Abstract:
Method, receiver and computer program product for processing a signal transmitted from a plurality of spatially separated transmit antennas using a Multiple-Input Multiple-Output transmission over a wireless network. The signal is received at a plurality of spatially separated receive antennas, the signal comprising a plurality of data streams and the quality/reliability of each of the data streams in the received signal is determined. Based on the determined quality/reliability of the data streams, a decoding technique is selected to be one of (i) a successive decoding technique for successively decoding data streams in which one of the data streams is decoded and a signal corresponding to said one of the data streams is removed from the received signal prior to decoding further data streams in the received signal, and (ii) a non-successive decoding technique in which each data stream is decoded from the received signal by treating the other data streams as noise in the received signal. The received signal is then decoded using the selected decoding technique.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to decoding a signal, and in particular to decoding a signal transmitted via a Multiple-Input Multiple-Output (MIMO) transmission over a wireless network. 
       BACKGROUND 
       [0002]    Recent advances of wireless communications have led to the emergence of new multi-user communication techniques, including multi-user diversity and Successive Interference Cancellation (SIC). SIC is the optimal multiple access scheme to achieve the uplink capacity, see, e.g., D. Tse and P. Viswanath, “Fundamentals of Wireless Communication”, Cambridge University Press, 2005. In a conventional single-decoder receiver, the interference from data streams associated with other users in the network is treated as noise. Differently, in an uplink receiver employing SIC, a data stream associated to a first user is decoded and the corresponding reconstructed data stream is removed from the aggregate received signal, before the next data stream is decoded. This process is repeated for each data stream until all of the data streams in the signal have been decoded. SIC techniques can also be used on the downlink, where data streams intended for different users are simultaneously transmitted and potentially interfere with each other. Using a SIC receiver at a particular user&#39;s device, the data streams with the highest signal quality (corresponding to the lowest probability of decoding error) are decoded first and the corresponding reconstructed signals are then successively removed from the received signal, before the data stream intended for the particular user is decoded. SIC processing is described for example in M. K. Varanasi and T. Guess, “Optimum Decision Feedback Multiuser Equalization with Successive Decoding Achieves the Total Capacity of the Gaussian Multiple-Access Channel”, in Proceedings of Thirty-First Asilomar Conference on Signals, Systems, and Computers, vol. 1, pp. 1405-1409, November 1997, and in D. Tse and P. Viswanath, “Fundamentals of Wireless Communication”, Cambridge University Press, 2005. The implementation of a SIC receiver requires a significant use of processing resources. In the downlink, the use of SIC techniques at the user equipment (UE) receiver is therefore limited by its complexity, which scales with the number of users. 
         [0003]    In a Multiple-Input Multiple-Output (MIMO) system, spatial multiplexing allows the transmission of multiple data streams (or data layers) over different spatial channels. As is known in the art, multiple transmit antennas can send different data streams over separate spatial channels, and the use of multiple receive antennas can allow the recovery of the different data streams, see, e.g., G. J. Foschini and M. J. Gans, “On Limits of Wireless Communications in a Fading Environment when Using Multiple Antennas”, Wireless Personal Communications, vol. 6, no. 3, pp. 311-335, March 1998, I. E. Telatar, “Capacity of Multi-Antenna Gaussian Channel”, European Transactions of Telecommunications, vol. 10, no. 6, pp. 585-595, November/December 1999, and D. J. Love and R. W. Heath, Jr., “Limited Feedback Unitary Precoding for Spatial Multiplexing Systems”, IEEE Transactions on Information Theory, vol. 51, no. 8, August 2005. As shown schematically in  FIG. 1 , each transmit antenna  14   1  and  14   2  transmits to each (both) receive antennas  16   1  and  16   2  at the receiver. Any number of transmit antennas and receive antennas may be used, and the maximum number of data streams that can be distinguished due to the spatial multiplexing is equal to the lower of the number of transmit antennas and the number of receive antennas. 
         [0004]    The transmission system shown in  FIG. 1  can be described by the equation 
         [0000]        r=Hx+n   (1)
 
