Abstract:
Methods are disclosed for improving communications on feedback transmission channels, in which there is a possibility of bit errors. The basic solutions to counter those errors are: proper design of the CSI vector quantizer indexing (i.e., the bit representation of centroid indices) in order to minimize impact of index errors, use of error detection techniques to expurgate the erroneous indices and use of other methods to recover correct indices.

Description:
TECHNICAL FIELD 
       [0001]    Wireless communication using multiple antennas. 
       BACKGROUND 
       [0002]    One of the most promising solutions for increased spectral efficiency in high capacity wireless systems is the use of multiple antennas on fading channels. The fundamental issue in such systems is the availability of the channel state information (CSI) at transmitters and receivers. While it is usually assumed that perfect CSI is available at the receivers, the transmitter may only have partial CSI available due to the feedback delay and noise, channel estimation errors and limited feedback bandwidth, which forces CSI to be quantized at the receiver to minimize feedback rate. 
       SUMMARY 
       [0003]    Methods are disclosed for improving communications on feedback transmission channels, in which there is a possibility of bit errors, The basic solutions to counter those errors are: proper design of the CSI vector quantizer indexing (i.e., the bit representation of centroid indices) in order to minimize impact of index errors, use of error detection techniques to expurgate the erroneous indices and use of other methods to recover correct indices (see pending US patent application “Quantized channel state information prediction in multiple antenna systems” Ser. No. 11/852,206.) The content of U.S. Ser. Nos. 11/754,965 and 11/852,206 are incorporated herein by reference. 
         [0004]    There is provided a method of reducing the effect of errors in the feedback of channel state information from a receiver to a transmitter. In an embodiment, the method comprises the steps of choosing multiple mappings of indices to channel states, estimating the effect on transmission quality of feedback errors for each of the mappings of indices to channel states, selecting a mapping of indices to channel states to reduce the effect of feedback errors; and transmitting feedback of channel state information from the receiver to the transmitter, the receiver representing a channel state using the codeword determined by the selected mapping of indices to channel states. 
         [0005]    These and other aspects of the method are set out in the claims, which are incorporated here by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]    Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which: 
           [0007]      FIG. 1  is a schematic diagram showing the division of the channel state space into Voronoi regions. 
           [0008]      FIG. 2  shows the basic structure of the system proposed in U.S. patent application Ser. No. 11/754,965, and which may be used as modified according to the disclosed algorithms. 
           [0009]      FIG. 3 a    shows a good mapping of indices to centroids. 
           [0010]      FIG. 3 b    shows a bad mapping of indices to centroids. 
           [0011]      FIG. 4  shows the general operation of an embodiment of an indexing optimization algorithm. 
           [0012]      FIG. 5  shows the operation of the initialization phase of the exemplary indexing design algorithm of  FIG. 4 . 
           [0013]      FIG. 6  shows the operation of the optimization phase of the exemplary indexing design algorithm of  FIG. 4 . 
           [0014]      FIG. 7  shows an embodiment of the system operation with error-detecting codes. 
           [0015]      FIG. 8  shows an embodiment of the system operation without error detecting codes. 
           [0016]      FIG. 9  shows a more general embodiment of an indexing algorithm. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the typical CSI vector quantizer (VQ), the quantization of the channel vector space is performed as in  FIG. 1 : the available space is tessellated by Voronoi regions  20  with corresponding centroids  22  that represent all vector realizations within each Voronoi region. The number of such regions (centroids) is defined by the number of available bits and each centroid is assigned an index  23  with the binary representation of length equal to the number of available feedback bits. The indices can also be represented by arbitrary sequences of symbols, so long as each index is represented by a unique sequence of symbols, and the symbols can be transmitted by the feedback channel. When the receiver transmits its channel state information to the transmitter, it is the bits (or symbols) representing the centroid indices that are physically sent over the feedback channel. When the term “binary index” is used, one could substitute “the sequence of symbols representing the index”. In the claims when a mapping of indices to channel states is mentioned, a mapping of symbol sequences to indices would serve the same purpose and so the claims should be construed to cover both. 
         [0018]    All presented solutions can be used for both eigenmode and singular value codebooks in systems ranging from only one active receiver at a time to systems with multiple receivers being active simultaneously (where we define being active as receiving transmissions). The design of the feedback encoding solutions can he applied to quantized matrices of orthogonal eigenmodes, subsets of eigenmodes and scalar singular values as necessary. The following descriptions will be generic in form so that they can easily be applied to any type of CSI quantizing solution. 
