Patent Publication Number: US-5528624-A

Title: DS/CDMA receiver using parallel-operating multi-purpose correlators

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to direct sequence spread spectrum (code division multiple access) receivers. This invention is particularly concerned with a sync acquisition and tracking technique for direct sequence spread spectrum receivers of cellular mobile communications systems in which transmitted signals are severely affected by Rayleigh fading and multipath fading. 
     2. Description of the Related Art 
     Commercial interest in direct sequence spread spectrum (code division multiple access) communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the cell-site station of the DS/CDMA system, a data symbol is spread by multiplying it with orthogonal pseudo-random number (PN) sequences assigned to the cell site as well as with orthogonal Walsh codes assigned to the channel over which the spread signal is transmitted. In order to enable the mobile station to provide sync acquisition and tracking operation, a pilot signal is superimposed on the data symbol sequence. At the mobile station, a sliding correlation technique is used to shift the phase timing of a local PN sequence by a predetermined amount each time a correlation is taken between the received and local sequences and determine the correct phase timing for the local sequence when the correlation exceeds some critical value. Such phase shifting is performed at predetermined &#34;window&#34; intervals. Once synchronization is established, the phase difference is monitored and maintained to within one half of the chip interval. During transmission, the signal undergoes reflections from various land structures, producing a complex pattern of standing waves due to mutual interference. As a result, the propagation path of the signal exhibits a field intensity distribution which is approximated by the Rayleigh distribution. Thus, the signal experiences a phenomenon called &#34;Rayleigh fading&#34; and the envelope of the signal at the mobile station as well as its phase violently fluctuate. 
     Under such unfavorable conditions, the sliding correlation technique is not ideal to achieve quick synchronization in response to the rapidly varying signal levels. In addition, there is often a need to update the mobile&#39;s phase timing due to the arrival of a strong reflection or a signal of significant level from an adjacent cell site. Under such circumstances, the prior art system sets the correlation circuitry to the new phase timing immediately in response to the arrival of a new strong signal without taking the old symbols, which may be left in the correlation circuitry, into account. The use of the old symbols for the new phase timing results in timing discontinuity and produces over-correlation or under-correlation. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is therefore to provide a direct sequence spread spectrum receiver capable of establishing quick synchronization under varying signal levels. 
     Another object of the present invention is to provide a direct sequence spread spectrum receiver which ensures seamless transition when updating the phase timing of correlation process. 
     According to the present invention, there is provided a direct sequence spread spectrum receiver which comprises a plurality of correlators for despreading a spread data signal and a spread pilot signal, a plurality of pilot demodulators, a plurality of data demodulators and an adder. The data demodulators and the adder constitute a RAKE receiver. During a sync acquisition mode, the pilot demodulators are connected to the correlators, respectively, and during a subsequent tracking mode, the data demodulators are connected to the correlators to produce a plurality of demodulated data signals which are combined by the adder to produce an output for the RAKE receiver. A plurality of despreading codes of different phase positions are generated and supplied to the correlators, respectively, causing the correlators to despread one of the spread data and pilot signals. During the sync acquisition mode, the phase position of each despreading code is successively shifted and high correlation values are determined from the outputs of the pilot demodulators. During the tracking mode, the phase positions of the despreading codes are set according to the high correlation values. Since multiple correlators operate in parallel during the sync acquisition mode, the time taken to search through the range of possible phase positions is significantly reduced. In practical aspect, the data demodulators are provided in number corresponding to the number of multipath fading channels. 
     The despreading codes are generated such that, during the sync acquisition mode, all the correlators provide correlations for the incoming pilot signal only, and during the tracking mode, one half of the correlators provide correlation for the data signal and the remainder provides correlation for the pilot signal. 
     The despreading code comprises a pseudo-random number (noise) sequence uniquely identifying a cell site area and a Walsh code identifying a communication channel. When the Walsh code is set to an all-zero code, the correlators supplied with this code is set to provide correlation with the pilot signal. 
