Abstract:
A method for determining shifting parameters p 1  and p 2  to be used by a first and a second telecommunication devices for mapping symbols on sub-carriers. The method comprises the steps of:
       allocating to the first and the second telecommunication device sub-carriers, at least a part of the sub-carriers allocated to the first telecommunication device being allocated to the second telecommunication device,   determining the shifting parameter p 2 , the shifting parameter p 2  being even and at least equal to the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices or the shifting parameter p 2  being even and at most equal to M 2  minus the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method and a device for determining shifting parameters to be used by at least a first and a second telecommunication devices for transferring symbols. 
     More precisely, the present invention is in the field of coding and decoding schemes used in the context of MIMO (Multiple Input Multiple Output) communications especially used in conjunction of OFDM or OFDMA-like transmission schemes. 
     Orthogonal Frequency-Division Multiplexing (OFDM) is based upon the principle of frequency-division multiplexing (FDM) and is implemented as a digital modulation scheme. The bit stream to be transmitted is split into several parallel bit streams, typically dozens to thousands. The available frequency spectrum is divided into several sub-channels, and each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example PSK, QAM, etc. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other, meaning that cross talk between the sub-channels is eliminated. 
     The primary advantage of OFDM is its ability to cope with severe channel conditions, for example, multipath and narrowband interference, without complex equalization filters. Channel equalization is simplified by using many slowly modulated narrowband signals instead of one rapidly modulated wideband signal. 
     A variation called DFT spread OFDM or SC-FDMA (Single Carrier Frequency-Division Multiple Access) has been developed. In this system each symbol to be transmitted is spread over a set of transmitted frequencies by a DFT (Discrete Fourier Transform), the resulting signal is sent over a conventional OFDMA transmission system. 
     Actual implementation of coding/decoding are made either in the frequency domain or in the time domain while the implementation in the frequency domain may be preferred. 
     It is known that the use of several antennas both at the emitter and the receiver, leading to MIMO systems allows the improvement of the robustness of the transmission. This improved robustness can be used to increase the range or the bandwidth by adjusting the classical range versus bandwidth tradeoff. Several types of diversity schemes could be used to take advantage of multiple antennas at the emitter. 
     Alamouti has developed an Orthogonal Space Time Block Code (OSTBC) wherein information to be transmitted are spread in space, by the different antennas, and in time, using different time slots. The reference paper regarding Alamouti codes is “A simple transmit diversity technique for wireless communications”,  IEEE J. Select. Areas Commun. , vol. 16, pp. 1451-1458, October 1998. In a first implementation of Alamouti code, two transmit antennas (FirstAnt and SecondAnt) are used for transferring two symbols a and b in two time slots (T 1  and T 2 ). At time T 1  antenna FirstAnt transmits symbol a when antenna SecondAnt transmits symbol b. At time T 2  antenna FirstAnt transmits symbol −b* when antenna SecondAnt transmits symbol a*, where “*” denotes the complex conjugate. This Alamouti code, let us call it classical Alamouti in time, has the advantage to offer simple coding and decoding, the increased diversity leading to better performance. It is to be noted that the throughput is not increased. The optimal MAP for Maximum A Posteriori decoding is very simple, it does not imply matrix inversion, log enumeration or sphere decoding as long as the channel does not vary between T 1  and T 2  and as long as the channel can be characterized by a simple multiplication. It is naturally well combined with OFDM or OFDM-like modulation schemes. 
     A second implementation of Alamouti code called OSFBC for Orthogonal Space Frequency Block Code is based on transmission of the data over two different frequencies (F 1  and F 2 ), and not over two different time slots. With two transmit antennas (FirstAnt and SecondAnt), two symbols a and b are respectively sent on two frequencies (F 1  and F 2 ) using an antenna FirstAnt transmits symbol a when antenna SecondAnt transmits symbol b. Through the antenna FirstAnt, the symbol −b* is sent on the frequency F 1  and the symbol a* is sent on the frequency F 2  through the antenna SecondAnt. 
     The two frequencies are adjacent, to limit the variations of the channel. 
     By definition, this scheme is applied to OFDMA or OFDMA-like modulation schemes. By OFDMA-like modulations, we denote for example some frequency-domain implementation of a single carrier scheme, in which preferably, but not strictly necessarily, a cyclic prefix has been added, like for example the described DFT-spread OFDM. Compared to OSTBC, the advantage is the use of only one modulation slot, which can be advantageous from the multiplexing point of view, and may lead to better performance in case of very fast variations of the channel like high Doppler. Alamouti codes, due to their simple implementation and good performance are very attractive schemes to be used in MIMO transmission. When applied to SC-FDMA like modulation schemes, these codes do not have the valuable feature to produce signals with the low variation envelope property for each antenna, the envelope being the modulus of the complex envelope. 
     In the published patent invention WO 2008/098672, it has been proposed a method of radio data emission, by an emitter comprising at least two transmit antennas. The signal transmitted on a first antenna being considered in the frequency domain as resulting from a DFT of size M leading to the emission of a symbol on each of the M sub carriers allocated to the emitter on the first antenna. A SC(p) relation is defined by S k   SecondAnt =(−1) k+1 S* (p-1-k)mod M  for k=0 to M−1 giving the signal to be emitted on a second antenna SecondAnt from the signal S to be emitted on the first antennaFirstAnt, where p is an even shifting parameter between 0 and M−1 and k is the index of each used sub carrier in the frequency domain. 
     The use of above mentioned technique is not adapted into systems wherein plural devices like mobile stations use different bandwidths for data transmission which overlap each other. 
     The  FIG. 1   a  shows an example wherein a first emitter comprises two transmit antennas Ant 11  and Ant 12 . The signal transmitted on the antenna Ant  11  being considered in the frequency domain as resulting from a DFT of size M=8 leading to the emission of a symbol on each of the M sub carriers on the antenna Ant  11 . The SC(p) relation, with p=4, defined by X′ Ant12   k =(−1) k+1 * (p-1-k)mod M  for k=0 to M−1 gives the signal to be emitted on the antenna Ant 12  from the signal X to be emitted on the antenna Ant 11 . 
     The  FIG. 1   b  shows an example wherein a second emitter comprises two transmit antennas Ant 21  and Ant 22 . The signal transmitted on the antenna Ant 21  being considered in the frequency domain as resulting from a DFT of size M=12 leading to the emission of a symbol on each of the M sub carriers on the antenna Ant  21 . The SC(p) relation, with p=6, defined by Y′ Ant22    k =(−1) k+1 Y* (p-1-k)mod M  for k=0 to M−1, gives the signal to be emitted on the antenna Ant 22  from the signal Y to be emitted on the antenna Ant 21 . 
     When the first and second emitters transmit simultaneously data on frequency bands which overlap, for example when the first emitter transmits data on the frequency band composed of sub-carriers noted 1 to 8 of the  FIG. 1   c  and the second emitter transmits data on the frequency band composed of sub-carriers noted 0 to 11, some impairment problems occur. On the sub-carrier  5 , the couples of data (X 4 , −X 7 *) and (Y 5 , Y 0 *) are transferred but on the carrier 8, the couple of data (X 7 , X 4 *) is transferred and the couple (Y 0 , −Y 5 *) is not transferred on that carrier. 
