Data transmission network

FIELD OF THE INVENTION

The present invention relates to data transmission network.

The present invention relates further to a data transmission method, a controller and a control method and to a computer program for implementing said method.

The invention relates particularly to a single frequency network employing a space-time block code.

BACKGROUND OF THE INVENTION

Single frequency networks (SFN) are networks of transmitters transmitting the same signal at the same frequency channel and at the same time. They are mainly used in broadcast terrestrial standards such as, for instance, the DVB-T standard. They have the advantage of allowing a frequency reuse factor of one and the disadvantage of creating artificially long channels. The wireless channel seen by the receiver does not only consist of the delay spread, but it is also influenced by the relative position of the transmitters with respect to the receiver. The DVB-T standard faces this challenging scenario by employing OFDM (Orthogonal Frequency Division Multiplex) modulation with a very long guard interval.

OFDM is known to be very robust with respect to delay spread at the cost of a loss in spectral efficiency (the additional guard interval). To give an idea, a propagation only scenario can be expected to generate a channel with a maximum delay spread of about 5- lOμs. In the presence of an SFN, the maximum expected delay spread is as high as about 224 μs, which corresponds to the longest guard interval defined by the DVB-T standard. In other words, in the presence of SFN, a channel about 22 times longer than in non-SFN network might be expected.

WO 99/14871 discloses a simple block coding arrangement in which symbols are transmitted over a plurality of transmit channels, in connection with coding that comprises only of simple arithmetic operations, such as negation and conjugation. The diversity created by the transmitter utilizes space diversity and either time or frequency diversity. Space diversity is effected by redundantly transmitting over a plurality of antennas,

time diversity is effected by redundantly transmitting at different times, and frequency diversity is effected by redundantly transmitting at different frequencies. Illustratively, using two transmit antennas and a single receive antenna, one of the disclosed embodiments provides the same diversity gain as the maximal-ratio receiver combining (MRRC) scheme with one transmit antenna and two receive antennas.

The coding scheme disclosed in WO 99/14871 is known in the art as Alamouti coding scheme and has also been described in S. M. Alamouti "A simple transmit diversity technique for wireless communications", IEEE J. SeI. Areas. Comm. vol. 16, pp. 1451-1458, October 1998.

Space-time block codes (STBCs) are potential candidates for future multiple input multiple output (MIMO) wireless systems because they provide significant gain in terms of reliability. They include the use of at least two transmit antennas which can be located in the same transmitter or belong to different transmitters in the same network. STBCs are currently under consideration for the second generation terrestrial digital video broadcasting standard (DVB-T2).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a data transmission network and a corresponding data transmission method, a controller and a corresponding control method, and a computer program which enable the deployment of STBCs in an SFN. Preferably, a uniform gain in the coverage area of the SFN shall be provided.

In a first aspect of the present invention a data transmission network is presented as defined in claim 1 comprising: a space-time block code encoder for encoding user data of a user data stream into channel symbols of two or more channel data streams for transmission, a plurality of transmitters in a transmission area for transmitting said channel data streams using the same frequency channel for reception by one or more receivers located in said transmission area, wherein the number of transmitters is larger than the number of channel data streams and wherein each channel data stream is transmitted by at least one transmitter, and control means for controlling, which channel data stream is transmitted by which transmitter, such that all channel data streams are simultaneously transmitted, that at least two transmitters are transmitting the same channel data stream and that at least one transmitter is changing which channel data stream it transmits.

In a further aspect of the present invention a controller for use in a data transmission network according to claim 1 is presented, said controller being adapted for controlling, which channel data stream is transmitted by which transmitter, such that all channel data streams are simultaneously transmitted, that at least two transmitters are transmitting the same channel data stream and that at least one transmitter is changing which channel data stream it transmits.

According to further aspects of the present invention corresponding methods and a computer program for implementing the control method on a computer are presented according to further independent claims.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the subject matter of all independent claims may have similar and/or identical preferred embodiments as defined in the dependent claims.

