Abstract:
Disclosed is a method, an apparatus and a mobile station embodiment to receive a CDMA signal from a radio channel. The method includes inputting a CDMA signal received through the radio channel to a searcher and processing the received signal in the searcher to obtain a multi-path profile of the radio channel. Processing includes at least partially removing an effect of at least one of a transmit and a receive filter on the multi-path profile. In one embodiment at least partially removing the effect of at least one of the transmit and receive filter on the multi-path profile involves passing the received CDMA signal through an N-tap Finite Impulse Response filter having a filter characteristic that approximates an inverted amplitude response of the at least one of the transmit and a receive filter, while in another embodiment software external to the searcher is used to process the searcher output data.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to wireless communications systems and receivers and, more specifically, relates to pseudo-noise (PN) code searchers used in code division multiple access (CDMA) communications systems, such as those that include mobile stations, such as cellular telephones, and related infrastructure equipment, such as base stations.  
       BACKGROUND  
       [0002]     In a CDMA system, such as a cdma2000/IS2000 system or Wideband CDMA (WCDMA) system, a searcher element is used in the mobile station receiver demodulator for acquisition, set maintenance and control of the demodulating element, also referred to as finger assignment. In the Idle State or in the Traffic State, the searcher derives multi-path amplitude profiles corresponding to the Active, Candidate and Neighbor base stations. A mobile station controller uses the derived multi-path amplitude profiles to assign demodulator fingers in order to demodulate and combine the signals received from the strongest ones of the multi-paths.  
         [0003]     In accordance with conventional operation the searcher derives multi-path profiles by correlating the incoming signal with a replica of the PN sequence over a number of PN time offset hypotheses. Typically, the searcher produces a two sample/chip multi-path profile. This profile corresponds to the amplitude of the complex correlation of the incoming data with the PN replica over a certain correlation length at two samples/chip.  
         [0004]     It should be noted that the reference to amplitude implies a magnitude calculation. The multi-path profile is typically implemented as an amplitude profile because the bit width of the information to transfer from the searcher hardware to the DSP/general processor is reduced. Note that instead of the amplitude or magnitude of the complex correlation, one could use the power or magnitude squared of the complex correlation, as shown in the prior art searcher embodiment of  FIG. 2 .  
         [0005]     In general, the multi-path profile is the result of a convolution of the base station signal with a radio channel, the base station transmit filter and the mobile station receive filter. That is, a conventional searcher produces the multi-path profile using a convolution of the transmit/receive filters and the radio channel. The Active Set profile generated by the searcher is then used by the controller to assign the fingers to demodulate the incoming CDMA signal.  
         [0006]     However, if the multi-path profile data is compromised, such as by the presence of excessive noise or artifacts (such as sidelobe peaks) in the profile data, there exists a potential that the controller will assign fingers incorrectly to some PN offset, whereby the overall demodulation performance of the receiver is degraded. Further, when artifacts are present they are also processed by the finger assignment algorithm, thereby increasing the overall complexity without deriving any additional benefit.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0007]     The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.  
         [0008]     This invention relates to searcher technology for a receiver operating in accordance with a CDMA standard, such as cdma2000 or WCDMA. The invention is relevant to a base station receiver and to a mobile station receiver, although it is described below primarily in the context of a mobile station receiver. A searcher uses a deconvolution technique to at least partially remove the blurring effect of the transmitter and receiver filters so that the searcher produces substantially only the radio channel multi-path profile.  
         [0009]     In one aspect this invention provides a method to receive a CDMA signal from a radio channel, and the method includes inputting a CDMA signal received through the radio channel to a searcher and processing the received signal in the searcher to obtain a multi-path profile of the radio channel, where processing includes at least partially removing an effect of at least one of a transmit and a receive filter on the multi-path profile.  
         [0010]     In one embodiment at least partially removing the effect of at least one of the transmit and receive filter on the multi-path profile involves passing the received CDMA signal through an N-tap Finite Impulse Response filter having a filter characteristic that approximates an inverted amplitude response of the at least one of the transmit and a receive filter. The filter taps of the FIR filter may be derived in several different manners.  
