Patent Publication Number: US-2007121753-A1

Title: Wireless communications apparatus

Description:
The present invention is in the field of wireless communication, and particularly, but not exclusively, the field of multiple input, multiple output (MIMO) communications systems.  
      Conventional communication systems can be represented mathematically as: 
 
 y=Hx+v  
 
 in which, for a MIMO communication system, y is an n-by-1 vector representing the received signal, H is an n-by-m channel matrix modelling the transmission characteristics of the communications channel, x is an m-by-1 vector representing transmit symbols, v is an n-by-1 noise vector and wherein m and n denote the number of transmit and receive antennas respectively. 
 
      It will be understood by the skilled reader that the same representation can be used for multi-user detection in CDMA systems.  
      Recent publications have demonstrated how the use of a technique called Lattice Reduction can improve the performance of MIMO detection methods.  
      For example, “Lattice-Reduction-Aided Detectors for MIMO Communication Systems”, (H. Yao and G. W. Womell,  Proc. IEEE Globecom,  November 2002, pp. 424-428) describes Lattice-reduction (LR) techniques for enhancing the performance of multiple-input multiple-output (MIMO) digital communication systems.  
      In addition, “Low-Complexity Near-Maximum-Likelihood Detection and Precoding for MIMO Systems using Lattice Reduction”, (C. Windpassinger and R. Fischer, in  Proc. IEEE Information Theory Workshop,  Paris, March, 2003, pp. 346-348) studies the lattice-reduction-aided detection scheme proposed by Yao and Womell. It extends this with the use of the well-known LLL algorithm, which enables the application to MIMO systems with arbitrary numbers of dimensions.  
      “Lattice-Reduction-Aided Receivers for MIMO-OFDM in Spatial Multiplexing Systems”, (I. Berenguer, J. Adeane, I. Wassell and X. Wang, in  Proc. Int. Symp. on Personal Indoor and Mobile Radio Communications,  September 2004, pp. 1517-1521, hereinafter referred to as “Berenguer et al.”) describes the use of Orthogonal Frequency Division Multiplexing (OFDM) to significantly reduce receiver complexity in wireless systems with Multipath propagation, and notes its proposed use in wireless broadband multi-antenna (MIMO) systems.  
      Finally, “MMSE-Based Lattice-Reduction for Near-ML Detection of MIMO Systems”, (D. Wubben, R. Bohnke, V. Kuhn and K. Kammeyer, in  Proc. ITG Workshop on Smart Antennas,  2004, hereinafter referred to as “Wubben et al.”) adopts the lattice-reduction aided schemes described above to the MMSE criterion.  
      The techniques used in the publications described above use the concept that mathematically, the columns of the channel matrix, H, can be viewed as describing the basis of a lattice. An equivalent description of this lattice (a so-called ‘reduced basis’) can therefore be calculated so that the basis vectors are close to orthogonal. If the receiver then uses this reduced basis to equalise the channel, noise enhancement can be kept to a minimum and detection performance will improve (such as, as illustrated in  FIG. 5  in Wubben et al.). This process comprises the steps described as follows:  
      y r , x r  and H r  are defined to be the real-valued representations of y, x, and H respectively, such that:  
           y   r     =     [           Re   ⁡     (   y   )                 Im   ⁡     (   y   )             ]       ,       x   r     =     [           Re   ⁡     (   x   )                 Im   ⁡     (   x   )             ]       ,       H   r     =     [           Re   ⁡     (   H   )             -     Im   ⁡     (   H   )                   Im   ⁡     (   H   )             Re   ⁡     (   H   )             ]           
 
 where Re( ) and Im( ) denote the real and imaginary components of their arguments. 
 
