Method for Transmitting a Digital Data Stream, Transmitter, Method for Receiving a Digital Data Stream and Receiver

Abstract

A method for transmitting a digital signal is described, wherein the digital signal is to be transmitted by a plurality of antennas and a 2-domain pre-transformation is carried out, i.e., in course of a pre-transformation modulation symbols assigned to subcarriers having different frequencies and assigned to subcarriers sent by different antennas are linearly combined.

Description

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 shows a transmitter according to an embodiment of the invention.

FIG. 2 shows a 2-D pre-transform unit according to an embodiment of the invention.

FIG. 3 shows a 2-D pre-transform unit according to an embodiment of the invention.

FIG. 4 shows a transmitter according to an embodiment of the invention.

FIG. 5 shows a transmitter according to an embodiment of the invention.

FIG. 6 shows a transmitter 600 according to an embodiment of the invention.

DETAILED DESCRIPTION

Illustratively, a 2-domain (2D) pre-transformation is carried out that combines modulation symbols of different subcarriers sent by the same antenna (this corresponds to a pre-transformation in the frequency domain) as well as modulation symbols of different antennas (this corresponds to a pre-transformation in the spatial domain). In particular, modulation symbols corresponding to different data streams (to be transmitted using different antennas) can be combined in this way.

In this way, performance of the transmission of the data stream can be increased with respect to prior art methods.

The invention can for example be used in communication systems according to WLAN 11n, WIMAX (Worldwide Interoperability for Microwave Access), B3G (beyond 3G), ADSL (asynchronous digital subscriber line). It can be used in combination with OFDM (orthogonal frequency division multiplexing), SC-FDE (single carrier with frequency domain equalization) and OFDMA (orthogonal frequency division multiple access).

The methods according to the invention can be carried out by a computer that is provided the necessary instructions e.g. in form of a computer program.

Embodiments of the invention arise from the dependent claims. Embodiments that are described in the context of the method for transmitting a digital data stream are also valid for the transmitter, the method for receiving a digital data stream and the receiver.

In one embodiment, the modulation symbols are grouped into at least one block.

A block of modulation symbols is for example transformed to a block of transformed modulation symbols according to the multiplication by a transform matrix. The transform matrix is, in one embodiment, unitary.

The transformed modulation symbols are for example used for OFDM modulation of the subcarriers.

A forward error correction coding can be carried out on the data stream before it is mapped to the modulation symbols. The data stream can be interleaved before it is mapped to the modulation symbols. The data stream can also be scrambled before it is mapped to the modulation symbols. An STBC (space time block code) encoding can be carried out.

FIG. 1 shows a transmitter 100 according to an embodiment of the invention.

The transmitter 100 is formed according to a S×S SM (spatial multiplexing) PT-FEC-coded (pre-transform forward error correction coded) OFDM (orthogonal frequency multiplexing) system. The data 101 to be transmitted (source bits) are at first de-multiplexed by a S/P (serial/parallel) unit 102 into two parallel streams of data.

In other embodiments, the data to be transmitted is mapped into more than two parallel streams.

Each of the information bits in the two parallel streams is processed by a scrambler 103 and is FEC (forward error correction) encoded by a FEC unit 104. This is done in parallel for the two data streams, such that the transmitter 100 comprises two scramblers 103 and two FEC units 104.

The two (scrambled and FEC-encoded) parallel data streams are then processed by a 2-D Bit interleaving and mapping unit 105.

The 2-D Bit interleaving and mapping unit 105 performs interleaving in each parallel stream and performs interleaving across both parallel streams.

In this embodiment, the interleaving in each parallel stream is a two-step permutation. The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. The second permutation ensures that adjacent coded bits are mapped alternately onto less and more significant bits of the constellation and, thereby, long runs of low reliability bits (LSB) are avoided.

The interleaving across both parallel streams is for example carried out according to the following.

Let N_S=96 denote the number of data symbols in one block of the parallel data streams (note that the parallel streams are subdivided into data symbols and blocks of data symbols). Let further N_T denote the number of transmit antennas, in this case N_T=2.