     Such impairments lead to situation wherein the decoding of the received symbols at the receiver side is not possible. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention aims at enabling the decoding of the symbols simultaneously transmitted by plural emitters on frequency bands which overlap. 
     To that end, the present invention concerns a method for determining shifting parameters p 1  and p 2  to be used by a first and a second telecommunication devices for mapping symbols on sub-carriers, the first telecommunication device comprising at least two transmit antennas, the symbols being transferred through each antenna of the first telecommunication device on at least an even number ‘M 1 ’, strictly greater than two, of used sub-carriers allocated to the first telecommunication device, the second telecommunication device comprising at least two transmit antennas, the symbols being transferred through each antenna of the second telecommunication device on an even number ‘M 2 ’, equal or greater than M 1 , of sub-carriers allocated to the second telecommunication device,
         the first telecommunication device transferring on a first antenna of the first telecommunication device during a time slot on each frequency ‘k1’, with k1=0 to M 1 −1, a signal representing a symbol ‘X k1 ’ in the frequency domain,   the first telecommunication device transferring on a second antenna of the first telecommunication device during the time slot, on each used frequency ‘k1’, with k1=0 to M 1 −1, a signal representing a symbol ‘X′ k1 ’ derived from the symbol X k1 , for each frequency k1, by the formula X′ secondAnt   k1 =ε(−1) k1+1 X* (p     1     -1-k1)mod M     1   , where ε is 1 or −1, X* means the complex conjugate of X, p 1 −1-k1 is taken modulo M 1  and p 1  is even,   the second telecommunication device transferring on a first antenna of the second telecommunication device during a given time slot on each frequency ‘k2’, with k2=0 to M 2 −1, a signal representing a symbol ‘Y k2 ’ in the frequency domain,   the second telecommunication device transferring on a second antenna of the second telecommunication device during the same time slot, on each frequency ‘k2’, with k2=0 to M 2 −1, a signal representing a symbol ‘Y′ k2 ’ derived from the symbol Y k2 , for each frequency k2, by the formula Y′ secondAnt   k2 =ε(−1) k2+1  Y* (p2-1-k2)modM2  where p 2 −1-k2 is taken modulo M 2  and p 2  even,       

     the method comprises the steps of:
         allocating to the first and the second telecommunication devices sub-carriers, at least a part of the sub-carriers allocated to the first telecommunication device being also allocated to the second telecommunication device,   determining the shifting parameter p 2 , the shifting parameter p 2  being even and at least equal to the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices or the shifting parameter p 2  being even and at most equal to M 2  minus the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices.       

     The present invention concerns also a device for determining shifting parameters p 1  and p 2  to be used by a first and a second telecommunication devices for mapping symbols on sub-carriers, the first telecommunication device comprising at least two transmit antennas, the symbols being transferred through each antenna of the first telecommunication device on at least an even number ‘M 1 ’, strictly greater than two, of sub-carriers allocated to the first telecommunication device, the second telecommunication device comprising at least two transmit antennas, the symbols being transferred through each antenna of the second telecommunication device on an even number ‘M 2 ’, equal or greater than M 1 , of sub-carriers allocated to the second telecommunication device,
         the first telecommunication device transferring on a first antenna of the first telecommunication device during a time slot on each frequency ‘k1’, with k1=0 to M 1 −1, a signal representing a symbol ‘X k1 ’ in the frequency domain,   the first telecommunication device transferring on a second antenna of the first telecommunication device during the time slot, on each frequency ‘k1’, with k1=0 to M 1 −1, a signal representing a symbol ‘X′ k1 ’ derived from the symbol X k1 , for each frequency k1, by the formula X′ secondAnt   k1 =ε(−1) k1+1  X* (p     1     -1-k1)modM     1   , where ε is 1 or −1, X* means the complex conjugate of X, p 1 −1-k1 is taken modulo M 1  and p1 even,   the second telecommunication device transferring on a first antenna of the second telecommunication device during a given time slot on each frequency ‘k2’, with k2=0 to M 2 −1, a signal representing a symbol ‘Y k2 ,’ in the frequency domain,   the second telecommunication device transferring on a second antenna of the second telecommunication device during the same time slot, on each frequency ‘k2’, with k2=0 to M 2 −1, a signal representing a symbol derived from the symbol Y k2 , for each frequency k2, by the formula Y′ seccondAnt   k2 =ε(−1) k2+1  Y* (p2-1-k2)modM2  where p 2 −1-k 2  is taken modulo M 2  and p2 even,       

     the device for determining shifting parameters comprises:
         means for allocating to the first and the second telecommunication device sub-carriers, at least a part of the sub-carriers allocated to the first telecommunication device being also allocated to the second telecommunication device,   means for determining the shifting parameter p 2 , the shifting parameter p 2  being even and at least equal to the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices or the shifting parameter p 2  being even and at most equal to M 2  minus the number of overlapping sub-carriers allocated to both the first and the second telecommunication devices.       

     Thus, it is possible to decode the symbols simultaneously transmitted by plural emitters on same sub-carriers when convenient bandwidth allocation is being made. 
     According to a particular feature, the method comprises further step of determining the shifting parameter p 1 , the shifting parameter p 1  being even and different from the shifting parameter p 2 . 
     According to a particular feature, each sub-carrier allocated to the first telecommunication device is also allocated to the second telecommunication device, M 2  is strictly upper than M 1 , p 1  is determined as equal to null value and p 2  is determined as equal to M 1 . 
     Thus, it is possible to decode the symbols simultaneously transmitted by plural emitters on same sub-carriers. 
     According to a particular feature, M 2 =2M 1  and the method comprises a further step of transferring to the second telecommunication device an information indicating that the second telecommunication device has to use a non null shifting parameter equal to M 2  divided by two. 
     Thus, the signalling is limited. 
     According to a particular feature, the method comprises further step of transferring to the first telecommunication device an information indicating that the first telecommunication device has to use a null shifting parameter. 
     Thus, the signalling is limited. 
     According to a particular feature, the method comprises a further step of transferring to the second telecommunication device an information indicating the value of the shifting parameter to be used by the second telecommunication device. 
     According to a particular feature, each sub-carrier allocated to the first telecommunication device is allocated to the second telecommunication device and the sub-carriers allocated to the second telecommunication device and not allocated to the first telecommunication device surround the sub-carriers allocated to both the first and second telecommunication devices. 
     Thus, the sub-carrier allocation is flexible and enable many combination of allocations to the telecommunication devices. 
     According to a particular feature, information representative of the sub-carriers allocated to the first and second telecommunication devices is transferred respectively to the first and second telecommunication device. 
     According to a particular feature, the present invention is executed by a base station and the base station:
         de maps symbols on sub-carriers allocated to the first telecommunication device using the shifting parameter determined for the first telecommunication device,   de maps symbols on sub-carriers allocated to the second telecommunication device using the shifting parameter determined for the second telecommunication device.       

