Abstract:
A wireless communication device includes a Generalized RAKE (G-RAKE) receiver circuit that is configured to determine a traffic-to-pilot gain scaling parameter as part of the impairment correlation determination process that underlies (G-RAKE) combining weight generation. In this manner, the receiver circuit conveniently and accurately accounts for gain differences between the pilot channel of a received CDMA signal, as used for channel estimation, and the traffic channel(s) of the CDMA signal, which carry received data to be recovered. The gain difference accounting enables proper demodulation of amplitude-modulated traffic signals. By way of non-limiting example, such gain scaling may be used for demodulating/decoding High Speed Downlink Packet Access (HSDPA) signals used in Wideband Code Division Multiple Access (W-CDMA) systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to wireless communication receivers, and particularly relates to demodulating amplitude-modulated signals.  
         [0002]     Higher-order modulation constellations represent one mechanism supporting the faster data rates of current and evolving wireless communication systems. For a given symbol rate, more bits in each modulation symbol translate into higher effective data rates, assuming acceptable demodulation performance at the receiver. In general, the use of more complex modulation constellations for data transmission results in greater demodulation challenges at the receivers.  
         [0003]     In part, such challenges arise because higher order modulation schemes commonly use amplitude changes to convey information. For example, Quadrature Amplitude Modulation (QAM) generally defines modulation constellation points arrayed around the origin in the real-imaginary plane, where each point represents a unique pairing of phase and amplitude. 16QAM, for example, defines four points in each quadrant, yielding sixteen points in the QAM constellation. Each such point represents one of sixteen modulation symbols and is recognizable at the receiver based on the unique pairing of phase and amplitude represented by the symbol. With sixteen unique symbols, each QAM modulation symbol represents four bits, and thus affords higher effective data rates than QPSK, for example, assuming the same modulation symbol rate.  
         [0004]     Sometimes different modulation formats are used at different times, with the choice often made as a function of available transmitter resources, prevailing radio conditions, and/or required data rates. For example, the High Speed Downlink Packet Access (HSDPA) extension of the Wideband CDMA (W-CDMA) standard offers a range of data rates on a High Speed Downlink Shared Channel (HS-DSCH), which is a shared packet data channel using either QPSK or QAM modulation, depending on data rate requirements.  
         [0005]     Use of 16QAM on the HS-DSCH enables higher data rates, but such use complicates data reception. Specifically, an amplitude reference is required at the receiver to detect received symbols and properly scale the soft information for decoding (e.g., by a turbo decoder). For HS-DSCH, and for CDMA-based pilot-and-traffic-channel transmissions in general, the needed amplitude reference represents the relative scaling between the code channel used for estimation (e.g., the pilot or reference channel) and the code channel(s) being demodulated (e.g., the traffic or information-bearing channels). The needed scale factor g can be written as 
 
 h   traf   =gh   pil   Eq. 1 
 
 where h pil  is a channel response vector, as defined in G. E. Bottomley, T. Ottosson, and Y.-P. E. Wang, “A Generalized RAKE receiver for interference suppression,” IEEE J. Select. Areas Commun., vol. 18, pp. 1536-1545, August 2000, for example, which can be estimated from the pilot channel, h traf  represents the channel response vector for the traffic channel, and g is the scale factor. To illustrate the usefulness of the scale factor, consider the Log-Likelihood Ratio (LLR) for bit b j  of the ideal QAM demodulator, which is given as  
               LLR   ⁡     (     b   j     )       =         ∑       s   i     ∈       S   0     ⁡     (   j   )           ⁢     exp   ⁢     {     γ   ⁡     (       2   ⁢     Re   ⁡     (         s   i   *     ⁢   z         w   H     ⁢     h   traf         )         -            s   i          2       )       }             ∑       s   i     ∈       S   1     ⁡     (   j   )           ⁢     exp   ⁢     {     γ   ⁡     (       2   ⁢     Re   ⁡     (         s   i   *     ⁢   z         w   H     ⁢     h   traf         )         -            s   i          2       )       }                   Eq   .           ⁢   2             
 
