In wireless communications, transmit diversity techniques are used in order to mitigate the detrimental effects of fading. A simple transmit diversity technique is delay di- versity where the same signal is transmitted from multiple antennas with different delay. This results in an equiva- lent of input channel with increased frequency selectivity, and therefore, increased frequency diversity-compared to the original sub-channel from each transmit to each receive antenna. In a orthogonal frequency division multiplexing (OFDM) frequency diversity introduced in a transmitter can be explci d by a forward error correcting decoder arranged in a receiver.

However, introducing additional delay in multicarrier transmission systems, which is often required for achieving a time diversity, requires a longer guard interval which, consequently, results in a reduced bandwidth efficiency. If the guard interval is not sufficiently long, inter-carrier interference may-occur. However, increasing the length of the guard interval results in a reduced bandwidth effi- ciency, since the guard interval cannot be applied to in- formation transmission.

An increased frequency selectivity without exceeding the guard interval can be achieved introducing a cyclic delay diversity, as described in: A. Dammann and S. Kaiser, "Standard conformable antenna diversity techniques for OFDM systems and its application to the DVB-T system", IEEE Globecom, pages 3100-3105, November 2001, in: A. Dammann and S. Kaiser,"Low complex standard conformable antenna diversity techniques for OFDM systems and its application to the DVB-T system", 4th International ITG Conference on Source and Channel Coding, pages 253-259, January 2002, and in: A. Dammann, R. Raulefs, and S. Kaiser,"Beamforming in combination with space-time diversity for broadband OFDM systems", IEEE Conference on Communications (ICC), pages 165-171, April 2002. In accordance with the teachings of the above documents, a delay is introduced in a cyclic man- ner such that the guard interval is not exceeded.

Transmit diversity techniques are traditionally applied in a multi-user scenario in order to transmit a signal through a plurality of channels, wherein the transmit signal com- prises a plurality of signal streams associated with a plu- rality of users. At a receiver, the user streams are sepa- rated by explicitly exploiting the transmit diversity pro- vided by a processing of the transmit signal.

In order to generate transmit diversity, cyclic delay di- versity may be applied for transmitting data simultaneously from different transmit antennas, wherein each data stream associated with a transmit antenna has a delay with respect to the other data streams. At the receiver, a single re- ceive antenna or a plurality of receive antennas can be ap- plied.

In order to efficiently exploit the available bandwidth, the characteristics of the applied transmit diversity tech-

nique should be taken into account for effectively exploit- ing the full spatial diversity. For example in a multi- carrier transmissions scenario, the applied transmit diver- sity technique may have an influence on correlation proper- ties between sub-carriers at the receiver. As mentioned be- fore, the spatial diversity can be transformed into the frequency diversity which is can be picked up by the de- coder.

In a broadband orthogonal frequency division multiple ac- cess system (OFDMA), the available bandwidth is shared by several users. Usually, a block frequency interleaver is applied, i. e. the transmit symbols of a user are assigned to sub-carriers with an equal spacing in order to exploit frequency diversity such that neighbouring sub-carriers are allocated to different users. However, in combination with the previously mentioned cyclic delay diversity, where spa- tial diversity is reflected in uncorrelated channel coeffi- cients of neighbouring sub-carriers, and hence in uncorre- lated carriers, a frequency block interleaving may result in a complete failure to exploit spatial diversity. Hence, the available bandwidth is not efficiently exploited, since, for a user stream, an increased bit error rates oc- curs after decoding.

In order to improve the system's performance, a more com- plex coding scheme may be applied for coding the different user streams by introducing an increased redundancy. How- ever, this approach suffers form the fact, that the in- creased redundancy reduces the bandwidth efficiency. The spatial diversity, when considering the standard diversity techniques, may also be exploited by increasing a number of transmit and receive antennas. However, especially in a multi-user scenario, the increased number of receive anten-

nas would increase the complexity of the overall system, in particular a complexity of a mobile receiver.

A further possibility of improving the systems performance of a conventional multi-carrier transmission system using diversity techniques is to increase a length of a guard in- terval in order to reduce interferences. However, this ap- proach suffers from a reduced bandwidth efficiency.

Fig. 14 shows a conventional OFDMA system (OFDMA = orthogo- nal frequency division multiple access). The OFDMA system of Fig. 14 comprises a plurality of FEC encoders 1401 hav- ing an input and an output, wherein the respective outputs of the plurality of the FEC encoders 1401 are connected to a plurality of interleavers 1403. Each of the plurality of the interleavers 1403 has an output connected to an associ- ated mapper 1405 for mapping the discrete values provided by the respective interleaver 1403 on signals space repre- sentatives according to a chosen signal space constellation scheme (modulation). In Fig. 14, the mapper 1405 performs for example a QAM (QAM = quadrate amplitude modulation) or PSK (PSK = phase shift keying) modulation. Each of the plu- rality of mappers 1405 has an output connected to a inter- leaver 1407 having a plurality of outputs connected to a IFFT block 1409 in (IFFT = inverse fast Fourier transform).

The IFFT block 1409 has a plurality of outputs 1411 for providing a multi-carrier modulated signal.

Each of the FEC encoders 1401 receives a corresponding user signal, wherein in Fig. 14 user 1 and user U are depicted.

In a conventional OFDMA system of Fig. 14, the available sub-carriers are allocated to a particular user by using the interleaver 1407 for distributing the symbols of a user signal on sub-carriers with a fixed spacing in order to ex- ploit diversity. If cyclic delay diversity is applied, such

a block interleaving may introduces an error since the cor- relation properties of sub-carries are not taken into ac- count. In Fig. 14, the sub-carriers s = 0,3, 7,... are allocated to user 1. However, user 1 can not completely pick up the spatial diversity, when the applied spatial di- versity technique introduces a correlation between the car- riers allocated to user 1. A further disadvantage of the system of Fig. 14 is that the frequency selectivity of the channel can not be taken into account since a fixed alloca- tion scheme is used. Therefore, the conventional OFDMA sys- tems do not fully exploit the frequency diversity which re- sults in a reduced performance with respect to bandwidth efficiency.

It is the object of the present invention to provide a con- cept for an efficient multi-user multi-carrier transmission scheme.

This object is achieved by an apparatus for providing a multi-carrier modulated signal in accordance with claim 1 or by an apparatus for generating a first and a second transmit signal in accordance with claim 15 or by an appa- ratus for extracting values corresponding to a first user signal in accordance with claim 19 or by a method for pro- viding a multi-carrier modulated signal in accordance with claim 32 or by a method for generating a first and a second transmit signal in accordance with claim 33 or by a method for extracting values corresponding to a first user signal in accordance with claim 34 or by a computer program in ac- cordance with claim 35.