     According to a particular feature, each telecommunication device maps symbols on sub-carriers allocated to the telecommunication device using the shifting parameter determined for the telecommunication device. 
     According to still another aspect, the present invention concerns a computer readable medium storing program instructions or portions of code for implementing the steps of the method according to the invention, when said program instructions or portions of code are executed by a programmable device. 
     Since the features and advantages relating to the computer program are the same as those set out above related to the method and apparatus according to the invention, they will not be repeated here. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics of the invention will emerge more clearly from a reading of the following description of an example embodiment, the said description being produced with reference to the accompanying drawings, among which: 
         FIG. 1   a  represents an example of symbol mapping for a first mobile station wherein the number of sub carriers allocated to the first mobile station equals eight and the shifting parameter to be used by the first mobile station is equal to four; 
         FIG. 1   b  represents an example of symbol mapping for a second mobile station wherein the number of sub carriers allocated to the second mobile station equals twelve and the shifting parameter to be used by the second mobile station is equal to six; 
         FIG. 1   c  represents a mapping of symbols of first and second mobile stations on sub-carriers with impairment problems; 
         FIG. 2  represents a wireless cellular telecommunication network in which the present invention is implemented; 
         FIG. 3  is a diagram representing the architecture of a base station in which the present invention is implemented; 
         FIG. 4  is a diagram representing the architecture of a mobile station in which the present invention is implemented; 
         FIG. 5  illustrates the architecture of the encoder comprised in a mobile station according to a particular embodiment of the invention in frequency domain; 
         FIG. 6  illustrates the architecture of the decoder of a base station having several receive antennas according to a particular embodiment of the invention; 
         FIGS. 7   a  and  7   b  disclose an example of an algorithm executed by a base station according to the present invention; 
         FIG. 8  represents a first example of mapping of symbols of first and second mobile stations on sub-carriers according to the present invention; 
         FIG. 9  represents a second example of mapping of symbols of mobile stations on sub-carriers according to the present invention; 
         FIG. 10  represents a third example of mapping of symbols of first and second mobile stations on sub-carriers according to the present invention; 
         FIG. 11  represents a fourth example of mapping of symbols of first and second mobile stations on sub-carriers according to a variant of the present invention; 
         FIG. 12  discloses an example of an algorithm executed by each mobile station according to the present invention; 
         FIG. 13  discloses an example of an algorithm executed by the base station when the base station receives symbols from plural mobile stations according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  have already been disclosed, they will be not be described any more. 
       FIG. 2  represents a wireless cellular telecommunication network in which the present invention is implemented. 
     The present invention will be described in an example wherein the telecommunication system is a wireless cellular telecommunication system. 
     The present invention is also applicable to wireless or wired telecommunication systems like Local Area Networks. 
     In  FIG. 2 , one base station BS of a wireless cellular telecommunication network and four mobile stations MS 0 , MS 1 , MS 2 , and MS 3  are shown. 
     The base station BS is a base station of a wireless cellular telecommunication network comprising plural base stations. 
     According to the invention, the base station BS determines, for each mobile station MS located in the cell of the base station BS, the frequency band through which each mobile station MS has to transfer data. 
     More precisely, the base station BS determines, for each mobile station MS the base station BS is in charge, the sub-carriers on which the mobile station MS maps data. 
     According to the invention, the base station BS determines, for each mobile station MS i , with i=0 to 3, the base station BS is in charge, the shifting parameter p i  the mobile station has to use in order to enable the decoding, by the base station BS of the symbols transferred simultaneously by the mobile stations MS on overlapping frequency bands. 
     According to the invention, the base station BS transfers to each mobile station MS, information representative of the frequency band allocated to the mobile station MS and may transfer information representative of the shifting parameter determined for the mobile station MS. 
     According to the invention, each mobile station MS maps data on sub-carriers allocated to it according to the shifting parameter determined by the base station BS for the mobile station MS. 
       FIG. 3  is a diagram representing the architecture of a base station in which the present invention is implemented. 
     The base station BS has, for example, an architecture based on components connected together by a bus  301  and a processor  300  controlled by the program as disclosed in  FIGS. 7 and 13 . 
     It has to be noted here that the base station BS may have an architecture based on dedicated integrated circuits. 
     The bus  301  links the processor  300  to a read only memory ROM  302 , a random access memory RAM  303 , a wireless interface  305  and a network interface  306 . 
     The memory  303  contains registers intended to receive variables and the instructions of the program related to the algorithm as disclosed in  FIGS. 7 and 13 . 
     The memory  303  may comprise predefined frequency bands to allocate to mobile stations and the corresponding shifting parameters. 
     The processor  300  controls the operation of the network interface  306  and of the wireless interface  305 . 
     The read only memory  302  contains instructions of the program related to the algorithm as disclosed in  FIGS. 7 and 13 , which are transferred, when the base station BS is powered on, to the random access memory  303 . 
     The base station BS may be connected to a telecommunication network through the network interface  306 . For example, the network interface  306  is a DSL (Digital Subscriber Line) modem, or an ISDN (Integrated Services Digital Network) interface, etc. 
     The wireless interface  305  comprises means for transferring information representative of the frequency band or sub-carriers allocated to each mobile station MS and information representative of the shifting parameter determined for the mobile station MS and to be used by the mobile station MS for mapping symbols on allocated sub-carriers. 
     The wireless interface  305  comprises a decoder as disclosed in the  FIG. 6  or as disclosed in the patent application published under the reference WO 2008/098672. 
       FIG. 4  is a diagram representing the architecture of a mobile station in which the present invention is implemented. 
     The mobile station MS has, for example, an architecture based on components connected together by a bus  401  and a processor  400  controlled by the program as disclosed in  FIG. 12 . 
     It has to be noted here that the mobile station MS may have an architecture based on dedicated integrated circuits. 
     The bus  401  links the processor  400  to a read only memory ROM  402 , a random access memory RAM  403  and a wireless interface  405 . 
     The memory  403  contains registers intended to receive variables and the instructions of the program related to the algorithm as disclosed in  FIG. 12 . 
     The processor  400  controls the operation of the wireless interface  405 . 
     The read only memory  402  contains instructions of the program related to the algorithm as disclosed in  FIG. 12 , which are transferred, when the mobile station MS is powered on, to the random access memory  403 . 
     The wireless interface  405  comprises means for mapping data on sub-carriers comprised in the frequency allocated to the mobile station MS according to the shifting parameter determined for the mobile station MS. 
     The wireless interface  405  comprises an encoder as disclosed in the  FIG. 5  or as disclosed in the patent application published under the reference WO 2008/098672. 
       FIG. 5  illustrates the architecture of the encoder comprised in a mobile station according to a particular embodiment of the invention in frequency domain. 
     Data to be transmitted are coded and organized as symbols by the coding and modulation module  50  giving a set of symbols x n . Then the signal is spread in the frequency domain by the DFT (Discrete Fourier Transform) module  51 . In a variant, the DFT module is replaced by a Fast Fourier Transform module or any other processing module. 
     In case of OFDMA, DFT module may not be needed. 