         [0006]     In the above equation, z is the estimated symbol, s i  is a candidate symbol from a normalized scale constellation, γ is the signal-to-noise ratio (SNR), which actually may be calculated as a signal-to-interference-plus-noise ratio (SINR), and w represents a vector of combining weights. Assuming a log-max turbo decoder, the log-likelihood ratio becomes  
               LLR   ⁡     (     b   j     )       =       γ   ⁡     [         max       s   i     ∈       S   0     ⁡     (   j   )           ⁢     (       2   ⁢     Re   ⁡     (         s   i   *     ⁢   z     μ     )         -            s   i          2       )       -       max       s   i     ∈       S   1     ⁡     (   j   )           ⁢     (       2   ⁢     Re   ⁡     (         s   i   *     ⁢   z     μ     )         -            s   i          2       )         ]       .             Eq   .           ⁢   3             
 
 The normalization factor μ in Eq. 3 is defined as 
 
μ= w   H   h   traf .  Eq. 4 
 
 Here, the normalization factor scales the symbol estimate z for comparison to the symbols s i  from a unitary average power constellation. Conventionally, receivers determine the scale factor through time estimation of the RMS value of the symbol estimate, which may be expressed as, 
 
√{square root over (         | z|   2           )}≈ w   H   h   traf =μ.  Eq. 5 
 
 From the above equations, one sees that the normalization factor provides a basis for properly evaluating the log-likelihood ratio used in decoding the estimated traffic symbol z. It may be observed that Eq. 4 can be rewritten in consideration of Eq. 1 as 
 
μ= gw   H   h   pil .  Eq. 6 
 
 Thus, as an alternative to calculating a normalization value from Eq. 5, received signal processing may be configured to the normalization factor given in Eq. 6. However, that calculation may be challenging in terms of implementing it efficiently and/or integrating the calculation into existing received signal processing calculations. 
 