The present invention is based on finding, that, in a multi-carrier transmission scenario, a multi-user transmis- sion using diversity techniques can efficiently be per- formed by explicitly exploiting the correlation properties

of the sub-carriers used by the multi-carrier modulation scheme. In particular, it has been found that the spatial diversity can efficiently be exploited when a number of values corresponding to a first user signal is assigned to a set of carrier frequencies so that, at the receiver, re- spective neighbouring carriers are uncorrelated. When cy- clic delay diversity scheme is used, then the cyclic delays may be chosen such that a set of successive carriers is em- ployed for transmitting the first user signal, and a second set of successive carriers is employed for transmitting a second user signal, provided that the respective neighbour- ing sub-carriers are uncorrelated.

In accordance with the present invention, each user signal values of a plurality of users are allocated to a predeter- mined sub-carriers, for example to a set of successive car- riers or to a plurality sets of successive carriers. Hence, each user signal is allocated to one or to several sets of carriers (carrier frequencies), each set or a number of successive carriers, wherein two neighbouring carriers are uncorrelated. When using cyclic delay diversity as a spa- tial diversity scheme for multi-user transmission, the spa- tial diversity is transformed into frequency diversity at the receiver and can fully be exploited since the respec- tive two neighbouring carriers allocated to a respective user signals are uncorrelated. Simultaneously, also fre- quency diversity can also be exploited, since the sets of carriers allocated to one user or to a plurality of users can be placed within the available bandwidth independently.

Moreover, depending on the channel characteristics, for ex- ample channel attenuation, the discrete values of a user signal can interleaved or coded prior to being allocated to the respective set of successive carriers. In this way, each set of carriers may be adapted to the channel charac-

teristics freely an independently. Therefore, at the re- ceiver, the frequency diversity can fully be exploited.

It is an advantage of the present invention, that the spa- tial diversity can fully be exploited for simultaneously transmitting a plurality of multi-user signals without in- creasing the receiver's complexity. Furthermore, even a single antenna can be used at the receiver, since spatial diversity is transformed into frequency selectivity ba the inventive carrier allocation scheme. Therefore, a number of received signals to be processed can be reduced, which yields a reduction of signal processing resources required for fetching the corresponding user signal.

It is a further advantage of the present invention that a user signal can be allocated to different sub-carriers such that each user can exploit frequency diversity and spatial diversity, simultaneously.

In accordance with the present invention, the cyclic delay may be chosen such that neighbouring sub-carriers are un- correlated. An interleaving strategy ensures that the full spatial diversity can be exploited using forward error cor- rection codes of limited constrained length.

It is a further advantage of the present invention, that the inventive carrier allocation scheme is simple, so that neither a complicated, i. e. recursive computations nor time consuming system adaptation is required.

The present invention provides further an interleaving strategy which guarantees that the maximum possible diver- sity advantage over the standard approach is obtained. The inventive interleaving scheme has low complexity, since the decision with respect to the carriers to be allocated is

derived upon evaluation of channel characteristics, which, usually, are known in the multi-carrier receiver.

In order to guarantee en efficient exploitation of the spa- tial diversity also in frequency selective channels with unknown delay spread, the cyclic delay may be adapted in order to achieve the required low correlation property be- tween sub-carriers. The inventive cyclic delay scheme cause that the effective channel coefficients of neighbouring sub-carriers have a low correlation whereas not neighbour- ing sub-carriers may have high correlation, so that full spatial diversity can efficiently be exploited.

Further embodiments of the present invention are described in detail with respect to the following Fig. s, in which: Fig. 1 shows a block diagram of an inventive apparatus for providing a multi-carrier modulated signal in accordance with a first embodiment of the present invention; Fig. 2 shows a block diagram of an apparatus for provid- ing a multi-carrier modulated signal in accor- dance with a further embodiment of the present invention ; Fig. 3 shows a block diagram of an apparatus for provid- ing a multi-carrier modulated signal in accor- dance with a further embodiment of the present invention ; Fig. 4 demonstrates the inventive carrier allocation scheme;

Fig. 5 demonstrates cyclic delay diversity in a coded OFDM (transmitter); Fig. 6 demonstrates cyclic delay diversity in a coded OFDM (receiver); Fig. 7A demonstrates a fading channel and the correspond- ing encoded error rate; Fig. 7B shows the channel of Fig. 7a transformed by cy- clic delay diversity and the corresponding en- coded error rate; Fig. 8A shows a model of a communication channel; Fig. 8B shows an equivalent SIMO channel model using cy- clic delay diversity; Fig. 8C shows an equivalent SIMO channel model using a further cyclic diversity; Fig. 9 shows a channel frequency response; Fig. 10 demonstrates a correlation function for cyclic delay diversity with two transmit antennas in ac- cordance with the present invention; Fig. 11 shows a correlation function for cyclic delay di- versity using two transmit antennas in the case of a frequency selective channel; Fig. 12 shows an apparatus for generating a first and a second transmit signal in accordance with the present invention;

Fig. 13 shows a block diagram of an apparatus for ex- tracting values in accordance with the present invention; Fig. 14 shows a block-diagram of a conventional OFDM scheme.

Fig. 1 shows a block diagram of an inventive apparatus pro- viding a multi carrier modulated signal from a first user signal and from a second user signal in accordance with a first embodiment of the present invention.

The apparatus of Fig. 1 comprises an input 101 for receiv- ing the first user signal and receiving the second user signal. The input 101 comprises a first input terminal 103 and a second input terminal 105, both arranged in parallel.

The first input terminal 103 has an input 107 for receiving the first user signal and an output 109 for outputting val- ues of the first user signal. The second input terminal 105 has an input 111 for receiving the second user signal and an output 113 for outputting values of the second user sig- nal.

The outputs 109 of the first input terminal 103 and 113 of the second input terminal 115 are coupled in parallel to an assigner 115. The assigner 115 has a plurality of outputs coupled to a multi-carrier modulator 117 having an output 119 for outputting a multi-carrier modulated signal.

Fig. 1 demonstrates the inventive approach, wherein two us- ers of a plurality of users is depicted. The first user signal provided to the first input terminal 103 is associ- ated with a first user and the second user signal provided to the second input terminal 105 is associated with a sec- ond user. The term"user"describes a scenario, where the

user signals are assigned to physical entities. Alterna- tively, the term"user"may define a scenario, where the different user signals are associated with different ser- vices, for example an e-mail or a video stream, to be transmitted. The first input terminal 103 operates only on the first user signal, and the second input terminal 105 operates only on the second user signal. In other words, the different signal streams associated with different user signals are separately processed and the processing is per- formed in a parallel manner.

The first input terminal 103 receives the first user signal and outputs values or processed values of the first user signal denoted by [x (l),..., x (n) ], where n denotes a num- ber of values or processed values of the first user signal.