     The symbols spread in the frequency domain are mapped on sub-carriers comprised in the allocated frequency band by a frequency mapping module  52  which maps data to be transferred on sub-carriers The frequency mapping module  52  may comprise zeros insertion and/or frequency shaping capabilities. The symbols Xk outputted by the frequency mapping module  52  are transformed back in the time domain by the IDFT (Inverse Discrete Fourier Transform) module  53 . 
     An optional cyclic prefix insertion module  54  can be applied before transmission through a first antenna, for example the antenna Ant 11  of the mobile station MS 1 . 
     A second antenna, for example the antenna Ant  12  of the mobile station MS 1 , is fed by data computed by the space frequency block code computation module  55  according to the shifting parameter p 1  determined for the mobile station MS 1 , leading to a new branch having IDFT module  56  and an optional cyclic prefix insertion module  57  as the IDFT module  53  and cyclic prefix insertion module  54 . 
       FIG. 6  illustrates the architecture of the decoder of a base station having several receive antennas according to a particular embodiment of the invention. 
     Several signals  67  are received from the receive antennas. The synchronization module  60  synchronizes all these received signals  67 . 
     The optional cyclic prefix removal modules  61   l  to  61   L , remove the cyclic prefix if used, in parallel to all the synchronized signals. 
     The DFT modules  62   l  to  62   L  execute a DFT on the synchronized signals on which the cyclic prefix has been removed or not. In a variant, the DFT module is replaced by a Fast Fourier Transform module or any other processing module. 
     In case of OFDMA, DFT module may not be needed. 
     L modules, possibly one complex module, of channel estimation  63   l  to  63   L  will work on the L signals and feeding one decoder module  64  comprising a L by two by two elementary space frequency block decoder serially processing the pairs of sub-carriers. An inverse DFT module  65  before a classical channel decoding module  66  treats the resulting signal. 
       FIGS. 7   a  and  7   b  disclose an example of an algorithm executed by a base station according to the present invention. 
     More precisely, the present algorithm is executed by the processor  300  of the base station BS. 
     The present algorithm is executed each time sub-carriers have to be allocated to mobile stations MS. 
     At step S 700 , the processor  300  sets the variable i to the null value. The variable i is an index indicating for which mobile station MS i  the present algorithm is executing. 
     At next step S 701 , the processor  300  sets the variable ka 0  to null value. 
     At next step S 702 , the processor  300  selects a value of the variable kb 0  to a value equal or upper than null value. 
     For example, the value of variable kb 0  is selected to be equal to null value. 
     At next step S 703 , the processor  300  sets the variable ka to the value ka 0  and sets the variable kb to the sum of variables ka 0  and kb 0 . 
     At next step S 704 , the processor  300  checks if the variable i is lower than the number Max-users of mobile stations MS the base station BS is in charge. 
     According to the example of the  FIG. 2 , Max-users equals four. 
     If the variable i is lower than the number Max-users of mobile stations MS the base station BS is in charge or handles, the processor  300  moves to step S 705 . Otherwise, the processor  300  moves to step S 780 . 
     At step S 705 , the processor  300  selects the mobile station MS i  and determines the even number of sub-carriers M 1  to be allocated to the mobile station MS i . 
     For example, the processor  300  determines that eight sub-carriers are allocated to the mobile station MS 0 . 
     At next step S 706 , the processor  300  checks if ka is strictly lower than kb. 
     If ka is strictly lower than kb, the processor  300  moves to step S 750  of the algorithm of the  FIG. 7   b.    
     If ka is not strictly lower than kb, the processor  300  moves to step S 710 . 
     At step S 710 , the processor  300  checks if ka equals kb. 
     If ka equals kb, the processor  300  moves to step S 720 . Otherwise, the processor  300  moves to step S 711 . 
     As ka=kb=0, the processor  300  moves to step S 720 . 
     At step S 720 , the processor  300  selects the mobile station MS i+1  and determines the even number of sub-carriers M i+1  to be allocated to the mobile station MS i+1 . 
     For example, the processor  300  determines that twelve sub-carriers are allocated to the mobile station MS 1 . 
     At next step S 721 , the processor  300  checks if M i  is equal to M 1+1 . 
     If M i  is equal to M i+1 , the processor  300  moves to step S 725 . Otherwise, the processor S 722 . 
     At step S 725 , the processor  300  sets the value of the variables Sc i  and Sc i+1  to the value of ka and sets the shifting parameters p i  and p i+1  to be used by the mobile stations MS i  and MS i+1  to any even value comprised between 0 and M i. −1. 
     The variables Sc i  and Sc i+1  are the index of the first sub-carriers allocated to the mobile stations MS i  and MS i+1 . 
     At next step S 726 , the processor  300  sets the variable ka to the sum of ka and M i  and increments the variable i by two. 
     After that, the processor  300  moves to step S 714  of the  FIG. 7   a.    
     As M 0  is different from M 1 , the processor  300  moves from step S 721  to S 722 . 
     At step S 722 , the processor  300  renames, if needed, the mobile stations MS i  and MS i+1  in order that M i  is strictly upper than M i+1 . 
     As M 0  equals eight and M I  equals twelve, the mobile station MS 0  is renamed as MS 1  and vice-versa and M 0  and M 1  values are exchanged. 
     At next step S 723 , the processor  300  sets the value of the variables Sc i  and Sc i+1  to the value of ka, sets the shifting parameter p i  to be used by the mobile station MS i  to M i+1  and sets the shifting parameter p i+1  to be used by the mobile station MS i+1  to the null value. 
     According to above mentioned example, Sc 0  and Sc 1  are set to null value, i.e. the subcarrier noted zero is the first sub-carrier allocated to the mobile stations MS 0  and MS 1 .The shifting parameter p 0  to be used by the mobile station MS 0  is set to eight and the shifting parameter p 1  to be used by the mobile station MS 1  is set to the null value. Such allocation is disclosed in the  FIG. 8 . 
     The shifting parameter p 0  is even and at least equal to the number of sub-carriers allocated to the mobile station MS 1 . 
     The shifting parameter p 1  may be different from the shifting parameter p 0 . 
       FIG. 8  represents a first example of mapping of symbols of first and second mobile stations on sub-carriers according to the present invention. 
     The frequency band allocated to the mobile station MS 0  comprises the sub-carriers noted 0 to 11. The frequency band allocated to the mobile station MS 1  comprises the sub-carriers noted 0 to 7. 
     The mobile station MS 0  transfers symbols on the sub-carriers noted 0 to 11. The sub-carriers noted 0 to 11 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     The mobile station MS 1  transfers symbols on the sub-carriers noted 0 to 7. The sub-carriers noted 0 to 7 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     When the first mobile station MS 0  and the second mobile station MS, transmit simultaneously data on the frequency bands allocated to them, there is no impairment problem. 
     The line  800  comprises the sub-carrier 0 on which the couples of data (X 0 , −X 7 *) and (Y 0 , −Y 7 *) are mapped. The line  807  comprises the sub-carrier 7 on which the couples of data (X 7 , X 0 *) and (Y 7 , Y 0 *) are mapped. No impairment exists for the sub-carriers 0 and 7. 
