Abstract:
A method and apparatus for frequency tracking is described. The present invention provides a tracking mechanism for removing the effects of error due to frequency offset as well as compensation for frequency error due to doppler in a plurality of multipath signals. Each finger of a RAKE receiver utilizing the present invention will compute a frequency error for that finger. The weighted average of all of these frequency errors is calculated and filtered to provide a control signal for varying the frequency of IF and RF frequency synthesizers, accounting for the common frequency offset seen at each finger. Additionally, each finger is equipped with a rotator for providing frequency adjustment specific to that finger. The frequency of each finger is adjusted through feedback of the frequency error for that finger.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for frequency tracking of multipath signals which have been subjected to doppler shifts. 
     II. Description of the Related Art 
     Frequency tracking loops are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated by reference herein. 
     FIG. 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units  10   a-d  conduct wireless communication by establishing one or more RF interfaces with one or more base stations  12   a-d using CDMA modulated RF signals. Each RF interface between a base station  12  and a subscriber unit  10  is comprised of a forward link signal transmitted from the base station  12 , and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO)  14  and public switch telephone network (PSTN)  16 . The links between base stations  12 , MTSO  14  and PSTN  16  are usually formed via wire line connections, although the use of additional RF or microwave links is also known. 
     Each subscriber unit  10  communicates with one or more base stations  12  by utilizing a rake receiver. A RAKE receiver is described in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled “MULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS”, filed Sep. 30, 1994, assigned to the assignee of the present invention and incorporated herein by reference. 
     In any passband digital communication system, such as that described above in relation to FIG. 1, there is a need for carrier synchronization. The sender modulates information onto a carrier at frequency f c , and the receiver must recover this frequency so that the received signal constellation does not rotate and degrade the signal to noise ratio (SNR) of the demodulated symbols. In the following discussion, the sender is a CDMA base station and the receiver is a CDMA subscriber unit. 
     Although the receiver knows the nominal carrier frequency, there are two main sources of error that contribute to the frequency difference between the received carrier from the base station and the carrier produced at the subscriber unit. First, the subscriber unit produces the carrier using a frequency synthesizer that uses a local clock as its timing reference. An example RF/IF section of a conventional heterodyne CDMA receiver is shown in FIG. 2. A signal received at antenna  200  is passed through low-noise amplifier (LNA)  202  and filtered in filter  204  before being mixed down to IF by RF mixer  206 . This IF signal is filtered in filter  208 , passed through variable-gain amplifier (VGA)  210  and is then mixed down to baseband by IF mixer  212 . The baseband signal is then filtered in filter  214  and passed through analog to digital converter  216  to produce IQ symbols at baseband. 
     The carrier waveforms sent to RF and IF mixers  206  and  208  are produced using frequency synthesizers  218  and  220 , respectively, that use the subscriber unit&#39;s local clock as a timing reference. This clock has an unknown timing error, typically expressed in parts per million (ppm). In the exemplary implementation, this clock is voltage-controlled temperature compensated crystal oscillator (VCTCXO)  222 , whose frequency is 19.68 MHz and is rated at +/−5 ppm. This means if the desired waveform is a cellular 800 MHz carrier, the synthesized carrier applied to the RF mixer can be 800 MHz+/−4000 Hz. Similarly, if the desired waveform is a 1900 MHz PCS carrier, the synthesized carrier can be 1900 MHz+/−9500 Hz. To correct this error, CDMA receivers use a frequency tracking loop that monitors the frequency error and applies a tuning voltage to VCTCXO  222  to correct it. 
     The second source of error is due to frequency doppler created from movement of the subscriber unit station. The doppler effect manifests as an apparent change in the frequency of a received signal due to a relative velocity between the transmitter and receiver. The doppler contribution can be computed as          f   D     =         v   λ        cos                 θ     =       vf   c        cos                 θ                              
     where v is the velocity of the subscriber unit, x is the wavelength of the carrier, f is the carrier frequency, and c is the speed of light. The variable θ represents the direction of travel of the subscriber unit relative to the direction of the received path from the base station. If the subscriber unit is travelling directly toward the base station, θ=0 degrees. If the subscriber unit is travelling directly away from the base station, θ=180 degrees. So the carrier frequency received at the subscriber unit changes depending on the speed and direction of the subscriber unit relative to the received signal path. 