         [0019]      FIG. 2  shows the basic structure of the system proposed in U.S. patent application Ser. No. 11/754,965, The system works as follows: 
         [0020]    1. Before the transmission epoch, each receiver  38  estimates  40  its channel matrix H for a feedforward channel  34  and uses this information to perform the singular value decomposition (SVD)  42  of the matrix, 
         [0021]    2. The eigenmode and singular value components are separately quantized  44  using two codebooks V  46  and D  48 , respectively. 
         [0022]    3. The indices of the selected codewords are encoded  52 ,  54  and fed back  56 ,  58  to the transmitter  36  on a feedback channel  50 . 
         [0023]    4. The transmitter includes an indexer and optimizer  66  which uses all decoded  60 ,  62  indices  64  from all receivers in the system to choose  68 ,  70  the preselected linear modulation and power allocation matrices B and S, respectively. The choice is based on a predefined set of rules (maximum fairness, throughput etc.). 
         [0024]    5. The input signal  72  is modulated using the modulation  74  and power allocation  76  matrices, transmitted to the receiver over the feedforward channel  34 , using antennas  32 , and the transmitted modulated signal is processed  78  by the receiver. 
         [0025]    The feedback channel  50  shown in  FIG. 2  will inevitably suffer from the transmission errors and the indices of the channel vectors reported by the receivers will be erroneously decoded at the transmitter, Even if the feedback information is protected by channel codes, in a multiple user scenario, it is possible that the interference will cause the indices to he detected with errors, which will lower the system&#39;s throughput, 
         [0026]    For example, in  FIG. 1 , during the first part of the actual vector trajectory  24 , the centroid index number  3  represented by bits  011  would be reported to the transmitter. However, if for sonic reason, the second index bit would be recovered with an error, the centroid index number  1  (represented by bits  001 ) would instead be received by the transmitter, which would cause it to choose improper modulation matrices B and S (see previous section). 
         [0027]    The basic transmission of the feedback indices  23  may comprise the following steps: 
         [0028]    1. Selection of the indices  23  representing the centroids  22  closest to the actual channel vectors. 
         [0029]    2. (Optional) Adding error detection check bits to the binary representation of the centroid indices. 
         [0030]    3. (Optional) Adding channel error correction check bits. 
         [0031]    4. Transmission of all the bits. 
         [0032]    5. (Conditioned on 3) Performing channel decoding of the received bits. 
         [0033]    6. (Conditioned on 2) Performing error detection of the received bits, 
         [0034]    7. Reporting the received channel vector indices to the optimizer/indexer in  FIG. 2 . 
         [0035]    8. (Optional) Reporting which indices contain errors to the optimizer/indexer in  FIG. 2 . This step can use results from 6 or any alternative method as outlined later on. 
         [0036]    Based on the above eight steps, the indexer will now be able to make decision on the choice of modulation matrices for the next transmission epoch. Three exemplary approaches to the problem are: 
         [0037]    1. If error detection codes or any alternative error detection method is used and the base station optimizer is aware of erroneous receiver indices, it may discard them and only use the correct ones in the optimization phase. 
         [0038]    2. If error detection codes or any alternative error detection method is used and the base station optimizer is aware of erroneous receiver indices, it may attempt to recover them and use all received indices (correct and recovered) in the optimization phase. 
         [0039]    3. If error detection is not used at all, the optimizer assumes that the received indices are very close to the actual transmitted indices and use them as if they were all correct. 
         [0040]    A basic difference between methods 1, 2 and 3 lies in whether the error detection methods are used to detect problems in channel information indices fed back to the base station. If such methods are used, the transmitter may recognize which indices are incorrect and can take appropriate actions. If no error detection may be performed and the received indices are used ‘as-is’, the vector quantizer indexing must be properly designed as shown in  FIG. 3   a.    
         [0041]    The mapping of the indices to the centroids in a quantizer is a complex task that can influence the system&#39;s performance tremendously when errors in the feedback link are not negligible.  FIGS. 3 a  and 3 b    shows the situation, where the identical vector quantizer has a different mapping of indices  23  and the results of one bit error in the transmission of the centroid indices. 
         [0042]    In  FIG. 3   a,  the mapping was done in a way that ensured that one bit error in the last position of the index moved the centroid received at the transmitter not far from the actual one. In other words, the small Hamming distance of the difference between the actual and received indices of the centroid corresponds to the small distance  26  between the actual centroids. The centroid distance  26  function will be discussed further in the document. 
         [0043]    In  FIG. 3   b,  the small Hamming weight of index error does not correspond to the small centroid distance. In this case, a 1-bit error forces the centroid to move far away from the actual one, which may have a very negative influence on the system&#39;s performance. 