     In a preferred aspect, a second despreading code of different phase position is supplied to a specified one of the correlators during the tracking mode to continue searching for best phase positions. The phase position of the second despreading code is shifted in the neighborhood of those previously set in the sync acquisition mode and high correlation values are newly determined from output signals from one of the pilot demodulators. The previously set phase positions are updated according to the new high correlation values during an update mode. 
     To ensure smooth transition when new phase positions are determined and old phase positions are to be updated, a second data demodulator is provided. This data demodulator is connected to the output of the specified correlator at the time the specified correlator is set to a new phase position. In response to the connection of the second data demodulator to the specified correlator, a ramp-up time is introduced to allow old symbols to be replaced with new symbols, and immediately following the ramp-up time, a successive one of the outputs of the other data demodulators is disconnected from the adder for a predetermined interval and, instead, the output of the second data demodulator is connected to the adder during that predetermined interval. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a direct sequence spread spectrum receiver according to the present invention; 
     FIG. 2 is a circuit diagram of each correlator of FIG. 1; 
     FIGS. 3A, 3B and 3C are block diagrams of the processor FIG. 1 during sync acquisition, tracking and update modes, respectively; 
     FIG. 4 is a circuit diagram of each pilot demodulator of FIGS. 3A, 3B and 3C; 
     FIGS. 5A and 5B are circuit diagrams of different embodiments of each data demodulator of FIGS. 3A, 3B and 3C; and 
     FIGS. 6A, 6B and 6C are block diagrams of the phase shift controller FIG. 1 during sync acquisition, tracking and update modes, respectively. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown a receiving circuit of a mobile station for a DS/CDMA cellular communications system according to the present invention. At a cell-site station, a baseband downlink (cell-to-mobile) signal is initially encoded by a channel encoder into a known coded form that is optimized for radio transmission. At the chip rate much higher than the symbol rate, the symbols of the encoded signal are spread with PN (pseudo-random number) spreading sequences PN i  and PN q  commonly assigned to the service area in which the mobile station is currently located and further spread with orthogonal Walsh codes uniquely assigned to the downlink channel. A pilot signal, which may be a series of all zeros or ones, is spread at the same chip rate with the same PN sequences PN i  and PN q  with orthogonal Walsh all-zero codes. The in-phase and quadrature components of spread spectrum downlink signal are combined with the corresponding components of spread spectrum pilot signal to produce an I signal and a Q signal which are modulated onto orthogonal radio-frequency carriers, amplified and transmitted. 
     The spread spectrum signal from the cell-site is received by the mobile station and supplied to a quadrature demodulator 10 which includes radio-frequency amplification and demodulation stages. Using orthogonal local carriers, the demodulator 10 recovers the original i and q signals. After analog-to-digital conversion by an A/D converter 11, the digital i and q signals are fed in parallel to correlators CR 1  to CR 8  of identical configuration. 
     As illustrated in FIG. 2, each of the correlators CR k  (where k=1, 2, . . . ,8) comprises a pair of multipliers 20i and 20q for multiplying the I and Q digital signals from A/D converter 11 with the pseudo-random despreading sequences PN i (k) and PN q (k) to produce an in-phase output i k  =I×PN ik  from multiplier 20i and a quadrature output signal q k  =Q×PN qk  from multiplier 20q. The PN sequences PN ik  and PN qk  are the two inputs to an exclusive-OR gate 30 producing a switching signal. The output signal i k  of multiplier 20i and the output signal q k  of multiplier 20q are the inputs to switches 21i and 21q. If the switching signal is a binary 1, the output of multiplier 21i appears at the output of switch 21i as a signal I dk . If the switching signal is a binary 0, the signal I dk  is equal to the output of multiplier 21q. Similarly, the output of multiplier 20q appears at the output of switch 21q as a signal Q dk  when the switching signal is 1. Otherwise, the output of multiplier 20i appears as Q dk . Correlator CR k  further includes a pair of multipliers 23i and 23q for despreading the signals I dk  and Q dk  with a Walsh code W k . As will be described, all the Walsh codes supplied to correlators CR 1  to CR 8   are all-zero Walsh codes when the receiver is in a sync acquisition mode and some of these codes change to distinct Walsh codes of different Walsh code numbers when it is in a tracking mode. Since the pilot signal was spread at the transmitter with the all-zero Walsh code, the outputs of multipliers 23i and 23q represent the in-phase and quadrature components of the despread pilot signal during the sync acquisition mode and represent the in-phase and quadrature components of the despread data signal during the tracking mode. The outputs of multipliers 23i, 23q are respectively integrated over a symbol interval to produce in-phase and quadrature pilot or data symbols I k , Q k . 