T_i(k) where i=1, . . . , N_T and k=1, 2, . . . , N_S denotes the mapped symbol before symbol interleaving at parallel stream i, data symbol k, and

S_m(n) where i=1, . . . , N_T and k=1,2, . . . , N_S denotes the symbol after symbol interleaving at stream m, data symbol n.

The symbol interleaver defines the mapping operation from T_i(k) to S_m(n) where the (stream, symbol) index pair after interleaving (m,n) is generated according to

m=(k−1)/(N_S/N_T)+1

n=1 +(N_T(k−1)+(i−1)) mod N_S.

A receiver corresponding to the transmitter 100 comprises a de-interleaver that removes the interleaving across the two parallel streams and accordingly carries out the inverse mapping operation from S_m(n) to T_i(k) that is given by

i=1+(n−1) mod N_T

k=(m−1)N_S/N_T+(n−1)/N_T+1.

This interleaver is chosen when convolutional code is chosen for FEC. When LDPC (low-density parity check) is used for forward error correction, the interleaver differs from the one given above.

After interleaving, the 2-D Bit interleaving and mapping unit 105 maps the data contained in the interleaved parallel streams onto data symbols, such that two streams of data symbols are generated.

The two streams of data symbols are supplied to a 2-D pre-transform unit 106, which is explained in the following with reference to FIG. 2.

FIG. 2 shows a 2-D pre-transform unit 200 according to an embodiment of the invention.

The 2-D pre-transform unit 106 receives two streams of data symbols as input. Each stream of data symbol is converted into a block of data symbols. Let s₁ denote one block of data symbols (data symbol vector) of one data symbol stream, let it for example be the data symbol stream corresponding to the “upper” input in FIG. 2 and let s₂ denote the one block of data symbols (data symbol vector) of the other data symbol stream, i.e. the data symbol stream corresponding to the “lower” input in FIG. 2.

Each of the data symbol vectors comprises in this embodiment 96 components (equal to the number of subcarriers used for the data transmission), such that

s ₁ =[s ₁(1), s₁(2), . . . , s₁(96)]^T

s ₂ =[s ₂(1), s₂(2), . . . , s₂(96)]^T

wherein the s_i(j) are complex numbers. s₁ and s₂ each correspond to one OFDM symbol.

s₁ and s₂ are each supplied to a respective first multiplication unit 202 by which s₁ and s₂ are multiplied with a matrix T_f.

T_f can be chosen according to different transforms. One example transform is the Walsh-Hadamard transform. (T_f according to a Walsh-Hadamard transform (according to this example of order 96) can be generated from the order-8 Hadamard matrix

${\underset{\_}{T}}_8=\sqrt{\frac{1}{8}}\times \left[\begin{array}{cccccccc}1& 1& 1& 1& 1& 1& 1& 1\\ 1& -1& 1& -1& 1& -1& 1& -1\\ 1& 1& -1& -1& 1& 1& -1& -1\\ 1& -1& -1& 1& 1& -1& -1& 1\\ 1& 1& 1& 1& -1& -1& -1& -1\\ 1& -1& 1& -1& -1& 1& -1& 1\\ 1& 1& -1& -1& -1& -1& 1& 1\\ 1& -1& -1& 1& -1& 1& 1& -1\end{array}\right]$