         [0044]    The following algorithms are presented: 
         [0045]    1. Design of the indexing for CSI MIMO vector quantizers. 
         [0046]    2. Actual operation of the system using encoded feedback link with CSI MIMO quantizers with or without error detection. 
         [0047]    The algorithm for the design of the indexing can be carried out in any suitable computing device, including for example pen and paper. Typically the design will be carried out prior to the initialization of the MIMO system. 
         [0048]    The following notation will be used:
       V k —the kth centroid in codebook V.   k,l—indices of centroids in codebook V.   d kl —the distance between the centroids specified by indices k and l.   d(k;e)—distance profile for centroid V k  and a given number of bit errors e.   GDP(e)—global distance profile for a given number of bit errors e.   i,j—binary indices of a codeword in a vector quantizer.   H ij  —the Hamming distance between indices i and j.   I—the number of iterations of the index optimization algorithm   N—the length of the binary representation of centroid indices i,j       
 
         [0058]    It is assumed that the indexing design follows the design of the channel vector quantizer V using any of the existing methods, for example the method shown in the patent application “Quantization of channel state information in multiple antenna systems” (U.S. patent application Ser. No. 11/754,965 pending). The input to the quantizer indexing algorithm is the distance matrix D with number or rows and columns equal to the number of all centroids V k  (with our notation the number of rows and columns is equal to 2 N ). The entries in the matrix are distances between the centroids—for example, the kth row and lth entry is given by d kl . In particular, the entries on the diagonal of the matrix are equal to 0. The methods used to calculate the distance matrix D as well as the centroid distances are immaterial in this patent application. However, some of the methods to calculate the centroid distances for the matrix D can be defined as follows: 
         [0059]    1. d kl  is the angle between the centroids V k  and V l . 
         [0060]    2. d kl  is the smaller of the angles between the centroids V k  and V l  and between the centroids V k  and −V l . 
         [0061]    3. d kl  is the Euclidean distance between the centroids V k  and V l . 
         [0062]    4. d kl  is the average system throughput loss when centroid V k  is chosen instead of V l . 
         [0063]    5. d kl  is the user system throughput loss when centroid V k  is chosen instead of V l . 
         [0064]    6. Any other distance definition depending on the system design parameters 
         [0065]    In addition to the distance metric d kl , representing a distance between two specified quantizer centroids, a set of distance profiles, d(k;e), and a global distance profile, GDP(e), are used to represent the distance profile of the indexed quantizer. A distance profile d(k;e) for a given centroid k and a number of errors e represents a set of numbers corresponding to the distances between all erroneous representations of the centroid and the actual centroid V k , assuming that e errors appeared during the transmission of its corresponding index i. In other words, 
         [0000]        d ( k;e ) ={d   1   , d   2   , d   3    . . . , d   n   , . . . , d   E }, 
         [0000]    where E is the number of e-element subsets in N-long binary representation of codebook indices and d n  corresponds to distances d kl  between the centroid V k  and its erroneous version V l  containing e index errors. 
         [0066]    Finally, to characterize the entire codebook, a global distance profile GDP(e) is defined as the union of all distance profiles d(k;e). 
       Indexing Design Algorithm. 
       [0067]    Design of the indexing based on the distance matrix D is performed using a heuristic algorithm operating in two phases: the initialization phase and optimization phase. Since the initialization phase of the algorithm depends on random initial choice of indices, it is recommended that both phases of the algorithm are repeated storing the index map after each optimization step for a given number of iterations I until the best solution has been found or the design constraint has been met. The general operation of the indexing design algorithm is shown in  FIG. 4  and described below. 
       General Algorithm: 
       [0068]    1. In step  80 , initialize iteration counter i and in step  82  the previous global distance profile GDP previous . 
         [0069]    2. In step  84 , run the Initialization phase of the algorithm (see below). 
         [0070]    3. In step  86 , run the Optimization phase of the algorithm (see below). 
         [0071]    4. In step  88 , store index map and calculate the new global distance profile GDP(e). 
         [0072]    5. In step  90  compare GDP(e) with GDP previous ; in step  92  check if GDP(e) improves GDP previous  and if so then in step  94  exchange GDP previous  with GDP(e) and store the current index map. 
         [0073]    6. In step  96  increase i by 1. 
         [0074]    7. in step  98  if i&lt;I go to 2. 
         [0075]    8. In step  100 , STOP. 
       Initialization Phase: 
       [0076]    1. In step  102 , generate the distance matrix D; in step  104  initialize the lists of unassigned indices, centroids and processed entries in D. 