      It will be noted that Berenguer et al. describes the equivalent method in the complex plane, though for the purpose of clarity the Real axis representation of the method is used herein.  
      A number of lattice reduction algorithms exist in the art. One suitable lattice reduction algorithm is the Lenstra-Lenstra-Lovasz (LLL) algorithm referred to above, which is disclosed in Wubben et al., and also in “Factoring Polynomials with Rational Coefficients”, (A. Lenstra, H. Lenstra and L. Lovasz, Math Ann., Vol. 261, pp. 515-534, 1982, hereinafter referred to as “Lenstra et al.”), and in “An Algorithmic Theory of Numbers, Graphs and Convexity”, (L. Lovasz, Philadelpia, SIAM, 1980, hereinafter referred to as “Lovasz”).  
      Any one of these can be used to calculate a transformation matrix, T, such that a reduced basis, {tilde over (H)} r , is given by 
 
{tilde over (H)} r =H r T 
 
      The matrix T contains only integer entries and its determinant is ±1.  
      After lattice reduction, the system is re-expressed as:  
               y   r     =         H   r     ⁢     x   r       +     v   r                   =         H   r     ⁢   T   ⁢           ⁢     T     -   1       ⁢     x   r       +     v   r                   =           H   ~     r     ⁢     T     -   1       ⁢     x   r       +     v   r                   =           H   ~     r     ⁢   z     +     v   r                 
 
 where z r =T −1 x r . The received signal, y r , in this redefined system is then equalised to obtain an estimate of z r . This equalisation process then employs, for example, a linear ZF technique, which obtains: 
 
{tilde over ( z )} r =( {tilde over (H)}   r   *{tilde over (H)}   r ) −1   {tilde over (H)}   r   *y   r  
 
      Since {tilde over (H)} r  is close to orthogonal, {tilde over (z)} r  should suffer much less noise enhancement than if the receiver directly equalised the channel H r .  
      Of course, other equalisation techniques could be used. For example, MMSE techniques, or more complex successive interference cancellation based methods, such as in the published prior art identified above, could be considered for use.  
      A receiver in accordance with the above operates in the knowledge that the transmitted symbols contained in x are obtained from an M-QAM constellation. With this constraint, {tilde over (z)} r  can then be quantised in accordance with the method indicated in Wubben et al.:  
           z   ^     r     =     a   (       Q   ⁢     {         1   a     ⁢       z   ~     r       -       1   2     ⁢     T     -   1       ⁢     1     2   ⁢           ⁢   m           }       +       1   2     ⁢     T     -   1       ⁢     1     2   ⁢           ⁢   m           )         
 
 where Q{ } is the quantisation function that rounds each element of its argument to the nearest integer, and where 1 2m  is a 2*m-by-1 vector of ones. 
 
      It will be understood from the above that, the quantisation function apart, the remaining operations are a result of M-QAM constellations being scaled and translated versions of the integer lattice. The integer quantisation therefore requires the same simple scaling and translation operations.  
      The scalar value a is obtained from the definition of the M-QAM constellation in use, and is equal to the minimum distance between two constellation points. In the present example, a 16-QAM constellation is used, having real and imaginary components of {±1, ±3}. Therefore, as shown in  FIG. 3 , a=2.  
      Finally, the estimate {circumflex over (x)} r  of x r  is obtained by this method as 
 