and order-12 Hadamard matrix

${\underset{\_}{T}}_{12}=\sqrt{\frac{1}{12}}\times \left[\begin{array}{cccccccccccc}1& 1& 1& 1& 1& 1& 1& 1& 1& 1& 1& 1\\ 1& -1& 1& -1& 1& 1& 1& -1& -1& -1& 1& -1\\ 1& -1& -1& 1& -1& 1& 1& 1& -1& -1& -1& 1\\ 1& 1& -1& -1& 1& -1& 1& 1& 1& -1& -1& -1\\ 1& -1& 1& -1& -1& 1& -1& 1& 1& 1& -1& -1\\ 1& -1& -1& 1& -1& -1& 1& -1& 1& 1& 1& -1\\ 1& -1& -1& -1& 1& -1& -1& 1& -1& 1& 1& 1\\ 1& 1& -1& -1& -1& 1& -1& -1& 1& -1& 1& 1\\ 1& 1& 1& -1& -1& -1& 1& -1& -1& 1& -1& 1\\ 1& 1& 1& 1& -1& -1& -1& 1& -1& -1& 1& -1\\ 1& -1& 1& 1& 1& -1& -1& -1& 1& -1& -1& 1\\ 1& 1& -1& 1& 1& 1& -1& -1& -1& 1& -1& -1\end{array}\right]$

by performing the Kronecker product as

T_f=T₈T₁₂

T_f can also be chosen according to

${\underset{\_}{T}}_f=\underset{\_}{F}\times \left[\begin{array}{cccccc}1& 0& 0& \cdots & & 0\\ 0& \alpha & 0& & & 0\\ 0& 0& {\alpha }^2& & & 0\\ ⋮& ⋮& ⋮& ⋰& & ⋮\\ ⋮& ⋮& ⋮& & ⋰& \\ 0& 0& 0& \cdots & \cdots & {\alpha }^{N_S-1}\end{array}\right]$

where F is the FFT matrix of size N_S and α=exp(−jπ/2N_S).

Each block of-data symbols, multiplied by the matrix T_f is then supplied to a respective P/S unit 203 where it is mapped to a stream of data symbols. The streams of data symbols generated in this way are both fed to a second multiplication unit 204.

The second multiplication unit 204 performs a multiplication with a matrix T_s, such that altogether, the 2-D pre-transform unit 106 generates two vectors y₁ and y₂ according to

$\underset{\_}{y}=\left[\begin{array}{c}{\underset{\_}{y}}_1\\ {\underset{\_}{y}}_2\end{array}\right]=\underset{\_}{T}\left[\begin{array}{c}{\underset{\_}{s}}_1\\ {\underset{\_}{s}}_2\end{array}\right]$

where the 2-D transform matrix T can be written as

$\underset{\_}{T}=\underset{{\underset{\_}{T}}_s}{\underset{}{\left[\begin{array}{cccc}{\underset{\_}{T}}_{s,0}& 0& 0& 0\\ 0& {\underset{\_}{T}}_{s,1}& 0& 0\\ 0& 0& ⋰& 0\\ 0& 0& 0& {\underset{\_}{T}}_{s,N-1}\end{array}\right]}}\underset{\_}{P}\left[\begin{array}{cc}{\underset{\_}{T}}_f& 0\\ 0& {\underset{\_}{T}}_f\end{array}\right].$

and P is the n_TN-by-n_TN permutation matrix, with its elements having the following values:

${\underset{\_}{P}}_{i,j}=\{\begin{array}{cc}1,& \mathrm{when}j=\lceil \frac{i}{n_T}\rceil +\left(i\text{mod}n_T\right)\times N\\ 0,& \mathrm{otherwise}\end{array}i,j=0,1,\dots ,n_TN-1.$

The transform matrix T_s is a square matrix when the number of streams at the output is the same as that at the input (as it is the case in this example).

When there are more streams at the output than that at the input, transform matrix T_s is a rectangular matrix having more rows than columns. The row number is the same as the output stream number, and the column number is the same as the input stream number. The columns shall be orthonormal.

When T_s is not generated from the channel knowledge, it can take the following format

${\underset{\_}{T}}_{s,k}=\underset{{\underset{\_}{D}}_k}{\underset{}{\mathrm{diag}\left(1,{}^{{\mathrm{j\varphi }}_{1,k}},\dots ,{}^{{\mathrm{j\varphi }}_{n_T,k}}\right)}}\underset{\_}{S}k=0,1,2,\dots ,N-1$

where N is the number of subcarriers per OFDM symbol, S is either a square unitary matrix or a rectangular matrix with orthonormal columns, and φ_k can take different values for the different subcarriers.