In DVB-T2 two possible approaches of STBC are considered. The first approach is more conventional and requires that each transmitter is upgraded with a second transmit antenna. This approach requires an upgrade in the existing infrastructure, and the STBC is not going to give a significant gain because of the expected high correlation between the almost co-located antennas. The second approach requires that DVB-T2 is deployed in a SFN and that each transmitter of the SFN acts as a different antenna of the STBC. This approach will be referred to as distributed STBC. The distributed STBC is more likely to be used since it does not require any upgrade in the transmit antennas and the STBC is going to provide a significant gain. In particular, the deployment of the above mentioned Alamouti code, which is the most famous STBC, is considered in DVB-T2.

The deployment of STBC in an SFN in a distributed fashion means that the transmitters in the same SFN act as the different antennas of the same STBC. Therefore, if a user sees two transmitters which are acting as two different antennas of the STBC, then it will experience a better signal than a user that sees two different antennas transmitting just the same signal. As a result, distributed STBCs do not provide a uniform gain in the coverage area. There will be regions that experience the STBC gain and regions that do not. Since broadcast operators usually design the network considering the worst-case scenario, the deployment of distributed STBC seems to have limits in the SFN scenario. The proposed invention solves this problem by proposing a new mapping of STBC to transmitters in the same SFN.

The present invention is based on the idea not to apply a fixed allocation of the transmitters to the transmit antennas of the SFN, but to make this allocation variable, i.e.

to change the STBC mapping used in each transmit antenna of the SFN. This change can be in time domain, in frequency domain, or in both. In general, changing the STBC mapping is not done since the different transmit antennas of the STBC have similar coverage and similar location. But this is not the case in SFN with distributed STBC. The invention thus provides a better worst-case scenario of SFN with distributed STBC.

It shall be noted that according to the invention generally at any time more than one transmitter transmit the same channel data stream. Since the allocation of the channel data streams to the transmitters is not fixed, the channel data stream(s) which is/are transmitted by more than one transmitter changes. It is, however, also possible that all channel data streams are transmitted by more than one transmitter. It shall be understood, on the other hand, that it is also possible that during some time periods each channel data stream is transmitted by only one transmitter, whereas during other time periods at least one data stream is transmitted by more than one transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings:

Fig. 1 shows a block diagram of a general channel on which noise is added to a transmitted signal,

Fig. 2 shows two mappings from x _kl to x _k2 , showing on the right the scaled- repetition mapping, on the left the ordinary-repetition mapping,

Fig. 3 shows a diagram illustrating the basic capacity C , the repetition capacity C ₁ . the maximum transmission rates achievable with 4-PAM in the ordinary- repetition case Ia and the maximum rates achievable using scaled-repetition mapping Ib,

Fig. 4 shows a model of a 2x2 MIMO channel,

Fig. 5 shows a general layout of a single frequency network,

Fig. 6 shows a block diagram of the general layout illustrating the application of an STBC and OFDM in a communications system,

Fig. 7 shows diagrams illustrating the Alamouti scheme and the simple repetition scheme,

Fig. 8 shows a diagram illustrating a SFN scenario using three transmitters,

Fig. 9 shows a block diagram of a first embodiment of a data transmission network according to the present invention,

Fig. 10 shows a block diagram of a second embodiment of a data transmission network according to the present invention,

Fig. 11 shows a third embodiment of an SFN according to the invention using a transmitter allocation which is changing in time,

Fig. 12 shows a fourth embodiment of an SFN according to the invention using a transmitter allocation which is changing in frequency, and

Fig. 13 illustrates a general embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before embodiments of the present invention are explained, some information theory shall be discussed. The real- valued output y _k for transmission k = 1 ,2, • • • , K, see Fig.