     The line  801  comprises the sub-carrier 1 on which the couples of data (X 1 , X 6 *) and (Y 1 , Y 6 *) are mapped. The line  806  comprises the sub-carrier 6 on which the couples of data (X 6 , −X 1 *) and (Y 6 , −Y 1 *) are mapped. No impairment exists for the sub-carriers 1 and 6. 
     The line  802  comprises the sub-carrier 2 on which the couples of data (X 2 , −X 5 *) and (Y 2 , −Y 5 *) are mapped. The line  805  comprises the sub-carrier 5 on which the couples of data (X 5 , X 2 *) and (Y 5 , Y 2 *) are mapped. No impairment exists for the sub-carriers 2 and 5. 
     The line  803  comprises the sub-carrier 3 on which the couples of data (X 3 , X 4 *) and (Y 3 , Y 4 *) are mapped. The line  804  comprises the sub-carrier 4 on which the couples of data (X 4 , −X 3 *) and (Y 4 , −Y 3 *) are mapped. No impairment exists for the sub-carriers 3 and 4. 
     The line  808  comprises the sub-carrier 8 on which the couple of data (Y 8 , −Y 11 *) is mapped. The line  811  comprises the sub-carrier 11 on which the couple of data (Y 11 , Y 8 *) is mapped. No impairment exists for the sub-carriers 8 and 11. 
     The line  809  comprises the sub-carrier 9 on which the couple of data (Y 9 , Y 10 *) is mapped. The line  810  comprises the sub-carrier 10 on which the couple of data (Y 10 , −Y 9 *) is mapped. No impairment exists for the sub-carriers 9 and 10. 
     The mobile station MS 1  comprises two transmit antennas which transfer M 1  equals eight symbols on sub-carriers of the frequency band allocated to the mobile station MS 1 . The symbols X 0  to X 7  are transferred through a first antenna. The symbols X 0  to X 7  are modified according to the following formula by X′ secondAnt   k =(−1) k+1 X* (p1-1-k)modM1  for k=0 to M 1 −1 and p 1 =0 giving the signal to be emitted on the second antenna from the signal X to be emitted on the first antenna. 
     The lines  800  and  807  are linked as they carry information related to same symbols X 0  and X 7 . The lines  801  and  806  are linked as they carry information related to same symbols X 1  and X 6 . The lines  802  and  805  are linked as they carry information related to same symbols X 2  and X 5 . The lines  803  and  804  are linked as they carry information related to same symbols X 3  and X 4 . 
     The second mobile station MS 0  comprises two transmit antennas which transfer M 0  equals twelve symbols on sub-carriers of the frequency band allocated to the mobile station MS 0 . The symbols Y 0  to Y 11  are transferred through a first antenna. The symbols Y 0  to Y 11  are modified according to the following formula by Y′ secondAnt   k =(−1) k+1 Y* p0-1-k)modM0  for k=0 to M 0 −1 and p 0  equals eight giving the signal to be emitted on the second antenna from the signal Y to be emitted on the first antenna. 
     The line  800  shows the couple (Y 0 , −Y 7 *) to be transmitted on the sub-carrier 0. The line  807  shows the couple (Y 7 , Y 0 *) to be transmitted on the sub-carrier 7. The lines  800  and  807  are linked as they carry information related to same symbols. 
     The lines  801  and  806  are linked as they carry information related to same symbols Y 1  and Y 6 . The lines  802  and  805  are linked as they carry information related to same symbols Y 2  and Y 5 . The lines  803  and  804  are linked as they carry information related to same symbols Y 3  and Y 4 . The lines  808  and  811  are linked as they carry information related to same symbols Y 8  and Y 11 . The lines  809  and  810  are linked as they carry information related to same symbols Y 9  and Y 10 . 
     At next step S 724  of the algorithm of  FIG. 7   a , the processor  300  sets the value of ka to the sum of ka and M 1 , sets the value of kb to the sum of kb and M i+1  and increments the variable i by two. 
     The variable ka is then equal to twelve and the variable kb is then equal to eight. 
     After that, the processor  300  returns to step S 704 . 
     At step S 704 , the processor  300  checks if the variable i is lower than the number Max-users of mobile stations MS the base station BS is in charge. 
     According to the example of the  FIG. 8 , Max-users equals two, the processor  300  moves to step S 780 . 
     According to the example of the  FIG. 2 , Max-users equals four. 
     At step S 705 , the processor  300  selects the mobile station MS i  and determines the number of sub-carriers M i  to be allocated to the mobile station MS i . 
     For example, the processor  300  determines that eight sub-carriers are allocated to the mobile station MS 2 . 
     At next step S 706 , the processor  300  checks if ka is strictly lower than kb. 
     If ka is strictly lower than kb, the processor  300  moves to step S 750  of the algorithm of the  FIG. 7   b.    
     If ka is not strictly lower than kb, the processor  300  moves to step S 710 . 
     As ka equals twelve and kb equals eight, the processor  300  moves to step S 710 . 
     At step S 710 , the processor  300  checks if ka equals kb. 
     If ka equals kb, the processor  300  moves to step S 720 . Otherwise, the processor  300  moves to step S 711 . 
     As ka equals twelve and kb equals eight, the processor  300  moves to step S 711 . 
     At step S 711 , the processor  300  checks if M i  is strictly upper than ka minus kb. 
     If M i  is strictly upper than ka minus kb, the processor  300  moves to step S 715 . Otherwise, the processor  300  moves to step S 712 . 
     At step S 712 , the processor  300  calculates a new variable Sc i  according to the following formula:
 
 Sc   i   =kb+ ( ka−kb−M   i )/2.
 
     At the same step, the processor  300  sets the variable p i  to null value. 
     At next step S 713 , the processor  300  increments the variable i by one. 
     At next step S 714 , the processor  300  sets the variable ka 0  to the value of maximum value among the variables ka and kb. 
     Such operation is equivalent at cutting the frequency band and the sub-carrier which is equal to maximum value among the variables ka and kb. 
     After that, the processor  300  returns to step S 702 . 
     As M 2  equals eight and ka minus kb equals four, the processor  300  moves to step S 715 . 
     At step S 715 , the processor  300  sets the variable Sc i  to the value of the variable kb. At the same step, the processor  300  sets the variable pi to ka minus kb. 
     At next step S 716 , the processor sets the variable kb to the sum of ka and M i . At the same step, the processor  300  increments the variable i by one. 
     According to the example of  FIG. 9 , Sc 2  is set to eight, i.e. the subcarrier noted eight is the first sub-carrier allocated to the mobile station MS 2 . The shifting parameter p 2  to be used by the mobile station MS 2  is set to four, kb is set to sixteen and i=3. Such allocation is disclosed in the  FIG. 9 . 
       FIG. 9  represents a second example of mapping of symbols of mobile stations on sub-carriers according to the present invention. 
     The lines  900  to  907  are identical to the lines  800  to  807 . 
     The frequency band allocated to the mobile station MS 2  comprises the sub-carriers noted 8 to 15. The frequency band allocated to the mobile station MS 0  comprises the sub-carriers noted 0 to 11. 