The second input terminal 105 operates similarly. The sec- ond user signal is transformed into values of the second user signal denoted by [y (l),..., y (m) ], wherein m denotes a number of values of the second user signal or the number of processed values of the second user signal. For the sake of generality, it is assumed, that the number of values of the first user signal or the number of values of the processed first user signal provided by the first input terminal 103 differs from the number of values or processed values of the second user signal provided by the second input termi- nal 105. The values of the first user signal and the values of the second user signal (or processed versions thereof) are provided to the assigner 115 which is operative for as- signing the values of the first user signal or processed version thereof to a first set of carriers having succes- sive carrier frequencies and for assigning values of the second user signal or processed values of the second user signal to a second set of carrier having successive carrier frequencies. The successive carrier frequencies are deter- mined by a number of carriers used by the underlying multi-

carrier modulation scheme. The number of carriers defines in available bandwidth for signal transmission.

The assigner 115 is operative to allocate the first set of carriers and the second set of carrier independently within the available bandwidth. As is depicted in Fig. 1, the val- ues of the second user signal are assigned to the first set of carrier having successive carrier frequencies starting with a first carrier frequency. Contrary thereto, the val- ues of the first user signal are assigned to the second set of carriers having successive carrier frequencies starting with a second carriers frequency, which may be higher or lower than the first carrier frequency. Alternatively, the values of the first user signal may be assigned to the first set of carriers and the values of the second user signal may be assigned to the second set of carriers. The outputs of the assigner provide a multi-carrier signal in- cluding values of the first user signal or processed values of the first user signal allocated on successive carrier frequencies of the second set of carriers and the values of the second user signal allocated to successive carrier fre- quencies of the first set of carriers.

The multi-carrier modulator 117 simultaneously modulates the multi-carrier signal, e. g. the values assigned to the first set of successive carrier frequencies and values as- signed to the second set of successive carrier frequencies.

After modulation, a multi-carrier modulated signal provided via the output 119. If the multi-carrier modulated signal of Fig. 1 is a base band multi-carrier modulated signal, then the output 119 may be coupled via an analog/digital converter for providing a band pass signal.

Fig. 2 shows a block diagram of an apparatus for providing a multi-carrier modulated signal in accordance with a fur- ther embodiment of the present invention.

In Fig. 2, a multi user scenario with a plurality of users is considered, wherein two user signals are explicitly de- picted. The apparatus of Fig. 2 comprises the first input terminal 203 and second input terminal 205 arranged in par- allel. The first input terminal 203 comprises a first en- coder 207 having an output connected to a first interleaver 209. The first interleaver 209 has an output connected to a mapper 211 having an output 213. The first input terminal determines a first signal path wherein a second signal path, parallel to the first one, is determined by the sec- ond input terminal 205.

The second input terminal 205 comprises a second encoder 215 having an input and an output, wherein the output of the second encoder 215 is coupled to a second interleaver 217. The second interleaver 217 has an output coupled to a second mapper 219 having an output 221.

The outputs 213 of the first input terminal and 221 of sec- ond terminal, representing the first and the second signal path associated with a first and a second user of a plural- ity of users, are coupled to an assigner 223. The assigner 223 has a plurality of outputs coupled to a multi-carrier modulator 225 having an output 227.

The first encoder 207 may be a forward error correction en- coder (FEC encoder) for encoding the values of the first user signal and for providing the encoded values of the first user signal as the values of the first user signal.

The second coder 215 (FEC encoder) is operative for encod- ing the values of the second user signal to provide the en-

coded values of the second user signal as the values of the second user signal.

The first coder 207 and the second coder 215 perform an en- coding scheme introducing redundancy into the respective user signal. The first and the second coders, 207 and 215, may be operative to perform for example a Reed Salomon (RS) coding or a convolutional coding. The encoded values pro- vided by the first coder 207 are provided to the first in- terleaver 209 for interleaving the values of the first user signal and for providing the interleaved values of the first user signal as the values of the first user signal.

Accordingly, the second input terminal 205 comprises the second interleaver 217 for interleaving the values of the second user signal in for providing the interleaved of the second user signal as the values of the second user signal.

Both interleaver 209 and 217 may be block interleavers.

The first input terminal 203 further comprises the mapper 211 for mapping successive values of the first user signal on a number of successive signal space representatives of a predetermined signal space constellation for obtaining the number of successive signals space representative of the first user signal as processed values of the first user signal. Accordingly, the second input terminal 205 com- prises the mapper 219 for mapping successive values of the second user signal on a number of successive signal space representatives of a predetermined signal space constella- tion to obtain the number of successive signal space repre- sentatives of the second user signal as processed values of the second user signal.

The predetermined signal space constellations used by the respective mapper 211 and/or 219 belong to a PSK scheme.

QAM scheme (QAM = quadrature amplitude modulation) or to a PAM scheme (PAM = pulse amplitude modulation). Hence, the successive values of the first user signal provided by the interleaver 209 are firstly divided into a number of groups containing successive values, and, secondly, each group consisting of for example four values is mapped on a signal space representative describing i. e. a phase and a ampli- tude of a resulting vector in the signal space. Each of the mapper 211 and/or 219 provides the number of successive signal space representatives as the number of processed values of the first user signal and as the number of proc- essed values of the second user signal. If a complex valued signal space constellation, for example QAM, is used, then each processed values may be complex, that means that each processed value contains two values, the first one charac- terising a real part and the second one characterising an imaginary part of the considered processed value.

It is to be remarked at this point that the interleaver 209 and/or 217 are optional. Also the coders 207 and 215 are optional, since coding an interleaving may be performed somewhere else, so that the first user signal and the sec- ond user signal depicted in Fig. 2 are received as already coded and interleaved user signals.

The assigner 223 operates similarly to the assigner 215 of Fig. 2. In addition, the assigner 223 of Fig. 2 assigns processed values provided by the mapper 211 to a plurality of sets of successive carriers, wherein a total number of carriers comprised by the plurality of sets of successive carriers is equal to the number of processed values associ- ated with the first user signal. In Fig. 2, two sets of successive carriers, set A and set B, each of which con- sisting of successive carriers, are depicted. For example, the assigner 223 is operative to assign a first number of

processed values to the carriers of set A and second number of processed values of the first user signal to set B. In order to exploit frequency diversity and to take channel fluctuations into account, set A and set B can freely be placed within the available bandwidth determined by the carrier frequencies of the used multi-carrier modulations scheme. The first number of processed values and the second number of processed values may be different. Alternatively, both numbers of processed values may be equal, so that set A and set B comprise the same number of carrier frequen- cies.

Accordingly, the assigner 223 allocates a first number of processed values provided by the mapper 219 to, for example set C of successive carriers and a second number of proc- essed values of the second user signal to set D of succes- sive carriers.