1, satisfies where x _k is the real- valued channel input for transmission k and n _k is a real- valued

Gaussian noise sample with mean E[N _k ] = 0, variance E[N]:] = σ ² , which is uncorrelated with all other noise samples. The transmitter power is limited, i.e. it is required that E[X] ] < P . It is well-known that an X which is Gaussian with mean 0 and variance P achieves capacity. This basic capacity (in bits per transmission) equals

When codewords are retransmitted (repeated), each symbol x _k from such a codeword (x _ι ,x ₂ ,- - -,x _κ ) is actually transmitted and received twice, i.e. x _kl = x _k2 = x _k , and

An optimal receiver can form

Now the variance of the noise variable (N _kl + N _k2 )/2 is σ ² /2. Therefore the repetition capacity for a single repetition in bit/transm. is

1 , _/1 2P

C _r = - _τ 1lOogg ₂₂ ((Il ++ - — ). (5) 4 σ

Fig. 3 shows the basic capacity C and repetition capacity C ₁ . as a function of the signal-to-noise ratio SNR which is defined as

A p

SNR = ^V. (6) σ

It is easy to see that always C _r ≤ C . For large SNR it can be written C _r » CIl + 1/4 , while for small SNR it is obtained C _r ~ C .

Next, ordinary repetition for 4-PAM (pulse-amplitude modulation) shall be discussed. When 4-PAM modulation is used, the channel inputs x _k assume values from A- _PAM ⁼ {—3 ,— 1 ,+ 1 ,+3 } , each with probability 1/4 . Ordinary repetition (see G. Benelli, "A new method for the integration of modulation and channel coding in an ARQ protocol," IEEE Trans. Commun., vol. COM-40, pp. 1594 - 1606, October 1992) leads to signal points (X ₁₅ X ₂ ) = (x,x) for xe A ₄ _ _FAM , see the left part of Fig. 2. For this case the maximum transmission rate I _a (X; Y ₁ , Y ₂ ) is shown in Fig. 3. Note that this maximum transmission rate is slightly smaller than the corresponding capacities C ₁ . , mainly because uniform inputs are used instead of Gaussians.

Benelli's method can be used to improve upon ordinary-repetition retransmission, i.e. by modulating the retransmitted symbol differently. It could e.g. be taken where M ₂ (α) = 2α -5 if α > 0 and M ₂ (α) = 2α +5 for α < 0 . This method is called scaled repetition since a symbol is scaled by a factor (2 here) and then compensated (add -5 or +5) in order to obtain a symbol from A ₄ _ _¥AM . This results in the signal points (x,M ₂ (x)) for xe A ₄ _ _FAM , see Fig. 2, right part. Also for the scaled-repetition case the maximum transmission rate I _b (X; Y ₁ , Y ₂ ) is shown in Fig. 3. Note that this maximum transmission rate is only slightly smaller than the basic capacity C Ordinary repetition is however definitively inferior to the basic transmission if the SNR is not very small.

Next, the demodulation complexity shall be discussed. Scaled repetition outperforms ordinary repetition, but also has a disadvantage. In an ordinary-repetition system the output y _k = (y _kl + y _k2 )/2 is simply sliced. In a system that uses scaled repetition it can only be sliced after having distinguished between two cases. More precisely note that x _k2 = M ₂ (x _k ) = 2x _k -D ₂ (x _k ), (8)

where D ₂ (Cc) = 5 if α > 0 and D ₂ (Cc) = -5 if α < 0 . Now a slicer can be used for y _kι ^{+ 2} y _k2 = x _k + n _kι + 2(2x _k -D ₂ (x _k ) + n _k2 ) = 5x _k -2D ₂ (x _k ) + n _kl +2n _k2 .

Assuming that x _k e {-3,-1} it is obtained that D ₂ (x _k ) = -5 and this implies that a threshold shall be put at 0 to distinguish between -3 and -1 . Similarly assuming that x _k € {+ 1,-1-3} it is obtained D ₂ (x _k ) = 5 and it must be sliced y _kl +2y _k2 again with a threshold at 0. Then the best overall candidate x _k is found by minimizing

(λi ^{~ x} _k f + (y _k2 ^~ M ₂ (x _k )) ² over the two candidates.