     As mentioned above, CDMA systems use RAKE receivers that combine symbol energy from different paths. Each strong path is tracked by a finger that performs despreading, walsh decovering and accumulation, pilot time and frequency tracking, and symbol demodulation. An exemplary finger architecture is shown in FIG. 3, where each of N fingers  3 A- 3 N outputs pilot and data symbols obtained for the path it is tracking to digital signal processor (DSP)  300 . DSP  300  performs symbol demodulation and implements the time and frequency tracking loops. IQ baseband samples are despread in PN despreaders  310 A- 310 N, and I and Q pilot and data samples are produced in walsh decover and accumulate blocks  320 A- 320 N and  330 A- 330 N, respectively. 
     An exemplary IS-95A CDMA receiver has four fingers to track four paths, whereas an exemplary cdma 2 OOO CDMA receiver has  12  fingers to handle the  3 x multicarrier case. Cdma 2 OOO is described in TIA/EIA/IS- 2000 - 2 , entitled “PHYSICAL LAYER STANDARD FOR CDMA 2000  SPREAD SPECTRUM SYSTEMS”, incorporated herein by reference. A subscriber unit can be tracking paths from different base stations (in soft handoff), as well as time-delayed paths from the same base station, created from reflections off of local objects. Since the angle θ can be different for each path that the subscriber unit is tracking, the frequency doppler seen by each finger can be different, as illustrated in FIG. 4, which shows subscriber unit  400  in 3-way soft handoff with base stations  410 A- 410 C. Subscriber unit  400  is traveling at velocity v and receiving signals from a variety of paths labeled Path  1  through Path  4 . Path  1  comes from base station  410 A at angle θ 1  equal to π. Path  2  comes from base station  410 B at angle θ 2 . Path  3  comes also comes from base station  410 B but reflects off building  420  and arrives with angle θ 3 . Path  4  comes from base station  410 C and arrives with angle θ 4  equal to  0 . 
     If we assume the subscriber unit has four fingers (labeled finger  1  through finger  4 ) and that finger i is tracking path i, we can see that the doppler seen by finger  1  is          -     vf   c       ,                          
     the doppler seen by finger  2             vf   c        cos                   θ   2       ,                          
     the doppler seen by finger  3  is            vf   c        cos                   θ   3       ,                          
     and the doppler seen by finger  4  is          +     vf   c       ,                          
     where v is the subscriber unit velocity, f is the carrier frequency, c is the speed of light, and θ 1  is the angle of incidence of the path with respect to the direction of subscriber unit  400 . 
     To reduce the frequency error, CDMA receivers typically use a frequency locked loop that can be modeled as shown in FIG.  5 . Frequency error detector  500  computes a measure of the difference between the received carrier frequency ω(n) and the synthesized carrier frequency ω(n). This error signal e(n) is filtered in loop filter  510  and fed back as c(n) to a voltage controlled oscillator (VCO)  520  that modifies the frequency of the synthesized carrier ω(n). This closed-feedback loop corrects the carrier error. 
     We can apply this principle to a CDMA receiver as shown in FIGS. 6A and 6B. IQ baseband samples are passed into N fingers, labeled  600 A- 600 N in FIG.  6 B. FIG. 6A details the frequency error discrimination function of finger  600  which produces frequency error measure e(n). This functionality is replicated in fingers  600 A- 600 N to produce frequency error measures e 1 (n)-e N (n), respectively. PN despreading and walsh accumulation to demodulate pilot symbols is performed in block  610 . The resulting I(n) and Q(n) are delayed in blocks  620  and  630 , respectively. The frequency error is measured by computing the phase rotation between successive pilot symbols in phase rotation measure block  640  to produce error measurement e(n). 
     Referring to FIG. 6B, frequency error measures e 1 (n)-e N (n) from fingers  600 A- 600 N are added together in summer  650  and the sum is passed through loop filter  660  with adjustable gain α. The result is sent to the voltage controlled oscillator  680  using pulse-density modulator (PDM)  670 . Pulse density modulation is a method, known in the art, of converting a digital signal into an analog control voltage. This method applies a single frequency correction (by changing the local clock frequency) that affects all the fingers. In doing so, it basically neglects the individual doppler frequency error component affecting each finger. 