         [0000]    where r denotes the received signal vector, x is the transmitted signal vector, H indicates the MIMO channel matrix, and n is the noise (noise-plus-interference) vector. The channel matrix H models the characteristics of the MIMO propagation channel. In the case of a frequency non-selective channel, Equation (1) can be expanded as 
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         [0000]    where r 1  and r 2  are the signals received at the respective receive antennas  16   1  and  16   2 ; x 1  and x 2  are the signals transmitted from the respective transmit antennas  14   1  and  14   2 ; h 11 , h 12 , h 21  and h 22  are the coefficients of the (frequency non-selective) MIMO channel; and n 1  and n 2  are the noise (noise-plus-interference) at the respective receive antennas  16   1  and  16   2 . The noise-plus-interference term typically includes noise generated inside the receiver, which is conventionally modelled by an equivalent stochastic process at the antenna connector. 
         [0005]    A successive interference cancellation (SIC) technique can be used to improve the decoding process in the MIMO system. Once a data stream is correctly decoded, the corresponding received signal component can be reconstructed and subtracted from the received signal. Assuming that each data stream is decoded perfectly and the corresponding reconstructed signal is subtracted at each stage of the SIC procedure, then decoding the final data stream is performed with no interference caused by the other data streams in the received signal. 
         [0006]    The SIC technique is applicable to both cases of Multi-User MIMO (MU-MIMO) transmission, where the multiple spatially separated data streams are simultaneously transmitted to multiple users, and Single-User MIMO (SU-MIMO) transmission, where the multiple parallel data streams are transmitted to a single user. 
         [0007]    The SIC technique is particularly sensitive to error propagation. If an error occurs in decoding or subtracting one of the data streams, then the subsequent data streams to be processed are likely to be affected by an increased probability of decoding errors. The ordering of the data streams can affect the error propagation. In a MIMO system where all of the data streams are equally protected by channel coding, the data streams are decoded starting from the strongest data stream (in a Signal to Noise Ratio sense or based on other suitable metrics) and continuing by decoding progressively weaker data streams in sequence. 
       SUMMARY 
       [0008]    The present invention relates to improvements in the decoding of signals transmitted via a MIMO transmission. In particular, we consider the conditions in which the use of a SIC decoding technique is advantageous and the conditions in which the use of a SIC decoding technique is not advantageous. 
         [0009]    According to a first aspect of the invention there is provided a method of processing a signal transmitted from a plurality of spatially separated transmit antennas using a Multiple-Input Multiple-Output transmission over a wireless network, the method comprising: receiving the signal at a plurality of spatially separated receive antennas, the signal comprising a plurality of data streams; determining the quality/reliability of each of the data streams in the received signal; based on the determined quality/reliability of the data streams, selecting a decoding technique to be one of (i) a successive decoding technique for successively decoding data streams in which one of the data streams is decoded and a signal corresponding to said one of the data streams is removed from the received signal prior to decoding further data streams in the received signal, and (ii) a non-successive decoding technique in which each data stream is decoded from the received signal by treating the other data streams as noise in the received signal; and decoding the received signal using the selected decoding technique. 
         [0010]    According to a second aspect of the invention there is provided a receiver for processing a signal transmitted from a plurality of spatially separated transmit antennas using a Multiple-Input Multiple-Output transmission over a wireless network, the receiver comprising: receiving means comprising a plurality of spatially separated receive antennas for receiving the signal, the signal comprising a plurality of data streams; determining means for determining the quality/reliability of each of the data streams in the received signal; selecting means for selecting, based on the determined quality/reliability of the data streams, a decoding technique to be one of (i) a successive decoding technique for successively decoding data streams in which one of the data streams is decoded and a signal corresponding to said one of the data streams is removed from the received signal prior to decoding further data streams in the received signal, and (ii) a non-successive decoding technique in which each data stream is decoded from the received signal by treating the other data streams as noise in the received signal; and decoding means for decoding the received signal using the selected decoding technique. 
         [0011]    According to a third aspect of the invention there is provided a wireless network comprising: a transmitter comprising a plurality of spatially separated transmit antennas for transmitting a signal using a Multiple-Input Multiple-Output transmission over the wireless network; and a receiver as described above in the second aspect of the invention for receiving and processing the signal. 
         [0012]    According to an fourth aspect of the invention there is provided a computer program product comprising computer readable instructions for execution at a receiver of a wireless network, the receiver comprising a plurality of spatially separated receive antennas for receiving a signal transmitted from a plurality of spatially separated transmit antennas using a Multiple-Input Multiple-Output transmission over a wireless network, the signal comprising a plurality of data streams, wherein the instructions comprise instructions for: determining the quality/reliability of each of the data streams in the received signal; based on the determined quality/reliability of the data streams, selecting a decoding technique to be one of (i) a successive decoding technique for successively decoding data streams in which one of the data streams is decoded and a signal corresponding to said one of the data streams is removed from the received signal prior to decoding further data streams in the received signal, and (ii) a non-successive decoding technique in which each data stream is decoded from the received signal by treating the other data streams as noise in the received signal; and decoding the received signal using the selected decoding technique 