         [0011]     For example, in one embodiment a least squares criterion can be used to derive the radio channel multi-path profile x from a searcher profile y, where y=F·x+v, where v is a noise vector and F is a transmit/receive matrix.  
         [0012]     The invention can be practiced primarily by searcher hardware (circuitry), or it can be practiced primarily by computer software, such as software executed by a mobile station Digital Signal Processor (DSP) or executed by a mobile station general purpose data processor.  
         [0013]     Apparatus for practicing the method is also disclosed, as is a mobile station that is constructed and operated in accordance with the invention to have a receiver adapted to receive a CDMA signal from a radio channel. The receiver includes a receiver front end for receiving the CDMA signal from the radio channel. The receiver from end includes at least one receiver filter and, in accordance with this invention, a searcher having an input coupled to an output of the receiver front end for inputting a received signal and an output for outputting a digital representation of a radio channel multi-path profile to a mobile station control function, such as a DSP or a general purpose data processor. In one embodiment the searcher includes a hardware unit for processing the received signal to at least partially remove an effect of at least said receiver filter on the multi-path profile, and to possibly also at least partially remove an effect of a base station transmitter filter on the multi-path profile. In another embodiment the effect of the receiver/transmitter filter is removed in software by operating on the conventional multi-path profile that is output from the searcher.  
         [0014]     Also disclosed is a method to reduce an amount of data provided to a finger assignment algorithm. The method includes inputting a CDMA signal received through a radio channel to a searcher, and processing the received signal in the searcher to generate output data for the finger assignment algorithm that represents a multi-path profile of the radio channel. Processing includes passing the received CDMA signal through a filter selected to have a filter characteristic that approximates an inverted response of at least one of a base station transmit filter and at least one mobile station receive filter so as to reduce an occurrence of multi-path sidelobes in the output data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:  
         [0016]      FIG. 1  is a simplified block diagram of a mobile station that is suitable for practicing this invention;  
         [0017]      FIG. 2  illustrates a prior art searcher unit, in combination with a portion of an analog receiver of the mobile station;  
         [0018]      FIG. 3A  is a block diagram of a searcher unit that includes a deconvolution processor block in accordance with this invention;  
         [0019]      FIG. 3B  shows an N-tap FIR filter embodiment of the deconvolution processor block of  FIG. 3A ;  
         [0020]      FIG. 3C  shows an IIR filter embodiment of the deconvolution processor block of  FIG. 3A ;  
         [0021]      FIG. 3D  shows a post-processor, least squares embodiment of the deconvolution processor block of  FIG. 3A ;  
         [0022]      FIG. 4   a  shows an example of a multi-path profile from the radio channel,  FIG. 4   b  shows a combined transmitter/receiver filter response, and  FIG. 4   c  shows the convolution of the multi-path profile from the radio channel and the combined transmitter/receiver filter response, which corresponds to the multi-path profile measured by a conventional searcher unit, such as the one shown in  FIG. 2 ; and  
         [0023]      FIG. 5  shows an embodiment of a transmit/receive matrix F. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIG. 1  is a block diagram showing portions of a mobile station  130  that includes a searcher/receiver  110  that is constructed and operated according to embodiments of the invention. Reference with regard to the overall exemplary mobile station  130  architecture shown in  FIG. 1  can be made to commonly assigned U.S. Pat. No. 6,144,691, “Method and Apparatus for Synchronizing to a Direct Sequence Spread Spectrum Signal”, by Thomas Kenney. The exemplary mobile station  130  architecture includes an antenna  100 , a duplexer  102 , a transmit power amplifier  104 , an analog receiver  106 , a transmit power controller  108 , the searcher/receiver  110 , a first digital data receiver  112 , a second digital data receiver  114 , a third digital data receiver, a diversity combiner/decoder  116 , a control processor  118 , a RAM  126 , a digital vocoder  120 , a transmit modulator  122  and a user interface  124 .  