{circumflex over (x)} r =T{circumflex over (z)} r  
 
      Occasionally, if errors are present in the estimate of {circumflex over (z)} r  then it is possible that some of the symbol estimates in {circumflex over (x)} r  may not be valid symbols. In such cases, these symbols are mapped to the nearest valid symbol. For example, for the present example employing 16-QAM, the values ±1, ±3 may define the valid entries in {circumflex over (x)} r . Therefore if a component of {circumflex over (x)} r  were, for example, equal to +5, then this would be mapped to a value of +3.  
       FIG. 1  demonstrates the advantages of techniques in accordance with the published art, including the above described example thereof, over other MIMO detection methods for an uncoded system. ‘ZF’ and ‘MMSE’ refer to the standard linear detection methods, ‘RL-ZF’ and ‘RL-MMSE’ refer to the lattice reduction aided methods, and ‘Sphere’ refers to results obtained using the Sphere decoding algorithm (almost identical to the performance of maximum-likelihood detection).  
      Such reduced lattice detectors (e.g. for MIMO systems) usually output hard decisions. The only mention in the literature of a technique that could be employed for obtaining soft-output is “From Lattice-Reduction-Aided Detection Towards Maximum-Likelihood Detection in MIMO Systems”, (C. Windpassinger, L. Lampe and R. Fischer, in Proc. Int. Conf. on Wireless and Optical Communications, Banff, Canada, July 2003, hereinafter referred to as “Windpassinger et al.”). The method that Windpassinger et al. proposes is complex, and the performance of this technique was not validated in the publication. Therefore it is an aim of aspects of the present invention to provide a MIMO detector capable of determining a soft output using a simple and proven approach.  
      U.S. Pat. No. 6,724,843 describes a detector in which received symbols are decoded in a multiple-antenna communication system using lattice-based decoding. The symbols are generated using a modulation constellation, e.g., a diagonal modulation constellation, and the constellation is characterized as a lattice for decoding purposes. For example, if a given communication link of the multiple-antenna communication system includes M transmitter antennas and a single receiver antenna, the diagonal modulation constellation can be characterized as a lattice in M dimensions. A differential decoding operation for received differential symbols involves a determination of the closest point in the lattice corresponding to the constellation. This determination may be made in an efficient manner using a basis reduction algorithm which generates an approximately orthogonal basis for the lattice, and then utilizes component-wise rounding to determine the closest point. The lattice-based decoding has a complexity which is polynomial rather than exponential in the particular number of antennas and the system rate, but is nonetheless able to deliver error rate performance which approximates that of maximum likelihood decoding.  
      Therefore, with the exception of Windpassinger et al., the reduced lattice detection schemes described in all other references only output hard decisions for the estimate of the transmitted symbol vector, {circumflex over (x)}. When used in a system with an outer channel code (i.e. in any practical system) the performance of the code can be substantially improved if it is supplied with soft-information, e.g. a log-likelihood ratio (LLR) for each bit.  
      An aspect of the invention provides a method for determining soft estimates of transmitted bit values from a received signal in a lattice-reduction-aided receiver based wireless communications system, the method comprising obtaining an estimate of the channel response, applying lattice reduction to said channel response and equalising said received signal in accordance with the reduced basis channel, and determining probabilities of transmitted bits having particular values by means of selecting a set of candidate vectors in the reduced basis, determining a corresponding transmitted symbol vector for each candidate vector and, on the basis of the received signal determining the probability of each transmitted bit value estimate having been transmitted.  
      Preferably, the method provides applying lattice reduction in accordance with the LLL algorithm.  
      The method is suitable for use in a MIMO wireless communications system. Further, it can be used with any other system wherein a received signal is the result of transmission from a plurality of antennas, which may or may not be collocated. Further, the method is applicable to CDMA systems, such as multi-user detection (MUD).  
      Another aspect of the invention provides a receiver for use in a lattice-reduction-aided receiver based wireless communications system, the receiver comprising means for obtaining an estimate of the channel response and a detector operable to process a received signal so as to determine soft estimates of transmitted bit values, the detector comprising means for applying lattice reduction to the channel response estimate and equalising said received signal in accordance with the reduced basis channel, means for selecting a set of candidate vectors in the reduced basis, transmitted symbol vector determining means for determining a transmitted symbol vector for each candidate vector and probability determining means operable to determine, on the basis of the received signal the probability of each transmitted bit value having been transmitted.  
      Another aim of the present invention is to provide a method for obtaining log-likelihood ratios (LLRs) for bits at the output of a lattice-reduction-aided MIMO receiver.  
      To this end, the probabilities determined in either the method above or by the detector above can be converted into LLRs. 
    
    
       FIG. 1  illustrates a graph of performance of prior art examples described above in comparison with standard MIMO detection methods for an uncoded system;  
       FIG. 2  illustrates a graph of performance of prior art examples described above in comparison with standard MIMO detection methods for a coded system;  
       FIG. 3  illustrates a graph of a lattice used in the wireless communications system of a specific embodiment of the invention, and used in the described examples of the prior art;  
       FIG. 4  illustrates schematically a MIMO system including a transmitter and a receiver;  
       FIG. 5  illustrates in further detail the receiver of  FIG. 4 ;  
       FIG. 6  illustrates a detecting method operable by means of the detector illustrated in  FIG. 5 . 
    