         [0013]    Embodiments of the present invention address the problem of ordering the data streams in a received MIMO signal for use in a SIC decoding technique. Typically, the modulation scheme and coding rate of the different data streams transmitted in the downlink may be individually adapted to the channel conditions based on feedback of the current channel quality from the receiver. 
         [0014]    Error propagation can be minimized by sending appropriate information on the quality of the data streams from the receiver to the transmitter. In a Single-User MIMO (SU-MIMO) system, i.e., a MIMO system where all of the parallel data streams are intended for a single receiver, the receiver can preferentially treat one of the data streams such that it is chosen to be decoded first in the SIC decoding. The channel quality of the preferentially treated data stream can be underreported to the transmitter (e.g. by reporting an adjusted value of the channel quality). In response the transmitter will follow the reported channel quality for that particular data stream, which will in turn reduce the error-rate of that particular data stream after decoding. Since that data stream is chosen to be the first data stream to be decoded in the SIC decoding, fewer errors propagate through to the decoding of the other data streams. In this way, the performance of the overall SIC decoding can be improved. 
         [0015]    Information on the quality/reliability of each data stream (e.g. the coding rate and modulation scheme of each data stream) can also be used to adaptively switch between a non-SIC decoding technique and a SIC decoding technique. When decoding one of the data streams in the received signal, the non-SIC decoding technique treats the other data streams as noise. Differently, as described above, the SIC decoding technique decodes one data stream and then removes the reconstructed signal corresponding to that data stream from the received signal before decoding subsequent data streams. Compared to the non-SIC decoding technique, the SIC decoding technique may result in improved performance (increased supported data rate or reduced error rate), but requires more processing power to implement. Depending on the conditions on the data streams and/or on the capabilities of the receiver, the receiver can dynamically choose between using the SIC decoding technique or the non-SIC decoding technique. 
         [0016]    The information on the channel quality for each data stream (e.g., the reported coding rate and modulation scheme of each data stream) can also be used to decide the number of data streams that can be supported in the MIMO transmission. This can be reported from the receiver to the transmitter using a rank indicator (RI), which indicates the rank of the channel matrix. The rank of the channel matrix is defined as the number of linearly independent columns of the channel matrix and can be used to indicate the number of spatial dimensions of the MIMO channel. For example, for a MIMO transmission with four transmit antennas (N T =4) and four receive antennas (N R =4), the 4×4 channel matrix can have rank equal to 4, 3, 2 or 1 (rank≦min (N T , N R )). The rank of the channel matrix also determines the size of a precoding matrix which is used by the transmitter, i.e., the number of columns of the precoding matrix. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the following drawings in which: 
           [0018]      FIG. 1  is a schematic representation of MIMO transmission with N T =2 transmit antennas and N R =2 receive antennas; 
           [0019]      FIG. 2  is a schematic representation of a transmitter and a receiver according to a preferred embodiment; 
           [0020]      FIG. 3   a  is a flow chart for a process of decoding a signal according to a SIC technique; 
           [0021]      FIG. 3   b  is a flow chart for a process of decoding a signal according to a first method; 
           [0022]      FIG. 4  is a flow chart for a process of decoding a signal according to a second method; and 
           [0023]      FIG. 5  is a flow chart for a process of decoding a signal according to a third method. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    With reference to  FIG. 2  there is now described a preferred embodiment of the present invention. A transmitter  202  comprises a transmit antenna block  204  having multiple antennas, such as antennas  14   1  and  14   2  shown in  FIG. 1 , for use in MIMO transmission. A receiver  206  comprises a receive antenna block  208  having multiple antennas, such as antennas  16   1  and  16   2  shown in  FIG. 1 , for use in MIMO transmission. The schematic representation of  FIG. 1  refers to the case of N T =2 transmit antennas and N R =2 receive antennas only for simplicity of illustration, while the validity of the present invention is maintained for a different number of transmit and receive antennas. The receiver  206  also comprises processing means  210  and a user interface  212 . The processing means  210  is connected to the receive antenna block  208  and to the user interface  212 . The user interface  212  is used to communicate data to and from a user of the receiver  206 . For example, the user interface may comprise a speaker and a visual display for outputting data to the user and a microphone, a camera and a keypad allowing the user to input data to the receiver  206 . 
         [0025]    In operation, data is transmitted wirelessly from the transmitter  202  to the receiver  206  with a MIMO transmission using the transmit antenna block  204  and the receive antenna block  208 . In this way multiple data streams are transmitted between the transmitter  202  and the receiver  206 . A Successive Interference Cancellation (SIC) technique is used to decode the data streams at the receiver  206 . In order to minimize error propagation in the SIC technique, it is advantageous to decode a full data stream before cancellation of the reconstructed signal corresponding to the considered data stream from the received signal. It is preferable to decode the different data streams in an order that matches their quality/reliability in the received signal. More specifically, the receiver  206  is adapted to first decode the most reliable data stream, where the most reliable data stream is the one producing the lowest number of errors at the decoder output. In this way, the error propagation is minimized in the SIC technique. 
         [0026]    With reference to  FIG. 3   a , there is now described a method of decoding a signal according to a SIC technique. In step S 302  the signal is received at the receive antenna block  208 . The signal comprises multiple data streams transmitted as described above from the transmit antenna block  204 . 