Next, fundamental properties for the 2x2 MIMO channel shall be described and a model description shall be introduced. A 2x2 MIMO channel is shown in Fig. 4. Both the transmitter T and the receiver R use two antennas. The output vector (y _u ,y _2k ) at transmission k relates to the corresponding input vector (x _lk ,x _2k ) as given by hλJh _n K ₂ Yx ₁ AJn ₁ A _(io)

where (n _lk ,n _2k ) is a pair of independent zero-mean circularly symmetric complex Gaussians, both having variance σ ² (per two dimensions). Noise variable pairs in different transmissions are independent.

It is assumed that the four channel coefficients Zz ₁₁₅ Zz ₁₂ , A ₂₁ , and Zz ₂₂ are independent zero-mean circularly symmetric complex Gaussians, each having variance 1 (per two dimensions). The channel coefficients are chosen prior to a block of K transmissions and remain constant over that block. The complex transmitted symbols (x _kV x _k2 ) must satisfy a power constraint, i.e.

E[x _kl x _kl +x _kl x _k2 ] ≤ P. (11)

If the channel input variables are independent zero-mean circularly symmetric complex Gaussians both having variance P/2, then the resulting mutual information (called Telatar capacity here, see I.E. Telatar, "Capacity of multi- antenna Gaussian channels" European Trans. Telecommunications, vol. 10, pp. 585-595, 1999. (Originally published as AT&T Technical Memorandum, 1995)) is

P/2

C _Telata (# ) = log ₂ det(/ ₂ +—HH'), (12) σ where

i.e. the actual channel-coefficient matrix and I ₂ the 2x2 identity matrix (here H^ denotes the Hermitian transpose of H . It involves both transposition and complex conjugation). Also in the 2x2 MIMO case the signal-to-noise ratio is defined as

A p

SNR = ^. (14) σ

It can be shown (see e.g. H. Yao, "Efficient Signal, Code, and Receiver Designs for MIMO Communication Systems," Ph.D. thesis, M. I. T., June 2003, p. 36) that for fixed R and SNR large enough

PrlC^^ ^l - γ - SNR- ⁴ , (15) for some constant γ .

The worst-case error-probabilities shall now be described. Consider M (one for each message) Kx 2 code-matrices c _v c ₂ ,- - -,c _M resulting in a unit average energy code. Then Tarokh, Seshadri and Calderbank, "Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction," IEEE Trans. Inform. Theory, Vol. 44, pp. 744- 765, March 1998, showed that for large SNR

Pr{c → c'} » y'(det((c'-c)(c'-cy X ² SNR- ⁴ . (16) for some γ' if the rank of the difference matrices c -c' is 2, and it is transmitted x = y[Pc_ . If this holds for all difference matrices it is said that the diversity order is 4. Therefore it makes sense to maximize the minimum modulus of the determinant over all code-matrix differences.

S. M. Alamouti, "A simple transmit diversity technique for wireless communications," IEEE J. SeI. Areas. Comm. vol. 16, pp. 1451-1458, October 1998 proposed a modulation scheme (space-time code) for the 2x2 MIMO channel which allows for a very simple detector. Two complex symbols S ₁ and S ₂ are transmitted in the first transmission (an odd transmission) and in the second transmission (the next even transmission) these symbols are more or less repeated. More precisely

The received signal is now

Rewriting this results in

or more compactly y = s _γ a + s ₂ b_ + n, (20) with y = (yu > y2i > yi ^* 2 > y2 ^* 2) ^τ >

Since a and b are orthogonal the symbol estimates ^ ₁ and S ₂ can be determined by simply slicing (a ' y)/(g? a) and (y^b)/(b?b) respectively.

