the available transmission time interval and the available frequency band.

Steps 2 and 3 provides the desired diversity gain. All users of the system will have a similar diversity improvement if the three steps described above are similar for the different users. If for instance the same interleaving is performed in the first two steps, and the mapping patterns used in the third step only differ be a shift in time and/or frequency.

First, after the constellation mapping, the resulting N_s constellation symbols are mapped onto N_0FDM OFDM units. Each OFDM unit is here simply a group of constellation symbols and, typically, all OFDM units contain an equal number of constellation symbols. The constellation symbols are grouped so that each group may be mapped on a specific frequency sub-band.

Secondly, the OFDM unit interleaver serves to optimize the performances of the bit-interleaver (appearing earlier in the transmitter chain, see Fig. 3) by taking into account the 2- dimensional correlation properties of the radio channel . The OFDM unit interleaver can be described by a permutation vector 1 of the N_OFDM consecutive OFDM units . The OFDM unit interleaver can be made transparent by setting the permutation vector J= [1

2 3 ... NOF M] ■

In accordance with the preferred embodiment, the real- and imaginary parts of the OFDM units are mapped differently.

Thereby a better diversity gain is obtained, and the effects of the noise present in the system are combated more efficiently, since presumably a deep fade will not hit both of the components. The real part of the signal is mapped onto one specific frequency sub-band and the imaginary part onto some other.

Finally, the interleaved sequence of OFDM units is mapped onto the physical channels. For this purpose, the whole available OFDM frequency band is divided into N_B frequency sub-bands, as was mentioned above. A single physical channel uses just l/N_B of the total available aggregate resources for the duration of N_sym_i_nt OFDM unit periods, meaning that it might be possible to have either up to N_B concurrent users, or up to N_B concurrent physical channels for a single user enabling very fast data rate for a single user.

The size (i.e. the number of constellation symbols) of the OFDM units is such that each frequency sub-band contains one OFDM unit during each OFDM symbol interval . It means that in each OFDM symbol interval at most N_B parallel OFDM units can be transmitted.

When performing the time-frequency (T-F) mapping of the third step each user may be assigned a specific pattern, see Figs. 6a-d. In Figs. 6a-d different patterns to be used in the third step is illustrated, and four different exemplary patterns are shown, with N_B=15 and N_sym_i_nt=12 and N₀FDM=12 . Note that N₀FDM= N_Sym_int is not necessarily the case. If these- parameters are different, the N_BN_sym_i_nt resources could fit more than N_BN_0FDM streams of OFDM units.

The randomised T-F mapping D in Fig. 6d is a preferred embodiment, and shows the best performance, in certain channel environments, of the T-F-mappings A-D shown in Figs. 6a-d for the ITU-channels VA120 (Vehicular-A at 120 km/h, see 3GPP TR 25.890 VI.0.0, 2002-05 for details) and PB3 (Pedestrian-B at 3 km/h, see 3GPP TR 25.890 VI .0.0 , 2002-05 for details) in terms of bit-error rate and frame-error rate. Fig. 7 shows some test results, illustrating the performance of the T-F-mappings of Figs. 6a-d. In Fig. 7 the bit error rate (BER) is plotted for the mappings shown in Figs. 6a-d for a channel environment specified by ITU (Vehicular A 120 km/h) . The randomised T-F mapping D in Fig. 6d is a preferred embodiment, and shows the best performance, in certain channel environments, as can be seen in Fig. 8. In Fig. 8 the bit error rate (BER) is plotted for the mapping shown in Fig. 6d, again for a channel environment specified by ITU (Vehicular A 120 km/h) . The test was performed for both coded and uncoded cases, as well as with and without modulation diversity. Eight iterations were carried through in a turbo decoder. As can be seen, the modulation diversity method in accordance with the present invention improved the performance considerably.

The time-frequency mapping of OFDM units is characterized by two parameters for each OFDM unit: one indicating the OFDM symbol time interval and one indicating the chosen frequency sub-band. These parameters can be collected in a T-vector containing the indexes of OFDM symbol intervals, in the range {l, N₀FDM] and an F-vector containing the indexes of the corresponding sub-bands for each OFDM unit, in the range {l, N_B} . The T-F mapping for the k-th input OFDM unit is determined by the pair of values at the J-th position in T and F.

The mapping being used should produce the best diversity performance in any channel environment. It should be such that every pair of OFDM units is separated in frequency and time domain as much as possible.

A typical way to divide the components of a signal is to divide them into real and imaginary parts, respectively, or equivalently, into in-phase and quadrature components. When the method in accordance with the present invention is used to improve the performance in connection with modulation diversity, it is important that the real and imaginary parts are interleaved separately. When the method in accordance with the present invention is used generally for improving the performance of a coded OFDM system, the interleaving of a sequence of OFDM units could be performed in some other way.

The mapping may in one embodiment be performed by means of a trial -and-error procedure, or may in another embodiment be done in dependence on the choice of channel .