T_s can also be generated from the channel knowledge.

The multiplication of

$\hspace{1em}\left[\begin{array}{c}{\underset{\_}{s}}_1\\ {\underset{\_}{s}}_2\end{array}\right]$

by the transformation matrix T effects a pre-transformation in the spatial domain and the frequency domain (therefore denoted by 2D pre-transformation).

The multiplication by the matrices T_f and T_s that is performed by the 2-D Pre-transform unit 200 can also take place in different order, as it is shown in FIG. 3.

FIG. 3 shows a 2-D pre-transform unit 300 according to an embodiment of the invention.

The functionality of the 2-D pre-transform unit 300 is similar to the functionality of the 2-D pre-transform unit 200 shown in FIG. 2, but the input is first processed by the second multiplication unit 301 and the two data streams generated by the second multiplication unit 301 are (after being mapped to blocks by a respective S/P unit 302) processed by the respective first multiplication unit 303.

Altogether, the 2-D pre-transform unit 106 generates two vectors y₁ and y₂, which are mapped to corresponding symbol streams by a respective P/S unit 304, wherein

$\underset{\_}{y}=\left[\begin{array}{c}{\underset{\_}{y}}_1\\ {\underset{\_}{y}}_2\end{array}\right]=\underset{\_}{T}\left[\begin{array}{c}{\underset{\_}{s}}_1\\ {\underset{\_}{s}}_2\end{array}\right]$

where the 2-D transform matrix T can be written as

$\underset{\_}{T}=\left[\begin{array}{cc}{\underset{\_}{T}}_f& 0\\ 0& {\underset{\_}{T}}_f\end{array}\right]\underset{\_}{P}\underset{{\underset{\_}{T}}_s}{\underset{}{\left[\begin{array}{cccc}{\underset{\_}{T}}_{s,0}& 0& 0& 0\\ 0& {\underset{\_}{T}}_{s,1}& 0& 0\\ 0& 0& ⋰& 0\\ 0& 0& 0& {\underset{\_}{T}}_{s,N-1}\end{array}\right]}}$

Where P is an n_TN-by-n_TN permutation matrix, with its elements having the following values:

${\underset{\_}{P}}_{i,j}=\{\begin{array}{cc}1,& \mathrm{when}j=\lceil \frac{i}{N}\rceil +\left(i\text{mod}N\right)\times n_T\\ 0,& \mathrm{otherwise}\end{array}i,j=0,1,\dots ,n_TN-1.$

As above, T_s can also be generated from the channel knowledge or independent of the channel knowledge. When the number of streams to be transmitted is the same as the number of antennas, T_s is a square unitary matrix. When there are more transmit antennas than the number of data streams, it is a rectangular matrix with orthonormal columns.

The output of the 2-D Pre-transform unit 106, i.e. symbol streams corresponding to the vectors y₁ and y₂, are each used for OFDM modulation by a respective OFDM unit 107, wherein inverse Fast Fourier Transform (IFFT) and cyclic prefix (CP) insertion is performed.

An alternative embodiment is shown in FIG. 4.

FIG. 4 shows a transmitter 400 according to an embodiment of the invention.

The functionality of the S/P unit 401, the scramblers 402, the FEC units 403 and the 2-D Bit interleaving and mapping unit 404 is the same as the one of the corresponding units of the embodiment described above with reference to FIG. 1.

In contrast to the embodiment described with reference to FIG. 1, the OFDM modulation is carried out by the OFDM units 406 in between of the processing by the respective first multiplication unit 405 (that performs multiplication by T_f) and the processing by the second multiplication unit 407 (that performs multiplication by T_s).

In this case, the spatial domain transform (corresponding to T_s) can be the same for each data symbol in one OFDM block. Note that no unified 2-D transform matrix T can be worked out as there is a OFDM modulation between the frequency domain transform (corresponding to T_f) and the spatial domain transform.

In the following, an embodiment is explained wherein the 2D-transform takes place before the data streams are separated for transfer by different antennas.

FIG. 5 shows a transmitter 500 according to an embodiment of the invention.