       SUMMARY OF THE INVENTION  
       [0007]     A Generalized RAKE (G-RAKE) receiver circuit suitable for use in a wireless communication device is configured to determine a traffic-to-pilot gain scaling parameter in a G-RAKE receiver as part of the impairment correlation determination process that underlies combining weight generation. In this manner, the receiver circuit conveniently and accurately accounts for gain differences between the pilot channel of a received CDMA signal, as used for channel estimation, and the traffic channel(s) of the received CDMA signal, which carry received data to be recovered. The gain difference accounting enables proper demodulation of amplitude-modulated signals, such as the 16QAM signals associated with High Speed Downlink Shared Channel (HS-DSCH) transmissions in Wideband Code Division Multiple Access (W-CDMA) systems.  
         [0008]     In one embodiment, the G-RAKE receiver circuit comprises one or more processing circuits configured to determine a traffic-to-pilot gain scaling parameter. More particularly the one or more processing circuits are configured to express data correlations of despread traffic values (i.e. information-bearing symbols) obtained from a received CDMA signal as a function of interference and noise correlations scaled by first and second model fitting parameters, respectively, and channel estimates scaled by a gain scaling parameter.  
         [0009]     With this expression, the processing circuit(s) determines the gain scaling parameter in conjunction with determining the first and second model fitting parameters via least squares fitting of the function to the data correlations. Further, in one or more embodiments, the gain scaling parameter provides the basis for calculating a normalization factor used in demodulating the traffic symbol estimates output by the G-RAKE receiver circuit, e.g., the normalization factor can be used to scale the estimated symbols for comparison to nominal symbols in a reference constellation.  
         [0010]     In a non-limiting example, receiver processing as taught herein may be embodied as a computer program, suitable for execution on general-purpose or specialized microprocessor circuits. In one embodiment, the computer program comprises program instructions to obtain despread traffic and pilot values from a received CDMA signal, generate channel estimates from the despread pilot values, and measure data correlations for the despread traffic values. The program further comprises program instructions to express the data correlations as a function of interference and noise correlations scaled by first and second model fitting parameters and the channel estimates scaled by a gain scaling parameter, and to determine the gain scaling parameter in conjunction with the first and second model fitting parameters via least squares fitting of the function to the data correlations.  
         [0011]     Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will appreciate additional features and advantages of the present invention upon reading the following discussion, and upon viewing the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of a wireless communication receiver, including a Generalized RAKE (G-RAKE) receiver circuit that is configured to determine traffic-to-pilot gain scaling for demodulation and decoding.  
         [0013]      FIG. 2  is a logic flow diagram of general processing logic that can be implemented by the G-RAKE receiver circuit of  FIG. 1 , for gain scaling determination.  
         [0014]      FIG. 3  is a logic flow diagram of a more detailed embodiment of gain scaling processing, such as can be used for a Wideband CDMA (W-CDMA) receiver receiving a High Speed Downlink Shared Channel (HS-DSCH) signal, for example.  
         [0015]      FIG. 4  is a block diagram of circuit details for one embodiment of the G-RAKE receiver circuit of  FIG. 1 .  
         [0016]      FIG. 5  is a block diagram of a wireless communication network, such as a cellular communication network based on W-CDMA standards, wherein a mobile station and/or a base station incorporate gain scaling as taught herein. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  illustrates a wireless communication receiver  10 . In the illustrated embodiment, the receiver  10  comprises a Generalized RAKE (G-RAKE) receiver circuit  12 , which includes one or more processing circuits  14  that are configured for traffic-to-pilot gain scaling as taught herein, a receiver front-end circuit  16 , an antenna  18 , and one or more additional processing circuits  20 , such as demodulator circuits and/or turbo decoding circuits.  
         [0018]     The receiver  10  may, depending upon its operating mode and/or depending upon the characteristics of its supporting wireless communication network, receive a CDMA signal having amplitude information carried within it. For example, the received CDMA signal may comprise a Wideband CDMA (W-CDMA) signal, including a multi-coded High Speed Downlink Shared Channel (HS-DSCH) signal transmitted using Quadrature Amplitude Modulation (QAM). Such systems “map” groups of information bits to be transmitted to points in a modulation constellation, wherein, for 16QAM, each point represents one of sixteen unique pairings of phase and amplitude. These phase/amplitude pairings are referred to as modulation symbols.  
         [0019]     With sixteen symbols (points) in the 16QAM constellation, each symbol represents a unique combination of four transmit bits, and recovery of those bits at the receiver depends on accurately identifying the received modulation symbols. In turn, accurate recovery of the received modulation symbols, i.e., the traffic symbols, depends on accurate channel estimation. As a general rule, CDMA receivers estimate the transmission channel(s) based on a received pilot signal, which commonly is sent in conjunction with the traffic channel(s) of interest, but using different spreading factors and transmit powers. As such, there is a definite, but usually unknown, amplitude relationship between the traffic signal(s) of interest and the pilot signal used for channel estimation.  
         [0020]     The G-RAKE receiver circuit  12 , which may be implemented in a variety of devices and systems, provides for the traffic-to-pilot gain scaling needed to properly demodulate 16QAM and other amplitude-modulated signals. The G-RAKE receiver circuit  12  may operate as a parametric G-RAKE. As used herein, the term “parametric G-RAKE” connotes a G-RAKE receiver that uses modeled terms for the interference and/or noise correlations that are used in its combining weight generation. As those skilled in the art will appreciate, a “standard” RAKE receiver combines multipath images of a received signal using propagation channel taps as combining weights, whereas a G-RAKE receiver combines multipath images of a received signal using interference-suppressing combining weights calculated from the interference and/or noise correlations measured or estimated across the RAKE fingers. For additional G-RAKE details, one may refer to U.S. Pat. No. 6,363,104 to Bottomley, which is incorporated herein by reference.  
         [0021]     In parametric G-RAKE receivers, one or more of the impairment correlations used to generate the combining weights are based on modeled impairments. More particularly, a parametric G-RAKE receiver commonly uses a parametric model to determine the noise correlations used in its combining weight generation.  
         [0022]     With the above in mind, according to a broad embodiment of received signal processing as taught herein, the G-RAKE receiver circuit  12  is configured to calculate Eq. 6 through a parametric G-RAKE solution for the combining weights w that are used to generate the symbol estimates z. This approach removes the need for estimating the RMS values given by Eq. 5 for use in symbol detection (i.e., comparing the estimated symbols with nominal symbols in a reference modulation constellation).  
         [0023]     Thus,  FIG. 2  illustrates processing logic according to one method of gain scaling as taught herein. According to the illustrated processing, the G-RAKE receiver circuit  12  obtains despread traffic and pilot values from a received CDMA signal (Step  100 ). The G-RAKE receiver circuit  12  generates channel estimates from the despread pilot values (Step  102 ). Notably, these channel estimates include the influence of pilot channel spreading factor and pilot channel transmit power.  
         [0024]     Processing continues with the G-RAKE receiver circuit  12  measuring data correlations from the despread traffic values (Step  104 ). The G-RAKE receiver circuit  12  then uses the measured data correlations to determine a traffic-to-pilot gain scaling parameter in conjunction with determining its parametric G-RAKE model fitting parameters (Step  106 ). While the following discussion explains this step in much more detail, the step generally comprises expressing the measured data correlations as a function of the modeled impairment correlations as scaled by model fitting parameters, and further as a function of the channel estimates scaled by a gain scaling parameter.  
         [0025]     With this expression, the G-RAKE receiver circuit  12  uses Least Squares (LS) fitting to determine the model fitting parameters and the gain scaling parameter that best fits the modeled impairment correlations to the measured data correlations (Step  108 ). That is, as will be detailed below, the G-RAKE receiver circuit  12  expresses the measured (short-term) data correlations as a function of its parametric model, which includes the gain scaling term to be determined, such that LS fitting of the parametric model to the measured data correlations yields the gain scaling term in conjunction with determining the model fitting parameters.  
         [0026]     To better understand the above method, it is helpful to begin with the assumption that the channel estimation and (combining) weight computation algorithms implemented by the G-RAKE receiver circuit  12  employ despread data from the pilot channel. With these assumptions in mind, the parametric noise correlation matrix, R u , used by the G-RAKE receiver circuit  12  to compute combining weights may be expressed as 
 