     The mobile station MS 0  transfers symbols on the sub-carriers noted 0 to 11. The sub-carriers noted 0 to 11 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     The mobile station MS 2  transfers symbols on the sub-carriers noted 8 to 15. The sub-carriers noted 8 to 15 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     When the mobile station MS 0 , the mobile station MS 1  and the mobile station MS 2  transmit simultaneously data on the frequency bands allocated to them, there is no impairment problem. 
     The line  908  comprises the sub-carrier 8 on which the couples of data (Z 0 , −Z 3 *) and (Y 8 , −Y 11 *) are mapped. The line  911  comprises the sub-carrier 11 on which the couples of data (Z 3 , X 0 *) and (Y 11 , Y 8 *) are mapped. No impairment exists for the sub-carriers 8 and 11. 
     The line  909  comprises the sub-carrier  9  on which the couples of data (Z 1 , Z 2 *) and (Y 9 , Y 10 *) are mapped. The line  910  comprises the sub-carrier 10 on which the couples of data (Z 2 , −Z 1 *) and (Y 10 , −Y 9 *) are mapped. No impairment exists for the sub-carriers 9 and 10. 
     The line  912  comprises the sub-carrier 12 on which the couple of data (Z 4 , −Z 7 *) is mapped. The line  915  comprises the sub-carrier 15 on which the couple of data (Z 7 , Z 4 *) is mapped. No impairment exists for the sub-carriers 12 and 15. 
     The line  913  comprises the sub-carrier  13  on which the couple of data (Z 5 , Z 6 *) is mapped. The line  914  comprises the sub-carrier 14 on which the couple of data (Z 6 , −Z 5 *) is mapped. No impairment exists for the sub-carriers 13 and 14. 
     The mobile station MS 2  comprises two transmit antennas which transfer M 2  equals eight symbols on sub-carriers of the frequency band allocated to the mobile station MS 2 . The symbols Z 0  to Z 7  are transferred through a first antenna. The symbols Z 0  to Z 7  are modified according to the following formula by Z′ secondAnt   k =(−1) k+1 Z* (p2-1-k)modM2  for k=0 to M 2 −1 and p 2 =4 giving the signal to be emitted on the second antenna from the signal Z to be emitted on the first antenna. 
     Once the step S 716  is executed, the processor  300  returns to step S 704 . 
     At step  5704 , the processor  300  checks if the variable i is lower than the number Max-users of mobile stations MS the base station BS is in charge. 
     According to the example of the  FIG. 2 , Max-users equals four. 
     At step S 705 , the processor  300  selects the mobile station MS i  and determines the number of sub-carriers M i  to be allocated to the mobile station MS i . 
     For example, the processor  300  determines that four sub-carriers are allocated to the mobile station MS 3 . 
     At next step S 706 , the processor  300  checks if ka is strictly lower than kb. 
     If ka is strictly lower than kb, the processor  300  moves to step S 750  of the algorithm of the  FIG. 5   b.    
     If ka is not strictly lower than kb, the processor  300  moves to step S 710 . 
     As ka equals twelve and kb equals sixteen, the processor  300  moves to step S 750 . 
     At step S 750 , the processor  300  checks if ka equals ka 0 . 
     If ka equals ka 0 , the processor  300  moves to step S 756 . Otherwise, the processor  300  moves to step S 751 . 
     As ka equals twelve the processor  300  moves to step S 751 . 
     At step S 751 , the processor  300  checks if M, is strictly upper than kb minus ka. 
     If M i  is strictly upper than kb minus ka, the processor  300  moves to step S 754 . Otherwise, the processor  300  moves to step S 752 . 
     At step S 754 , the processor  300  sets the variable Sc, to the value of the variable ka. At the same step, the processor  300  sets the variable pi to kb minus ka. 
     At next step S 755 , the processor sets the variable kb to the sum of kb and M i . At the same step, the processor  300  increments the variable i by one. 
     After that, the processor  300  returns to step S 704 . 
     As M 3  equals four and ka minus kb equals four, the processor  300  moves from step S 751  to step S 752 . 
     At step S 752 , the processor  300  calculates a new variable Sc i  according to the following formula:
 
 Sc   i   =ka+ ( kb−ka−M   i )/2.
 
     At the same step, the processor  300  sets the variable p i  to null value. 
     According to the example of  FIG. 9 , Sc 3  is set to twelve, i.e. the subcarrier noted twelve is the first sub-carrier allocated to the mobile station MS 3 . The shifting parameter p 3  to be used by the mobile station MS 3  is set to null. 
     At next step S 753 , the processor  300  increments the variable i by one. 
     The frequency band allocated to the mobile station MS 3  comprises the sub-carriers noted 12 to 15. The frequency band allocated to the mobile station MS, comprises the sub-carriers noted 8 to 15. 
     When the mobile station MS 0 , the mobile station MS 1 , the mobile station MS 2  and the mobile station MS 3  transmit simultaneously data on the frequency bands allocated to them, there is no impairment problem. 
     The line  912  comprises the sub-carrier 12 on which the couples of data (Z 4 , −Z 7 *) and (T 0 , −T 3 *) are mapped. The line  915  comprises the sub-carrier 15 on which the couples of data (Z 7 , Z 4 *) and (T 3 , T 0 *) are mapped. No impairment exists for the sub-carriers 12 and 15. 
     The line  913  comprises the sub-carrier 13 on which the couples of data (Z 5 , Z 6 *) and (T 1 ,T 2 *) are mapped. The line  914  comprises the sub-carrier 14 on which the couples of data (Z 6 , −Z 5 *) and (T 2 , −T 1 *) are mapped. No impairment exists for the sub-carriers 13 and 14. 
     The mobile station MS 3  comprises two transmit antennas which transfer M 3  equals four symbols on sub-carriers of the frequency band allocated to the mobile station MS 3 . The symbols T 0  to T 3  are transferred through a first antenna. The symbols T 0  to T 3  are modified according to the following formula by T′ secondAnt   k =(−1) k+1 T* p3-1-k)modM3  for k=0 to M 3 −1 and p 3 =0 giving the signal to be emitted on the second antenna from the signal T to be emitted on the first antenna. 
     After that, the processor  300  returns to step S 714  and returns to S 702 . 
     The processor  300  executes the steps S 702  to S 704  and moves to step S 780 . 
     At next step S 780 , the processor  300  commands the transfer to each mobile station MS of information representative of the sub-carriers allocated to the mobile station MS. 
     At next step S 781 , the processor  300  commands the transfer to at least one mobile station MS of information representative of the determined shifting parameter p determined for the mobile station MS. 
     If each sub-carrier allocated to a mobile station MS is also allocated to another mobile station MS and the number of allocated sub-carriers to the mobile station is twice lower than the number of sub-carriers allocated to the other mobile station MS, a single information, like a bit value equal to one, is transferred to the other mobile station indicating that the other mobile station MS has to use a non null shifting parameter which is equal to half the number of sub-carriers allocated to the mobile station. 
     For example a bit value equal to one is transferred to the mobile station MS 0  indicating that the mobile station MS 0  has to use a non null shifting parameter which is equal to half the number of sub-carriers allocated to the mobile station MS 1 . 
