Abstract:
In accordance with some embodiments, a communication system is described that comprises a soft-output detector configured to receive a transmit signal and output reliability information regarding the received signal. In accordance with such embodiments, the detector comprises a symbol combiner configured to operate in both a repetition mode and a non-repetition mode, wherein repetition mode comprises receiving a plurality of signals for the transmit signal.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to wireless communications, and more particularly, to performing SISO detection in a WiMAX environment. 
     BACKGROUND 
     Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. To improve a given link&#39;s robustness, WiMAX (Worldwide Interoperability for Microwave Access) systems based on the IEEE 802.16 standard may provide for the repetition of data over a SISO (single-input single-output) channel. Various modes may be supported, such as a classical repetition protocol where exact copies of a QAM signal sequence or codeword are resent. Other modes include Hybrid ARQ (HARQ), which is a variation of the ARQ error control method. Both protocols involve at least some level of repetition involving transmission of QAM signals. 
     Those skilled in the art will appreciate that there is a need for optimized implementations of SISO detectors for application in shared communications medium, such as, but not limited to, a wireless network whereby computational resources are efficiently utilized. The wireless network may conform, for example, to the WiMAX standard. Furthermore, there is a need for improving the SNR (signal-to-noise ratio) associated with received signals and ultimately for improving the overall error performance when a repetition channel is considered involving, for example, WiMAX&#39;s repetition and HARQ modes. 
     SUMMARY 
     Briefly described, one embodiment, among others, is a communication system. The system comprises a soft-output detector configured to receive a transmit signal and output reliability information regarding the received signal. In accordance with such embodiments, the detector comprises a symbol combiner configured to operate in both a repetition mode and a non-repetition mode, wherein repetition mode comprises receiving a plurality of signals for the transmit signal. 
     Another embodiment is a method that comprises receiving a transmit signal in a WiMAX (Worldwide Interoperability for Microwave Access) environment. In response to receiving a plurality of signals for the transmit signal, the method further comprises combining the plurality of signal into a single signal. The method also comprises demodulating the single signal and providing reliability information regarding the demodulated symbol. 
     Another embodiment is communication system that comprises means for receiving a transmit signal in a WiMAX (Worldwide Interoperability for Microwave Access) environment, means for combining a plurality of signals into a single signal when a plurality of signals is received for the transmit signal, and means for performing zero-forcing (ZF) equalization on the transmit signal when a single signal is received. 
     Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a wireless communication system incorporating a detector. 
         FIG. 2  is an embodiment of the detector shown in  FIG. 1 . 
         FIG. 3  is an alternative embodiment of the SISO detector shown in  FIG. 1 . 
         FIG. 4  is an equivalent model for a wireless communication system incorporating a SISO detector whereby the same signal is transmitted over R independent channels. 
         FIGS. 5-6  provide exploded views of various components in the symbol combiner shown in  FIG. 4 . 
         FIG. 7  is an exemplary embodiment of the detector shown in  FIG. 4  where the real and imaginary parts of the received signal are processed in parallel. 
         FIG. 8  is an embodiment for performing detection in the wireless communication system shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Having summarized various aspects of the present disclosure, reference will now be made in detail to the description of the disclosure as illustrated in the drawings. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure as defined by the appended claims. 
     Various embodiments are described herein for performing soft-output detection that may be used in SISO (single-input single-output) communication systems. In particular, the systems and methods described herein may be used within a shared communications medium, such as, but not limited to, a wireless network. The wireless network may conform, for example, to the WiMAX (Worldwide Interoperability for Microwave Access) standard. Those skilled in the art will appreciate that the embodiments described herein are directed to an optimized approach for hardware implementation of SISO detectors. 
     Various embodiments described include a symbol combiner that efficiently improves the received SNR (signal-to-noise ratio) and ultimately the overall error performance when a repetition channel is incorporated that utilizes, for example, WiMAX&#39;s repetition and HARQ modes. The exemplary symbol combiners described herein are based on a maximum ratio combining (MRC) algorithm, which reverts back to a classical zero-forcing (ZF) equalizer when a non-repetition channel is involved. According to some embodiments, the overall architecture for the detector may be implemented using low-complexity ASK (amplitude shift key) demodulators, which are used to demodulate QAM signals transmitted over a SISO channel. 
     Reference is made to  FIG. 1 , which depicts a wireless communication system incorporating a SISO detector in accordance with some embodiments. The system includes a FEC (forward error correction) encoder  104  and a QAM modulator  106 . In the system shown, FEC (forward error correction) coded QAM signals are transmitted over a SISO channel with a complex gain h  108 . The received QAM signal y is received by the SISO detector  108 , which then forwards the signal to an FEC decoder  110 , which outputs the received bits  112 . 
     In accordance with some embodiments, the SISO detector  108  receives QAM signal y and outputs a bit-wise soft-reliability information represented by Λ. The FEC decoder  110  receives this vector of bit-wise soft-reliability information for received signals. For purposes of nomenclature, the soft-information for the individual bits is represented as Λ(b 0 ), . . . , Λ(b M−1 ). Generally, an approximation for the m th  bit may be represented by the LLR (log likelihood ratio) as follows: 
               Λ   ⁡     (     b   m     )       =       -     1     2   ⁢           ⁢     σ   2           ⁢     (              y   -     hx     m   ,   1              2     -            y   -     hx     m   ,   0              2       )                 where                 x     m   ,   z       =         arg   ⁢           ⁢   min           x   ∈   Ω     |     b   m       =   z       ⁢            y   -   hx          2     ⁢           ⁢     ∀     m   ∈     {     0   ,   …   ⁢           ,     M   -   1       }             ,     z   ∈     {     0   ,   1     }       ,         
and where Ω|b m =z is the subset of Ω formed by signals with b m =z. As the gain h is generally not available, an estimate of h, denoted as ĥ, may be utilized. For purposes of this disclosure, a perfect channel estimation (i.e., h=ĥ) will be assumed.
 