Another advantage of the Alamouti method is that the densities of a? a and t?b_ are (identical and) chi-square with 8 degrees of freedom. This results in a diversity order 4, i.e.

PrRSμ ,S ₂ ) ≠ tS, . S ₂ ⁾ ) SNR -4

(22) for fixed rate and large enough SNR .

Fig. 5 illustrates the typical layout of a single frequency network having, in this embodiment, three transmitters TxI, Tx2, Tx3 and one receiver Rx in the transmission area. All transmitters transmit the same signal at the same time using the same frequency.

Fig. 6 shows a block diagram illustrating the application of an STBC and OFDM in a data transmission system / communications system, in particular an SFN. In this transmission system the space-time block code is combined with OFDM to achieve spatial diversity gain over frequency-selective fading channels. In particular, space-time coding on blocks of data symbols instead of individual symbols is applied.

On the transmission side a serial-to-parallel converter 10 collects K serial data symbols X(ni) into a data block or vector X(n). A space-time encoder 11 takes two data

vectors X(n) and X(/?+l) and encodes them into two channel data streams using a space-time block code transmission matrix

The channel data streams are provided to respective modulator 12, 13 for modulating by an IDFT (Inverse Discrete Frequency Transformer) and cyclic prefix adding for adding of a length G cyclic prefix resulting in OFDM symbol vectors which are transmitted by the transmitters 14, 15. Thus, transmitter 14 transmits X(n) and -X (n+1), and transmitter 15 transmits X(/?+l) and X (n).

On the receiver side a single receiver 20 receives the transmitted signals and provides it to a demodulator 21 for cyclic prefix removing and demodulating by a DFT (Discrete Frequency Transformer) into Y(n). In parallel, the received signals are provided to a channel estimator 22, whose result is provided to a combiner and detector 23, which also obtains the demodulated signal Y(n). Finally, a parallel-to-serial converter 24 transforms the detected data stream into serial output data symbols X{m) . It shall be noted that X(n) and X(n+1) play the same role as S ₁ and S ₂ in the above equation (17).

As mentioned above space time block codes (STBCs) are widely used in Multiple Input Multiple Output (MIMO) systems, and in MISO (Multiple Input Single Output) systems. They allow for an improvement in reliability, and they are also used in cooperative communications. The most famous STBC is the above explained Alamouti code. According to the present invention, an allocation method is proposed for STBC suited for a broadcast SFN.

Before the invention will be explained in more detail the Alamouti scheme shall be explained again compared to a simple repetition scheme. As shown in Fig. 7A the Alamouti scheme uses two transmit antennas Al, A2, each transmitting all symbols, but changed in order, sign and complex conjugation. This form of coding allows the receiver with a single antenna for an optimal and simple detection. The available capacity CAk is indicated with "STBC".

The simple repetition shown in Fig. 7B also uses two transmit antennas Al, A2, but each antenna is transmitting the same symbol. The channel capacity is lower. The available capacity C _rep is indicated with "no STBC".

In particular, it holds for these two schemes:

C _AI , -I ₀₈ I ₊ (K ⁺ Kf ff) >-C, ψ" ₊ H-fjψj

In distributed STBC, the two antennas Al, A2 are not located in the same transmitter. Multiple transmitters are synchronized and act as different antennas of the STBC.

A simple example of a SFN scenario using three transmitters TxI, Tx2, Tx3 is shown in Fig. 8. In this normal distributed STBC the first transmitter TxI is used as first antenna Alof the STBC, the second transmitter Tx2 is used as second antenna A2 of the STBC and the third transmitter Tx3 is also used as first antenna Alof the STBC. The regions A(I), A(2) and A(3) are border regions of the SFN, having no SFN gain anyhow. The other regions have the following gains:

Regions Gain

A(1, 2, 3) STBC, antenna Al has a stronger contribution

A(1, 2) STBC

A(2,3) STBC

A(1, 3) no STBC

Region A(1, 3) has the worst performance, because it sees two transmitters which transmit exactly the same symbol (i.e. TxI and Tx3 which act as the same antenna of the STBC). Therefore, receivers in that region do not experience any STBC gain, but a simple repetition gain having a lower capacity than provided by the Alamouti scheme as shown above. The SFN is thus generally designed around this region. If the gain of this region can be improved, then the overall design of the SFN can be relaxed.