     In a variant, a single information, like a bit value equal to zero, is transferred to the mobile station indicating that the mobile station MS has to use a null value shifting parameter. 
     For example a bit value equal to null is transferred to the mobile station MS 1  indicating that the mobile station MS, has to use a non null shifting parameter. 
     If each sub-carrier allocated to a mobile station MS is also allocated to another mobile station MS and the number of allocated sub-carriers to the mobile station is lower than the number of sub-carriers allocated to the other mobile station MS, a single information is transferred to the other mobile station indicating the value of the shifting parameter to be used by the other mobile station. 
     After that, the processor  300  interrupts the present algorithm. 
     If we consider another example wherein two mobile stations MS 0  and MS 1  are handled by the base station BS and kb 0  is selected as being equal to two at step S 702 , ka is equal to null value and kb is equal to two at step S 704 . 
     The processor  300  selects at step S 705  the mobile station MSO and determines for example M 0  as equal to eight. 
     As ka is lower than kb, the processor  300  moves from step S 706  to step S 750 . 
     At step S 750 , the processor  300  checks if ka equals ka 0 . 
     If ka equals ka 0 , the processor  300  moves to step S 756 . Otherwise, the processor  300  moves to step S 751 . 
     As ka equals null value and kb equals two, the processor  300  moves to step S 756 . 
     At step S 756 , the processor  300  selects the mobile station MS i+1  and determines the number of sub-carriers M i  to be allocated to the mobile station MS i+1 . 
     For example, the processor  300  determines that eight sub-carriers are allocated to the mobile station MS 1 . 
     At next step S 757 , the processor  300  checks if M i  is lower than the sum of kb and M i+1 . 
     If M i  is lower than the sum of kb and M i+1 , the processor  300  moves to step S 761 . Otherwise, the processor  300  moves to step S 758 . 
     At step S 758 , the processor  300  sets the value of the variables Sc i  to the value of ka, Sc i+1  to the value of kb and sets the shifting parameters p i  as equal to p i =2 (kb−ka)+M i+1  and p i+1  to null value. 
     At next step S 759 , the processor  300  checks if p i , is equal or upper than M i . 
     If p i , is equal or upper than M i , the processor  300  moves to step S 770 . Otherwise, the processor  300  moves to step S 771 . 
     At step S 770 , the processor  300  sets the variables ka and kb to M i , sets p i  to p i  modulo M i , and increments the variable i by two. 
     After that the processor  300  moves to step S 760 . 
     At step S 771 , the processor  300  sets the variable kb to the sum of ka and p i , sets the variable ka to the sum of ka and M i , and increments the variable i by two. 
     After that the processor  300  moves to step S 760 . 
     At step S 760 , the processor  300  checks if ka is equal to kb. 
     If ka is equal to kb, the processor  300  moves to step S 714 . Otherwise, the processor  300  moves to step S 704 . 
     As M 0  is lower than the sum of kb and M 1 , the processor  300  moves from step S 757  to S 761 . 
     At step S 761 , the processor  300  sets the value of the variables Sc i  the value of ka, Sc i+1  to the value of kb, sets the shifting parameter p i  to be used by the mobile station MS i  to kb and sets the shifting parameter p i+1  to be used by the mobile station MS i+1  to the value M i  minus p i . 
     According to above mentioned example, Sc 0  is set to null value, Sc 1  is set to two. The shifting parameter p 0  to be used by the mobile station MS 0  is set to two and the shifting parameter p 1  to be used by the mobile station MS 1  is set to the value six. Such allocation is disclosed in the  FIG. 10 . 
     After that, the processor  300  returns to step S 704 . 
       FIG. 10  represents a third example of mapping of symbols of first and second mobile stations on sub-carriers according to the present invention. 
     The frequency band allocated to the mobile station MS 0  comprises the sub-carriers noted 0 to 7. The frequency band allocated to the mobile station MS 1  comprises the sub-carriers noted 2 to 9. 
     The mobile station MS 0  transfers symbols on the sub-carriers noted 0 to 7. The sub-carriers noted 0 to 7 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     The mobile station MS 1  transfers symbols on the sub-carriers noted 2 to 9. The sub-carriers noted 2 to 9 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     When the first mobile station MS 0  and the second mobile station MS 1  transmit simultaneously data on the frequency bands allocated to them, there is no impairment problem. 
     The line  1000  comprises the sub-carrier 0 on which the couple of data (X 0 , −X 1 *) is mapped. The line  1001  comprises the sub-carrier 1 on which the couple of data (X 1 , X 0 *) is mapped. No impairment exists for the sub-carriers 0 and 1. 
     The line  1002  comprises the sub-carrier 2 on which the couples of data (X 2 , −X 7 *) and (Y 0 , −Y 5 *) are mapped. The line  1007  comprises the sub-carrier 7 on which the couples of data (X 7 , X 2 *) and (Y 5 , Y 0 *) are mapped. No impairment exists for the sub-carriers 2 and 7. 
     The line  1003  comprises the sub-carrier 3 on which the couples of data (X 3 , −X 5 *) and (Y 1 , Y 4 *) are mapped. The line  1006  comprises the sub-carrier 6 on which the couples of data (X 6 , −X 3 *) and (Y 4 , −Y 1 *) are mapped. No impairment exists for the sub-carriers 3 and 6. 
     The line  1004  comprises the sub-carrier  4  on which the couples of data (X 4 , −X 5 *) and (Y 2 , −Y 3 *) are mapped. The line  1005  comprises the sub-carrier  5  on which the couples of data (X 5 , X 4 *) and (Y 3 , Y 2 *) are mapped. No impairment exists for the sub-carriers 4 and 5. 
     The line  1008  comprises the sub-carrier 8 on which the couple of data (Y 6 , −Y 7 *) is mapped. The line  1009  comprises the sub-carrier 9 on which the couple of data (Y 7 , Y 6 *) is mapped. No impairment exists for the sub-carriers 8 and 9. 
     The mobile station MS 1  comprises two transmit antennas which transfer M 1  equals eight symbols on sub-carriers of the frequency band allocated to the mobile station MS 1 . The symbols X 0  to X 7  are transferred through a first antenna. The symbols X 0  to X 7  are modified according to the following formula by X′ secondAnt   k =(−1) k+1 X* (p1-1-k)modM1  for k 32  0 to M 1 −1 and p 1 =2 giving the signal to be emitted on the second antenna from the signal X to be emitted on the first antenna. 
     The lines  1000  and  1001  are linked as they carry information related to same symbols X 0  and X 1 . The lines  1002  and  1007  are linked as they carry information related to same symbols X 2  and X 7  . The lines  1003  and  1006  are linked as they carry information related to same symbols X 3  and X 6 . The lines  1004  and  1005  are linked as they carry information related to same symbols X 4  and X 5 . 
     The second mobile station MS 0  comprises two transmit antennas which transfer M 0  equals twelve symbols on sub-carriers of the frequency band allocated to the mobile station MS 0 . The symbols Y 0  to Y 11  are transferred through a first antenna. The symbols Y 0  to Y 11  are modified according to the following formula by Y′ secondAnt   k =(−1) k+1 Y* (p0-1-k)modM0  for k=0 to M 0 −1 and p 0  equals 6 giving the signal to be emitted on the second antenna from the signal Y to be emitted on the first antenna. 