         [0027]    In step S 304  the data streams are analysed to determine an ordering for decoding the data streams and then in step S 306  the data streams are ordered in accordance with the analysis performed in step S 304 . In step S 308  the first data stream of the ordered data streams is selected and decoded. 
         [0028]    In step S 310  the remaining data streams in the signal are decoded using a SIC technique. In this way, the ordering performed in steps S 304  and S 306  determines which data stream is decoded first. Once the first data stream has been decoded, the reconstructed signal corresponding to the first data stream is removed from the received signal. The SIC technique continues by successively decoding the remaining data streams in accordance with the ordering established in step S 306 . 
         [0029]    Following the analysis of the data streams in step S 304  the method also passes to step S 312  in which the quality of the data streams is computed. The quality of the data streams is reported to the transmitter in step S 314 . 
         [0030]    With reference to  FIG. 3   b , there is now described a method of decoding a signal according to a first method. In step S 322  the signal is received at the receive antenna block  208 . The signal comprises multiple data streams transmitted as described above from the transmit antenna block  204 . 
         [0031]    In step S 324  the data streams are analysed. However, in contrast to the analysis performed in step S 304  of  FIG. 3   a , an ordering for decoding the data streams is not determined. 
         [0032]    Following step S 324  the method passes to step S 326  in which the quality of the data streams is computed. In step S 328  a data stream is selected and the value of the quality of the selected data stream to be reported to the transmitter is adjusted. In step S 328  more than one data stream may be selected and the values of the quality of the selected data streams to be reported to the transmitter may be adjusted. It can be particularly useful to select more than one data stream in step S 328  when there are more than two data streams in total. In step S 330  the receiver  206  then sends a report of the quality of the data streams to the transmitter  202 , which includes the adjusted quality value for the selected data stream(s). Each data stream is encoded separately at the transmitter  202  such that each data stream is coded and mapped to a number of modulation symbols by the transmitter  202  depending on the information on the reception quality fed back by the receiver  206  on each data stream. The information describes the channel quality and it is often reported in terms of a corresponding Modulation and Coding Scheme (MCS). 
         [0033]    The analysis of the data streams performed in steps S 324  and S 326  is implemented in software in the preferred embodiment in the processing means  210 . In alternative embodiments the analysis is carried out in hardware rather than in software. 
         [0034]    In steps S 322  and S 324  the signal is decoded using a SIC technique. In this way, the data stream selected in step S 328  is decoded first in step S 332 . Once the selected data stream has been decoded, the reconstructed signal corresponding to the selected data stream is removed from the received signal. The SIC technique continues by decoding the remaining data streams. In this way, the number of errors in the decoded signal, across all the data streams, is reduced. Where more than one data stream is selected in step S 328  those selected data streams are decoded before the remaining data streams are decoded. 
         [0035]    Step S 332  uses the information on which data stream has had an adjusted quality value reported to the transmitter  202  in previous Transmission Time Interval(s) (TTI). This means that if in step S 328  in a particular TTI a new data stream is selected for reporting an adjusted value of the quality to the transmitter, then step S 332  will not switch to decoding that new data stream first in that particular TTI. Only in subsequent TTIs will step S 332  switch to decoding the new data stream first. 
         [0036]    In the first method, in the case of Single-User MIMO (SU-MIMO) transmission, the receiver  206  determines that a particular data stream will be decoded first in the SIC decoding. Furthermore, the receiver  206  reports a lower MCS to the transmitter  202  in step S 330  for the particular data stream. In other words, the receiver  206  reports to the transmitter  202  that one of the data streams is received with a lower quality than it actually is. This is referred to as ‘underreporting’ the quality of the data stream. In this way, the receiver reports an adjusted value of a characteristic (in this case the quality) of a particular data stream to the transmitter. In response to receiving this report, the transmitter  202  will transmit a more robust MCS (requiring a lower Signal-to-Noise Ratio for a given error probability), which in turn will decrease the probability of error for the considered data stream, when decoded by the receiver  206 . The information of the bias in the reported channel quality of the particular data stream will be taken into account in the ordering and selection of the data streams in step S 332 . In this way the receiver  206  will in practice determine the ordering of the data streams in the SIC decoding. Furthermore, by underreporting the quality of the first data stream to be decoded, the block error rate of that particular data stream is reduced, such that the effect of error propagation in the SIC decoding is reduced. In this way the overall performance of the receiver  206  is improved. In fact, any errors in decoding the data stream which is selected to be decoded first in the SIC decoding technique would propagate through to the decoding of all of the other data streams, and it is therefore particularly important to reduce the decoding errors on the first data stream to be decoded. The approach can be extended to the reporting and ordering of the remaining data streams. 
         [0037]    This scheme is particularly useful in the case where the data streams experience similar propagation conditions. In this case, all the data streams will have a similar probability of error, which in turn results in a lack of ordering of the reception quality of the data streams at the receiver  206 . This lack of ordering will cause a degradation of the performance of a conventional SIC decoding technique. In the preferred embodiment described above, an adjusted value of the channel quality of a particular data stream is reported to the transmitter  202 , and the receiver chooses the particular data stream to be the first to be decoded and subtracted from the received signal. In this way, the overall performance of the receiver is improved by increasing the reliability of SIC processing. 