         [0025]     The antenna  100  is coupled to the analog receiver  106  through duplexer  102 . Signals received at antenna  100  are input to analog receiver  106  through duplexer  102 . The received signals are converted to an IF frequency and then filtered and digitized in analog receiver  106  for input to the digital data receiver  112 , digital data receiver  114  and searcher/receiver  110 . The digitized IF signal input to digital data receiver  112 , digital data receiver  114  and searcher/receiver  110  may include signals from many ongoing calls, together with the pilot carriers transmitted by the base station of the cell site in which the mobile station is currently located, plus the pilot carriers transmitted by the base stations in all neighboring cell sites. Digital data receiver  112 , digital data receiver  114 , and digital data receiver  115  perform a correlation on the IF signal with a PN sequence of a desired received signal. The output of digital data receivers  112 ,  114  and  115  is a sequence of encoded data signals from three independent paths.  
         [0026]     Searcher/receiver  110  searches the pilot channel PN phase offset space for pilot channel signals transmitted from a base station. Searcher/receiver  110  is also used to search for other signals transmitted from different base stations. Searcher/receiver  110  generates correlation results for a desired waveform and generates signals to control processor  118  indicating the correlation results of the searched signals.  
         [0027]     The encoded data signals output from digital data receivers  112 ,  114  and  115  are input to the diversity combiner/decoder  116 . In diversity combiner/decoder  116  the encoded data signals are aligned and combined, the resultant data signal is then decoded using error correction, and is then input to digital vocoder  120 . Digital vocoder  120  outputs information signals to the user interface  124 . The user interface  124  may be a handset with a keypad or another type of user interface, such as a laptop computer monitor and keyboard.  
         [0028]     For transmission of signals from mobile station  130 , a signal received at user interface  124  is input to user digital vocoder  120  in digital form, as for example, data or voice that has been converted to digital form at user interface  124 . In digital vocoder  120  the signal is encoded and output to transmit modulator  122 . Transmit modulator  122  Walsh encodes the signal and then modulates the Walsh encoded signal onto a PN carrier signal, with the PN carrier sequence being the PN carrier sequence of the CDMA channel to which the mobile station is assigned. The PN carrier information is transmitted to mobile station  130  from the telecommunications system and transferred to control processor  118  from digital data receivers  112  and  114 . Control processor  118  sends the PN carrier information to transmit modulator  122 . A PN modulated signal is output from transmit modulator  122  to transmit power control  108 . Transmit power control  108  sets the level of the transmission power of mobile station  130  according to commands received from control processor  118 . The power control commands may be generated by control processor  118  according to commands received from the system, or may be generated by software of control processor  118 , according to predetermined criteria, in response to data received from the system through digital data receivers  112 ,  114  and  115 . The modulated signal is then output from transmit power control  108  to transmit power amplifier  104  where the signal is amplified and converted to an RF carrier frequency signal. The RF carrier frequency signal is then output from power amplifier  104  to duplexer  102  and transmitted from antenna  100  to the base station (not shown).  
         [0029]     For reference purposes,  FIG. 2  shows a block diagram of a conventional searcher unit that could be used to implement the searcher/receiver  110  of  FIG. 1  in a conventional mobile station  130 . Reference with regard to this conventional searcher architecture can be made to commonly assigned U.S. Pat. No. 6,269,075 B1, “Finger Assignment in a CDMA RAKE Receiver”, by Jean-Marie Tran (the named inventor of this patent application).  
         [0030]     The searcher unit  110  includes a search engine element having in-phase (I) and quadrature-phase (Q) samplers  110 A and  110 B, respectively, a de-spreader element  110 C, I and Q accumulators  110 D and  110 E, respectively, and a magnitude squaring unit  110 F. Note that the magnitude squaring unit  110 F could be replaced by a simple magnitude calculation unit so that the prior art searcher can produce either the magnitude squared (the power profile) or the magnitude (the amplitude profile) of the radio channel.  