     Returning to FIG. 1, the orthogonal output signals from each of the correlators CR are supplied to a data processor 12 where they are processed in a manner to be described and applied to a phase shift controller 13 as correlation sum signals S1˜S8. During the sync acquisition mode, pilot symbols are demodulated from the outputs of all correlators and applied to the phase shift controller. During the tracking mode, data symbols are demodulated from the outputs of correlators CR 1  to CR 6  and a sum of these signals is supplied to a channel decoder 14, while pilot signals are demodulated from the outputs of correlators CR 7  and CR 8  and applied to the phase shift controller as signals S7 and S8 and binary 0&#39;s are fed to the phase shift controller as signals S1 to S6. The CDMA receiver enters an update mode in which the phase timing obtained during the tracking mode is used to update the phase timing of the PN sequences applied the correlators CR 7  and CR 8  which were used during the tracking mode to produce pilot symbols. In this update mode, the outputs of all correlators are used to recover data symbols which are summed together and applied to the channel decoder 14. 
     In response to signals S1˜S8, the phase shift controller 13 produces phase position signals P1˜P8 and Walsh code numbers N1˜N8. During the sync acquisition mode, the phase shift controller produces eight sequences of different phase position signals and supplies these sequences simultaneously as sequences P1˜P8 to PN sequence generators PG 1  ˜PG 8  and Walsh code generators WG 1  ˜WG 8 , respectively, and supplies zero Walsh code numbers (ω 0 ) to all the Walsh code generators. During this acquisition mode, the phase shift controller makes a search through the correlation sum signals S1 to S8 generated as a result of successive phase shifts through all the range of the PN sequence (corresponding to the shifts of 0 to 2 16  bits) and determines the highest three values and identifies corresponding phase positions Δθ 1  to Δθ 3 . 
     During the tracking mode, these phase position signals are applied as signals P1 to P6 to PN generators PG 1  to PG 6  in pairs so that the PN generators of each pair are driven by the same phase position. PN generators PG 7  and PG 8  are then sequentially driven by signals P7 and P8 at phase positions which are located in the neighborhood of Δθ 1  to Δθ 3  to continue the search to precisely determine the optimum phase positions from the signals P7 and P8. At the same time, Walsh code numbers ω 1 , ω 3  and ω 5  are respectively supplied to Walsh code generators WG 1 , WG 3  and WG 5  corresponding to phase positions Δθ 1 , Δθ 2  and Δθ 3 , respectively, and zero code numbers Δθ 0  are supplied to Walsh code generators WG 2 , WG 4 , and WG 6  to WG 8 . As a result, during the tracking mode, the outputs of correlators CR k  (where k=1, 3 and 4) are orthogonal data symbols, while the outputs of correlators CR k+1  are orthogonal pilot symbols. 
     During the update mode, the phase position signals Δθ 1  to Δθ 3  are respectively updated with the optimum phase positions represented by signals P7 and P8. 
     By using the phase position signals P1˜P8, PN generators PG 1  ˜PG 8  respectively produce pairs of PN sequences PN i1 , PN q1 , PN i2 , PN q2 , . . . , PN i8 , PN q8  for coupling to correlators CR 1  ˜CR 8 , respectively. Each Walsh code generator WG k  uses a phase position signal P(k) and a corresponding code number N(k) to produce a Walsh code at the phase timing of the signal P(k) for coupling to the corresponding correlator CR k . 