Since border regions are not changed and since there will always be border regions, it is proposed according to the present invention to change the allocation of transmitters to antennas of the STBC, preferably in time, in frequency or in both.

Fig. 9 shows a block diagram illustrating a data transmission system including a first embodiment of the data transmission network according to the present invention. The system is basically identical to the system shown in Fig. 6, but now comprises a common controller 16 between the space-time encoder 11 and the modulators 12, 13 and at least one additional modulator 17 and transmitter 18.

The controller 16 is adapted for directing the channel data streams Cl, C2 outputted from the space-time encoder 11 to the respective transmitters 14, 15, 18 (via their modulators 12, 13, 17) such that all channel data streams Cl, C2 are simultaneously transmitted and that at least one of the channel data streams is simultaneously transmitted by more than one transmitter. For instance, the directing can be controlled such that channel data

stream Cl is transmitted by transmitters 14 and 15, and channel data stream C2 is transmitted by transmitter 18. However, according to the present invention this allocation of the channel data streams to the antennas is not fixed (in time and/or frequency) but is changed by the controller, so that, for instance, in another period and/or frequency sub-channel the channel data stream Cl is transmitted by transmitter 14, and channel data stream C2 is transmitted by transmitters 15 and 18.

Fig. 10 shows a block diagram illustrating a data transmission system including a second embodiment of the data transmission network according to the present invention. The system is basically identical to the system shown in Fig. 9, but now comprises separate controllers 161, 162, 163 for each particular transmitter 14, 15, 18. Different from the embodiment shown in Fig. 9 the controllers 161, 162, 163 are provided with the channel data streams Cl, C2 and then control which bits from the channel data streams are transmitted in which OFDM symbol and in which frequency. Preferably, an additional master controller (not shown) for controlling the controllers 161, 162, 163 is additionally provided.

In a further embodiment, instead of a single common Alamouti encoder 11 it is possible to have separate Alamouti encoder for each transmitter. Also hybrid embodiments are possible, in which, for instance, groups of transmitters have their own Alamouti encoders and/or controllers.

A third embodiment of the SFN scenario according to the present invention, according to which the allocation of the transmitters to two antennas is changed in time, is shown in Fig. 11. According to this embodiment, the allocation of the transmitters TxI, Tx2, Tx3 to the two antennas Al, A2 is different for odd blocks and even blocks of the STBC. For instance, the following two different allocations are applied:

Odd blocks: TxI <-> Al, Tx2 <-> A2, Tx3 <-> Al

Even blocks: Txl <-> Al, Tx2 <-> Al, Tx3 <-> A2 This leads to the following gains:

Regions Gain (odd blocks) Gain (even blocks)

A(1,2,3) STBC STBC

A(1,2) STBC no STBC

A(2,3) STBC STBC

A(1,3) no STBC STBC

As shown in Fig. 11, the change of the allocation in this example is equivalent to swapping the rows of the second block of the STBC compared to the first block. On average, the worst case scenario has improved with this assignment.

A fourth embodiment of the SFN scenario according to the present invention, according to which the allocation of the transmitters to two antennas is changed in frequency, is shown in Fig. 12. According to this embodiment, the allocation of the transmitters TxI, Tx2, Tx3 to the two antennas Al, A2 is different for even sub-carriers and odd sub-carriers of the OFDM (or multi-carrier) system. Most of the broadcast systems use OFDM (DVB), which is going to be used also in DVB-T2 and which provides orthogonal sub-carriers.