         [0077]    2. In step  106 , find the smallest unprocessed entry d kl &gt;0 in the matrix  0 . 
         [0078]    3. Mark the entry d kl  as processed. 
         [0079]    4. In step  108  search for centroids V k  and V l  corresponding to d kl ; in step  110  check if any indices have been assigned to either centroid. If neither centroids V k  nor V l  corresponding to d kl  have been assigned any indices i or j, in step  112  ( a - c ), step  118  ( d - e ) and step  120  ( f ):
       a) Choose a random index i from the list of the unused indices.   b) Assign the index Ito the centroid   c) Mark the ith entry in the list of used indices list as taken.   d) Choose a random index j from the list of the unused indices in such a way that the corresponding H ij  is minimized.   e) Assign the index j to the centroid V l .   f) Mark the jth entry in the list of used indices list as taken, and V l  as having an index assigned to it in the list of used centroids.       
 
         [0086]    5. In step  114 , check if only one of the centroids V k  or V l  have been assigned an index, and if so in step  118  ( a - b ) and step  120  ( c ):
       a) Choose a random index j from the list of the unused indices in such a way that H ij  is minimum, where it is assumed (step  116 ) that I is the binary index of the already indexed centroid.   b) Assign the index j to the unassigned centroid.   c) Mark the fill entry in the list of used indices as taken, and mark the unassigned centroid as assigned.       
 
         [0090]    6. If both centroids V k  or V l  have been assigned an index, go to 7. 
         [0091]    7. In step  122 , if there are still unassigned indices, go to 2. 
         [0092]    8. in step  124 , STOP. 
         [0093]    The operation of the initialization phase is presented in  FIG. 5 . 
         [0094]    After the completion of the initialization phase, all centroids V k  in the codebook V have been assigned the binary indices i, with the majority of smallest distances d kl  in matrix D coupled to the binary indices i and j with small Hamming distances. However, the initialization phase can only reach locally optimum solutions and, in the next step, an improved solution is iteratively searched for. 
       Optimization Phase: 
       [0095]    1. In step  130 , generate the distance matrix D and initialize the global distance profile GDP(e). 
         [0096]    2. In step  132 , initialize the list of processed entries in D. 
         [0097]    3. In step  134 , set the previous global error distance as GDP previous =GDP(e). 
         [0098]    4. In step  136 , find the smallest unprocessed entry d kl &gt;0 in the matrix D, and mark it as processed. 
         [0099]    5. In step  138 , find the binary indices i and j assigned to the centroids k and l.
       Choose whether to swap them as follows:   a) In step  140 , calculate the global distance profile GDP original (e) with mapping of centroid V k  to binary index i and centroid V l  to binary index j.   b) In step  142  and  144 , calculate the global distance profile GDP swapped (e) with mapping of centroid V k  to binary index j and centroid V l  to binary index i.   c) In step  146  choose the mapping corresponding to better global distance profile from 6a) and 6b) and in step  148  assign the indices of centroids accordingly.       
 
         [0104]    6. In step  150 , check the list of the processed entries d kl , and if there are still unprocessed entries, go to 4. 
         [0105]    7. In step  152 , calculate the GDP(e) with the current mapping of centroids and indices. If it is better than GDP previous  then go to 2. 
         [0106]    8. In step  154 , if there are no improvements over GDP previous  STOP. 
         [0107]    The operation of the optimization algorithm is presented in  FIG. 6 . 
         [0108]    The optimization phase iteratively searches for better mapping between centroids and indices by swapping the binary representation of the closest pairs. After each such swap, the global distance profile for swapped mapping is compared to the unswapped mapping and the globally better solution is chosen. The algorithm is repeated iteratively through all centroids and stops when no improvement can be achieved by consecutive swapping of the indices. 
         [0109]    A more general version of this approach to indexing is shown in  FIG. 9 . In  FIG. 5 , pairs of centroids (that is, channel states representing a region of channel state space for purposes of quantization) near one another in terms of the chosen distance measure are chosen and are assigned indices also near one another in Hamming distance, Hamming distance being a proxy for the probability of one index being mistakenly received as another (the less the distance, the more likely they will be confused). Since the feedback bandwidth is limited, there will have to be indices near one another in Hamming distance and the effect of feedback errors is reduced if such nearby indices are assigned to nearby channel states, so as to reduce the effect on transmission quality if they are mistaken for one another. Hence the initialization phase depicted in  FIG. 5  can be regarded as choosing multiple mappings of indices to channel states in step  220  (the different possible assignments of indices to a pair of channel states), estimating the effect of feedback errors of each in step  222  (using Hamming distance as a proxy in this case), and selecting one in step  224  (in this case one with the minimum Hamming distance) so as to reduce the effect of feedback errors. Similarly the optimization phase shown in  FIG. 6  includes choosing multiple mappings of indices to channel states in step  220  (in this case differing from one another by the swapping of pairs of indices), estimating the effect of feedback errors of each mapping (in this case using the global distance profile), and selecting a mapping to reduce the expected effect of feedback errors (by choosing the one with the best global distance profile in this case). No matter how the mapping is selected, it is used in step  226  to send feedback of information concerning a channel state from a receiver to a transmitter, the receiver representing the channel state using the index mapped to the exemplary state (eg. centroid) for the region of channel state space in which the channel state lies. 