     Details of the data processor 12 are shown in FIGS. 3A, 3B and 3C for sync acquisition, tracking and update modes, respectively. As illustrated in FIG. 3A, the processor 12 includes three groups of switches 31-i, 32-i and 33-j (where i=1˜4, j=1˜3). All of these switches operate in response to a switching signal supplied from a mode controller 22. The outputs of correlators CR 1  to CR 8  are coupled in pairs via corresponding switches 31-1, 31-2, 31-3 and 31-4 to pairs of pilot demodulators (PD 1 , PD 2 ), (PD 3 , PD 4 ),(PD 5 , PD 6 ) and (PD 7 , PD 8 ) or data demodulators DD 1 , DD 2 , DD 3  and DD 4 . The outputs of pilot demodulators of each pair are supplied via switch 32 corresponding to that pair to phase shift controller 13 as correlation sum signals S1 and S2 and the output of the data demodulator associated with that pair is supplied via corresponding switch 33 to an adder 34 as a signal &#34;r i  &#34;. Therefore, the outputs of correlators CR.sub. 3 and CR 4 , for example, are coupled via switch 31-2 to pilot demodulators PD 3 , PD 4  or data demodulator DD 2  and the outputs of these pilot demodulators are coupled via switch 32-2 to phase shift controller 13 as correlation sum signals S3 and S4 and the output of data demodulator DD 2  being coupled via switch 33-2 to adder 34 as a signal r 2 . 
     Mode controller 30 supplies a mode indicating signal to phase shift controller 13 and responds to a signal from phase shift controller 13 by operating the switches to change the operating mode to the next. 
     During the sync acquisition mode, mode controller 30 set all the switches so that the orthogonal output signals I k  and Q k  (where k=1, 2, . . . , 8) are coupled to corresponding pilot demodulator PD k  to supply correlation sum signals S1 to S8 to phase shift controller 13, and all the inputs to adder 34 are set to zero. 
     During the tracking mode (FIG. 3B), the outputs of correlators CR 1  to CR 6  are switched to the corresponding data demodulators DD 1  ˜DD 3  producing output signals r 1 , r 2  r 3 , which are connected via switches 33-1, 33-2, 33-3 to the adder 34. Switches 32-1, 32-2 and 32-3 are switched to the binary 0 position to set the output level of signals S1 to S6 to zero. On the other hand, the outputs of correlators CR 7  and CR 8  remain connected to pilot demodulators PD 7  and PD 8  to produce correlation sum signals S7 and S8. 
     During the update mode (FIG. 3C), the outputs of all the correlators are connected in pairs to the corresponding data demodulators to generate output signals r 1  ˜r 4 . As will be described, switches 33-1, 33-2 and 33-3 are controlled to combine two of the signals r 1 , r 2  and r 3  with the signal r 4  by adder 34 for a brief period immediately following a &#34;ramp-up&#34; time. 
     As shown in FIG. 4, each pilot demodulator includes a pair of integrators 40i and 40q, a pair of squaring circuits 41i and 41q and an adder 42. The in-phase and quadrature signals I k  and Q k  from correlator CR k  are respectively summed by integrators 40i, 40q during the period of &#34;m&#34; symbols to produce orthogonal sum signals which are squared respectively by the squaring circuits 41i and 41q. The sum-and-squared signals are combined together by adder 42 to produce the correlation sum signal S k  for the correlator CR k . 