     Embodiments of a low-complexity architecture are described herein for computing the bit-wise LLR Λ(b m ) based on an input signal {tilde over (y)}. In accordance with some embodiments, the SISO detector  108  may include an equalization module  114  and a demodulation module  116 . The equalization module  114  filters each received QAM signal. This may be performed, for example, utilizing a single-tap equalizer, represented by f, where f is a complex scalar. The equalized signal is represented by {tilde over (y)} and is equal to the product, f·y. It should be noted that the equalization steps involves approximately just one complex multiplication per received QAM signal. 
     The demodulation module  116  demodulates the equalized signal into soft-reliability metrics on a bit-by-bit basis. As described earlier, these soft-reliability metrics are denoted by Λ(b 0 ), . . . , Λ(b M−1 ). As the QAM modulator  106  utilizes independent Gray coding for each dimension of a signal, the QAM demodulator  106  operates on the real and imaginary parts of each equalized QAM signal ({tilde over (y)} i  and {tilde over (y)} q ) independently. Accordingly, ASK demodulators are utilized in exemplary embodiments of the SISO detector  108 . 
     Reference is now made to  FIG. 2 , which depicts an embodiment of the SISO detector shown in  FIG. 1 . The non-limiting example depicted in  FIG. 2  assumes serial demodulation of {tilde over (y)} i  and {tilde over (y)} q . However, the SISO detector can be adapted to also support a parallel demodulation scheme. As described earlier, the received signal y is first filtered by the equalization module  114 . The equalized signal is represented by {tilde over (y)} and is equal to the product, f·y. For the embodiment shown in  FIG. 2 , a zero forcing (ZF) equalizer f=h −1    202  is considered. As described earlier, it should be noted that exemplary embodiments of SISO detectors revert to a classical zero-forcing (ZF) equalizer when a non-repetition channel is involved. 
     As depicted in  FIG. 2 , the real  204  and imaginary  206  components of {tilde over (y)} are processed separately. The real  204  and imaginary  206  components are passed to a rescaling block  208  where the real  204  and imaginary  206  components are rescaled by an inverse scaling factor λ to de-normalize the received equalized signal and rescale the signal to a point Y in the odd integer 1-D grid. The inverse scaling factor λ is related to the value M, which varies as a function of the constellation size and represents the number of bits per constellation. The inverse scaling factor λ as a function of the modulation order M is shown in the table below. 
                                         TABLE 1                       M   2   4   6                           λ             2     4           {square root over (10)}   {square root over (42)}                        
Once the equalized received signal is rescaled or normalized, the received signal is forwarded to saturation block  210  where the received signal is saturated to a closed range. In the non-limiting example shown in  FIG. 2 , the received signal is saturated to ±√{square root over (2 M )}. The signal Y is then sent to an appropriate normalized LLR computation module  212 .
 
