Abstract:
A spread spectrum receiver in a communication system compensating channel response, and the method thereof is disclosed. The spread spectrum receiver comprises a multiplier, a filter, a data channel correlator, a delay, and a channel compensator. The multiplier ( 322 ) multiplies despread pilots ( 320 ) with conjugate of pilot pattern ( 324 ) to provide channel estimates. The filter ( 323 ) filters the channel estimates from the multiplier ( 322 ). The data channel correlator ( 30 ) despreads data symbols in a data channel. The delay module ( 38 ), coupled to the data channel correlator ( 30 ), delays the despread data symbols for a period. The channel compensator ( 34 ), coupled to the filter ( 323 ) and the delay module ( 38 ), compensates the delayed despread data symbols with the filtered channel estimates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application Ser. No. 60/785,909 filed Mar. 24, 2006, which is incorporated herein by reference. This application also claims the benefit of European application No. 06017993.4 filed Aug. 29, 2006, the subject matter of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to channel estimation, and in particular to channel estimation in a spread spectrum receiver. 
     2. Description of the Related Art 
     A spread spectrum system combats multipath interference by multiplying narrowband data by a wideband pseudo noise (PN) code in a spread spectrum transmitter to produce a wideband transmitted signal with nearly the spectrum of the PN code. Examples of spread spectrum systems can be found in Code Division Multiple Access (CDMA) and Wideband CDMA (WCDMA) systems conforming to the UMTS/IMT-2000 (Universal Mobile Telecommunications System/International Mobile Telecommunications-2000) specifications. Typically, spread spectrum systems such as WCDMA systems employ an orthogonal spreading code spreading a data symbol over a desirable spectrum and pseudo noise (PN) scrambling codes separating different cells in downlink transmission and user in uplink transmission. The spreading codes, also known as orthogonal channelization codes, transform a data symbol into a number of chips as the spread-spectrum signal prior to data transmission. Spread spectrum receivers such as RAKE receivers, frequently used in CDMA systems, descramble, and/or despread the received spread spectrum, reducing signal multipath reception to improve reception quality. Spreading and despreading codes may be Gold codes, Walsh codes, orthogonal variable spreading factor (OVSF) channelization codes and/or equivalent code sequences. 
     A pilot channel is typically used for channel estimation to control various parameters of a received spread spectrum signal at reception, compensating various channel conditions in a particular transmission channel. In wireless communications, channel conditions vary over both time and frequency, thus wireless communications systems, such as spread spectrum communication systems, must estimate and compensate for varying channel conditions to the received spread spectrum signal. In WCDMA downlink transmission, the pilot channel is referred to as common pilot channel (CPICH). Since pilot symbols in CPICH are transmitted with a higher power then other dedicated traffic channels, the result is a better reception, whereby the channel condition can be accurately estimated. 
     There is a need for a spread spectrum receiver and a method thereof accurately estimating channel conditions for spread spectrum signals. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     According to an embodiment of the invention, a channel compensation method for uses in a spread spectrum receiver ( 3 ) for receiving a pilot channel and a data channel is disclosed, comprising despreading pilots ( 602 ) from the pilot channel, multiplying ( 603 ) despread pilots with conjugate of pilot pattern to provide channel estimates, filtering ( 604 ) the channel estimates, despreading ( 606 ) data in the data channel, delaying ( 608 ) the despread data for a period corresponding to the filtering, and compensating ( 610 ) the delayed data with the filtered channel estimates. 
     According to another embodiment of the invention, a spread spectrum receiver ( 3 ) in a communication system compensating channel response is disclosed, comprising a multiplier, a filter, a data channel correlator, a delay, and a channel compensator. The multiplier ( 322 ) multiplies despread pilots ( 320 ) with conjugate of pilot pattern ( 324 ) to provide channel estimates. The filter ( 323 ) filters the channel estimates from the multiplier ( 322 ). The data channel correlator ( 30 ) despreads data symbols in a data channel. The delay module ( 38 ), coupled to the data channel correlator ( 30 ), delays the despread data symbols for a period. The channel compensator ( 34 ), coupled to the filter ( 323 ) and the delay module ( 38 ), compensates the delayed despread data symbols with the filtered channel estimates. 
     In the embodiments, the period could be proportional to the group delay corresponding to the filter as well as the filtering. The period could be also proportional to a ratio of spreading factors of the pilot channel to the data channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of a part of a conventional spread spectrum receiver. 
         FIG. 2  channel signal level against time incorporating the spread spectrum receiver in  FIG. 1 . 
         FIG. 3  is a block diagram of an exemplary spread spectrum receiver in the invention. 
         FIG. 4  shows an exemplary WCDMA data format. 
         FIG. 5  channel signal level against time incorporating the spread spectrum receiver in  FIG. 3 . 
         FIG. 6  is a flowchart of a channel compensation method according to the invention, incorporating the spread spectrum receiver in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Scope of the invention may include, but is not limited to, Code Division Multiple Access (CDMA), TD-S-CDMA, CDMA-2000 and wideband CDMA (WCDMA) cellular radio telephone receivers receiving spread spectrum signals. 