 R   u   =αR   I   +βR   n   Eq. 7 
 
 The interference matrix (R I ) captures the effect of own-cell interference while the white noise matrix (R n ) models the effect of other-cell interference plus thermal noise. Note that the weighting coefficients α and β reflect the relative contribution of own-cell versus other-cell interference plus noise. Eq. 7 may be computed explicitly or implicitly. For parametric calculation, the interference correlations R I  and the noise correlations R n  may be modeled terms, and α and β are referred to as first and second model fitting parameters, respectively. 
 
         [0027]     These first and second model fitting parameters are functions of system or environmental factors as shown below  
             α   =       E   c       E   pil               Eq   .           ⁢   8               β   =     N   0             Eq   .           ⁢   9             
 
 Here, E c  represents the total energy of the serving base station per chip, E pil  is the energy given to the pilot channel, and N 0  is the one-sided power spectral density of the white noise that models other-cell interference plus noise. 
 
         [0028]     For an example of model fitting parameter calculation based on short-term estimations of the noise correlations as determined from despread pilot values, one may refer to U.S. application Ser. No. 10/800,167, which was filed on 12 Mar. 2004, is entitled, “Method and Apparatus for Parameter Estimation in a Generalized RAKE Receiver,” and which is incorporated herein by reference. In the incorporated reference, determining impairment correlations from the despread pilot values requires an explicit gain scaling calculation as an additional processing step.  
         [0029]     In contrast, one or more of the methods taught herein perform parametric model fitting using short-term data correlation estimates determined from despread traffic values, such that the gain scaling parameter needed for proper demodulation of received QAM symbols is determined as part of the model-fitting process.  
         [0030]     In more detail, one notes that the noise covariance R u  is identical for all code channels of the received CDMA signal. Thus, the noise covariance matrix can be expressed as 
 
 R   u   =R   traf   −h   traf   h   traf   H   Eq. 10 
 
 where R traf  is the data correlation matrix of the despread values of the traffic channel. Knowing that h traf =gh pil , where g is the traffic-to-pilot gain scaling factor to be determined, Eq. 10 can be rewritten as 
 
 R   u   =R   traf   −g   2   h   pil   h   pil   H   Eq. 11 
 
         [0031]     With the above expression, one may rewrite Eq. 7 as 
 
 {circumflex over (R)}   traf   =αR   I   +βR   n   +δh   pil   h   pil   H   Eq. 12 
 
 where δ=g 2  (i.e., the square of the desired gain scaling parameter g), β=N 0  as before, and  
       α   =         E   c       E   CPICH       .         
 