The data 501 to be transmitted (source bits) are processed by a scrambler 502, an FEC unit 503 and a Bit interleaver 504. A mapper 505 maps the data 501 to be transmitted to a stream of modulation symbols, in this example to modulation symbols suitable for modulating the subcarriers transmitted by two antennas according to an OFDM modulation. The stream of modulation symbols is fed to a 2-D Pre-transform unit 506.

The stream of modulation symbols are first mapped into blocks of modulation symbols by a S/P unit 507. Each block of modulation symbols is processed by a first multiplication unit 508 that performs a multiplication of each block of modulation symbols by a matrix T_f. T_f is a unitary matrix and can be chosen as described-above.

The blocks of modulation symbols processed by the first multiplication unit 508 are processed by a P/S unit 509 and a second multiplication unit 510 which performs a multiplication by a matrix T_s, which, using the above denotations, is for example given by

${\underset{\_}{T}}_{s,k}=\left[\begin{array}{c}1\\ {}^{{\mathrm{j\varphi }}_k}\end{array}\right]k=0,1,2,\dots ,N-1$

Altogether, similar to above, the processing of the 2-D Pre-transform unit 506 corresponds a matrix T which can in this example be written as

$\underset{\_}{T}=\left[\begin{array}{cc}{\underset{\_}{T}}_f& \\ & \underset{\_}{D}{\underset{\_}{T}}_f\end{array}\right],D=\mathrm{diag}\left(1,{}^{{\mathrm{j\varphi }}_1},\dots ,{}^{{\mathrm{j\varphi }}_{N-1}}\right)$

T_s can also be given by

${\underset{\_}{T}}_{s,k}=\left[\begin{array}{c}{\alpha }_{1,k}\\ {\alpha }_{2,k}\end{array}\right]k=0,1,2,\dots ,N-1$

with angles α_i,k. In this case, the 2-D transform matrix T is given by

$\underset{\_}{T}=\left[\begin{array}{cc}{\underset{\_}{D}}_1{\underset{\_}{T}}_f& \\ & {\underset{\_}{D}}_2{\underset{\_}{T}}_f\end{array}\right],{\underset{\_}{D}}_s=\mathrm{diag}\left({\alpha }_{s,1},{\alpha }_{s,2},\dots ,{\alpha }_{s,N-1}\right),s=1,2$

The output of the 2-D Pre-transform unit 506 is a vector of dimension 2N (compare the dimension of the matrix T). The output vector is separated into two blocks of dimension N and each block is fed to a respective OFDM unit 511.

Between the 2-D Pre-transform unit 506 and the OFDM unit 511, there is in one embodiment a STBC unit such that the output of the 2-D Pre-transform unit 506 is STBC coded before it is used for OFDM modulation. Alternatively, STBC encoding is performed after OFDM modulation. Note that this embodiment is for example used when the channel conditions deteriorate and STBC is used to improve the signal link quality. Only one stream of data is processed and no data rate is gained through the MIMO structure in this case. Since in this embodiment, there are two antennas, the architecture would be a 2×2 STBC PT-FEC-OFDM.

In one embodiment, the transmitter has a 4×2 GSTBC (group-wise STBC) PT-FEC-OFDM architecture. In this case, four antennas would be provided that are used to transmit two parallel streams, wherein each stream is transmitted according to 2×2 STBC PT-FEC-OFDM. Therefore, the data rate attained is the same as the 2×2 SM PT-FEC-OFDM (see the embodiment described with reference to FIG. 1, but transmit diversity is achieved. A transmitter according to this embodiment would have the structure of the transmitter shown in FIG. 1 but each of the OFDM units 107 would be coupled with a STBC unit such that from the output of each OFDM unit 107, two data streams are generated. The resulting four data streams are then transferred simultaneously.

An alternative to the transmitter 100 shown in FIG. 1 is shown in FIG. 6.

FIG. 6 shows a transmitter 600 according to an embodiment of the invention.