     As stated above, the frequency error has a local clock error component that is the same across all fingers, and a doppler component that is different across fingers. The conventional approach just discussed does not address the doppler component. Although the frequency doppler is not a serious problem at low speeds, it can become a problem when travelling at high speeds, such as on a bullet train. For bullet trains travelling at 500 km/hr, the maximum doppler is around 880 Hz, which can severely degrade the demodulated symbols and lead to dropped calls. So, for travelling on fast moving vehicles such as bullet trains, and in any other application where doppler effects on individual paths vary, there is a need for an improved frequency tracking loop in a CDMA receiver that considers the effect of doppler on each finger. 
     SUMMARY OF THE INVENTION 
     A novel and improved method and apparatus for frequency tracking is described. Frequency tracking is commonly utilized to provide for synchronization between locally generated carriers in a receiver and the carriers used at the base station to modulate the signals which are received. Two main sources of error that contribute to frequency difference include frequency offset between the two timing sources and doppler effects due to movement of a mobile receiver. In a CDMA system utilizing a RAKE receiver to demodulate multipath signals, each received multipath signal can contain a unique doppler effect as well as a common frequency offset component. The present invention provides a tracking mechanism for removing the effects of error due to frequency offset as well as compensation for frequency error due to doppler in a plurality of multipath signals. 
     Each finger of a RAKE receiver utilizing the present invention computes a frequency error for that finger. The weighted average of all of these frequency errors is calculated and filtered to provide a control signal for varying the frequency of IF and RF frequency synthesizers. This feature of the invention accounts for the common frequency offset seen at each finger. 
     Additionally, each finger is equipped with a rotator for providing frequency adjustment specific to that finger. The frequency of each finger is adjusted through feedback of the frequency error for that finger. One embodiment of the invention accomplishes this by subtracting the frequency error component of a finger from the overall weighted average, filtering the remainder, and with that filtered remainder controlling the rotator for that finger. In this way the weighted average of all the errors is used to drive the common frequency synthesis and the difference between the average and the specific error for each finger is used to drive each finger&#39;s individual rotator and thus its doppler frequency compensation. 
     An alternative embodiment uses the independent frequency error for each finger directly by filtering it and using it to drive the rotator for that finger. Thus, the independent frequency errors are used to directly compensate for the doppler on each finger. These frequency errors, now doppler adjusted, are then weighted and averaged, the result of which is filtered and used to drive the frequency synthesizers. This weighted average accommodates the frequency offset component of the frequency error as well as the average of the doppler components of the various fingers. The gains of the individual loop filters can be adjusted in relation to the gain of the loop filter driving the frequency synthesizers so that the tracking speeds of the individual doppler compensation loops is appropriate in relation to the speed of the overall frequency offset tracking loop. 
     This alternative embodiment can be further refined to provide a system which ensures that the frequency synthesizers are tracking the average frequency error of all the fingers. In the previous embodiments, if the rotator loop compensates for some of the frequency error before its contribution is included in the synthesizer loop, the synthesizer loop may not be tracking the true weighted average. The refinement is to compute a second weighted average of the filtered versions of the individual frequency errors. This second weighted average is then multiplied by a factor and summed with the weighted average calculated as described above. The sum is used to drive the frequency synthesis loop. Therefore, even if the frequency error for a finger is driven to zero, effectively removing that finger&#39;s contribution to the weighted average of frequency errors, its filtered frequency error will contribute to the second weighted average, and the synthesizer loop will be driven by it. Thus, the synthesizer loop will be driven according to the true weighted average of the finger frequency errors. Timing based on the average frequency error is a useful feature when used in other parts of a system. For example, a receiver&#39;s timing may be useful for timing a transmitter to which it is coupled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
     FIG. 1 is a block diagram of cellular telephone system; 
     FIG. 2 is a prior art RF/IF section of a conventional heterodyne CDMA receiver; 
     FIG. 3 is a block diagram of a prior art finger architecture of a RAKE receiver; 
     FIG. 4 depicts a subscriber unit utilizing multipath in a RAKE receiver; 
     FIG. 5 is a prior art frequency locked loop; 
     FIGS. 6A and 6B describe the frequency locked loop of FIG. 5 as applied to a CDMA system employing a RAKE receiver; 
     FIG. 7 depicts a frequency tracking loop configured in accordance with the present invention; 
     FIG. 8 depicts an alternate embodiment of a frequency tracking loop configured in accordance with the present invention; and 
     FIG. 9 is a refinement of the alternate embodiment described in reference to FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A block diagram configured in accordance with the present invention is shown in FIG. 7. I and Q baseband samples are delivered to rotators  706 A- 706 N. The rotated I and Q samples are delivered to fingers  700 A- 700 N, respectively. Frequency errors, e 1 (n)-e N (n) respectively, are computed in each of fingers  700 A- 700 N, in accordance with finger  600  in FIG. 6A, described above. 