Each output of the plurality of outputs of the assigner 223 provides a signal space representative as a processed value to the multi-carrier modulator 225. In case of an OFDM transmission scheme, the multi-carrier modulator 225 is op- erative to perform an inverse fast Fourier transform (IFFT) in order to transform the frequency domain signal provided by the assigner 223 into a time domain signal provided by the output 227 of the multi-carrier modulator 225. In this case, the time domain signal is the multi-carrier signal to be transmitted. The multi-carrier modulator 225 may be op- erative to perform an inverse Fourier transform (IFT), an inverse fast Fourier transform (IFFT) or an inverse dis- crete Fourier transform (IDFT) to obtain the multi-carrier modulated signal.

In order to pick up spatial diversity, the apparatus of Fig. 2 allocates groups of for example S neighbouring sub-

carriers to each user, wherein S denotes the first number and the second number when both numbers are equal. The num- ber S of sub-carriers per group is determined by the number of different channel states which will be described later.

An additional group-wise frequency interleaving performed by the interleavers 209 and 217 enables an exploitation of frequency diversity due to the frequency selectivity of the channel.

In Fig. 3, an apparatus for providing a multi-carrier sig- nal in accordance with a further embodiment of the present invention is shown. For convenience, components in Fig. 3 that are similar to previously described components., are as- signed the same reference numerals, and different compo- nents are assigned different reference numerals.

Unlike the apparatus of Fig. 2, the apparatus shown in Fig.

3 comprises a first selector 301 coupled to the output of the decoder 307. The selector 301 has a first output 303 and a second output 305. The first output 303 is coupled to a interleaver 307 and the second output 305 is coupled to a further interleaver 309. The interleaver 307 has an output coupled to a mapper 311, and the further interleaver 309 has an output coupled to a further mapper 313. The mapper 311 has an output 315 and the mapper 313 has an output 317.

Accordingly, the output of the decoder 205 is coupled to a second selector 309 having a first output 321 and a second output 303. The first output 321 is coupled to an inter- leaver 325, and the second output 323 is coupled to an in- terleaver 327. Each of the interleavers 325 and 327 has an output, wherein the output of the interleaver 325 is cou- pled to a mapper 329, and wherein the output of the inter- leaver 327 is coupled to a mapper 331. The mapper 329 has an output 333 and the mapper 331 has an output 335.

The outputs 325,327, 333 and 335 of the respective mapper are coupled to an assigner 337 having a plurality of out- puts coupled to the multi-carrier modulator 225 having the output 227.

The first decoder 207 receives the first user signal and provides decoded first user values as first user values.

The first user values are provided to the first selector 301, which is operative for providing a stream of selected values of the first user signal via the first output 303 as the values of the first user signal by selecting every Sth value of the first user signal starting with a value having an ordering index and for providing a further stream of se- lected values of the first user signal as further values of the first user signal via the second output 305 by select- ing every Sth value of the first user signal starting with a further value having a further ordering index, which is different from the first ordering index.

Accordingly, the second selector 319 provides a stream of selected values of the second user signal as the values of the second user signal by selecting every Sth values of the second user signal starting with a value having an ordering index and provides a further stream of selected values of the second user signal as further values of the second user signal by selecting every Sth value of first user signal starting with a further value having a further ordering in- dex being different from the ordering index.

In other words, the first selector 301 and the second se- lector 319 are respectively operative for proving a number of streams of selected values of the first user signal and a number of streams of selected values of the second user signal, respectively, by collecting every Sth value of the

considered user signal to an associated stream. In Fig. 3, only two streams, namely the stream and the further stream of the values of the first and second user signal are de- picted.

For example, the first selector 301 and the second selector 319 are multiplexers.

The stream provided by the first output 303 is interleaved by the interleaver 307, and the further stream provided by the second output 305 is interleaved by the interleaver 309. Accordingly, the stream and the further stream associ- ated with the second user signal are interleaved by the in- terleaver 325 and the further interleaver 327. The inter- leaver 307 is operative for interleaving the values of the first user signal associated with the stream of values, and the further interleaver 309 is operative for interleaving the further values of the first user signal associated with the further stream. The interleaved values and further val- ues provided by the interleavers 307 and 309 are separately provided to the mapper 311 and 313 as values of the first user signal and as further values of the user signal.

Similar operation is performed by the interleaver 325 and further interleaver 327 associated with a signal path as- signed to the second user.

The mappers 311,313, 329 and 339 are operative to provide a number of successive signal space representatives of the values of the first user signal as processed values of the first user signal, the further values of the first user signal as further processed values of the first user sig- nal, as values of the second user signal as processed val- ues of the second user signal and as further values of the

second user signal as further processed values of the sec- ond user signal.

It is to be noted that the first input terminal of the ap- paratus shown in Fig. 3 is constituted by the first selec- tor 301, the interleaver 307 and a further interleaver 309 and by the mapper 311 and the further mapper 313. Accord- ingly, the second input terminal comprises the second se- lector 319, the interleaver 325 and the further interleaver 327, the mapper 329 and the further mapper 331. Further- more, the mapper 311, 313,329 and 339 operate in the same way as the mapper discussed in connection with embodiment shown in Fig. 2.

The assigner 337 is operative for assigning a number of values of the first user signal (the number of successive signal space representatives provided by the mapper 311) to a first set of carrier frequencies having the number of successive carrier frequencies. Accordingly, the assigner 337 assigns the number of successive further value of the first user signal (the number of successive signals space representatives provided by the mapper 313) to a third set of carrier frequencies comprising the number of successive carriers frequencies. As depicted in Fig. 3, the first set of carrier frequencies and the third set of carrier fre- quencies, although employed for transmission of the first user signal, are placed independently form each other within the available bandwidth determined by the used car- rier frequencies of the underlying multi-carrier modulation scheme.

Accordingly, the assigner 337 assigns the number of values of the second user signal (successive signal space for presentations provided by the mapper 329) to a second set of carrier frequencies comprising the number of successive

carrier frequencies. The number of further values of second user signal (the number of successive signal space repre- sentatives provided by the mapper 331) is allocated on the number of successive carrier frequencies of the fourth set of carrier frequencies.

At a receiver, for example a convolutional code picks up the maximum diversity, if successive code bits are trans- mitted via uncorrelated sub-carriers, as discussed in con- nection with the embodiments shown in Fig. 1, 2 and 3. In accordance with the present invention, the stream of code bits is split into S streams. The bits in each stream are optionally interleaved and mapped on constellation elements of a modulation scheme, i. e. QAM. The number S of streams is given by the number of different channel states.

The apparatus shown in Fig. 3 demonstrates a transmitter exploiting the above described inventive concept. The bits of each user are encoded by a forward error correcting code. A serial to parallel converter splits the stream of code bits into S-streams, where S denotes the number of carrier frequencies comprised by a respective set of car- rier frequencies. The first bit is assigned to stream 1, the second to stream 2 etc. , wherein stream 1 and stream 2 denote the stream and the further stream mentioned before.