In contrast to the transmitter 100 shown in FIG. 1, the data to be sent (source bits) is not mapped to parallel streams before scrambling, FEC encoding and bit interleaving. Instead, the data is processed by only one scrambler 601, one FEC unit 602 and one 2-D bit interleaver 603.

The output of the 2-D bit interleaver 603 is separated into two parallel streams of data, which are each mapped to modulation symbols by a respective mapper 604.

Similar to the transmitter 100, the parallel streams of data are transformed according to a 2-D transform by a 2-D pre-transform unit 605 and each transformed parallel stream of data is used for OFDM modulation by a respective OFDM unit 606.

According to one embodiment, a receiver is used, wherein an iterative subcarrier reconstruction is performed before the PIC+MRC (parallel interference cancellation and maximal ratio combining) process. The rational behind this is that iterative reconstruction provides the PIC+MRC module with a much better estimate of the transmitted signal and hence the interference estimate. Therefore, the error propagation of the PIC+MRC module can be significantly reduced and hence, better error performance is produced. If complexity permits, the output of the PIC+MRC module can be fed back to the iterative subcarrier reconstruction module to improve the reconstruction as well. The iteration between these two modules produces even better performance.

Two different schemes can be used:

Scheme 1: Subcarrier Reconstruction+PIC+MRC

1) Perform linear ZF-IS (zero forcing interference suppression) or MMSE-IS (minimum mean squared error interference suppression).

2) For each transmit data stream, perform iterative subcarrier reconstruction for p subcarriers and q iterations on each subcarrier

3) Use the output of the subcarrier reconstruction to perform PIC+MRC for r iterations

4) Perform inverse PT on the output from PIC+MRC. Demodulate the signal and pass it to output.

Scheme 2: Iterative Subcarrier Reconstruction+PIC+MRC

1) Perform linear ZF-IS or MMSE-IS and inverse PT to obtain the estimate of the transmitted signal

2) For each transmit data stream, perform iterative subcarrier reconstruction, using the estimate of the transmitted signal from the previous step.

3) Using the output of the subcarrier reconstruction to perform PIC+MRC for r iterations, go to step 3

4) Repeat Step 3 and 4 until the iteration number exceeds the chosen value

5) Inverse PT the output form PIC+MRC. Demodulate the signal pass to output.

It should be noted that in the embodiments described in connection with the FIGS. 1 to 6, the functional blocks 103 and 104 in FIG. 1, 202 in FIG. 2, 303 in FIG. 3, 402, 403 and 405 in FIG. 4 and 604 in FIG. 6 in the respective data streams may have the same or different implementation.

In the above, the following documents are cited:

G. J. Foschini, G. D. Golden, R. A. Valenzuela, and P. W. Wolnianski, “Simplified processing for high spectral efficiency wireless communication employing multi-element arrays”, IEEE Journal on Selected Areas of Communications, vol. 17, No. 11, pp. 1841-1852, November 1999.

Zhongding Lei, Yan Wu, Chin Keong Ho, Sumei Sun, Ping He, and Yuang Li, “Iterative detection for Walsh-Hadamard transformed OFDM”, IEEE Vehicular Technology Conference Spring, 2003, pp. 637-640, vol. 1, April 2003

Claims

1. A method for transmitting a digital data stream, wherein: the data stream is mapped to a plurality of modulation symbols;each modulation symbol is associated with a subcarrier of a multiplicity of subcarriers;the multiplicity of subcarriers comprises at least a first plurality of subcarriers and a second plurality of subcarriers;the subcarriers of the first plurality of subcarriers have different frequency and are sent by a first antenna;the subcarriers of the second plurality of subcarriers have different frequency and are sent by a second antenna;the modulation symbols are transformed such that a multiplicity of transformed modulation symbols is formed from the modulation symbols wherein; at least two modulation symbols associated with sub-carriers having different frequency are linearly combined to form a transformed modulation symbol; andat least one modulation symbol associated with a sub-carrier sent by the first antenna and at least one modulation symbol associated with a sub-carrier sent by the second antenna are linearly combined to form a transformed modulation symbol; andeach transformed modulation symbol is associated with a subcarrier; andeach transformed modulation symbol is used to modulate the subcarrier associated with the transformed modulation symbol.