    
      The present invention will now be described with reference to an implementation thereof for the equalization of a wireless communication system.  FIG. 4  illustrates such a system, comprising a MIMO data communications system  10  of generally known construction. New components, in accordance with a specific embodiment of the invention, will be evident from the following description.  
      The communications system  10  comprises a transmitter device  12  and a receiver device  14 . It will be appreciated that in many circumstances, a wireless communications device will be provided with the facilities of a transmitter and a receiver in combination but, for this example, the devices have been illustrated as one way communications devices for reasons of simplicity.  
      The transmitter device  12  comprises a data source  16 , which provides data (comprising information bits or symbols) to a channel encoder  18 . The channel encoder  18  is followed by a channel interleaver  20  and, in the illustrated example, a space-time encoder  22 . The space-time encoder  22  encodes an incoming symbol or symbols as a plurality of code symbols for simultaneous transmission from a transmitter antenna array  24  comprising a plurality of transmit antennas  25 . In this illustrated example, three transmit antennas  25  are provided, though practical implementations may include more, or less antennas depending on the application.  
      The encoded transmitted signals propagate through a MIMO channel  28  defined between the transmit antenna array  24  and a corresponding receive antenna array  26  of the receiver device  14 . The receive antenna array  26  comprises a plurality of receive antennas  27  which provide a plurality of inputs to a lattice-reduction-aided decoder  30  of the receiver device  14 . In this specific embodiment, the receive antenna array  26  comprises three receive antennas  27 .  
      The lattice-reduction-aided decoder  30  has the task of removing the effect of the MIMO channel  28 . The output of the lattice-reduction-aided decoder  30  comprises a plurality of signal streams, one for each transmit antenna  25 , each carrying so-called soft or likelihood data on the probability of a transmitted bit having a particular value. This data is provided to a channel de-interleaver  32  which reverses the effect of the channel interleaver  20 , and the de-interleaved bits output by this channel de-interleaver  32  are then presented to a channel decoder  34 , in this example a Viterbi decoder, which decodes the convolutional code. The output of channel decoder  34  is provided to a data sink  36 , for further processing of the data in any desired manner.  
      The specific function of the lattice-reduction-aided decoder  30  will be described in due course.  
       FIG. 5  illustrates schematically hardware operably configured (by means of software or application specific hardware components) as the receiver device  16 . The receiver device  16  comprises a processor  110  operable to execute machine code instructions stored in a working memory  112  and/or retrievable from a mass storage device  116 . By means of a general purpose bus  114 , user operable input devices  118  are capable of communication with the processor  110 . The user operable input devices  118  comprise, in this example, a keyboard and a mouse though it will be appreciated that any other input devices could also or alternatively be provided, such as another type of pointing device, a writing tablet, speech recognition means, or any other means by which a user input action can be interpreted and converted into data signals.  
      Audio/video output hardware devices  120  are further connected to the general purpose bus  114 , for the output of information to a user. Audio/video output hardware devices  120  can include a visual display unit, a speaker or any other device capable of presenting information to a user.  