     Details of each data demodulator DD j  according to one embodiment of this invention are illustrated in FIG. 5A (where j=1, 2, . . . 4). Each data demodulator DD j  includes a pair of multipliers 43i and 43q and an adder 44. Multiplier 43i takes correlation between the output signal I k  of correlator CR k  and the output signal I k+1  of correlator CR k+1  and multiplier 43q takes correlation between the output signals Q k  and Q k+1 . As described above, during the tracking mode, output signals I k  and Q k  are orthogonal data symbols, while output signals I k+1  and Q k+1  are orthogonal pilot symbols. Therefore, the multiplication of a data symbol of each in-phase or quadrature component with a pilot symbol of the corresponding orthogonal component at multiplier 43 causes the data symbol to be rotated to the reference axis of the I-Q plane and weighted by the amplitude of the pilot symbol. The outputs of multipliers 43i and 43q  are summed together by adder 44 to produce an output signal r j . The output of adder 44 is supplied to the channel decoder 14 where it undergoes a process inverse to that of the transmitter&#39;s channel encoder. 
     The amplitude of the pilot signal is usually much higher than that of the data signal, and the phase difference between these signals is negligibly small. In order to obtain the output signal r j  the data signal vector is projected onto the I axis of the I-Q complex plane by multiplying each data signal component with a complex conjugate of the corresponding pilot signal component and summing the products as follows: ##EQU1## where r d  is the amplitude of the data symbol, r p  is the amplitude of the pilot symbol, θ d  and θ p  are the angle of phase of the data and pilot symbols, respectively. Equation (1) indicates that a unit vector of the data signal is rotated clockwise to the I axis of the I-Q complex plane and weighted by the scalar product r p  r d . 
     Rotation of the data vector onto the I-axis of the I-Q complex plane allows for the simple summation of signals at the inputs of the adder 34 of processor 12. The use of the pilot vector in the rotation method described above reduces the effect of fading on the signal outputted from adder 24 as the pilot vector weights the data symbol and stronger multipath signals will have stronger moving-averaged pilot strong signals which indicate low levels of fading. 
     Another, but preferred embodiment of each data demodulator is shown in FIG. 5B in which each data demodulator further includes a pair of delay circuits 45i, 45q for introducing a delay of one half of &#34;s&#34; data symbols to the output signals I k  and Q k , respectively, and a pair of moving average circuits 46i, 46q for successively integrating &#34;s&#34; symbols of the output signals I k+1  and Q k+1 , respectively, to produce a moving average value of pilot signals. Each of the moving average circuits is implemented with an s-stage shift register and an adder. The shift register receives an input signal from the preceding circuit and the adder is connected to all the stages of the register for successively summing the values of their contents so that the adder produces a signal representing the moving average of the values of &#34;s&#34; pilot symbols (corresponding to &#34;s&#34; data symbols) for each phase component. Thus, for each delayed data symbol, a moving average value is derived from s/2 pilot symbols that precede the delayed symbol as well as from s/2 pilot symbols that succeed it. If several data symbols are severely corrupted by noise during transmission, the corresponding pilot symbols are moving-averaged and the effect of the noise on the pilot signal is reduced in this manner, leading to a reduction in the effect of the noise on the signal r j . 
     The outputs of delay circuit 45i and 45q are supplied respectively to the multipliers 43i and 43q and the moving average values of in-phase and quadrature components are supplied respectively to multipliers 43i and 43q. Since the incoming data symbols are always subjected to multipath fading and Rayleigh fading, an incoming symbol may be corrupted by random noise during the time the corresponding pilot symbol is moving-averaged over the period of &#34;s&#34; symbols. The introduction of s/2-symbol delays has the effect of reducing noise-related problems to a minimum. 
     Referring to FIGS. 6A to 6C, details of the phase shift controller 13 are illustrated for the sync acquisition, tracking and update modes of operation, respectively. The phase shift controller comprises a controller 60, a phase position data source 61, a selector 62 and a Walsh code number generator 63. 
     In FIG. 6A, controller 60 is responsive to a mode indicating signal from processor 12. During a sync acquisition mode, controller 60 commands the phase position data source 61 to simultaneously generate phase position sequences Pk (where k=1, 2, . . . , 8) each comprising position signals Δθ k1  to Δθ kn . The phase position signals P1˜P8 are applied through selector 62 to the PN generators PG 1  ˜PG 8  and Walsh code generators WG 1  ˜WG 8 , respectively. At the same time, controller 60 commands the Walsh code number generator 63 to generate zero Walsh code numbers ω 0  and applies these numbers as signals N1˜N8 to the Walsh code generators WG 1  ˜WG 8 , respectively. 