     One of ordinary skill in the art will appreciate that scaling to the odd integer grid using rescaling block  208  removes the need for complex operations such as multiplication operations while computing the normalized LLR {tilde over (Λ)}(b m ). It should be noted, however, that a scaling operation is required to convert the normalized LLR back to its true value. As depicted in the normalized LLR computation module  212 , the LLR for a modulation order of 2 (M=2) can be derived without the use of an ASK demodulator, thereby further reducing the need for complex operations while computing the LLR. For a modulation order of 4, the normalized signal is routed to the 4 ASK demodulator  216 , which has 2 outputs, whereas the 8 ASK demodulator  218  has 3 outputs. 
     The normalized LLR {tilde over (Λ)}(b m ) is scaled back to its true value in block  214  such that 
               Λ   ⁡     (     b   m     )       =         Λ   ~     ⁡     (     b   m     )       ×     (       -          h        2         2   ⁢           ⁢     λ   2     ⁢     σ   2         )             
to obtain the LLR value, Λ(b m ). For some embodiments, this “un-scaling” (or resealing) factor may be given by:
 
                 -          h        2         (     2   ⁢     λ   2     ⁢     σ   2       )       .         
It should be noted that unlike the scaling factor λ, the un-scaling factor can generally be approximated with no performance degradation. For example, for some embodiments, shift operations may be performed in place of the multiplication operation to approximate the un-scaling/rescaling function. As a non-limiting example, for a convolutional coded AWGN transmission, the un-scaling factor can be approximated by −1 while achieving no degradation in performance. It should be emphasized that for exemplary embodiments such as the embodiment depicted in  FIG. 2 , the use of scaling and saturation modules  208 ,  210  allows the ASK demodulators  216 ,  218  in the LLR computation module  212  to be implemented without the use of multiplication operations, which are computationally intensive in nature.
 
     The SISO detector depicted in  FIG. 2  operates based on the assumption that serial demodulation of {tilde over (y)} i  and {tilde over (y)} q  is used. In accordance with other embodiments, however, the demodulation of {tilde over (y)} i  and {tilde over (y)} q  can also be performed in a parallel fashion. Reference is made to  FIG. 3 , which depicts an alternative embodiment of the SISO detector shown in  FIG. 1 . The SISO detector shown is similar to the embodiment shown in  FIG. 2 . In  FIG. 3 , however, the processing of the real component of the equalized received signal is performed separately from the processing of the imaginary component, as depicted in blocks  308 ,  310 , and  312 . While the embodiment shown in  FIG. 3  generally requires more computational resources, it should be emphasized that the throughput of the detector  300  is also increased. 
     To improve the robustness of a given link, WiMAX compliant systems provide for the repetition of data. As known by those skilled in the art, various modes are supported, including a classical repetition mode where exact copies of a QAM signal sequence or codeword is resent. Another mode relates to the Hybrid ARQ (HARQ) mode, which involves resending only a portion of the codeword&#39;s redundancy. Such repetition modes can be incorporated into the receiver side in order to improve the received SNR, and hence the performance in decoding signals. Based on the foregoing, alternative embodiments are described below wherein the embodiment depicted in  FIG. 3  is modified to take into account signals transmitted in accordance with a repetition mode. 
     An alternative embodiment of the SISO detector depicted in  FIG. 2  is now described. The alternative embodiment may be utilized in communications systems incorporating a repetition protocol or mode with respect to the transmit signals. Reference is made to  FIG. 4 , which is an equivalent model for a wireless communication system incorporating a SISO detector whereby the same signal is transmitted over R independent channels. It should be emphasized that for the model depicted in  FIG. 4 , the use of R independent channels to transmit the same signal is essentially equivalent to repeating the same signal over the same channel. The model in  FIG. 4  is used to illustrate the operation of the SISO detector shown. Assuming a repetition factor of R, the system  400  shown in  FIG. 4  transmits R identical QAM signals over R independent channels. The system  400  includes a QAM modulator  402  and a SISO detector  404 , which includes a QAM demodulator  406 . Block  408  represents the complex channel gain for each transmission (1 to R). Block  410  represents the complex circular AWGN (additive white Gaussian noise) with variance σ 2  for each transmission. The received QAM signal for the k th  transmission is derived according to:
 
 y   k   =h   k   x+n   k .
 