According to this embodiment, within the same OFDM symbol a first set of sub-carriers, in which a transmitter acts as an antenna of the STBC, and another set of sub- carriers, in which the same transmitter acts as the other antenna of the STBC, is used. An example of a possible allocation is as follows:

TxI : Odd sub-carriers: antenna 1 Even sub-carriers: antenna 1

Tx2: Odd sub-carriers: antenna 2 Even sub-carriers: antenna 2

Tx3: Odd sub-carriers: antenna 1 Even sub-carriers: antenna 2

The sub-carriers are obtained via OFDM modulation. OFDM modulation allows the transmitter and receiver to see the channel as a set of orthogonal sub-carriers. When STBC is applied in presence of OFDM (as it is in most of the cases, if not all), it is basically applied sub-carrier by sub-carrier. This means that, usually, S ₁ is transmitted on one sub-carrier, -S ₂ * in the same sub-carrier of the following OFDM symbol, S ₂ in the same sub- carrier of the other antenna, S ₁ * in the same sub-carrier of the other antenna of the following OFDM symbol. This is then repeated for each sub-carrier. According to the present invention the mapping of the STBC to sub-carriers within the same OFDM symbol is varying, so that the receiver can always experience a sub-set of sub-carriers with a STBC gain. This leads to the following gains:

Regions Gain

A(1, 2) STBC

A(2,3) STBC/2+ "no STBC'72

A(1, 3) "no STBCV2+STBC/2

The present invention deals with the general problem of an STBC with two antennas and L transmitters in the SFN, and solves this problem by assigning different roles in the STBC (i.e. to act as different antennas) to any transmitter in the SFN.

An embodiment of a general solution is illustrated in Fig. 13 showing a matrix C having - as an example - four rows representing four time/frequency blocks and eight columns representing eight transmitters. The elements cfj of this matrix C thus indicate that in time/frequency block i (i being an integer from 1 to 4) the transmitter y (j being an integer from 1 to 8) acts as STBC antenna cij. As can be seen the assignment of the transmitters to the two antennas of the STBC is different for each of the four time/frequency blocks.

The matrix C is built such that the columns differ in many positions, i.e. the two transmitters often have different roles in the STBC. This corresponds to building the matrix C from columns of an error correcting code with a large minimum Hamming distance.

Each transmitter can use a distinct column, i.e. the order of columns of C is irrelevant. If less than L (in this example L=8) transmitters are used, it is possible to use an advantageous sub-set of columns of C. For example, with two transmitters, columns 1 (1111) and 4 (2222) are preferred so that they always act as different STBC antennas.

Using coding theory, it can be concluded that the matrix C is optimum. By increasing the number of time/frequency blocks, better combinations are possible. However, the gain is upper bounded as follows from results in coding theory. In particular, results from coding theory show the optimum solution for L=2 ^m transmitters: the Simplex code of length L-I has L words and minimum Hamming distance L/2 (cf. F.J. Mac Williams and N. J. A. Sloane, "The Theory of Error-Correcting Codes", Elsevier, 1977, Ch. 1, Sec. 9); the Plotkin bound (cf. Ch. 2, Sec. 2 of the publication of Mac Williams), implies that for a code of length n with minimum distance d and L codewords, L < 2d/(2d-n), so d/n < L / (2L- 1).

As mentioned, the matrix C shown in Fig. 13 is an example, but with four blocks, no better result can be obtained than that. The matrix C is built like this. The first three rows represent all the possible configurations of the STBC with two antennas in the three blocks used by the eight transmitters (2 ³ =8). The fourth block (last row) is then used to get an even number of "1" or "2" per column to ensure the maximum possible STBC gain. Getting the even number of "1" or "2" is equivalent to add a simple parity check bit.

Even more generally, the matrix C is a binary b x T matrix, where b is the number of blocks and T is the number of transmitters in the STBC. The columns of the matrix are selected (designed) such that the minimum numbers in which two distinct columns differ is large. It can be started with a binary error-correcting code of length b with T words and a large minimum Hamming distance, and these words can be taken as the columns of C. In the specific example, a code of length b=4 with T=8 words and minimum Hamming

distance equal to 2 is used, and it is not possible to have a larger minimum distance with b=4 and T=8.