2. A method according to claim 1, wherein the modulation symbols are grouped into at least one block.

3. A method according to claim 2, wherein a block of modulation symbols is transformed to a block of transformed modulation symbols according to the multiplication by a transform matrix.

4. A method according to claim 3, wherein the transform matrix is unitary.

5. A method according to claim 1, wherein the transformed modulation symbols are used for OFDM modulation of the subcarriers.

6. A method according to claim 1, wherein a forward error correction coding is carried out on the data stream before it is mapped to the modulation symbols.

7. A method according to claim 1, wherein the data stream is interleaved before it is mapped to the modulation symbols.

8. A method according to claim 1, wherein the data stream is scrambled before it is mapped to the modulation symbols.

9. A method according to claim 1, wherein an space-time block code encoding is carried out.

10. A transmitter for transmitting a digital data stream, comprising: a mapping unit adapted to map the data stream to a plurality of modulation symbols;an association unit adapted to associate each modulation symbol with a subcarrier of a multiplicity of subcarriers; wherein the multiplicity of subcarriers comprises at least a first plurality of subcarriers and a second plurality of subcarriers;the subcarriers of the first plurality of subcarriers have different frequency and are sent by a first antenna;the subcarriers of the second plurality of subcarriers have different frequency and are sent by a second antenna;a transforming unit adapted to transform the modulation symbols such that a multiplicity of transformed modulation symbols is formed from the modulation symbols; wherein at least two modulation symbols associated with sub-carriers having different frequency are linearly combined to form a transformed modulation symbol; andat least one modulation symbol associated with a sub-carrier sent by the first antenna and at least one modulation symbol associated with a sub-carrier sent by the second antenna are linearly combined to form a transformed modulation symbol; andeach transformed modulation symbol is associated with a subcarrier; anda modulating unit adapted to use each transformed modulation symbol for modulating the subcarrier associated with the transformed modulation symbol.

11. A method for receiving a digital data stream, wherein: a multiplicity of subcarriers comprising a first plurality of subcarriers and a second plurality of subcarriers is received, wherein the first plurality of subcarriers is received by a first antenna and the second plurality of subcarriers is received by a second antenna;the multiplicity of subcarriers is de-modulated to generate a multiplicity of transformed modulation symbols, wherein each transformed modulation symbol is associated with the subcarrier it is de-modulated from;an inverse transformation is carried out on the multiplicity of transformed modulation symbols to form a multiplicity of modulation symbols; wherein at least two transformed modulation symbols associated with subcarriers having different frequency are linearly combined to form a modulation symbol andat least one transformed modulation symbol associated with a sub-carrier sent by the first antenna and at least one transformed modulation symbol associated with a sub-carrier sent by the second antenna are linearly combined to form a modulation symbol; andthe data stream is generated from the multiplicity of modulation symbols.

12. A receiver for receiving a digital data stream, comprising: a receiving unit adapted to receive a multiplicity of subcarriers comprising a first plurality of subcarriers and a second plurality of subcarriers, wherein the first plurality of subcarriers is received by a first antenna and the second plurality of subcarriers is received by a second antennaa de-modulation unit adapted to de-modulate the multiplicity of subcarriers to generate a multiplicity of transformed modulation symbols, wherein each transformed modulation symbol is associated with the subcarrier it is de-modulated from;a transformation unit adapted to carry out an inverse transformation on the multiplicity of transformed modulation symbols to form a multiplicity of modulation symbols; wherein at least two transformed modulation symbols associated with subcarriers having different frequency are linearly combined to form a modulation symbol; andat least one transformed modulation symbol associated with a sub-carrier sent by the first antenna and at least one transformed modulation symbol associated with a sub-carrier sent by the second antenna are linearly combined to form a modulation symbol; anda generation unit adapted to generate the data stream from the multiplicity of modulation symbols.