The system  400  further includes a symbol combiner  410 , the function of which may be represented by:
 
 F=[f   1   f   2    . . . f   R ] T .
 
The received equalized signal {tilde over (y)} is output from the symbol combiner  410  and forwarded to the QAM demodulator  406 , which eventually outputs the LLR value. With reference back to  FIGS. 1-3 , the primary difference depicted in  FIG. 4  with respect to the embodiments shown in  FIGS. 2 and 3  involves the replacement of the ZF equalizer  202 ,  302  with the symbol combiner  412 . The symbol combiner  412  takes the R received signals associated with the same transmit signal and outputs a single symbol which correlates with the equalized symbol {tilde over (y)} depicted in  FIGS. 2 and 3 .
 
     The output of the symbol combiner  412  may be expressed as: 
               F   =       H   H       HH   H         ,         
where H H  represents the transpose conjugate of H, which may be represented by H=[h 1  h 2  . . . h R ]. Accordingly, the output of the symbol combiner  412 , {tilde over (y)}, may be expressed by the following equation:
 
               y   ~     =         ∑     k   =   1     R     ⁢       y   k     ⁢     h   k   *             ∑     k   =   1     R     ⁢       h   k     ⁢     h   k   *                 
where h k * denotes the complex conjugate of h k . It should be noted, however, that the expression for the combined signal above is a non-limiting example and may change based on the matched filters incorporated. As another non-limiting example, the following expression for F may be utilized:
 
     
       
         
           
             F 
             = 
             
               
                 
                   H 
                   H 
                 
                 
                   
                     HH 
                     H 
                   
                 
               
               . 
             
           
         
       
     
       FIGS. 5-6  provide exploded views of various components in the symbol combiner shown in  FIG. 4 . With reference to  FIG. 5 , the sum Σ k y k h k * can be computed recursively each time a new version y k  of the repeated symbol (and the corresponding channel gain h k ) is received. The sum Σ k h k h k * can be computed using different methods. For some embodiments, the sum may be pre-computed (e.g., in the channel estimation module) and fed to the detector. Module  500  is an exemplary circuit for calculating the sums Σ k h k h k * and Σ k y k h k *. The module  500  includes a memory reset circuit  502  for the memory  504  shown in  FIG. 5 . Memory  504  stores the sum Σ k h k h k * while this value is updated recursively. Upon reaching k iterations, the final sum Σ k h k h k * is output. 
     Referring now to  FIG. 6 , the ZF equalizer  202  shown in  FIG. 2  is replaced with a maximum ratio combiner (MRC)  600 . It should be noted that for a single repetition (R=1), the output of the MRC  600  and ZF equalizer  202  in  FIG. 2  are equivalent. The MRC  600  depicted in  FIG. 6  includes a memory reset circuit  602  and a QAM symbol memory  604 , which is configured to store the sum Σ k y k h k * (which may be calculated using the exemplary circuit shown in  FIG. 5 ), which is divided by the sum Σ k h k h k * (also derived using the exemplary circuit shown in  FIG. 5 ) to derive the MRC  500  output. 
     Having described a SISO detector for communication systems deploying a repetition of transmit signals, reference is made to  FIG. 7 , which is an exemplary embodiment of a SISO detector for repetition channels where the real and imaginary parts of the received signal are processed in parallel. Assuming again a repetition factor of R, the system  700  transmits R identical QAM signals over R independent channels. The exemplary SISO detector depicted in  FIG. 7  is based on the concepts described in the preceding figures. Referring back briefly to  FIGS. 2 and 3 , the real and imaginary components of {tilde over (y)} are initially processed separately. In  FIG. 2 , the real and imaginary components are multiplexed before being passed on to the rescaling block  208  and the saturation block  210 , whereby the real and imaginary components are fed alternatively into the rescaling block  208 . For the alternative embodiment shown in  FIG. 3 , however, the processing of the real and imaginary components of the equalized received signal is performed entirely separately (and concurrently), as depicted in blocks  308 ,  310 , and  312 . 
     While the embodiment shown in  FIG. 3  generally requires more computational resources, it should be emphasized that the throughput of the detector  300  is also increased. Referring now to  FIG. 7 , the embodiment shown incorporates the parallel processing architecture depicted in  FIG. 3 . In stage A in  FIG. 7 , the real and imaginary components of the received signal are extracted. The real and imaginary components for the channel gain parameter, h k , are also extracted for later processing. In stage B, the sums Σ k h k h k * and Σ k y k h k * are calculated in a recursive fashion similar to that shown earlier in  FIGS. 5 and 6 . 
     In stage C, rescaling is performed on the real and imaginary components by an inverse scaling factor λ to de-normalize the received equalized signal and rescale the signal to a point Y in the odd integer 1-D grid. The inverse scaling factor λ is related to the value M, which varies as a function of the constellation size and represents the number of bits per constellation. The inverse scaling factor λ as a function of the modulation order M is shown in Table 1 described earlier. 
     Once the equalized received signal is rescaled or normalized, the received signal is forwarded to stage D (saturation stage), where the received signal is saturated to a closed range. In the non-limiting example shown in  FIG. 7 , the received signal is saturated to ±√{square root over (2 M )}. The signal Y is then sent to an appropriate normalized LLR computation stage E. It should be emphasized that for exemplary embodiments such as the embodiment depicted in  FIG. 7  (and  FIG. 2 ), the use of scaling and saturation in stages C and D allows the ASK demodulators  716 ,  718 ,  720 ,  722  in stage E to be implemented without the use of multiplication operations. In stage F, the normalized LLR {tilde over (Λ)}(b m ) is scaled back to its true value in block  214  such that 
               Λ   ⁡     (     b   m     )       =         Λ   ~     ⁡     (     b   m     )       ×     (       -          h        2         2   ⁢           ⁢     λ   2     ⁢     σ   2         )             
to obtain the LLR value, Λ(b m ). For some embodiments, this “un-scaling” (or rescaling) factor may be given by:
 
