Abstract:
Signal received in a wireless network are detected by arranging samples of the signal in a set of windows. For each window, estimating interference, an average signal, noise and interference ratio (SINR), and a channel. For each window, also classifying a type of a process of a set of processes of a receiver. Then, selecting the process for each windows according to the type to detect the signal in the window.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to detecting signals in a wireless network, and more particularly to detecting signals using a set of adaptive receiver processes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Signal Classification for Noise-Limited Scenario 
         [0003]    In a baseband, a wireless communication network has the following transmit-receive signal model with complex values 
         [0000]    where y is a channel output, h is a channel gain, s is a modulation input, and n is channel noise. 
         [0004]    With Gaussian channel noise and perfect receiver channel, state information (CSI), an optimal receiver for the above signal model is a zero-forcing, (ZF) receiver followed by a signal slicer. The ZF receiver can be expressed in a compact-form as 
         [0000]    
       
         
           
             
               
                 
                   s 
                   ^ 
                 
                 ZF 
               
               = 
               
                 
                   argmin 
                   
                     x 
                     ∈ 
                     S 
                   
                 
                  
                 
                   
                      
                     
                       
                         y 
                         h 
                       
                       - 
                       x 
                     
                      
                   
                   2 
                 
               
             
             , 
           
         
       
     
         [0000]    where S is a signal constellation, i.e., the set of valid signal points. 
         [0005]    If a minimum mean-square error (MMSE) receiver is used, a decision produced by the MMSE receiver is 
         [0000]    
       
         
           
             
               
                 s 
                 ^ 
               
               MMSE 
             
             = 
             
               
                 argmin 
                 
                   x 
                   ∈ 
                   S 
                 
               
                
               
                 
                    
                   
                     
                       
                         yh 
                         * 
                       
                       
                         
                           
                              
                             h 
                              
                           
                           2 
                         
                         + 
                         
                           σ 
                           N 
                           2 
                         
                       
                     
                     - 
                     x 
                   
                    
                 
                 2 
               
             
           
         
       
     
         [0000]    where σ N   2  is a variance of the noise n 
         [0000]      σ N   2   =E└|n|   2 ┘.
 
         [0006]    In the absence of noise, the MMSE estimate is 
         [0000]    
       
         
           
             
               
                 
                   yh 
                   * 
                 
                 
                   
                     
                        
                       h 
                        
                     
                     2 
                   
                   + 
                   
                     σ 
                     N 
                     2 
                   
                 
               
               = 
               
                 
                   ( 
                   
                     
                       
                          
                         h 
                          
                       
                       2 
                     
                     
                       
                         
                            
                           h 
                            
                         
                         2 
                       
                       + 
                       
                         σ 
                         N 
                         2 
                       
                     
                   
                   ) 
                 
                  
                 s 
               
             
             , 
           
         
       
     
         [0000]    which is strictly less than s. That is, the estimate is biased, and signal scaling leads to performance degradation for higher order constellations. 
         [0007]    Signal Classification in the Presence of Noise and Interference 
         [0008]    In the presence of interference, the received signal model is 
         [0000]    
       
      
       y=h×s+g×u+n, 
      
     
         [0000]    where, y, h and s are defined above, u an interfering modulation symbol, and g is the channel between an interfering transmitter and the receiver. 
         [0009]    If the receiver knows the interfering channel and the interfering modulation symbol, then an optimal detector cancels the contribution of g×u from y before performing a zero-forcing detection. 
         [0010]    That is, 
         [0000]    
       
         
           
             
               
                 s 
                 ^ 
               
               ZF 
             
             = 
             
               
                 argmin 
                 
                   x 
                   ∈ 
                   S 
                 
               
                
               
                 
                    
                   
                     
                       
                         y 
                         - 
                         
                           g 
                           × 
                           u 
                         
                       
                       h 
                     
                     - 
                     x 
                   
                    
                 
                 2 
               
             
           
         
       
     
         [0000]    is the ZF detector output. 
         [0011]    If the interfering channel and the interference constellation type, i.e., u coming from QPSK or 16-QAM or 64-QAM constellation, are known, then the optimal detector performs joint detection of s and u 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   
                     
                       s 
                       ^ 
                     
                     ML 
                   
                   , 
                   
                     
                       u 
                       ^ 
                     
                     ML 
                   
                 
                 ) 
               
               = 
               
                 
                   argmin 
                   
                     
                       x 
                       ∈ 
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                     , 
                     