      Communications hardware devices  122 , connected to the general purpose bus  114 , are connected to the antenna  26 . In the illustrated embodiment in  FIG. 5 , the working memory  112  stores user applications  130  which, when executed by the processor  110 , cause the establishment of a user interface to enable communication of data to and from a user. The applications in this embodiment establish general purpose or specific computer implemented utilities that might habitually be used by a user.  
      Communications facilities  132  in accordance with the specific embodiment are also stored in the working memory  112 , for establishing a communications protocol to enable data generated in the execution of one of the applications  130  to be processed and then passed to the communications hardware devices  122  for transmission and communication with another communications device. It will be understood that the software defining the applications  130  and the communications facilities  132  may be partly stored in the working memory  112  and the mass storage device  116 , for convenience. A memory manager could optionally be provided to enable this to be managed effectively, to take account of the possible different speeds of access to data stored in the working memory  112  and the mass storage device  116 .  
      On execution by the processor  110  of processor executable instructions corresponding with the communications facilities  132 , the processor  110  is operable to establish communication with another device in accordance with a recognised communications protocol.  
      The function of the lattice-reduction aided decoder  30  will now be described in further detail in accordance with  FIG. 6 . This method as illustrated commences once the quantised estimate of the transmitted lattice point in the reduced basis, i.e. {circumflex over (z)} r , has been determined as outlined in the introduction and discussion of the prior art above. The manner in which this estimate is obtained is immaterial: any appropriate lattice reduction algorithm may have been used, and any of a number of equalisation methods may have been applied.  
      In step S 1 - 2 , the vector {circumflex over (z)} r  is taken as the first entry in a list of candidate vectors. Other candidate vectors are then obtained in step S 1 - 4  by modifying one or more elements of the vector {circumflex over (z)} r  and adding these as new candidate vectors to the list.  
      Whilst any of these additional candidate vectors may differ from {circumflex over (z)} r  in more than one element, the example described herein generates candidates by only ever allowing these to vary one element of {circumflex over (z)} r . Creating candidate vectors by allowing perturbations to multiple elements of {circumflex over (z)} r  can slightly improve performance, but at the expense of increasing the length of the candidate list and hence increasing complexity.  
      For the purpose of this description, the ith candidate vector in this list is defined as c (i) , and hence c (1) ={circumflex over (z)} r .  
      For the purpose of this description of a specific embodiment, a particular method of generating a list of candidates is to perturb each element of {circumflex over (z)} r  in turn by ±a (where a is the minimum distance between 2 constellation points, as defined above with reference to  FIG. 3 ).  
      For example, if {circumflex over (z)} r  is a 2-by-1 vector, then there would be 4 additional candidate vectors, giving a total of 5 candidates as follows:  
                 c     (   1   )       =     [             z   ^       r   ⁢           ⁢   1                   z   ^       r   ⁢           ⁢   2             ]       ,                   c     (   2   )       =     [               z   ^       r   ⁢           ⁢   1       +   a                 z   ^       r   ⁢           ⁢   2             ]       ,                   c     (   3   )       =     [               z   ^       r   ⁢           ⁢   1       -   a                 z   ^       r   ⁢           ⁢   2             ]       ,                   c     (   4   )       =     [             z   ^       r   ⁢           ⁢   1                     z   ^       r   ⁢           ⁢   2       +   a           ]       ,                 c     (   5   )       =       [             z   ^       r   ⁢           ⁢   1                     z   ^       r   ⁢           ⁢   2       -   a           ]     .               
 