       FIG. 1  is a block diagram of a part of a conventional spread spectrum receiver  1 , comprising data channel correlator  10 , channel estimator  12 , channel compensator  14 , and code generator  16 . Code generator  16  is coupled to data channel correlator  10 , in conjunction with channel estimator  12  couples to channel compensator  14 . Channel estimator  12  comprises pilot channel correlator  120 , multiplier  122 , filter  123 , spreading code generator  121  and conjugate pilot pattern generator  124 . Pilot channel correlator  120  is coupled to spreading code generator  121  and multiplier  122 . In the WCDMA environment, the pilot channel may refer to a Common Pilot Channel (CPICH) and the pilot pattern generator  124  generates specified CPICH pilot pattern. Ordinary in the skilled would understand that the pilot channel is not confined and restricted as the CPICH. It also means the characteristics, such as spreading factor, of pilot signal are not confined with the CPICH of WCDMA standard in this disclosure. 
     In one embodiment, spread spectrum receiver  1  is a WCDMA receiver, including several data channel correlators  10  correlating data Dr I, Q  transmitted over corresponding downlink physical channels (DPCH), and pilot channel correlator  120  correlating pilot Pr I, Q  in a common pilot channel (CPICH). Each data channel correlator  10  despreads and/or descrambles data Dr I, Q  with a different despreading code generated by code generator  16 . Pilot channel correlator  120  despreads and/or descrambles pilot Pr I, Q  with a known spreading code sequence. Pilot Pr I, Q  has a constant spreading factor of 256 and a spreading code of all ones, and the CPICH pilot pattern is (1+j). Pilot Pr I, Q  are used in spread spectrum receiver  1  for channel estimation. Multiplying the despread pilots by the conjugate of CPICH pilot pattern (1−j) generates channel estimate, for estimating noisy channel due to the presence of Additive White Gaussian Noise (AWGN) and interferences. Filter  123  receives and reduces noise in channel estimate Pr I, Q . Filtered channel estimate Cr I, Q  is then delivered to channel compensator  14  to be multiplied by correlated data Dr I, Q  to generate a channel compensated output data Do I, Q . Code generators  16  and  121  may comprise Linear Feedback Shift Registers (LFSR) compliant with standards defined in 3GPP. 
     However, when conventional channel compensator  14  obtains filtered channel estimate Pr I, Q  and correlated data Dr I, Q , to perform channel compensation, a finite group delay D g  is found in filtered channel estimate Pr I, Q  owning to filter  123 , in comparison to correlated data Dr I, Q . Consequently channel compensator  14  performs channel compensation for correlated data Dr I, Q  with delayed channel estimate Pr I, Q . 
       FIG. 2  plots signal level against time, incorporating the spread spectrum receiver in  FIG. 1 , comprising real channel  20  and filtered channel estimate  22 . Channel estimate passes filter  123  to remove undesirable noise and interferences and generate filtered channel estimate  22 . Group delay D g  is observed between real channel  20  and filtered channel estimate  22 . Since correlated data Dr I, Q  is concurrent with real channel  20 , a time difference occurs between correlated data Dr I, Q  and filtered channel estimate  22 , such that channel compensator  14  performs channel compensation on correlated data Dr I, Q  with delayed channel estimate  22 , leading to inaccurate channel compensation. 
       FIG. 3  is a block diagram of an exemplary spread spectrum receiver in the invention, comprising data channel correlator  30 , channel estimator  32 , channel compensator  34 , code generator  36 , and compensation delay  38 . Code generator  36  is coupled to data channel correlator  30 , compensation delay  38 , in conjunction with channel estimator  32  couples to channel compensator  34 . Channel estimator  32  comprises pilot channel correlator  320 , spreading code generator  321 , multiplier  322 , filter  323 , and conjugate pilot pattern generator  324 . Pilot channel correlator  320  is coupled to spreading code generator  321  and multiplier  322 . 
     Data channel correlator  30  despreads and/or descrambles a data channel with an appropriate spreading and/or descrambling code generated by code generator  36 . In an embodiment, the spread spectrum receiver is a WCDMA receiver, and each data channel correlator  30  despreads and/or descrambles spread spectrum data Dr I, Q  in a downlink physical channel (DPCH) with a difference spreading code generated by code generator  36 . Spread spectrum data Dr I, Q  may comprise In-phase and Quadrature components to be carried in each DPCH. The spreading factors may range from 4 to 256 or above depending on transmission data bit-rate. DPCH may be Downlink Physical Data Channels (DPDCH), or Downlink Physical Control Channel (DPCCH). 
       FIG. 4  shows an exemplary WCDMA data format, comprising DPDCH data channels  400  and  406 , and DPCCH control channels  402 ,  404 , and  408 . In a WCDMA downlink physical channel (DPDCH/DPCCH), pilot symbols (2 to 8 symbols) and control symbols are transmitted in every slot. There are 15 slots per WCDMA frame, and each frame is 10 ms long and has 38400 chips (3.84M cps). Different data rate may be achieved by different spreading factors. In WCDMA systems, the spreading factor ranges between 4 to 256, corresponding to 960 kbps to 15 kbps data rate. 
     Referring back to  FIG. 3 , pilot channel correlator  320  in channel estimator  32  despreads and/or descrambles a pilot channel with an appropriate spreading code, while data channel correlator  30  despreads and/or descrambles data Dr I, Q  concurrently. In an embodiment, spread spectrum receiver in a WCDMA receiver, each pilot channel correlator  32  despreads and/or descrambles spread spectrum pilot Pr I, Q  in a Common Pilot Channel (CPICH) with a fixed spreading code and spreading factor 256. The spreading factors may be fixed at 256. Pilot channel correlator  320  despreads and/or descrambles pilot by the fixed spreading code and spreading factor of 256, thereby generating despread pilot symbols Then the multiplier  322  multiplies the despread pilots and gets the channel estimates Cr I, Q . Filter  323  filters noise and interference off channel estimate to produce filtered channel estimate with group delay D g  to channel compensator  34 . Filter  323  may be an infinite impulse filter (IIR). 
     Compensation delay  38  receives and delays correlated data Dr I, Q  for period N corresponding to group delay D g  arising from filter  323 , such that delayed data Dr I, Q  and filtered channel estimate Cr I, Q  are concurrent upon arrival at channel compensator  14 . Period N is determined as follows: 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       
                         D 
                         g 
                       