As is well known within the art, soft-decision decoding gives a greater certainty to the decoding, in that both the sign of the received signal (hard decision) as well as a measure of the amplitude is taken into account. The data are modulated onto several carriers in an OFDM system, and the different carriers will have different signal-to-noise ratios and will thus experience differing fading. As the method in accordance with the present invention preferably interleaves the real- and imaginary parts, respectively, on different sub-bands, the probability that both of the components are lost is low, or put differently, the probability that at least one uncorrupted component is received is large. Consequently, when applying the present invention to a turbo-coded system (or some other system using soft-decision) , the soft bits fed to the first stage of the iterative decoder are improved. Therefore the decoded bits will have lower error probability than systems not capturing this diversity.

The present invention can potentially be used in a multi-user OFDM-based transmission system for high-speed downlink shared channel (HS-DSCH) in the third generation cellular systems.

The present invention provides a method which can easily be coalesced with, or added to, the existing standards, see for example ETSI 3GPP TS 25 212 V5.2.0 (2009-09), 3 GPP TSG RAN: Multiplexing and channel coding (FDD) (Release 5) . The present invention is thus a highly attractive and usable alternative.

The invention has been described in conjunction with preferred embodiments. It is evident that numerous alternatives, modifications, variations and uses will be obvious to a person skilled in the art in light of the foregoing description. For example, the interleaving method in accordance with the present invention has been described in conjunction with a channel mapping stage. The invention is however not limited to this, but the interleaving method in accordance with the present invention may be used as the first (and possibly only) interleaving as well, or as an interleaving step elsewhere in the transmission chain. A person skilled in the art realises that a corresponding deinterleaving is performed in the reception chain.

Claims

1. A method for interleaving in an OFDM system, characterized in that the method exploits the two-dimensional time-frequency structure of an OFDM system, and comprises the steps of:

- dividing the available frequency band into several sub-bands, each sub-band comprising essentially only adjacent frequencies,

- mapping a sequence of constellation symbols to a sequence of OFDM units, where one OFDM unit is defined as a group of constellation symbols, after which step the constellation symbols are only processed group-wise.

- interleaving a sequence of OFDM units, where each OFDM unit may be mapped onto one of said sub-bands.

2. Method for interleaving as claimed in claim 1, characterized in that said step of dividing the available OFDM frequency band into several sub-bands comprises dividing the frequency band into equally sized sub-bands.

3. Method for interleaving as claimed in claim 1, characterized in that said dividing of the available OFDM frequency band into

• several sub-bands comprises dividing the frequency band into non-equally sized sub-bands .

4. Method for interleaving as claimed in any of claims 1-3, characterized in that said step of interleaving a sequence of OFDM units, comprises interleaving the signal components separately.

5. Method as claimed in claim 4, characterized in that said step of interleaving a sequence of OFDM units comprises interleaving the real and imaginary parts, respectively, separately.

6. Method for interleaving as claimed in any of claims 1-5, characterized in that the method is performed after a constellation mapping step.

7. Method for interleaving as claimed in any of claims 1-6, characterized in that said step of mapping the sequence of constellation symbols comprises the mapping of constellation symbols onto OFDM units after a rotation of the constellation symbols .

8. Method for interleaving as claimed in any of claims 1-7, characterized in that said step of interleaving is a time and frequency mapping of the sequence of OFDM units, where each OFDM unit is allocated to a unique time and frequency-band position within the available transmission time interval and available frequency spectrum.

9. Method for interleaving as claimed in any of claims 1-8, characterized in that the mapping pattern used is a randomised time-frequency mapping.

10. Method for interleaving as claimed in any of claims 1-9, characterized in that the method is used in a multi-user environment.

11. A transmitter for use in an OFDM system, characterized in that the transmitter comprises :

- means for dividing the available frequency band into several sub-bands, each sub-band comprising essentially only adjacent frequencies,

- means for mapping a sequence of constellation symbols to a sequence of OFDM units, where one OFDM unit is defined as a group of constellation symbols, after which the constellation symbols are only processed group-wise;

- means for interleaving a sequence of OFDM units and separately for the real and imaginary parts, respectively, of OFDM units, where each OFDM unit may be mapped onto one of said sub-bands .

12. Transmitter as claimed in claim 11, characterized in that said means for dividing the available OFDM frequency band into several sub-bands comprises dividing the frequency band into equally sized sub-bands.

13. Transmitter as claimed in claim 11, characterized in that said means for dividing of the available OFDM frequency band into several sub-bands comprises dividing the frequency band into non-equally sized sub-bands.

14. Transmitter as claimed in any of claims 11-13, characterized in that said means for interleaving a sequence of OFDM units, comprises means for interleaving the signal components separately.

15. Transmitter as claimed in claim 14, characterized in that said means for interleaving of a sequence of OFDM units comprises means for interleaving the real and imaginary parts, respectively, separately.

16. Transmitter as claimed in any of claims 11-15, characterized in that said means for mapping the sequence of constellation symbols comprises means for mapping constellation symbols onto OFDM units after a rotation of the constellation symbols .

17. Transmitter as claimed in any of claims 11-16, characterized in that said means for interleaving comprises means for time and frequency mapping of the sequence of OFDM units, where each OFDM unit is allocated to a unique time and frequency-band position within the available transmission time interval and available frequency spectrum.

18. Transmitter as claimed in any of claims 11-17, characterized in that the mapping pattern used is a randomised time-frequency mapping.

19. An OFDM radio communication system for communication of signals between at least one transmitter and one receiver, said communication system including at least one transmitter as claimed in any of claims 11-18.