         [0031]     If one assumes perfect match filtering with I and Q matched filters  106 A,  106 B, that form part of the analog receiver  106  of  FIG. 1  (also referred to herein as the receiver front end), the I and Q signal inputs to de-spreader element  110 C have the form shown in  FIG. 2 , wherein h′(t) is the impulse response of the matched filters  106 A,  106 B. The de-spreader element  110 C is a complex de-spreader that multiplies each chip sample by the term shown in  FIG. 2 . Letting H(t−kT c )=h(t−kT c )*h′(t), then the signal output from the accumulators  110 D and  110 E (neglecting interference terms) is N c H(delay)e −jφ , where delay is a small delay in the generated PN timing in the complex de-spreader  110 C. The magnitude squaring unit  110 F (or magnitude unit, in the amplitude embodiment referred to above) eliminates the arbitrary initial phase such that the output measurements from the searcher unit  110  are proportional to H(delay) 2 . The searcher unit  110  may measure the multipath environment with 1/½ chip resolution to avoid unacceptable signal loss due to sampling errors. Also shown in  FIG. 2  is a base station  50  having an antenna  52 , a transmitter  54  and a transmitter filter  56  for transmitting a CDMA signal through a radio channel.  
         [0032]      FIG. 3A  illustrates a searcher unit  200  in accordance with this invention. The searcher unit  200  receives the CDMA signal that is applied to a correlator  202  that also receives the PN signal. The output of the correlator  202  is applied to an amplitude calculation block  204 . In accordance with an embodiment of this invention, the output of the amplitude calculation block  204  is applied to a deconvolution processing block  206  that also receives a filter signal  206 A. The output of the deconvolution processing block  206  is applied to a data reduction/sorting block  208 , and thence to output buffers  210  that are readably coupled to a digital signal processor (DSP) and/or a general data processor of the mobile station or the base station, depending on where the CDMA receiver that employs this invention is used.  
         [0033]     Discussing  FIG. 3A  now in further detail, the correlator  202  may be a conventional correlator capable of performing coherent integration for a given number of chips. Note that another suitable conventional correlator implementation uses additional non-coherent integration after the magnitude or magnitude squared block. In block  204  the amplitude is derived, that is, block  204  computes the magnitude of the complex correlation: |correlation|=square root (real part*real part+imag part*imag part). Block  208  typically reduces the data transfer from the ASIC, that contains the illustrated circuitry, to the DSP. In one implementation the block  208  performs a partial sorting of the data and reduces the profile to include only the 16 or 32 highest amplitude data points. In addition to the amplitude, block  208  keeps track of the PN offsets of the values selected. Block  208  may also include peak finder circuitry, that is, block  208  could also include means to identify and provide the PN and the amplitude of the arrival peaks. The Filter input  206 A conveys a set of N coefficients which are the taps of the filter, such as the N-tap FIR filter. What is filtered by the FIR filter is the raw amplitude profile derived by the searcher.  
         [0034]     Since it may be reasonably assumed that the receive filtering response of the matched filters  106 A and  106 B ( FIG. 2 ), or filters that, once implemented, are an approximation of the matched filters  106 A,  106 B, is approximately known, in accordance with an aspect of this invention the transmit and receive filtering response can be removed from the multi-path amplitude profile using the deconvolution processing block  206 . As a result, instead of observing the convolution of the discrete CDMA signal arrivals from the radio channel and a combined transmitter/receiver filter response, the discrete radio channel arrival paths alone can be recovered from the received CDMA signal.  
         [0035]      FIG. 4   a  shows an example of a multi-path profile from the radio channel,  FIG. 4   b  shows a combined transmitter/receiver filter response, and  FIG. 4   c  shows the convolution of the multi-path profile from the radio channel and the combined transmitter/receiver filter response, which corresponds to the multi-path profile measured by a conventional searcher unit, such as the searcher unit  110  shown in  FIG. 2 .  