                       × 
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         F 
                         pilot 
                       
                     
                     
                       S 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         F 
                         data 
                       
                     
                   
                 
               
               
                 
                   ( 
                   F1 
                   ) 
                 
               
             
           
         
       
     
     where
         N is the period;   D g  is group delay D g  corresponding to the filter;   SF pilot  is a spreading factor of the pilot; and   SF data  is a spreading factor of the data.       

     Channel compensator  34  comprises conjugate calculator  340  and multiplier  342 . Conjugate calculator  340  receives and calculates a conjugate of filtered channel estimate Cr I, Q , and multiplier  342  multiplies delayed data Dr I, Q  by the conjugate of filtered channel estimate Cr I, Q  to output compensated output data Do I, Q . Since delayed data Dr I, Q  and filtered channel estimate Cr I, Q  are concurrent, channel compensator  34  performs channel compensation on data Dr I, Q  at accurate timing, leading to an accurate compensation result Do I, Q . 
       FIG. 5  plots signal level against time incorporating the spread spectrum receiver in  FIG. 3 , comprising the real channel  50  multiplied on delayed data and filtered channel estimate  52 . Channel estimate Cr I, Q  passes filter  323  to remove undesired noise and interference, generating filtered channel estimate  52 . Referring to  FIG. 5 , the real channel  50  multiplied on delayed data is synchronous with filtered channel estimate  52 , thus channel compensator  34  is able to perform channel compensation to delay data with filtered channel estimate  52  at accurate timing, resulting in accurate channel compensation. 
       FIG. 6  is a flowchart of a channel compensation method according to the invention, incorporating the spread spectrum receiver in  FIG. 3 . 
     Upon initialization, spread spectrum receiver  3  receives pilot Pr I, Q  from CPICH and data Dr I, Q  from several DPCH simultaneously. 
     In step S 602 , pilot channel correlator  320  despreads pilot Pr I, Q  with a known spreading code, the spreading factor is fixed at 256. And in step S 603 , a multiplier multiplies the despread pilots with conjugate of CPICH pilot pattern to generate channel estimate Cr I, Q . In step S 604 , filter  323  receives and filters channel estimate Cr I, Q  to remove undesired noise and interface, resulting in filtered channel estimate Cr I, Q  with group delay D g . to channel compensator  36 . 
     Step S 606  operates parallel to step S 602 , with data channel correlator  30  despreads data Dr I, Q  with a spreading code generated by code generator  36 . The spreading factor may range from 4 to 256 for WCDMA systems. Next, in step S 608 , compensation delay  38  delays despreaded data Dr I, Q  for period N corresponding to the group delay D g  due to filter  323 , such that both despreaded data Dr I, Q  and filtered channel estimate Cr I, Q  are delayed by group delay D g  when arriving at channel compensator  34 . Period N may be determined by F1. 
     In step S 610 , channel compensator  34  compensates the delayed data Dr I, Q  with filtered channel estimate Cr I, Q  by calculating the conjugate of filtered channel estimate Cr I, Q , then multiplying the delayed data Dr I, Q  by the conjugate. Since now both delayed data Dr I, Q  and filtered channel estimate Cr I, Q  are delayed by identical period N, channel compensator  32  produces an accurately compensated output data Do I, Q . 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.