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                       ∈ 
                       
                         S 
                         u 
                       
                     
                   
                 
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                       - 
                       
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                         × 
                         x 
                       
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                         g 
                         × 
                         v 
                       
                     
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                   2 
                 
               
             
             , 
           
         
       
     
         [0000]    where S u  is the signal constellation of the interference. 
         [0012]    If the constellation of u is unknown, then a linear MMSE solution produces an estimate of s that is mean-square optimal. That is, 
         [0000]    
       
         
           
             
               
                 s 
                 ^ 
               
               MMSE 
             
             = 
             
               
                 argmin 
                 
                   x 
                   ∈ 
                   S 
                 
               
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                           yh 
                           * 
                         
                         
                           
                             
                                
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                             2 
                           
                           + 
                           
                             
                                
                               g 
                                
                             
                             2 
                           
                           + 
                           
                             σ 
                             N 
                             2 
                           
                         
                       
                       - 
                       x 
                     
                      
                   
                   2 
                 
                 . 
               
             
           
         
       
     
         [0013]    A signal classifier that classifies the received signal according to whether the channel and/or the modulation symbol of the interference is known can enable the receiver perform optimal detection on a symbol-by-symbol basis. 
         [0014]    Signal Classification with Multiple Receiver Antennas 
         [0015]    Unlike the scenarios above, a single antenna transmission with multiple receiver antennas in the presence of multiple interferers is now described. The received signal model is 
         [0000]    
       
         
           
             
               y 
               _ 
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         y 
                         1 
                       
                     
                   
                   
                     
                       
                         y 
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                       ⋮ 
                     
                   
                   
                     
                       
                         y 
                         L 
                       
                     
                   
                 
                 ] 
               
               = 
               
                 
                   
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                     ] 
                   
                   × 
                   s 
                 
                 + 
                 
                   
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                       = 
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                       l 
                     
                   
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                    
                   
                     
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                                 ( 
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                                 ( 
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                     × 
                     
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                       k 
                     
                   
                 
                 + 
                 
                   [ 
                   
                     
                       
                         
                           n 
                           1 
                         
                       
                     
                     
                       
                         
                           n 
                           2 
                         
                       
                     
                     
                       
                         ⋮ 
                       
                     
                     
                       
                         
                           n 
                           L 
                         
                       
                     
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    where L is the number of receiver antennas, the channel gains h 1 ,h 2 , . . . , h L  are the channel coefficients between the desired transmitter and the desired receiver, u k  is the k-th interfering transmitter, g 1 (k),g 2 (k), . . . , g L (k) are the channel gains between the k th  interferer and the desired receiver, and n 1  is the additive noise on the l th  receiver antenna. 
         [0016]    The receiver that completely ignores the interference component is the maximal-ratio combining (MRC) receiver. The soft signal estimate of the MRC receiver is given by 
         [0000]    
       
         
           
             
               
                 
                   s 
                   ^ 
                 
                 
                   soft 
                   , 
                   MRC 
                 
               
               = 
               
                 
                   
                     ∑ 
                     
                       l 
                       = 
                       1 
                     
                     L 
                   
                    
                   
                       
                   
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                       l 
                     
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                       = 
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                        
                       
                         h 
                         l 
                       
                        
                     
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             , 
           
         
       
     
         [0017]    and the corresponding received SINR is 
         [0000]    
       
         
           
             
               Γ 
               MRC 
             
             = 
             
               
                 
                   
                      
                     
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                       _ 
                     
                      
                   
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                           h 
                           _ 
                         
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                        
                       
                         ( 
                         
                           
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                               = 
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                               l 
                             
                           
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                            
                           
                             
                               
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                                 ( 
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                                 ( 
                                 k 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
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                       n 
                       2 
                     
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                          
                         
                           h 
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                          
                       
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               . 
             
           
         
       
     
         [0018]    Note that in the absence of any interference, the above SINR simplifies to 
         [0000]    
       
         
           
             
               
                 Γ 
                 MRC 
               
               = 
               
                 
                   
                      
                     
                       h 
                       _ 
                     
                      
                   
                   2 
                 
                 
                   σ 
                   n 
                   2 
                 
               
             
             , 
           
         
       