      The effect of perturbing elements of {circumflex over (z)} r  is to generate other points in the reduced lattice. The perturbations by ±a, give the closest points in the lattice as a is the distance between any two neighbouring points. An implementation may alternatively choose to increase the list of candidates though perturbing elements of {circumflex over (z)} r  by multiples of a (i.e. to not just the closest point, but the closest few points), and/or by perturbing multiple elements of {circumflex over (z)} r  simultaneously rather than just one element at a time.  
      It will be appreciated that other methods of generating suitable candidates are possible, and would provide a suitable list for further processing in accordance with the method. One alternative method would be to perturb not just one element at a time, but to perturb two simultaneously. Therefore, for each pair of elements, the decoder would cycle through the four combinations of perturbing by ±a. Then, the decoder would pick the next pair of elements and repeat in order to generate more candidates.  
      Once a list of candidate vectors in the reduced lattice has been obtained then, in step S 1 - 6 , each candidate is converted to a transmitted symbol vector estimate. The list of transmitted symbol vector estimates is {circumflex over (x)} r   (i) , giving: 
 
x r   (i) =Tc (i)  
 
 where T is the lattice reduction transformation matrix as defined above. 
 
      Just as for the hard-output detector outlined in the introduction and discussion of the prior art above, occasionally it is possible that some of the elements of the vector {circumflex over (x)} r   (i)  may not be valid symbols. Therefore, step S 1 - 8  seeks to determine if this is the case, and, if so, in step S 1 - 10 , these symbols are mapped to the nearest valid symbol. (e.g. for 16-QAM, if the values ±1, ±3 define the valid entries as illustrated in  FIG. 3 , then if an element were for example equal to +5, this would be mapped to a value of +3.)  
      For each candidate symbol vector {circumflex over (x)} r   (i)  (corrected, if required), in step S 1 - 12 , the detector calculates its probability of being transmitted, as:  
         p     (   i   )       =       1       π   ⁢           ⁢     σ   v   2           ⁢     exp   (       -            y   r     -       H   r     ⁢       x   ^     r     (   i   )                    σ   v   2       )           
 
      These probabilities are then used to calculate, in step S 1 - 14 , the probability of symbol x′ having been transmitted from antenna k, where x′εX and X defines the set of symbols in the chosen constellation.  
           P   ⁡     (     k   ,     x   ′       )       =         ∑     {       i   ❘       x   ^     k     (   i   )         =     x   ′       }       ⁢       p     (   i   )       ⁢           ⁢   for   ⁢           ⁢   k       =   1       ,   …   ⁢           ,       m   ⁢           ⁢   and   ⁢           ⁢     x   ′       ∈   X         
 
      Depending on the list of candidates, according to the above definition P may not be specified for all values of k and x′. In these cases P is set to a default (small) value. This default can be a fixed value or it could varied according to a method such as that described in “Adaptive Selection of Surviving Symbol Replica Candidates Based on Maximum Reliability in QRM-MLD for OFCDM MIMO Multiplexing” (K. Higuchi, H. Kawai, N. Maeda and M. Sawahashi, in Proc. IEEE Globecom, Dallas, December 2004), or by any other appropriate method.  
      Now that the receiver has information on the probability of different symbols having been transmitted, these are processed in the conventional way in step S 1 - 16  to obtain a log-likelihood ratio for each transmitted bit. In the present example, this is done as follows:  
         L   ⁡     (     b     k   ,   i       )       =     log   (         ∑       x   ′     ∈     X     (   1   )           ⁢     P   ⁡     (     k   ,     x   ′       )             ∑       x   ″     ∈     X     (   0   )           ⁢     P   ⁡     (     k   ,     x   ″       )           )         
 
 where L(b k,i ) is the log-likelihood ratio of bit b k,i , k indicates the transmit antenna, i=1, . . . , M where M is the number of bits per symbol, and where X(1) and X(0) are the sets of symbols for which b k,i =1 and b k,i =0 respectively. 
 
      The graph of  FIG. 2  sets out experimental performance data of the present method in comparison with prior art decoding methods aiming to provide hard information for the channel decoder.  FIG. 2  demonstrates the benefit that can be obtained by providing a lattice reduction detection scheme to output soft information for the channel decoder.  
      It will be appreciated that the foregoing disclosure of specific embodiments of the invention can be applied to any communications product employing MIMO transmission techniques, to take advantage of the benefits of the invention. Further, the invention is applicable to any circumstance in which the detection of symbols which may be based on multiple input is required. This could arise in systems where a plurality of antennas are provided in separate locations. Further, CDMA MUD may be a suitable basis for use of the method of the present invention.  
      The invention has been described by way of a software implementation. This software implementation can be introduced as a stand alone software product, such as borne on a storage medium, e.g. an optical disk, or by means of a signal. Further, the implementation could be by means of an upgrade or plug-in to existing software.  
      Whereas the invention can be so provided, it could also be by way exclusively by hardware, such as on an ASIC.  
      The reader will appreciate that the foregoing is but one example of implementation of the present invention, and that further aspects, features, variations and advantages may arise from using the invention in different embodiments. The scope of protection is intended to be provided by the claims appended hereto, which are to be interpreted in the light of the description with reference to the drawings and not to be limited thereby.