Within each stream, an optional bit interleaver permutes the code bits before mapping them to constellation elements of a digital modulation method such as for example QAM or PSK. The modulation symbols (signal space representations) are assigned to S neighbouring sub-carriers, where a stream s always assigned to the Sth sub-carrier within a group. A spacing between groups of a particular user may be fixed.

In this case, the assigner 223 is a group-wise frequency block interleaver. Therefore, the frequency diversity of the original channel is obtained by distributing groups

over the entire bandwidth whereas spatial diversity is ex- ploited by allocating the streams within one group (one set) which are transmitted on neighbouring sub-carriers.

Optionally, the assignment performed by the assigner 223 can be changed from one OFDM symbol (multi-carrier modu- lated signal) to the next. I. e. , sub-carriers 0,..., S-1 can be assigned to a first user (user 1) in a first OFDM symbol but to a second user (user 2) in the second OFDM symbol.

The receiver performs inverse operations to those performed by the receiver. For example, the receiver performs the re- verse interleaving operations of the transmitter.

As mentioned above, the inventive assigner may further be operative to assign the value associated with the first and/or second user to a respective set of carrier frequen- cies depending on channel characteristics, for example on a channel transfer function.

Fig. 4 shows an embodiment of a assignment of the first set of carrier frequencies and of the second set of carrier frequencies to carrier frequencies subject to a channel transfer function, which is depicted as a dashed line in Fig. 4.

The first set of carrier frequencies is allocated within a sub-bandwidth, where the channel transfer function has a low attenuation. Accordingly, the second set of carrier frequencies is allocated within a second sub-bandwidth, where the channel transfer function also has a low attenua- tion. Between the first and the second sub-bandwidth, the general transfer function is characterised by a significant attenuation. If, for example, a first set of carrier fre- quencies would be allocated within this bandwidth, then a

significant increase of a bit error ratio would result. In order to avoid this scenario, the sets of carrier frequen- cies may be allocated upon evaluation of a channel informa- tion.

In accordance with a further embodiment to the present in- vention, the inventive apparatus for providing a multi- carrier signal, as for example considered in Fig. s 1 to 4, may further comprise a means for providing channel informa- tion with respect to a channel characteristic, for example the channel transfer function, within a bandwidth deter- mined by a number of carriers of the multi-carrier modula- tion scheme. In this case, the assigner is operative to de- termine a first carrier frequency of the first set of suc- cessive carrier frequencies and/or to determine a first carrier frequency of the second set of successive carrier frequencies on the basis of the channel information.

If the channel transfer function is known, then each sub- bandwidth can be determined by for example thresholding for channel transfer function in order to determine carrier frequencies which are expected to be less disturbed by the channel influence then other carrier frequencies.

As discussed in connection with the embodiment of Fig. 3, the inventive assigner is operative to assign values of the first user signal to the first set of carriers having suc- cessive carrier frequencies, to assign further values of a first user signal to a third set of carriers having succes- sive carriers frequencies, to assign values of the second user signal to second set of carrier frequencies and to as- sign the further values of the second user signal to a fourth set of carrier frequencies having successive carrier frequencies. If a plurality of sets of carriers frequencies are to be assigned to a plurality of sub-bandwidths, then

the inventive assigner may exploit the channel information provided by the means for providing channel information for determining the first carrier frequencies of the respective set of successive carrier frequencies of the first, second, third and fourth set of carrier frequencies on the basis of the channel information.

As depicted in Fig. 3, the multi-carrier modulator 225 is operative for simultaneously modulating values assigned to the first set of successive carrier frequencies, to the second set of successive carrier frequencies, to the third set of successive carrier frequencies and to the fourth set of successive carrier frequencies to obtain the multi- carrier modulated signal. Generally speaking, the inventive assigner may assign a plurality of sets containing succes- sive carrier frequencies. This plurality of sets of succes- sive carrier frequencies is, in accordance with the present invention, simultaneously modulated in order to obtain a multi-carrier modulated signal to be transmitted, wherein the multi-carrier modulated signal contain all user streams in a superimposed manner.

In the following, the cyclic approach with respect to the inventive concept will be described in detail, wherein a multiple-input-multiple-output (MIMO) channel with NT transmit antennas and NR receive antennas will be consid- ered.

An impulse response from transmit antenna n to receive an- tenna m at time t is given by the 1 x N5 vector wherein D denotes a memory of the channel.

In order to explain the principle of cyclic delay diver- sity, reference is made to Fig. 5.

The apparatus in Fig. 5 shows the encoder 401 coupled to the interleaver 403. The output of the interleaver 403 is coupled to the inventive apparatus 501 for providing the multicarrier signal. Compared to the embodiment shown in Fig. 4, the inventive apparatus 501 includes the demulti- plexer 405, the number of interleavers 407, the number of encoders 409 and the assigner 411. The multi-carrier signal is provided to the transformer 413. The transformed signal is multiplied at the multiplying point 415 into a number of copies of the transformed signal, the number of copies cor- responding to the number of transmit antennas 425.

As depicted in Fig. 5, the signal provided via the signal path 417 is identical to the transformed signal provided by the transformer 413. Due to the shift introduced by the means 422, the copy of the transformed signal provided via the signal path 421 is shifted by one coefficient. Accord- ingly, the copy of the transformed signal corresponding to the signal path 423 is shifted by one coefficient with re- spect to the shifted copy corresponding to the signal path 421. As depicted in Fig. 5, the means 422 for introducing the delay are respectively operative to perform a left- shift. Alternatively, the means 422 may be operative to perform a right-shift. Moreover, a number of coefficients, the signals are shifted by, is variable and may be greater than 2.

Generally, the data is encoded by for example a forward er- ror correcting encoder FEC 401 and interleaved. After the (optional) interleaver 403, the code bits are modulated, e. g. on QAM (QAM=quadrature amplitude modulation) or PSK (PSK= phase shift keying) symbols. OFDM is then implemented by using the transformer 413 being operative to perform the inverse fast Fourier transform (IFFT) of size Ns, where Ns is the number of sub-carriers. The output symbols of the

IFFT 413 are denoted by xt, t = 0,..., Nus-1. Each antenna introduces a different cyclic delay Ant n = 1,..., nT, i. e. the transmit symbol from antenna in a time t is given by <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> ion)<BR> <BR> = x(t-#n)mod Ns' t = 0.-.., Ns-l, n = 1,... nT..

Prior to transmission, a cyclic guard interval (GI) is in- cluded by the respective means 419 at each transmit an- tenna.