     
         [0000]    which is the SNR corresponding o the best possible receiver. 
         [0019]    Note that the presence or absence of interference completely changes the distribution properties of the SINR. In the absence of interference, the SINR (or SNR) is distributed as Chi-squared (or Gamma random variable) with 2L degrees of freedom for uncorrelated zero-mean and unit-variance complex-Gaussian channel gains. The peak of SNR probability distribution function (pdf) shifts to the right as the number of antennas increases. On the other hand, in the presence of interference the SINR is no more Chi-square distributed. With increasing number of interferers, the SINR pdf shifts to the left, and the average SINR also decreases. 
         [0020]    Instead of MRC, if an optimal receiver weight w is applied, the soft symbol estimate with this weight is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       s 
                       ^ 
                     
                     
                       soft 
                       , 
                       
                         rx 
                         - 
                         weight 
                       
                     
                   
                   = 
                     
                    
                   
                     
                       
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                         _ 
                       
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                      
                     
                       y 
                       _ 
                     
                   
                 
               
             
             
               
                 
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                         ( 
                         
                           
                             
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                            
                           
                             h 
                             _ 
                           
                         
                         ) 
                       
                        
                       s 
                     
                     + 
                     
                       
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                           k 
                           = 
                           1 
                         
                         
                           N 
                           l 
                         
                       
                        
                       
                           
                       
                        
                       
                         
                           
                             w 
                             _ 
                           
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                             ( 
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                             ) 
                           
                         
                          
                         
                           u 
                           k 
                         
                       
                     
                     + 
                     
                       
                         
                           w 
                           _ 
                         
                         Herm 
                       
                        
                       
                         n 
                         _ 
                       
                     
                   
                 
               
             
           
         
       
     
         [0021]    As an example, the interference-rejection combining linear receiver has the following weight vector: 
         [0000]    
       
         
           
             
               w 
               _ 
             
             = 
             
               
                 
                   ( 
                   
                     
                       
                         
                           h 
                           _ 
                         
                         Herm 
                       
                        
                       
                         h 
                         _ 
                       
                     
                     + 
                     
                       
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                           k 
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                           1 
                         
                         
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                        
                       
                           
                       
                        
                       
                         
                           
                             
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                             ( 
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                             ( 
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                         L 
                       
                     
                   
                   ) 
                 
                 
                   - 
                   1 
                 
               
                
               
                 h 
                 _ 
               
             
           
         
       
     
         [0022]    The SINR with optimally chosen w is 
         [0000]    
       
         
           
             
               Γ 
               
                 rx 
                 - 
                 weight 
               
             
             = 
             
               
                 
                    
                   
                     
                       
                         w 
                         Herm 
                       
                        
                       h 
                     
                     _ 
                   
                    
                 
                 2 
               
               
                 
                   
                     
                       
                         w 
                         _ 
                       
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                      
                     
                       ( 
                       
                         
                           ∑ 
                           
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                             = 
                             1 
                           
                           
                             N 
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                          
                         
                             
                         
                          
                         
                           
                             
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                               ( 
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                              
                             
                               ( 
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                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                    
                   
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                     _ 
                   
                 
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                     n 
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                    
                   
                     
                        
                       
                         w 
                         _ 
                       
                        
                     