     In response to a sequence P(k) of phase position signals Δθ k1  to Δθ kn , each PN generator PN k  (FIG. 1) successively supplies a sequence of in-phase pseudo-random numbers PN i (k) and a sequence of quadrature pseudo-random numbers PN q (k) to the corresponding correlator CR k . At the same time, the correlator CR k  is supplied with a Walsh code sequence W k  that is produced by the Walsh code generator WG k  according to the corresponding Walsh code number N(k) at successive phase positions Δθ k1  to Δθ kn . 
     All the correlators CR operate in parallel to take correlations between the incoming signal and the respective despreading codes (each comprising PN sequences and a Walsh sequence) during the sync acquisition mode, using divided sequences of phase position signals to all phase shift controller 13. Since the Walsh code sequences are all-zero sequences during the sync acquisition mode, mode controller 30 at data processor 12 initially sets all the switches A, B, C, D in a manner as illustrated in FIG. 3A, so that all pilot demodulators PD 1  ˜PD 8  are connected to the outputs of corresponding correlators CR 1  ˜CR 8  to produce correlation sum signals S1 through S8. 
     Returning to FIG. 6A, controller 60 is responsive to the signals S1˜S8 for selecting the highest three values of correlation sums from all phase positions Δθ 11  to Δθ 8n  and the first, second and third highest values are stored into registers RS1, RS2 and RS3, respectively. Phase position signals Δθ 1 , Δθ 2  and Δθ 3  corresponding respectively to the first, second and third highest values are detected by controller 60 and stored into registers RP1, RP2 and RP3, respectively. Controller 60 returns an end-of-search signal signifying the end of a sync acquisition mode to the processor 12 to allow it to proceed to a tracking mode. As a result, the amount of time taken to complete a search through the whole range of phase positions is therefore 1/8 of the sync acquisition time which would be taken by a single correlator using a non-divided sequence of phase positions Δθ 11  to Δθ 8n . 
     In response to the end-of-search signal, the data processor 12 operates the switches 31, 32 and 33 as shown in FIG. 3B for coupling the inputs of data demodulators DD 1  to DD 3  to the outputs of corresponding correlators CR 1  to CR 6  and their outputs to adder 34 and disconnecting pilot demodulators PD 1  to PD 3  from their associated correlators CR to set the level of signals S1 to S6 to zero. The outputs of pilot demodulators PD 7  and PD 8  continue producing the signals S7 and S8. Mode controller 30 supplies a tracking mode signal to the controller 60 of phase shift controller 13. 
     In FIG. 6B, controller 60 responds to the tracking mode signal by controlling the selector 62 to read the phase position signals stored in registers RP1 to RP3 and delivered from the selector as signals P1 to P6. Specifically, the first phase position signal Δθ 1  in register RP1 is coupled to PN generators PG1 and PG2 as signals P1 and P2, the second phase position signal Δθ 2  in register RP2 being coupled to PN generators PG3 and PG4 as signals P3 and P4, and the phase position signal Δθ 3  in register RP3 being coupled to PN generators PG5 and PG6 as signals P5 and P6. Concurrently, controller 60 selects phase positions (Δθ 11  &#39; to Δθ 1m  &#39;) and (Δθ 11  &#34; to Δθ 1m  &#34;) in the neighborhood of the phase position Δθ 1 , phase positions (Δθ 21  &#39; to Δθ 2m  &#39;) and (Δθ 21  &#34; to Δθ 2m  &#34;) in the neighborhood of the phase position Δ θ 2 , and phase positions (Δθ 31  &#39; to Δθ 3m  &#39;) and (Δθ 31  &#34; to Δθ 3m  &#34;) in the neighborhood of the phase position Δθ 3 . Controller 60 directs the phase position data source 61 to simultaneously generate a first sequence of phase position signals (Δθ 21  &#39; to Δθ 2m  &#39;), (Δθ 21  &#39; to Δθ 2m  &#39;) and (Δθ 31  &#39; to Δθ 3m  &#39;) and a second sequence of phase position signals (Δθ 11  &#34; to Δθ 1m  &#34;), (Δθ 21  &#34; to Δθ 2m  &#34;) and (Δθ 31  &#34; to Δθ 3m  &#34;) and supply the first and second sequences as signals P6 and P7 to the PN generators PG 7  and PG 8 , respectively. 