     The line  1000  shows the couple (Y 0 , −Y 5 *) to be transmitted on the sub-carrier 0. The line  1007  shows the couple (Y 5 , Y 0 *) to be transmitted on the sub-carrier 7. The lines  1000  and  1007  are linked as they carry information related to same symbols. 
     The lines  1001  and  1006  are linked as they carry information related to same symbols Y 1  and Y 4 . The lines  1002  and  1003  are linked as they carry information related to same symbols Y 3  and Y 2 . The lines  1008  and  1009  are linked as they carry information related to same symbols Y 6  and Y 7 . 
       FIG. 11  represents a fourth example of mapping of symbols of first and second mobile stations on sub-carriers according to a variant of the present invention. 
     According to that variant, the processor  300  sets at steps S 701  and S 702  ka equal to null value and kb to one. 
     Four sub-carriers are allocated to the mobile stations MS 1  and twelve sub-carriers are allocated to the mobile station MS 0 . 
     The mobile station MS 0  transfers symbols on the sub-carriers noted 0 to 11. The sub-carriers noted 0 to 11 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     The mobile station MS 0  transfers symbols on the sub-carriers noted 1 to 4. The sub-carriers noted 1 to 4 are the sub-carriers at the input of the frequency mapping module  52  of the  FIG. 5 . According to the frequency mapping module  52 , these subcarriers may be different from the ones provided at the output of the frequency mapping module  52  and thus at the input of the IDFT modules  53  and  56  of the  FIG. 5 . 
     When the first mobile station MS 0  and the second mobile station MS, transmit simultaneously data on the frequency bands allocated to them, there is no impairment problem. 
     The line  1100  comprises the sub-carrier 0 on which the couple of data (Y 0 , −Y 5 *) is mapped. The line  1105  comprises the sub-carrier 5 on which the couple of data (Y 5 , Y 0 *) is mapped. No impairment exists for the sub-carriers 0 and 5. 
     The line  1101  comprises the sub-carrier 1 on which the couples of data (X 0 , −X 3 *) and (Y 1 , Y 4 *) are mapped. The line  1104  comprises the sub-carrier 4 on which the couples of data (X 3 , X 0 *) and (Y 4 , −Y 1 *) are mapped. No impairment exists for the sub-carriers 1 and 4. 
     The line  1102  comprises the sub-carrier 2 on which the couples of data (X 1 , X 2 *) and (Y 2 , −Y 3 *) are mapped. The line  1103  comprises the sub-carrier 3 on which the couples of data (X 2 , −X 1 *) and (Y 3 , Y 2 *) are mapped. No impairment exists for the sub-carriers 2 and 3. 
     The line  1106  comprises the sub-carrier 6 on which the couple of data (Y 6 , −Y 11 *) is mapped. The line  1111  comprises the sub-carrier 11 on which the couple of data (Y 11 , Y 6 *) is mapped. No impairment exists for the sub-carriers 6 and 11. 
     The line  1107  comprises the sub-carrier 6 on which the couple of data (Y 7 , Y 10 *) is mapped. The line  1110  comprises the sub-carrier 10 on which the couple of data (Y 10 , −Y 7 *) is mapped. No impairment exists for the sub-carriers 7 and 10. 
     The line  1108  comprises the sub-carrier 8 on which the couple of data (Y 8 , −Y 9 *) is mapped. The line  1109  comprises the sub-carrier 9 on which the couple of data (Y 9 , Y 8 *) is mapped. No impairment exists for the sub-carriers 8 and 9. 
     The mobile station MS 1  comprises two transmit antennas which transfer M 1  equals four symbols on sub-carriers of the frequency band allocated to the mobile station MS 1 . The symbols X 0  to X 3  are transferred through a first antenna. The symbols X 0  to X 3  are modified according to the following formula by X′ secondAnt   k =(−1) k+1 X* (p1-1-k)modM1  for k=0 to M 1 −1 and p 1 =0 giving the signal to be emitted on the second antenna from the signal X to be emitted on the first antenna. 
     The lines  1101  and  1104  are linked as they carry information related to same symbols X 0  and X 3 . The lines  1102  and  1103  are linked as they carry information related to same symbols X 1  and X 2 . 
     The second mobile station MS 0  comprises two transmit antennas which transfer M 0  equals twelve symbols on sub-carriers of the frequency band allocated to the mobile station MS 0 . The symbols Y 0  to Y 11  are transferred through a first antenna. The symbols Y 0  to Y 11  are modified according to the following formula by Y′ secondAnt   k =(−1) k+1 Y* (p0-1-k)modM0  for k=0 to M 0 −1 and p 0  equals 6 giving the signal to be emitted on the second antenna from the signal Y to be emitted on the first antenna. 
     The line  1100  shows the couple (Y 0 , −Y 5 *) to be transmitted on the same sub-carrier. The line  1105  shows the couple (Y 5 , Y 0 *) to be transmitted on the sub-carrier 5. The lines  1100  and  1105  are linked as they carry information related to same symbols. 
     The lines  1101  and  1104  are linked as they carry information related to same symbols Y 1  and Y 4 . The lines  1102  and  1103  are linked as they carry information related to same symbols Y 2  and Y 3 . The lines  1106  and  1111  are linked as they carry information related to same symbols Y 6  and Y 11 . The lines  1107  and  1110  are linked as they carry information related to same symbols Y 7  and Y 10 . The lines  1108  and  1109  are linked as they carry information related to same symbols Y 8  and Y 9 . 
       FIG. 12  discloses an example of an algorithm executed by each mobile station according to the present invention. 
     At step S 1200 , the mobile station MS receives information representative of the sub-carriers allocated to the mobile station MS. 
     At next step S 1201 , the mobile station MS receives information representative of the determined shifting parameter p determined for the mobile station MS. At next step S 1202 , the symbols to be transferred are mapped on the allocated sub-carriers according to the received shifting parameter and transferred to the base station BS. 
       FIG. 13  discloses an example of an algorithm executed by the base station when the base station receives symbols from plural mobile stations according to the present invention. 
     More precisely, the present algorithm is executed by the processor  300  of the base station. 
     At step S 1300 , the processor  300  obtains information representative of the sub-carriers allocated to each mobile station MS the base station BS handles. 
     Information representative of the sub-carriers allocated to each mobile station BS the base station BS handles is as the one determined according to the algorithm disclosed in the  FIGS. 7 . 
     At step S 1301 , the processor  300  obtains information representative of the shifting parameter determined for each mobile station MS the base station BS handles. 
     Information representative of the shifting parameter determined for each mobile station MS the base station BS handles is as the one determined according to the algorithm disclosed in the  FIG. 7 . 
     At next step S 1302 , the received symbols are de-mapped on the allocated sub-carriers according to the received shifting parameters. 
     Naturally, many modifications can be made to the embodiments of the invention described above without departing from the scope of the present invention.