         [0038]    In conventional SIC decoding techniques the first step is often to analyse the data streams to thereby select the data stream which has the highest quality to be the first data stream to be decoded. However, in preferred embodiments of the present invention the SIC decoding does not require a step of analysing the data streams to thereby order the data streams (i.e. a step similar to step S 306  of  FIG. 3   a  is not required in the method described above in relation to  FIG. 3   b ). Instead a particular data stream is chosen by the receiver to be the first data stream to be decoded, and the quality (or some other characteristic, e.g. the reliability) of the chosen data stream is underreported to the transmitter, such that the chosen data stream is preferentially treated to have an improved quality (or an improved characteristic, e.g. reliability). This removes the need for the analysing and selecting steps in the SIC decoding which can reduce the processing required to perform the SIC decoding. 
         [0039]    The data stream chosen to be decoded first in the SIC decoding and to be underreported to the transmitter can be chosen based on the quality of the received data stream. For example, the data stream may be chosen because it has the highest quality of the received data streams. The data stream may be chosen because it has the lowest quality of the data streams, hence reducing the complexity of encoding and subtracting that particular stream. Alternatively, the data stream can be chosen independently of the quality of the data streams, e.g. the data stream can be chosen randomly. It will be apparent to the skilled person that different methods for choosing a data stream may be employed in different embodiments of the invention. 
         [0040]    The receiver could additionally use the information provided by the cyclic redundancy check (CRC) of each decoded block of the first data stream, and decide depending on the CRC which block is to be regenerated and subtracted from the received signal. That is, the technique works at the block level but also at the codeword level. One transport block of one stream may contain multiple coded blocks each with its own CRC. 
         [0041]    In the preferred embodiment described above the ordering of the data streams depends on the reports generated at the receiver  206  and on the channel quality. This is advantageous in that the receiver  206  has control over the ordering of the data streams. 
         [0042]    With reference to  FIG. 4  there is now described a preferred method of decoding a signal. Steps S 402  and S 404  correspond to steps S 312  and S 314  described above. Therefore in step S 402  a signal comprising multiple data streams is received at the receive antenna block  208  and in step S 404  the data streams are analysed to determine information on the received quality (supported MCS) of each data stream. The information on supported channel quality/reliability and the MCS for each data stream is used to adaptively switch between a conventional decoding technique and a SIC decoding technique. In step S 406  a SIC decoding technique or a conventional decoding technique is selected. As described above, the SIC technique involves successively decoding one of the data streams and then removing the data stream from the received signal before decoding subsequent data streams. In this way the data streams are decoded in order and the interference caused by a data stream is reduced when decoding a subsequent data stream. However, in a conventional decoding technique (i.e. a non-SIC decoding technique) each data stream is decoded from the received signal separately and the other data streams are treated as noise in the received signal. If the analysis of step S 404  suggests that all the data streams experience communication channels with similar quality/reliability, and specifically with similar probability of error, then the ordering of the data streams is difficult, and the improvement in using a SIC technique as compared to using a conventional decoding technique would be limited. In that case, in order to save in complexity, in step S 406  a conventional decoding technique would be selected because a conventional decoding technique uses less of the processing resources of the receiver  206  than would be used by a SIC decoding technique. The selection in step S 406  may be based, at least in part, on the processing resources available at the receiver  206 . 
         [0043]    In step S 408  the decoding technique selected in step S 406  is used to decode the received signal, and the final step is to report the quality of the data streams to the transmitter in step S 410 . 
         [0044]    In this way, where the use of a SIC decoding technique is advantageous, the SIC decoding technique is selected in step S 406 . However, where the detriment of the extra processing resources required to perform the SIC decoding technique outweighs the advantages of the improved quality of decoded data streams produced by using the SIC decoding technique, then a conventional decoding technique is used instead of the SIC decoding technique. This provides the receiver  206  with added flexibility for adapting the decoding technique to the specific conditions on the communication channel. The decision between a SIC decoding technique and a conventional decoding technique can be made dynamically in response to changing channel conditions. 
         [0045]    By way of example, the case of two data streams is now considered. The reception quality of the data streams in the received signal is determined, and the difference in the probability of error in decoding the two data streams is estimated. If the difference in the estimated error probability of the two data streams is greater than a threshold value, then the SIC decoding technique is used, because it is advantageous to remove the more reliable data stream before decoding the less reliable data stream. However, if the difference in the estimated error probability of the two data streams is less than the threshold value, then the conventional decoding technique is used, because it is advantageous to avoid the extra processing required by a SIC decoding technique. 
         [0046]    In pseudo-code this example can be described as follows: 
         [0000]                                                                                    if (Delta_P e  &gt; θ )                use MMSE-SIC                else                use MMSE                end                        
where Delta_P e  denotes the difference between the estimated error probability of the second data stream and the estimated error probability of the first data stream, θ is a design parameter indicating a threshold value, MMSE-SIC is a minimum mean square error SIC decoding technique and MMSE is a conventional minimum mean square error decoding technique.
 