The system of Fig. 5 is equivalent to the transmission of the sequence x = [#0,...,#Ns-1] over a frequency-selective channel with one transmit antenna yet impulse response given by to receive antenna m, m = 1,..., nR, with As depicted in Fig. 5, the transmit antennas 425 transmit signals to the receive antenna 503 considered above.

Fig. 6 shows a corresponding OFDM-receiver structure having the receive antenna 503 at which received signals are im- pinging. The received signals are then provided via a plu- rality of further processing means not depicted in Fig. 6 to a means 601 for removing the guard interval. The means 601 for removing the guard interval is coupled to a time- frequency transformer 603 inoperative to perform a fast Fourier transform (FFT). The transformed signals at an out- put of the transformer 603 are provided to a means 605 for demodulating. The means 605 for demodulating is coupled to the interleaver 607 having an output connected to a forward error correction decoder 609. Particularly, the means 605 for demodulating is operative to perform operations inverse to those performed in the transmitter.

Basically, the cyclic delay diversity transforms the multi- ple-input-multiple-output (MIMO) channel to a single-input- multiple-output (SIMO) channel having an increased fre- quency selectivity, i. e. the spatial diversity is trans- formed to frequency diversity. This effect is demonstrated in Fig. 7a and 7b.

In the upper diagram of Fig. 7a, an absolute value of the channel coefficients H (f) over a frequency is shown, wherein a flat fading channel scenario is considered. In the lower diagram of Fig. 7a, a corresponding uncoded error rate over a frequency is depicted. Since in Fig. 7a a flat fading channel is considered, the uncoded error rate fol- lows, by the way of example only, a vertical line over fre- quency.

In Fig. 7b, the transformation introduced by the cyclic de- lay diversity is demonstrated.

In the upper diagram of Fig. 7b, an absolute value of the channel coefficients is shown over frequency. Obviously, the channel has been transformed from a flat fading channel to a frequency selective channel having coefficients with increased energy and coefficients with decreased energy.

The corresponding uncoded error rate is shown in the lower diagram of Fig. 7b. As can be seen, the uncoded (bit) error rate is not constant over the sub-carriers. However, the average bit error rate for an uncoded transmission will be the same as in the case of a flat fading channel considered in Fig. 7a. Nevertheless, an outer forward error correction decoder can pick up the available frequency diversity.

The inventive carrier frequency allocation scheme is based on an efficient exploitation of channel correlation proper- ties. To be more specific, the inventive assigner assigns successive values to successive carrier frequencies,

wherein neighbouring carrier frequencies are uncorrelated or almost uncorrelated. In particular, in a OFDMA scenario, an efficient choice of cyclic delay may improve or even de- termine the correlation properties of the carrier frequen- cies, or in other word, of the effective channel frequency response.

In accordance with a further aspect of the present inven- tion, the cyclic delay should be chosen such that a FEC de- coder can exploit the full spatial diversity which is in- herent in a channel. In order to preserve the maximum di- versity level, the equivalent channel taps should not con- tain sums of taps at different delays d. This concept is depicted in Figs. 8a, 8b and 8c for a frequency selective channel with a memory D=1 and nit = 2 transmit antennas. Ts denotes a sampling instant. For 2 = 1, an equivalent chan- nel according to the previously discussed channel with three taps is obtained (Fig. 8c). However, since the second tap is a sum of two independent channel coefficients h (lm> (0) and h (2m (0), the full diversity which is inherent in the original channel can not be resolved. A cyclic delay of 2 = D + 1 = 2 yields an equivalent channel with four taps and, consequently, enables exploitation of the full diver- sity (Fig. 8c). Therefore, the cyclic delays preferably meet An > #n-1 where An is normalized to the FFT sample spacing. Ts and D is the channel memory.

However, the channel memory D can vary and is unknown to the transmitter. Therefore, the cyclic delay is preferably

as large as possible. Since D is unknown, the cyclic delay is preferably The above equation determines the inventive cyclic delay which guarantees that two neighbouring sub-carriers are un- correlated.

This inventive choice of the cyclic delays has the conse- quence that the channel coefficients Hm (d) of neighbouring sub-carriers have low correlation or, ideally, are even un- correlated. The resulting channel coefficient of the d-th sub-carrier at receive antenna m in the k-th OFDM symbol is given by is the FFT of the channel impulse response.

In order to describe the resulting scenario n, = 2 transmit antennas and a cyclic delay of A2 = will be considered in the following.

From the above follows that

Fig. 9 demonstrates a resulting channel frequency response for a cyclic delay diversity with two transmit antennas and the cyclic delay #2 = Ns/2 in the case of a flat fading channel. As can be seen from Fig. 9, the absolute values of the resulting channel frequency response alternate between a first value and a second value, wherein the second value is smaller than the first value.

Given the correlation matrix the associated correlation function R (md) for a first subcar- rier is shown in Fig. 10. It can be seen, that the values of the correlation function vary between 0 (characterizing non-correlated channel coefficients, wherein 1 denotes a relative value) and 1 (characterizing fully-correlated channel coefficients). Hence, the channel coefficients of every other subcarrier are correlated whereas neighboring subcarriers are uncorrelated. Consequently, a standard dif- ferential modulation in frequency direction without consid- ering these effects will fail.

The characteristics of the correlation function shown in Fig. 10 demonstrates the inventive concept for avoiding the problems described above. For example, in the case of two antennas, the data can be divided into two streams, which are separately differentially modulated. One of the stream is then transmitted over the even numbered subcarriers, the other one is transmitted over the odd numbered subcarriers, so that the values of the corresponding stream are always transmitted via correlated carriers, e. g. via carrier fre- quencies associated with correlated channel coefficients in a frequency domain.

Fig. 13 shows a correlation function Rld for cyclic delay diversity with two transmit antennas and cyclic delay #2 = Ns for the frequency slective channel of Fig. 12. It 2 can be seen, that the correlation function has the values characterizing the correlated channel coefficients, wherein these values are greater than the values of the correlation function characterizing the uncorrelated channel coeffi- cients.

If a cyclic delay of for A2 is chosen, then the term ej2ffin/Ns can take four different values, i. e. in a flat channel four different channels states of the resulting frequency domain channel H (m) can be observed.

Fig. 12 shows an embodiment of an apparatus for generating a first and a second transmit signal. Essentially, the ap- paratus of Fig. 12 is operative for providing the first and the second transmit signal on the basis of the multi- carrier modulated signal provided by the inventive appara- tus for providing the multi-carrier modulated signal, which apparatus has been described in detail above.

The apparatus shown in Fig. 12 comprises a means 1201 for generating the first and the second transmit signal. The means 1201 has an input 1203, which input is divided into a plurality of paths by, for example, a divider not explic- itly depicted in Fig. 12. For the sake of simplicity, Fig.