       System Operation With Error Detection in the Feedback Link 
       [0110]    If the system uses error detecting codes in the feedback link, its operation can be summarized as follows: 
         [0111]    1. In step  160 , initialize transmission epoch to t=1. 
         [0112]    2. In step  162 , each receiver estimates its channel matrix H[t]. 
         [0113]    3. In step  164 , each receiver performs the vector quantization of the channel state information. 
         [0114]    4. In step  166 , the channel state information indices are encoded using error detecting code (such as CRC). 
         [0115]    5. (Optional) In step  168 , the channel state information indices with the error-detection redundancy are encoded using error correcting code (such as convolutional or turbo codes). 
         [0116]    6. In step  170 , the encoded channel state information indices are fed back to the transmitter. 
         [0117]    7. (Conditional on 5) In step  172 , the received channel state information indices are decoded in a channel decoder that attempts to correct possible channel transmission errors. 
         [0118]    8. In step  174 , the decoded indices are checked by an error-detecting decoder. 
         [0119]    9. In step  176 , the receiver counts the number of erroneously decoded indices, 
         [0120]    10. Depending (in step  178 ) on the implementation, the receiver may then:
       a) In step  180 , expurgate the erroneous indices, if there are still enough indices for optimization process, and process only the remaining ones   b) In step  184 , ignore the errors and use the erroneous indices.   c) In step  182 , regenerate the erroneous indices, for example, by using the channel prediction techniques from pending US patent application “Quantized channel state information prediction in multiple antenna systems” Ser. No. 11/852,206.       
 
         [0124]    11. In step  186 , the transmitter performs the selection of active users using any method (maximum fairness, maximum throughput etc.) and chooses the corresponding modulation matrices. 
         [0125]    12. The signal is transmitted to the selected active receivers. 
         [0126]    13. In step  188 , increase transmission epoch as t=t+1. 
         [0127]    14. Go to 2. 
         [0128]    The operation of the algorithm is presented in  FIG. 7 . 
       System Operation Without Error Detection in the Feedback Link 
       [0129]    If the system uses no error detecting codes in the feedback link, its operation can be summarized as follows: 
         [0130]    1. In step  190 , initialize transmission epoch to t=1. 
         [0131]    2. In step  192 , each receiver estimates its channel matrix H[t]. 
         [0132]    3. In step  194 , each receiver performs the vector quantization of the channel state information. 
         [0133]    4. (Optional) In step  196 , the channel state information indices with the error-detection redundancy are encoded using error correcting code (such as convolutional or turbo codes). 
         [0134]    5. In step  198 , the encoded channel state information indices are fed back to the transmitter using the method described in previous sections. 
         [0135]    6. (Conditional on 4) In step  200 , the received channel state information indices are decoded in a channel decoder that attempts to correct possible channel transmission errors. 
         [0136]    7. if (step  202 ) the index error detection is possible using alternative methods, for example, channel prediction techniques such as the one presented in the pending US patent application “Quantized channel state information prediction in multiple antenna systems” Ser. No. 11/852,206, the transmitter may:
       a. In step  206 , expurgate the erroneous indices if there are still enough indices for optimization process,   b. In step  204 , regenerate the erroneous indices using, for example, channel prediction techniques.       
 
         [0139]    8. If (step  202 ) the index error detection is impossible, in step  208  the receiver uses the received indices as correct ones, 
         [0140]    9. In step  210 , the transmitter performs the selection of active users using any method (maximum fairness, maximum throughput etc.) and chooses the corresponding modulation matrices. 
         [0141]    10. The signal is transmitted to the selected active receivers. 
         [0142]    11. in step  212 , increase transmission epoch as t=i+1. 
         [0143]    12. Go to 2. 
         [0144]    The operation of the algorithm is presented in  FIG. 8 . 
         [0145]    Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before a claim feature does not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.