 The term “CPICH” denotes the common pilot channel received as part of the composite received CDMA signal. On this basis, the G-RAKE receiver circuit  12  uses a short-term slot estimate of data correlation of despread traffic values, denoted as {circumflex over (R)} traf , as opposed to determining the short-term correlation estimate from the despread pilot values. 
 
         [0032]      FIG. 3  illustrates processing logic according to the above details, wherein processing begins with the G-RAKE receiver circuit  12  computing a short-term estimate of data correlations for the despread traffic values, {circumflex over (R)} traf , as obtained from the received CDMA signal over a given window of time, e.g., over one slot of a given TTI in a received W-CDMA signal (Step  110 ). Note that the illustrated start of processing assumes that received signal samples are available—e.g., buffered by the G-RAKE receiver circuit  12 —and that channel estimates have been calculated using despread pilot values obtained from the received CDMA signal.  
         [0033]     Processing continues with the LS fitting of Eq. 12 (Step  112 ), which determines values for the first model fitting parameter α, which scales the modeled same-cell interference correlations R I , the second model fitting parameter β, which scales the modeled other-cell plus noise correlations R n , and the gain scaling parameter δ, which scales the h pil h pil   H  product of the channel estimates determined from the despread pilot values. With this approach, the G-RAKE receiver circuit  12  is configured to calculate a LS solution of Eq. 12 to obtain the gain scaling parameter δ=g 2  in conjunction with obtaining the model fitting parameters α and β.  
         [0034]     The G-RAKE receiver circuit  12  then computes Maximum Likelihood (ML) combining weights (Step  114 ) as 
 
 w   pil   =R   u   −1   h   pil   Eq. 13 
 
 where R u  may be computed according to Eq. 11, for example, using {circumflex over (R)} traf . With the combining weights thus calculated, the G-RAKE receiver circuit  12  continues its processing by computing the traffic SNR (Step  116 ) as 
 
γ=δ w   pil   H   h   pil ,  Eq. 14 
 
 and estimating the received traffic symbols (Step  116 ) as, 
 
 z=w   pil   H   y   Eq. 15 
 
         [0035]     Then, in one or more embodiments, the RAKE receiver circuit  12  is configured to pass the estimated symbols z, SNR y, and the normalization factor μ to the one or more additional processing circuits  20 . In at least one embodiment, these additional processing circuits  20  include a QAM demodulation circuit and a decoding circuit, such as a turbo decoder (Step  118 ). In practice, the estimated traffic symbols z are normalized to a unitary average power before comparing to a unitary average power QAM constellation template. The term  
       z       gw   H     ⁢     h   pil           
 
 represents a normalization for comparing the estimated traffic symbol z to the nominal traffic symbol s i , where E{|s i | 2 }=1. (Note that s i  is from the set of nominal symbol values in the unitary-power modulation constellation template.) Remembering that δ=g 2 , one may define a normalization factor μ as 
 