                 -          h        2         (     2   ⁢     λ   2     ⁢     σ   2       )       .         
It should be noted that all multiplication operations shown in  FIG. 7  are real. Furthermore, the sum Σ k  h k h* k (or √{square root over (Σ k  h k h* k )}) is assumed to be pre-computed in the channel estimation module, and fed to the SISO detector  700 . The SISO detector  700  complexity can be further reduced by considering the following approximations. First, the multiplication by
 
             -         ∑     k   =   1     R     ⁢       h   k     ⁢     h   k   *           2   ⁢     λ   2     ⁢     σ   2               
shown in stage F can be replaced with a shift operation by the closet power-of-two number. While this may result in some performance degradation, replacement of the multiplication operation by the shift operation can reduce the computational resources needed. Another approximation involves the normalization by the noise variance σ 2 . As those skilled in the art will appreciate, when Viterbi or ML-MAP decoders are used, the normalization by noise variance σ 2  is generally not useful and can therefore be ignored, while yielding no performance degradation.
 
     Reference is made to  FIG. 8 , which is an embodiment for performing SISO detection in a wireless communication system. In block  802 , a transmit signal is received. For some embodiments, the transmit signal may be a SISO signal received over a WiMAX environment. As described earlier, the transmit signal may be sent using repetition protocol or mode such as HARQ. In step  820 , if the transmit signal is sent using a repetition protocol (e.g., where the transmit signal is sent in a repetitive fashion), the signals are combined (step  820 ). If the transmits signal is not sent using a repetition protocol, then the transmit signal undergoes zero-forcing equalization (step  830 ). 
     In step  840 , the real and imaginary components of the equalized signal are scaled by a factor to de-normalize the received signal and rescale the signal to a point in an odd integer 1-D grid. In accordance with some embodiments, the (inverse scaling) factor λ is related to the value M, which varies as a function of the constellation size and represents the number of bits per constellation. In step  850 , the signal undergoes saturation where the signal is saturated to a closed range. For some embodiments, the signal may be saturated to ±√{square root over (2 M )}. In step  860 , the signal is demodulated. In particular, the signal may be demodulated using the ASK demodulators described earlier. In step  870 , a bit-wise LLR Λ(b m ) based on the received signal is provided. 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.