         [0047]    In a third method of decoding a signal the CQI information is used to indicate the rank of the MIMO communication channel, i.e. the number of data streams that can be supported on the communication channel. The actual value of the CQI of the first data stream and the difference Delta_CQI between the reported Channel Quality Indicator (CQI) of the two data streams, are used by the receiver  206  to form an indication of the condition of the communication channel. This indication can be used to determine the number of independent data streams, i.e., the rank of the communication channel. The Delta_CQI gives a relative measure of the quality of the two data streams in contrast to the actual value of the CQI which gives an independent measure of the quality of a data stream. It is also possible to use the actual value of the CQI for the first and second data streams rather than using the actual value of the CQI of the first data stream and the difference Delta_CQI between the reported Channel Quality Indicator (CQI) of the two data streams. The receiver and the transmitter use the same scheme for defining the CQIs of the data streams. 
         [0048]    The third method is described with reference to  FIG. 5 . Similarly, to steps S 302  and S 304 , in step S 502  the signal is received at the receiver  206  from the transmitter  202  via a MIMO transmission. In step S 504  the received signal is analysed to determine the quality of the data streams. In this way a Channel Quality Indicator (CQI) is produced for each of the data streams. 
         [0049]    In step S 506  the rank of the channel is determined at the receiver  206 . The number of data streams that can be supported in a MIMO transmission is linked to the condition number of the channel matrix H (i.e. the rank of the channel matrix H). The rank of the channel matrix provides the number of independent data streams that can be supported in the MIMO transmission. A channel matrix of full rank will provide the maximum number of independent data streams, whereas an ill-conditioned channel matrix will give a lower number of independent data streams. 
         [0050]    As an example, for two transmit antennas and two receive antennas (as shown in  FIG. 1 ) a channel matrix of 
         [0000]    
       