12 shows a first path 1205, a second path 1207 and a fur- ther path 1209 of the plurality of paths.

The means 1201 for generating the first and the second transmit signal has a plurality of outputs, each output be- ing associated with a signal path. The first signal path 1205 is directly coupled to a first output 1211. The second

path 1207 is coupled to an input of a shift element 1213, the shift element 1213 having an output connected to a sec- ond output 1215 of the means 1201 for generating the first and the second transmit signal. The further path 1209 is coupled to a further shift element 1217 having an output connected to a further output 1219 of the means 1201 for generating the first and the second transmit signal.

Each of the outputs of the plurality of outputs of the means 1201 for generating the first and the second transmit signal is associated with the transmit antenna. For gener- ating the signals to be transmitted by the plurality of the transmit antennas, each output of the means 1207 for gener- ating the first and the second transmit signal may be cou- pled to a further means for generating a high frequency signal to be transmitted.

Generally, the inventive means 1201 for generating the first and the second transmit signal from the multi-carrier modulated signal provided via the input 1203 is operative, to generate a first multi-carrier modulated signal as the first transmit signal, and to generate a cyclically shifted version of the multi-carrier modulated signal as the second transmit signal.

As depicted in Fig. 12, the means 1201 generates a plural- ity of exact copies of the multi-carrier modulated signal, each copy being associated with one of the signal path, wherein a plurality of versions of the multi-carrier modu- lated signal, each version being obtained from a copy of the multi-carrier modulated signal, may be transmitted by a total number nT of transmit antennas, wherein each version is assigned to a single transmit antenna. To be more spe- cific, the first transmit signal may be transmitted by a transmit antenna of the total number of transmit antennas,

and the second transmit signal depicted in Fig. 12 may be transmitted by the further transmit antenna of the total number of transmit antennas, wherein each antenna of the total number of transmit antennas is associated with a num- bering index. The numbering index may is equal to or greater than one and equal to or smaller than nT.

In order to generate the second transmit signal depicted in Fig. 12, the means 1201 for generating the first and the second transmit signal is operative to generate a copy of the multi-carrier modulated signal as a version of the multi-carrier modulated signal and to cyclically shift the copy of the multi-carrier modulated signal by a shift fac- tor An to obtain the second transmit signal, wherein Ns denotes a number of carriers of the multi-carrier modulation scheme and n denotes the numbering index of the further antenna associated with the second transmit signal.

The means 1201 for generating the transmit signals may be operative to generate the first transmit signal by generat- ing a copy of multi-carrier modulated signal as the version of the-multi-carrier modulated signal. This scenario is demonstrated by the path 1205, wherein the first transmit signal is an identical copy of the multi-carrier modulated signal provided by the input 1203. However, the first transmit signal may also be a cyclically shifted further copy of the multi-carrier signal by a shift factor Ak

wherein k denotes the numbering index of the transmit an- tenna applied for transmitting the first transmit signal.

In this case, the first transmit signal would be provided by the output 1219 of Fig. 12.

The shift elements 1217 and 1213 are operative for cycli- cally shifting the respective copy of the multi-carrier modulated signal, wherein the cyclic shift introduces the previously mentioned cyclic delay. For example, each copy of the multi-carrier modulated signal comprises a set of discrete values associated with numbering indices starting with a value having a lowest numbering index and ending with a value having a highest ordering index. The delay element of the plurality of the delay elements is operative for cyclically shifting the copy of the multi-carrier modu- lated signal by a number of values, or in the number of values is determined by the shift factor. The cyclical shift is performed such that the last value is placed be- fore the first value to obtain for example the second transmit signal. In other words, the shift elements are op- erative to perform right shift or left shift to introduce the desired cyclic delay. Hence, the delay elements may be for example shift registers having an input and an output coupled to the input for providing cyclic shift property.

In accordance with the present invention, the correlation between two neighbouring carriers is minimized, when the cyclic shift factor is chosen for each signal to be trans- mitted as described above. However, in order achieve de- sired the correlation property, the transmit signals may be cyclically delayed with respect to each other, so that, as mentioned before, the cyclic shift associated with the nth_ path is depending of the cyclic shift associated with the kth-path.

After transmission, a receiver receives a superposition of the signals transmitted by the plurality of antennas. After band pass/basis band conversion, a received multi-carrier modulated signal is obtained, wherein the received multi- carrier modulated signal comprises a super position of the plurality of user signals. In order to extract the user signals, the inventive receiver is operative to perform op- erations which are exact counterparts to the operations performed by the inventive apparatus discussed above.

Fig. 13 shows a first embodiment of an apparatus for ex- tracting values corresponding to a first user signal from a received multi-carrier modulated signal. The apparatus shown in Fig. 13 may be, for example, implemented in a com- munication receiver for processing the received multi- carrier signal. To be more specific, if the received multi- carrier modulated signal is formed (at the transmitter) by assigning values or processed values of the first user sig- nal to a first set of carriers having successive carrier frequencies and by assigning values of a second user signal to a first set of carriers having successive carrier fre- quencies in multi-user scenario, then the values assigned to the first set of successive carrier frequencies as the values assigned to the second set of successive carrier frequencies are simultaneously modulated using a multi- carrier modulation scheme to obtain a multi-carrier modu- lated signal to be transmitted from a plurality of trans- mitting points, each point comprising a transmit antenna.

In other words, the received multi-carrier signal results from a transmission of the multi-carrier signal provided by the inventive concepts described in detail above.

The apparatus shown in Fig. 13 comprises a multi-carrier demodulator 1313 having an input and a plurality of out-

puts, wherein the plurality of outputs is connected to a selector 1315. The selector 1315 has a further input 1317 and a number of outputs 1318.

The apparatus depicted in Fig. 13 further comprises a means 1319 for providing user indication, or in the means 1319 has an output coupled to the selector 1315 via the further input 1317 of the selector 1315.

The multi-carrier modulator 1313 is operative for demodu- lating the received multi-carrier signal comprising a set of values associated with the first set of successive car- rier frequencies and a second set of values associated with the second set of successive carrier frequencies. The multi-carrier demodulator 1313 is operative to perform a multi-carrier demodulation in accordance with a multi- carrier modulation scheme used in a transmitter. To be more specific, the multi-carrier demodulator 1313 performs an operation which is inverse to that performed by for example the multi-carrier modulator of Fig. 1.

The multi-carrier demodulator 1313 provides, after demodu- lation, the received multi-carrier signal. The selector 1315 is operative for selecting one set of the first or second set of values to obtain extracted values of the first or of the second set of values. The extracted values (the selected set of values) is then provided via the num- ber of outputs 1318 for a further processing. In Fig. 13, the selector 1315 provides the selected set of values in parallel. However, the selector 1315 may further comprise a parallel to serial converter so that the selected set of values is provided as a serial stream of values.