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         [0023]    Clearly, by building knowledge of the distribution of the received SINR over a large observation window, the receiver can make an intelligent decision in using the right receiver type for the prevailing channel conditions. 
       SUMMARY OF THE INVENTION 
       [0024]    Signal received in a wireless network are detected by arranging samples of the signal in a set of windows. For each window, estimating interference, an average signal, noise and interference ratio (SINR), and a channel. For each window, also classifying a type of a process of a set of processes of a receiver. Then, selecting the process for each windows according to the type to detect the signal in the window 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic of an OFDM signal processed by embodiments of the invention; and 
           [0026]      FIG. 2  is a flow diagram of a method for detecting signals according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Signal Classification for Adaptive Signal Reception 
         [0028]    The embodiments of the invention provide a method for classifying a signal observed adaptively over time-frequency windows in a receiver, and then to detect the signal in each Window based on a receiver process type. 
         [0029]    We focus on a signal that is encoded using orthogonal frequency-division multiplexing (OFDM), wherein the time-frequency window corresponds to a set of OFDM symbols in a time domain, and a set of sub-carriers in a frequency domain. 
         [0030]    The first step to signal classification is the estimation of both instantaneous (short term) as well as average (long term) channel parameters, including an average signal power, an average noise power, and an average interference power. Instantaneous channel parameters include a complex-valued channel gain between a transmitter and the receiver. 
         [0031]    Estimation of Noise Power 
         [0032]    The null-subcarriers, i.e., the subcarriers that are not used for data or pilot transmission, in the OFDM signal can be used to estimate the average noise power per subcarrier. 
         [0033]    Estimation of Noise and Interference Power Some subcarriers are not allowed for modulation. That is, no modulated data are transmitted over these subcarriers. By measuring the total received power on these subcarriers, one can estimate the interference and noise Power. 
         [0034]    Estimation of Signal, Noise, and Interference Power 
         [0035]    The signal, noise, and interference power is the total power received on modulated data subcarriers. 
         [0036]    Using the above three estimates, it is possible to individually estimate the signal power, noise power and interference power. 
         [0037]      FIG. 1  shows a time-frequency view of the OFDM signals. Here, the OFDM symbols  102  are modulated in the frequency-domain over subcarriers  102 . A signal classifier  210  (see  FIG. 2 ) has access to a. base-band received signal R(k,  201 , where k is the frequency index and n is the OFDM symbol index, over a number of OFDM symbols. 
         [0038]    Samples of the signal are partitioned into a number of windows  110 , each window contains predetermined number subcarriers over a pre-determined number of OFDM symbols. Within each window, some known data symbols (also referred to as “pilots P”) are used for signal estimation purposes. It should be noted that the invention can also be used with other signaling modalities, such as Orthogonal Frequency-Division Multiple Access (OFDMA), and other estimation techniques. 
         [0039]      FIG. 2  shows the operation of a receiver including an adaptive signal classifier according to embodiments of the invention. As used herein, adaptive mean the receiver adjusts to the dynamics of the received signal over time, as the channel varies. 
         [0040]    The classifier based signal detection method is described as follows. The step of the method can be performed in a receiver including a processor, memory, and input/output interfaces as known in the art. 
         [0041]    Samples of a received base-band signal R(k, n)  201 , where k is the sub-carrier index and n is the OFDM symbol index, are arranged in a set of observation windows W, where within each window includes a predetermined number of samples of the signal R(k, n). 
         [0042]    For each window, we obtain an estimate  210  of the channel, an estimate of the interference, an average signal, noise and interference power (SINR). We also obtain the estimate of the SINR from these average signal power, noise power and interference power estimates. 
         [0043]    The estimated SINR on window w is denoted by Γ(w), and the estimated channel over window w is denoted by g(w). 
         [0044]    With W windows, the SINRs Γ( 1 ), . . . , Γ(W) are used to construct a histogram of the SINR stored in a memory. 
         [0045]    The SINR histogram enables us characterize the channel experienced by the received samples R(k, n) during the observation window. 
         [0046]    We also obtain various short and long terms statistics  221  of the SINR, for example, the average SINR and the probability that the SINR is less than a predefined threshold, also-called the SINR outage probability, 
         [0047]    The histogram enables us to estimate the types  211  of an optimal process of a set of process  231 - 233  be used for detecting the signals  251 - 252 . The process types are fed to an adaptive switch  240 , as each of the samples in each window are received and processed. 
         [0048]    The parameters  221  are fed to the processes associated with various types of processes. 
         [0049]    For example, we have a set of three process types:
       a) ZF process  231 ;   b) MMSE process  232 ; and   c) interference rejection (IR) process  233 .       
 
         [0053]    Additional process types can also be defined. 
         [0054]    ZF process 
         [0055]    We select two SINR thresholds. We denote these thresholds by Γ — 1 and Γ — 2, such that Γ — 1 is less than Γ — 2. 
         [0056]    We maintain a list of all the windows for which Γ(w) is not less than Γ — 2. The samples of these windows and the corresponding channel estimates are fed to the ZF process  231 . The ZF process operates on each of these windows separately. For each window, the corresponding channel estimate is used to detect the signals  251  as described above. 
         [0057]    MMSE Process 
         [0058]    We maintain a list all the windows for which Γ(w) is between Γ — 1 and Γ — 2. The samples of these windows and the corresponding channel estimates are fed to the linear MMSE process  232 . The MMSE process operates on each of these windows separately. For each window, the corresponding channel estimate and the noise variance are used to detect the signals  252 . 
         [0059]    Interference Rejection Process 
         [0060]    We maintain a list of all the windows for which Γ(w) is less than Γ — 1. The samples of these windows and the corresponding channel estimates are fed to the interference rejection (IR) process. This process can use interference rejection combining (IRC), a maximum likelihood (ML) process, or any other receiver process with optimal interference cancelation. The IR process operates on each of these windows separately. For each window, the corresponding channel estimate of the signal, an interfering user, the noise and interference power are used to detect the signals. 
         [0061]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.