     A weighted average of frequency errors e 1 (n)-e N (n) is computed in block  710 . In the exemplary implementation, the weight each finger&#39;s frequency error is in proportion to the strength of the finger&#39;s pilot, although other weightings are possible, such as a uniform weighting to each. This weighted average is passed through loop filter  720 , with adjustable gain α, and is sent via PDM  730  to voltage controlled oscillator  740 , producing a frequency which has been corrected for local clock error as well as the average doppler which has been computed in block  710  (note that this is not necessarily the true average frequency error—a modification with this feature is described below). The output of voltage controlled oscillator  740  is used in the RF and IF frequency synthesizers (not shown). 
     To correct the balance of the error on each finger, the difference between finger frequency error and the weighted average is computed in summers  702 A- 702 N, respectively. These differences are filtered, with adjustable gain β, in loop filters  704 A- 704 N, the outputs of which control rotators  706 A- 706 N, respectively, at the front of each finger  700 A- 700 N. As is known in the art, the loop filters  704 A- 704 N may be simply accumulators. Each rotator  706 A- 706 N rotates the input IQ samples to correct the rest of the frequency error due to doppler. By accounting for the doppler on each individual finger, the quality of symbol demodulation when travelling at high speeds is improved. 
     An alternative embodiment is shown in FIG.  8 . In like manner to that described in FIG. 7, I and Q baseband samples are delivered to rotators  806 A- 806 N. The rotated I and Q samples are delivered to fingers  800 A- 800 N, respectively. Frequency errors, e 1 (n)-e N (n) respectively, are computed in each of fingers  800 A- 800 N, in accordance with finger  600  in FIG. 6A, described above. 
     A weighted average of frequency errors e 1 (n)-e N (n) is computed in block  810 , in like manner to that described above with repect to block  710 . This weighted average is passed through loop filter  820 , with adjustable gain α, and is sent via PDM  830  to voltage controlled oscillator  740 , producing a frequency which has been corrected for local clock error as well as the average doppler which has been computed in block  810  (note, again, that this is not necessarily the true average frequency error—a modification with this feature is described below). The output of voltage controlled oscillator  840  is used in the RF and IF frequency synthesizers (not shown). 
     To correct the balance of the error on each finger, the frequency errors e 1 (n)-e N (n) are used directly. No difference between finger frequency error and the weighted average is computed, such as was done in summers  702 A- 702 N above. Frequency errors e 1 (n)-e N (n) are filtered, with adjustable gain β, in loop filters  804 A- 804 N, the outputs of which control rotators  806 A- 806 N, respectively, at the front of each finger  800 A- 800 N. Each rotator  806 A- 806 N rotates the input IQ samples to correct the rest of the frequency error due to doppler. 
     In this case, there are several frequency tracking loops running simultaneously. Each finger has its own frequency tracking loop that operates using its own rotator, and there is an overall frequency tracking loop that adjusts the local clock based on a weighted average of all the finger frequency errors. To allow these loops to operate simultaneously, we adjust the loop gains α and β so that each finger&#39;s individual frequency tracking loop operates much faster than the overall tracking loop that uses the weighted average (β&gt;α). In this way, the overall tracking loop gradually adjusts to the correct value to correct the local clock, while the individual tracking loops adjust quickly to account for the change introduced by the overall tracking loop. This implementation will converge to the same solution as the implementation shown in FIG. 7, so it provides the same benefits. 