         
           
             H 
             = 
             
               [ 
               
                 
                   
                     1 
                   
                   
                     0 
                   
                 
                 
                   
                     0 
                   
                   
                     1 
                   
                 
               
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         [0000]    has a rank of two because there are two independent columns. This means that two independent data streams can be used in the MIMO transmission. In fact, with this channel matrix, the data stream transmitted from each transmit antenna can be recovered independently. 
         [0051]    However, a channel matrix of 
         [0000]    
       
         
           
             H 
             = 
             
               [ 
               
                 
                   
                     1 
                   
                   
                     1 
                   
                 
                 
                   
                     1 
                   
                   
                     1 
                   
                 
               
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         [0000]    has a rank of one because the columns are not independent. This means that only one independent data stream can be used in the MIMO transmission. 
         [0052]    The channel matrix varies dynamically as the conditions of the channel vary. Therefore the rank of the channel matrix is determined dynamically in order to optimize the number of independent data streams in accordance with the current conditions for the MIMO transmission. 
         [0053]    When the rank of the channel matrix has been determined, in step S 508  a Rank Indicator (RI) message is reported from the receiver  206  to the transmitter  202  to inform the transmitter  202  of the determined channel rank to be used on the communication channel. 
         [0054]    As another example, in the case of two transmit antennas and two receive antennas (as shown in  FIG. 1 ), a situation where the Signal-to-Noise Ratio (SNR) is relatively low and the CQI difference Delta_CQI is relatively high, would indicate that there is one strong data stream and one weak data stream. In these conditions, the communication channel cannot realistically support spatial multiplexing of two streams, and so the message sent to the transmitter  202  instructs the transmitter  202  to use a channel with a rank of one. 
         [0055]    Alternatively, the third method of decoding a signal may be implemented at the transmitter after having received a channel quality indicator for each of the data streams from the receiver. In this case, the transmitter determines the number of independent data streams that can be supported in the MIMO transmission of the signal based on the channel quality indicator received from the receiver. The transmitter can then transmit the signal to the receiver using the determined number of independent data streams. 
         [0056]    There are described above new methods of decoding a received signal in a MIMO system. The methods described above introduce an adaptation criterion for SIC processing; simplify the data stream ordering operation; improve the system performance; provide a solution for the case where all the data streams are experiencing similar quality/reliability; do not require any further processing with respect to the implementation of the SIC receiver; and do not impact other end-users, in contrast to power-controlled systems, where by changing the power level one may modify the interference level or decrease the reliability. 
         [0057]    In the methods described above, the functional steps shown in  FIGS. 3 to 5  are performed in software by the processing means  210 . In alternative embodiments, these steps are performed by hardware in the receiver  206 . 
         [0058]    While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.