This embodiment ensures that for any pair of transmitters of the SFN, the transmitters play the role of different antennas in the STBC relatively often. As shown above error-correcting codes can be used to generate such an advantageous embodiment if the STBC has two antennas. If the number L of transmitters in the SFN is a power of two, then it can be assured that any pair of transmitters plays a different role in a fraction L/2(L-1) of the cases by employing the so-called simplex code of length L-I, and it can be shown that no larger fraction can be obtained.

In the case of L=8 antennas, the optimum can be achieved by using 7 blocks (in time and/or frequency) and it provides a STBC gain in at least four of the 7 available blocks. According to Fig. 13 it is proposed to use 4 blocks instead of 7. With four blocks it can be ensured that there is a STBC gain in at least 2 of the available 4 blocks. This is slightly worse than 4 out of 7, but having the number of blocks equal to a power of 2 gives significant advantages in terms of scheduling.

In summary, STBCs (in particular Alamouti codes) are likely to be used in DVB-T2 SFN. The direct deployment of distributed STBCs in SFN creates regions with different capacities. According to the present invention an approach has been proposed to improve the worst-case capacity region so that the SFN design can exploit the STBC gain. There are many possible mappings of STBC to SFN transmitters which can be realized without departing from the general idea of the present invention, not to use a fixed mapping of transmitters to the antennas.

The present invention solves a problem generated by the use of STBC in a distributed fashion in an SFN, where the multiple transmitters in the SFN do not transmit the same data, but they act as different antennas of the STBC. If STBC is used in a conventional way, i.e. with a fixed role of the antenna with respect to the STBC, then the distributed STBC in SFN will not provide a uniform gain within the coverage area. This problem has been solved by changing the STBC role of the transmitters in the SFN. The changing role can be achieved in time domain or in frequency domain.

In principle any SFN with distributed STBC can use the present invention. For instance, in 3GPP LTE (long term evolution) a broadcast mode is defined where the base stations act in an SFN fashion and in which the invention could be applied. Further, the invention can be used in DVB-T2, if DVB-T2 will support distributed STBC.

The invention can be used, for instance, in a SFN as described in European patent application 07109509.5 (PH008405) using a distributed pilot scheme. In current digital TV standards, e.g. DVB-T, the transmitters in the SFN transmit the exact same signal in the same frequency and at the same time. As a consequence, the channel seen by the receiver is the channel created by the SFN characterized by a very long delay spread, which can be challenging to estimate. According to this application, different, preferably orthogonal, pilot sequences are assigned to different transmitters in the same SFN so that the receiver can estimate the propagation channels from each transmitter to itself independently.

Further, the invention can be used, for instance, together with an Alamouti encoder as described in European patent application 07102772.6 (PH008030). In this application an encoder is described for encoding incoming symbols of an incoming data stream into channel symbols of a channel data stream for transmission over a transmission channel as well as to a corresponding decoder. To improve the error rate compared to a known Alamouti encoder, a scaled (and further preferred, rotated) Alamouti encoder is proposed comprising: mapping means for block by block mapping incoming symbols onto pairs of channel symbols, a block comprising two incoming symbols, the mapping being arranged for mapping the block onto two pairs of channel symbols such that said two pairs of channel symbols include scaled versions of said two incoming symbols and/or of the complex conjugate of at least one of said two incoming symbols, said scaled versions being obtained by applying a scaling function having a scaling factor with an absolute value different from one and being piece-wise linear with at least two pieces, and output means for outputting said channel symbols.

Details of the distributed pilot scheme for SFN and of the scaled and rotated Alamouti encoder can be found in the above mentioned European patent applications, the details of which are herewith incorporated by reference.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain

measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.