     FIG. 9 depicts a refined embodiment of that described in reference to FIG.  8 . This refinement is introduced to assure that the timing created by voltage controlled oscillator  940  is corrected for the average of frequency errors across all the fingers. This is a useful feature when the timing is used for more than demodulation in the receiver we have been describing. For example, it is common for a transmitter and receiver both to be included in a subscriber unit, and it is often advantageous for the transmitter to rely on system time as derived by the receiver. 
     To see why the weighted average computed in block  810  (and also block  710 ), described above, is not necessarily the average of the true frequency errors, consider a simple example. It is common in a RAKE receiver for a finger to be assigned a path and given a chance to track it before the results of that path are included in the overall tracking and demodulation. In this case, the frequency errors e 1 (n)-e N (n) are selectively included in the computation of the weighted average in block  810 . For this example, assume that initially no fingers are currently demodulating, and no frequency errors are included in the weighted average. Now a finger,  800 A for example, is assigned a path to begin tracking. It is possible that loop filter  804 A in conjunction with rotator  806 A will drive error e 1 (n) to zero before it is determined to include finger  800 A in the weighted average in block  810 . Once e 1 (n) is included in the average (of only one signal in this example), the weighted average will remain zero and the RF and IF frequency synthesizers (not shown) will not be adjusted by voltage controlled oscillator  840 . So it is clear that overall timing produced by voltage controlled oscillator  840  is not indicative of the average frequency error of all the fingers (the average in this example is for only finger  800 A and the average error is indicated by the output of loop filter  804 A). 
     Turn now to FIG. 9 to see the modifications which can be made to provide a timing reference which is based on the average frequency error. As before, I and Q baseband samples are delivered to rotators  906 A- 906 N. The rotated I and Q samples are delivered to fingers  900 A- 900 N, respectively. Frequency errors, e 1 (n)-e N (n) respectively, are computed in each of fingers  900 A- 900 N, in accordance with finger  600  in FIG. 6A, described above. Frequency errors e 1 (n)-e N (n) are filtered, with adjustable gain β, in loop filters  904 A- 904 N, the outputs of which control rotators  906 A- 906 N, respectively, at the front of each finger  900 A- 900 N. Each rotator  906 A- 906 N rotates the input IQ samples to correct the finger specific frequency error due to doppler. The components of FIG. 9 described thus far have not changed from their counterparts in FIG.  8 . 
     As above, a first weighted average of frequency errors e 1 (n)-e N (n) is computed in block  910 . However, this embodiment also includes a second weighted average, computed in block  914 , which averages the filtered versions of e 1 (n)-e N (n) produced in loop filters  904 A- 904 N, respectively. The method for computing each weighted average can be the same as those described above. The second weighted average, computed in block  914 , is modified by adjustable gain y in block  916 . This result is added in summer  918  to the first weighted average, computed in block  910 . 
     The remainder of FIG. 9 is similar to FIG.  8 . The sum from summer  918  is passed through loop filter  920 , with adjustable gain α, and is sent via PDM  930  to voltage controlled oscillator  940 , producing a frequency which has been corrected for the true average frequency error across all the fingers. The output of voltage controlled oscillator  940  is used in the RF and IF frequency synthesizers (not shown). 
     Owing to the additional connections just described, the design of FIG. 9 will always ensure that the voltage controlled oscillator settles at the average of the doppler errors from all fingers—not always the case with the previous two implementations as demonstrated in the previous example. In the previous example, before a finger was included, it was likely that its frequency error was already driven to zero by its rotator and loop filter. When finally added to the system described in FIG. 8, that implementation did not ensure that the voltage controlled oscillator moved to a value equal to the average frequency error based on all fingers. In the embodiment depicted in FIG. 9, on the other hand, the new connections from the outputs of loop filters  904 A- 904 N are non-zero and so will contribute to the averaging process. As a result, eventually voltage controlled oscillator  940  will move to its intended value of the average frequency of all N fingers. 
     Thus, a method and apparatus for frequency tracking has been described. The description is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.