In order to only extract the set of values corresponding to the associated user, the selector 1315 receives a user in-

dication provided by the means 1317 for providing user in- dication, whether the first or the second user signal is to be extracted.

The means 1319 for providing user indication may be opera- tive to receive an information, which set of carriers com- prises the associated set (or a plurality of sets) of val- ues of the first or of the second user.

The first set of carriers (to which the first set of values is assigned) may comprise successive carrier frequencies starting with a first carrier frequency of the first set of successive carrier frequencies, and the second set of car- riers (to which the second set of values is assigned) com- prise successive carrier frequencies starting with a first carrier frequency of the second set of successive carrier frequencies. In this case, the selector 1315 may be opera- tive to select the first set of carriers of the first user signal by selecting successive carriers starting with a first carriers frequency of the set of carriers or to se- lect the second set of carriers of the second user signal by selecting successive carriers starting with the first carrier frequency of the second set of carriers associated with the second user signal.

The means 1319 for providing user indication may provide the first carrier frequency of the first set of carrier frequencies, when signalling that the first user signals to be extracted or to provide the first carrier frequency of the second set of carrier frequencies in order to signal that the second user signal is to be extracted.

The values assigned to the first set of successive values are mapped on successive signals space representatives of values corresponding to the first user signal, and/or if

the values assigned to the second set of successive values are successive signal space representatives of values cor- responding to the second user signal, wherein the succes- sive signal space representatives belong to predetermined signal space constellation, i. e. QAM, then the selector may further comprise a de-mapper for de-mapping the values of the first set to obtain values corresponding to the first user for de-mapping the values of the second set to obtain values corresponding to the second user. In other words, the de-mapper is operative to perform an operation which is inverse to that of the mapper shown in Fig. 2.

Accordingly, if the values corresponding to the first user and/or if the values corresponding to the second user are interleaved versions of successive values corresponding to the first user and/or to the second user, then the selector may further comprise a de-interleaver for obtaining the successive values corresponding to the first user as the extracted values corresponding to the first user or for ob- taining the successive values corresponding to the second user as the values corresponding to the second user. In other words, the de-interleaver performs an inverse opera- tion to the operation of the de-interleavers shown in Fig.

2.

If, as discussed above, the values corresponding to the first user are encoded on the basis of an encoding scheme, i. e. a convolutional code, and/or if the values correspond- ing to the second user are encode to the basis of the en- coding scheme, then the selector may further comprise a de- coder for decoding the encoded values of the first or of the second user signal. The inventive decoder performs a decoding operation which is inverse to the coding operation of the coders shown in Fig. 2.

In reference to the embodiment of Fig. 3, the received multi-carrier signal may further comprise a third set of frequencies to each further values of the first user are assigned and/or a fourth set of values associated with the fourth set of frequencies, to which further values of the second'user signal are assigned. In this case, the selector may be operative to select the third set or the fourth set of values, in order to collect the values of the first user signal, by collecting for example the fourth set of carrier frequencies in the third set of carrier frequencies or by collecting the second set of carrier frequencies and the fourth set of carrier frequencies.

According to the embodiments described above, the third set of carriers may comprise successive carrier frequencies starting with fourth carrier frequency of the third set of successive carrier frequencies, and the fourth set of car- riers may comprise successive carrier frequencies starting with a first carrier frequency of the fourth set of carrier frequencies. Accordingly, the selector is operative to se- lect the third set of carriers of the first user signal by selecting successive carriers starting with the first car- rier frequency of the third set of carriers or to select the fourth set of carrier of the second user signal by se- lecting successive carriers starting with the first carrier frequency of the fourth set of carriers. In this case, the means for providing user indication may be operative to provide the first carrier frequency of the third set of carrier frequencies when signalling that the first user signal to be extracted, or to provide the fourth carrier frequency of the fourth set of carrier frequencies when signalling that the second user signal is to be extracted.

Accordingly, the further values assigned to the third set of successive values may be mapped on successive signal space representative or further values corresponding to the

first user signal, and/or the further values assigned to the fourth set of successive values may be successive fur- ther signal space representatives of further values corre- sponding to the second user signal. As described above, the successive signal space representatives may belong to a predetermined signal space constellation, for example QAM.

Accordingly, the inventive selector may further comprise a further demapper for demapping the values of the third set to obtain values corresponding to the first user or for de- mapping the values of the fourth set to obtain values cor- responding to the second user, wherein the further demapper operate in the previously described manner.

Accordingly, the further values corresponding to the first user and/or the further values corresponding to the second user may be interleaved versions of further successive val- ues corresponding to the first user and/or to the second user. In this case, the selector comprises a further de- interleaver for obtaining further obtaining a further suc- cessive values corresponding to the first user or for ob- taining the further successive values corresponding to the first user. In other words, the further de-interleaver de- interleaves the interleaved versions by performing an op- eration which is inverse to the operation of the previously described interleaver.

Furthermore, the further values corresponding to the first user may be encoded on the basis of an encoding scheme, for example a convolutional encoding, and/or wherein the fur- ther values corresponding to the second user may be encoded on the basis of the encoding scheme. In this case, the se- lector may comprise a further decoder for decoding the en- coded values corresponding to the first or to the second user, as described above.

Referring again to Fig. 3, the values corresponding to first set of values may be a stream of selected values of the first user signal obtained by selecting every Sth value of the first user signal starting with a value having an ordering index, wherein the further values corresponding to the third set of values are a further stream of selected values of the first user signal obtained by selecting every Sth value of the first user signal starting with a value having a further ordering index, and/or wherein the second set of values is a stream of selected values of the second user signal and wherein the fourth set of values is a fur- ther stream of selected values of the second user stream, the both streams being obtained by selecting the values of the first user signal in the same way as performed with re- spect to the first user signal, wherein S is, as mentioned above, a number equal to or greater than 2, then the selec- tor may further comprise a means for collecting the values of the stream and of the further stream of the first user signal or of the second user signal to obtain the first user signal or the second user signal. In other words, the means for collecting merges the stream and the further stream such that successive values of the first stream be- come every S-th value of the first user signal or of the second user signal.

Depending on certain implementation requirements of the in- ventive method for providing a multi-carrier modulated sig- nal, for generating a first and a second transmit signal, for extracting values corresponding to a first user signal can be implemented in hardware or in software. The imple- mentation can be'performed using a digital storage medium, in particular a disk or a CD having electronically readable control signals toward thereon, which can cooperate with a programmable computer system such that the inventive method or performed. Generally, the present invention is there-

fore, a computer program product with a program code stored on a machine readable carrier, the program code performing the inventive method, when the computer program product runs on a computer. In other words, the inventive methods are, therefore, a computer program having a program code for performing the inventive methods, when the computer program runs on a computer.