         [0036]     The deconvolution searcher  200 , in accordance with this invention, produces (approximately) the multi-path profile shown in  FIG. 4   a , as opposed to the multi-path profile shown in  FIG. 4   c , since the “blurring” function of the combined transmitter/receiver filter response of  FIG. 4   b  is essentially removed.  
         [0037]     There are a plurality of embodiments that can be used to realize the deconvolution processing unit  206  of the deconvolution searcher  200 . What follows are descriptions of two primarily hardware embodiments, and one primarily software embodiment. These various embodiments are not intended, however, to be read in an limiting sense upon the practice of the teachings of this invention.  
         [0038]     In general, in one hardware embodiment the deconvolution processor block  206  is implemented as an N-tap Finite Impulse Response (FIR) filter  300 , shown in  FIG. 3B , and in another hardware embodiment shown in  FIG. 3C  the deconvolution processor block  206  is implemented as an Infinite Impulse Response (IIR) filter  400  (having a number of taps at the numerator and at the denominator). The derivation of the filter taps for either of these embodiments can be performed in a variety of ways. For example, one suitable technique is to generate an inverse filter of the a-priori known convolution of the transmit and receive filter. Note in this regard that the transmit filter  56  model in the base station  50  may be a filter specified by the IS-95 CDMA standard, while the receive filter is preferably the actually implemented receive filter, or model of the receive filter.  
         [0039]     In the FIR filter  300  embodiment the amplitude response of the transmit/receive filters, such as the matched receive filters  106 A,  106 B and the corresponding base station transmit filter  56 , is inverted and the resulting inverse filter is approximated by the N-tap FIR filter  300  shown in  FIG. 3B . The FIR filter  300  is used to filter the PN-series generated by the searcher  200  (the output of correlator  202  via the amplitude (or power) calculation block  204 ). The FIR data need be provided by the control processor  118 , such as by the DSP, but once as a fixed model of the blurring function (e.g., that corresponding to  FIG. 4   b ). For example, the FIR filter  300  is the inverse of the convolution of a model of at least one of, and preferably both of, the base station transmit filter  56  (such as the transmit FIR filter defined in the IS-95 standard) and the mobile station receiver filters  106 A,  106 B. Note that the model of the mobile station  130  receiver filter may actually be the convolution of several filters, partitioned in accordance with the specific architecture of the receiver (e.g., analog baseband filter/digital filter), and may or may or may not include a fixed equalizer.  
         [0040]     In an alternate embodiment shown in  FIG. 3D  a post-processor unit  500  operates on the profile output from block  204 , and uses a least-squares criterion to derive the radio channel multi-path profile x from the searcher profile y, where y=F·x+v, where v is a noise vector and F is a transmit/receive matrix. In this embodiment the actual operation that is performed is a matrix multiplication, that is, x is derived from y by a relationship x=R·y. R is preferably derived through least squares, and can be fixed and computed off-line.  
         [0041]     Assuming that the blurring function has a span of 2k+1 samples, and is given by f 0 , f 1 , . . . , f2k+1, the transmit/receive matrix F has the form shown in  FIG. 5 .  
         [0042]     The least squares criterion is to minimize the L2 norm of the vector v, and the problem then becomes one of finding x so that ||y−F·x|| 2 . The vector x may be derived as x=(F T ·F) −1 ·F T ·y, where T denotes a transpose operation and −1 denotes an inverse matrix operation. Note that the matrix=(F T ·F) −1 ·F T  is preferably computed off-line and stored. A pre-whitening term may be added to stabilize the inverse with the solution x=(F T ·F+epsilon·I) −1 ·F T ·y, where epsilon is a fixed number. Note that the matrix=(F T ·F+epsilon·I) −1 ·F T  is also preferably computed off-line and stored. This second embodiment has the benefit of improved performance in a noisy environment.  
         [0043]     A variation on this technique uses L1 norm instead of L2 norm in the least squares derivation, and thus finds x as the solution to the minimization problem of ||y−F·x|| (note that the vectors are real), where L1 norm ||y−F·x||=sigma|v n |.  