     Controller 60 commands the Walsh code number generator 63 to supply the following code numbers: 
     N1=ω 1  (corresponding to Δθ 1 ) 
     N2=ω 0   
     N3=ω 3  (corresponding to Δθ 2 ) 
     N4=ω 0   
     N5=ω 5  (corresponding to Δθ 3 ) 
     N6=ω 0   
     N7=ω 0   
     N8=ω 0   
     As a result, correlators CR 1 , CR 3 , CR 5  produce orthogonal data signals (I 1 , Q 1 ), (I 3 , Q 3 ), (I 5 , Q 5 ), respectively, while correlators CR 2 , CR 4 , CR 6  produce orthogonal pilot signals (I 2 , Q 2 ), (I 4 , Q 4 ), (I 6 , Q 6 ), respectively. It is seen in FIG. 5A (or 5B), data signals I 1  and Q 1  are multiplied with pilot signal I 2  and Q 2  to produce an output signal r 1 . Similarly, data signals I 3  and Q 3  are multiplied with pilot signal I 4  and Q 4 , producing a signal r 2  and data signals I 5  and Q 5  are multiplied with pilot signal I 6  and Q 6 , producing a signal r 3 . The signals r 1 , r 2  and r 3  are summed by adder 34 and fed to the channel decoder 14. 
     On the other hand, correlators CR 7  and CR 8  use the despreading sequences successively generated as a result of new phase positions to cause processor 12 to supply correlation sum signals S7 and S8 to controller 60 to allow it to search for the highest value of .these correlation sums during this tracking mode. The first, second and third highest values are registers RS1, RS2 and RS3, respectively, and corresponding phase position signals Δθ u1 , Δθ u2  and Δθ u3  are stored into register RP4, RP5 and RP6, respectively. Controller 60 then supplies a start-of-update signal to the processor 12. 
     In response to the start-of-update signal, the phase shift controller 13 enters an update mode. To ensure seamless transition of phase positions from those detected in the sync acquisition mode to those detected in the tracking mode (without over-correlation or under-correlation), mode controller 30 (FIG. 3C) introduces a &#34;ramp-up&#34; time at the instant the PN generators PG 7  and PG 8  are set to a new phase position according to each of the new phase position signals Δθ u1 , Δθ u2  and Δθ u3 . 
     More specifically, mode controller 30 receives the start-of-update signal and directs the switch 31-4 to couple the outputs of correlators CR 7  and CR 8  to data demodulator DD 4  and introduces a ramp-up time (RUT) in response to the data demodulator DD 4  being coupled to the correlators CR 7 , CR 8 . If the circuit of FIG. 5A is used for the data demodulators, the ramp-up time corresponds to one symbol interval and the transient period (i.e., t 2  -t 3 , t 4  -t 5 , t 6  -t 7 ) in which the signal r 4  is connected to the adder also corresponds to one symbol interval. If use is made of the circuit of FIG. 5B both of the ramp-up time and the transient period correspond to the &#34;s&#34; symbol interval. 
     As shown in FIG. 6C, new phase position signals Δθ u1 , Δθ u2  and Δθ u3  are respectively set into registers RP4, RP5 and RP6 and the selector 62 sets the phase position signals P7 and P8 to the new phase position Δθ u1  at time t 1  and Walsh code number generator 63 sets the code number N7 to ω u1  corresponding to Δθ u1 . 