μ= gw   H   h   pil   =√{square root over (δ)}w   H   h   pil   Eq. 16 
 
         [0036]     Thus, the normalization factor needed for demodulation of QAM or other amplitude-modulated traffic symbols may be obtained from the gain scaling parameter δ, which is determined as part of the parametric G-RAKE receiver solution, e.g., it is obtained from the LS fitting of Eq. 12.  
         [0037]     The instantaneous soft bit information is then obtained by the additional processing circuit(s)  20  based on the distance of the normalized symbol estimate to the points in template constellation. Restoring the channel-dependent soft scaling via the SNR estimate γ provides the final soft bit information, i.e., the instantaneous soft bits are “de-normalized by multiplication with the SNR value.  
         [0038]     Those skilled in the art will appreciate that the G-RAKE receiver circuit  12  may be implemented in a variety of receiver architectures.  FIG. 4  illustrates one embodiment, wherein the one or more processing circuits  14  of the G-RAKE receiver circuit  12  include (or are associated with) a delay searcher/finger placement circuit  30 , a pilot correlation circuit  32 , a traffic correlation circuit  34 , a channel estimation circuit  36 , a (data) correlation measurement circuit  38 , a (parametric) model calculation circuit  40 , memory  42 , a combining weight calculation circuit  44 , a (traffic) symbol estimation circuit  46 , and a SNR and normalization factor calculation circuit (SNR/norm calculator)  48 .  
         [0039]     In operation, the delay searcher/finger placement circuit  30  provides finger placement values for setting the delays used by the pilot and traffic correlation circuits  32  and  34 , such that the received CDMA signal is despread at delays corresponding to, or related to, the multipath images of the received CDMA signal. The pilot correlation circuit  32  obtains despread pilot values from the received CDMA signal, which it provides to the channel estimation circuit  36  for calculation of the pilot channel estimates h pil . Similarly, the traffic correlation circuit  34  obtains despread traffic values from the received CDMA signal, which it provides to the correlation measurement circuit  38  for measurement of the short-term data correlations {circumflex over (R)} traf .  
         [0040]     In turn, the model calculation circuit  40  uses the channel estimates and the data correlations to carry out the LS fitting for determination of the model fitting parameters α and β, and the gain scaling parameter δ. Note that the parametric modeling and other information used for model fitting may be stored in the memory  42 , which may be included in, or associated with the model calculation circuit  40 .  
         [0041]     Regardless of such details, the combining weight calculation circuit  44  uses the channel estimates h pil  and interference correlation matrix R u  to calculate the combining weights w pil  for combining the despread traffic values from the different correlator outputs of the traffic correlator circuit  34 . Additionally, presuming that the normalization by μ is done as part of QAM demodulation, rather than in the combining step, the symbol estimation circuit  46  uses the combining weights, w pil , and the despread traffic values, y, to calculate estimates of the traffic symbols z. The SNR/norm calculator  48  also uses the channel estimates, combining weights, and gain scaling parameter to determine the traffic SNR γ. The values (γ, μ, and z) may be passed from the G-RAKE receiver circuit  12 , to the additional processing circuits  20  for using demodulating and decoding the estimated traffic symbols z.  
         [0042]     In addition to the variability of receiver architecture, those skilled in the art should appreciate that the gain scaling determination methods taught herein may be embodied in receivers intended for a variety of applications, such as shown in the context of  FIG. 5 , depicting a wireless communication network  50 . By way of non-limiting example, the network  50  may comprise a cellular communication network based on the W-CDMA standards.  
         [0043]     The illustrated embodiment of the network  50  includes at least one Radio Access Network (RAN)  52 , which communicatively couples a plurality of mobile stations  54 —one is shown for clarity—to one or more external networks  56 —such as the Internet-through one or more Core Networks (CNs)  58 . In more detail, according to the illustrated embodiment, the RAN  52  comprises one or more Base Station Controllers (BSCs)  60 , each supporting a number of Base Transceiver Stations (BTSs)  62 , which provide sectorized radio coverage, e.g., cell C 1  including sectors S 1 , S 2 , and so on.  
         [0044]     In the context of  FIG. 5 , an embodiment of the G-RAKE receiver  12  described herein may be implemented, for example, in the mobile stations  54 . Such an implementation may be advantageous in W-CDMA embodiments of the network  50 , at least for mobile stations for which HS-DSCH reception is desired. Of course, this implementation example should be understood as non-limiting.  
         [0045]     Indeed, the gain scaling parameter determination methods taught herein permit G-RAKE receivers to determine requisite traffic-to-pilot gain scaling as part of the LS fitting process used to determine the impairment correlations for combing weight generation. As such, in W-CDMA and other embodiments, the scaling aspect of QAM and other amplitude-dependent demodulation becomes part of the parametric/explicit G-RAKE weight computation process, and avoids additional gain determination processing steps. Therefore, the present invention is not limited by the foregoing discussion, or by the accompanying drawings. Rather, the present invention is limited only by the following claims and their legal equivalents.