         [0044]     The third, essentially software embodiment is now described.  
         [0045]     In a first step, and assuming a mobile station  130  embodiment of the invention, the searcher is commanded to search the desired Active Set member in a non-sorted mode.  
         [0046]     This particular Active Set member may be, for example, the serving sector for the Forward Packet Data Channel (F-PDCH) in IS2000 Release C or IS2000 Release D. In a second step, the data is retrieved from the searcher hardware to the control processor  118 , such as to the DSP of the mobile station  130 . In the third step one of the above-described deconvolution/filtering or post-processing embodiments ( FIG. 3B, 3C  or  3 D) is implemented in the DSP software, as opposed to the searcher hardware. The resulting multi-path profile solution is then fed to the finger assignment process that also operates in the DSP. In this case the searcher can be conventional in construction, as the invention is implemented in software outside of the searcher. In the above-noted non-sorted mode of operation there is no data reduction, and the data returned is the raw data, either the magnitude or the magnitude squared multi-path profile at the 1/½ chip sampling (typical) interval. This is opposed to the sorted mode of operation, where the raw data is pre-processed such that, typically, the 16 or 32 largest amplitude/power and associated PN offsets are returned.  
         [0047]     It is noted that the finger assignment process typically begins with a further reduction of the data returned by the searcher. That is, after the sort process the top 16 or top 32 amplitudes and corresponding PN offsets are available, from which the finger assignment module or algorithm needs to identify peaks that correspond to the multi-paths. These peaks may be filtered further, and the corresponding profiles are further processed to determine if there is a new path to assign a free finger to, or a new path that is stronger than a path currently being demodulated by a finger. When there is a strong multi-path present there may be sidelobes present as well that have the appearance of peaks, which can result an assignment of fingers to the sidelobes. One advantage of the deconvolution searcher  200  is that it can remove the sidelobe peaks or artifacts prior to the finger assignment module or algorithm, thereby minimizing a possibility that a finger will be assigned to a sidelobe.  
         [0048]     Thus, in that there can be sidelobes in the combined response of the transmit and receive filters, the use of the searcher deconvolution processing block  206  makes it possible for the searcher  200  to pass to the sorting routine only the main radio channel paths. In either the hardware or software embodiments of this invention the control processor  118  that uses the sorted data is inhibited from assigning a demodulator finger to a sidelobe of a path, since the sidelobe(s) have been removed from the “raw” searcher output by the operation of the deconvolution processing block  206 .  
         [0049]     It can thus be noted that there additional data reduction is made possible by the use of this invention, since the operation of the deconvolution processor  206  serves to remove some “peaks” as the sidelobes of strong peaks, and thus these sidelobes do not find their way in to the finger assignment process.  
         [0050]     It should be noted that there is no utility to be gained by assigning a finger to a sidelobe of a radio path, as there is no new information contained in the sidelobe. Actually, assigning a finger to a path and another to a sidelobe can create a performance degradation, since sidelobe sampling can result in a higher level of Inter-Chip Interference (ICI). The use of the searcher deconvolution processing block  206  can beneficially avoid the assignment of a finger to the sidelobe of a strong radio path, since the use of the searcher deconvolution processing block  206  makes it possible for the searcher  200  to pass to the sorting routine only the main radio channel paths.  
         [0051]     The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the best method and apparatus presently contemplated by the inventors for carrying out the invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. For example, and as was noted above, this invention can be used in downlink or forward receiver, such as in the mobile station receiver, or it can be used in the uplink or reverse receiver, such as in a base transceiver station or more simply base station. In addition, the deconvolution searcher of this invention will function with either type of multi-path profile, i.e., with one based on the amplitude or magnitude of the complex correlation, or one based on the power or magnitude squared of the complex correlation. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.  
         [0052]     Furthermore, some of the features of the present invention could be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the present invention, and not in limitation thereof.