     In response to the start-of-update signal, mode controller 30 (FIG. 3C) introduces the ramp-up time corresponding to several symbols (&#34;s&#34; symbols in the case of FIG. 5B) which prevails until time t 2 . The introduction of the ramp-up time allows previous residual data which may be left in correlators CR 7  and CR 8  and data demodulator DD 4  to be cleared and replaced with symbol or symbols resulting from the new phase position. Thus, the previous phase position Δθ 1  is still used and adder 34 produces a signal R=r 1  +r 2  +r 3  during the time prior to time t 2 . 
     During the interval t 2  to t 3 , mode controller 30 (FIG. 3C) controls the switch 33-1 to connect the output signal r 4  of demodulator DD 4  to adder 34, instead of signal r 1 , producing a signal r 4  +r 2  +r 3 . 
     At time t 3 , register RP1 (FIG. 6C) is updated with the new phase position signal Δθ u1  so that PN generators PG 1  and PG 2  are thereafter driven at the new phase position Δθ u1  and Walsh code number generator 63 updates the code number N1 with ω u1  replacing the previous number ω 1  and sets the code number N7 to ω u3  corresponding to Δθ u2 . Switch 33-1 (FIG. 3C) is returned to the uppermost position for coupling the output r 1  of demodulator DD 1  so that the output signal R of adder 34 during the interval t 3  to t 4  is equal to R=r 1  +r 2  +r 3  (see FIG. 6C). 
     Although the new phase position is used for the correlators CR 1  and CR 2 , the signal r 2  resulting from the previous phase position Δθ 2  is still used during the interval t 3  to t 4 . Therefore, the interval t 3  to t 4  is the ramp-up time for correlators CR 7  and CR 8  and demodulator DD 4  to allow their residual signals resulting from the phase position Δθ u1  to be cleared. During the subsequent interval t 4  to t 5 , switch 33-2 is moved to the lowermost position for coupling the signal r 4 , instead of signal r 2 , to adder 34, producing a signal r 1  +r 4  +r 3 . At time t 5 , register RP2 (FIG. 6C) is updated with the new phase position signal Δθ u2  so that PN generators PG 3  and PG 4  are thereafter driven at the new phase position Δθ u2 . Concurrently, Walsh code number generator 63 updates the code number N3 by replacing ω  3  with ω u3  corresponding to the new phase position Δθ u2 . 
     During the subsequent interval t 5  to t 6 , a ramp-up time is introduced for correlators CR 7 , CR 8  and demodulator DD 4  to allow their residual signals resulting from the phase position signal Δθ u2  to be cleared in preparation for the new phase position Δθ u3 . During interval t 6  to t 7 , switch 33-3 is moved to the lowermost position for coupling the signal r 4  to adder 34, instead of signal r 3 , producing a signal r 1  +r 2  +r 4 . At time t 7 , register RP3 (FIG. 6C) is updated with the new phase position signal Δθ u3  so that PN generators PG 5  and PG 6  are thereafter driven at the new phase position Δθ u3 . Concurrently, Walsh code number generator 63 updates the code number N5 by replacing ω 5  with ω u5  corresponding to the new phase position Δθ u3 . 
     At time t 7 , controller 60 controller 60 (FIG. 6B) directs the phase position data source 61 to regenerate the previously mentioned first and second sequences of phase position signals corresponding to those in the neighborhood of phase positions Δθ 1 , Δθ 2  and Δθ 3  and apply these sequences as phase position signals P7 and P8 to PN generators PG 7  and PG 8  to resume the search for optimum phase positions and commands the Walsh code number generator 63 to set the code number N7 to ω 0 . At the same time, controller 60 supplies an end-of-update signal to processor 12. In response, mode controller 30 of the processor operates switch 33-3 to return to the uppermost position, producing a signal r 1  +r 2  +r 3 . The receiver now resumes the tracking mode.