WO2001045288A1

WO2001045288A1 - Methods and apparatus for signal searching using correlation

Info

Publication number: WO2001045288A1
Application number: PCT/RU1999/000494
Authority: WO
Inventors: Rui R. Wang; Mohamed G. El-Tarhuni; Alexandr V. Garmonov; Galina S. Kravtsova; Nikolai I. Schukin
Original assignee: Nortel Networks Limited
Priority date: 1999-12-17
Filing date: 1999-12-17
Publication date: 2001-06-21
Also published as: KR20020055451A; CA2394510A1; AU3338800A; JP2003517247A; CN1398460A; CN1254927C

Abstract

A signal searcher for a CDMA communications system has a reference signal generator and a correlator to correlate a reference signal with a received signal in short (signal-coherent) correlation intervals. The code phase of the reference signal is changed through all of N possible code phases in N successive correlation intervals constituting one signal scan cycle. The correlation results for the same code phase for a plurality of the correlation intervals are accumulated over successive signal scan cycles using a combiner and buffer, and the presence and code phase of one or more desired signals in the received signal are determined by a decision unit in dependence upon the accumulated correlation results. In one embodiment all of the correlation results are accumulated; in other embodiments memory requirements are reduced by accumulating only larger values of the correlation results.

Description

METHODS AND APPARATUS FOR SIGNAL SEARCHING USING CORRELATION

This invention relates to methods and apparatus for searching for signals using correlation of a received signal with a reference signal, in particular for detecting the presence and code phase of wideband signals using CDMA (code division multiple access) in CDMA wireless cellular communications systems.

Background

Communications systems using CDMA signals have advantages in terms of their capacity, frequency planning, communications quality, security from unauthorized access, and immunity to interference. However, a significant challenge in CDMA system design arises from a need to achieve precise synchronization between a desired signal, contained in a received signal, and a locally generated reference signal. A first step in this synchronization is a signal searching process in which one or more parameters, such as the code phase and frequency of a pseudo-noise (PN) signal which constitutes the reference signal, are varied and hypotheses on the presence of a desired signal are progressively evaluated. This uses significant time and hardware resources of a CDMA system receiver.

More particularly, for each possible set of values of the parameters of the reference signal, referred to as an offset position, or state of the reference signal, a correlation is performed with a received signal and resulting correlation values are evaluated to determine the likely presence or absence of a desired signal with the respective offset or position. It is known that communicated signals in CDMA wireless cellular communications systems are subject to fading, in which reduced RF signal amplitude and variations in phase cause substantial degradation of the signal searching process.

To combat fast signal phase variance, or fast fading, it is known for example from "DS-SS Code Acquisition in a Rapid Fading Environment" by Manabu Mukai and Mutsumu Serizawa, IEEE 0-7803-2742-X/95, pages 281-285, 1995, to divide an accumulation interval TA into a plurality of m consecutive short intervals TCOH, so that TA = mTCOH. The duration of each interval TCOH is selected to be sufficiently small that the signal phase does not change appreciably during this interval, which accordingly is referred to as a signal coherence interval. During the accumulation interval TA correlation results for a respective offset or position of the reference signal are non-coherently accumulated. However, slow fading can cause the accumulated correlation result, which is used for determining the presence or absence of a desired signal with the respective offset, to differ appreciably from its long-term value, resulting in increased missed or false signal detection probability. Increasing the length of the accumulation interval TA to reduce this disadvantage would undesirably increase the signal searching time.

Effects of slow fading can be reduced by using diversity methods, for example spatial diversity techniques in which signals having relatively independent fading characteristics are received by two or more spaced antennas and the correlation results for these signals are combined. However, this undesirably increases complexity of the receiver, and the use of plural spaced antennas may not be practical, especially for portable receivers of small size. United States Patent No. 5,550,811 issued August 27, 1996 and entitled "Sync Acquisition and Tracking Circuit for DS/CDMA Receiver" discloses a time diversity arrangement for compensating for slow fading in which a selector switch supplies correlation results for each position or parameter set periodically to a respective combiner for performing a noncoherent accumulation. The switching period is determined in accordance with a fading period, so that the accumulated correlation results are averaged relative to signal fading. This arrangement has the disadvantage of being complex to implement, requiring the selector switch, its control arrangement, and as many combiners as there are parameter sets or positions. For example, for a mobile station searching for a pilot signal from a base station in an IS-95 CDMA wireless cellular communications system there are N = 32768 possible PN code phases or positions.

"CDMA. Principles of Spread Spectrum Communication" by Andrew J. Viterbi, Addison- esley Communication Series, 1995, Section 3.4.1, "Single-Pass Serial Search" discloses a signal searching method in which m correlation estimates in consecutive adjacent signal coherence intervals TCOH are accumulated during an accumulation interval TA = mTCOH. In an IS-95 CDMA system with a signal bandwidth of 1.25 MHz at a frequency of 800 MHz, the Rayleigh fading period is about 20 to 50 ms whereas the access channel signal accumulation interval length may be from 1.2 to 2.4 ms . Accordingly, substantial signal amplitude variations due to fading can occur within the accumulation interval TA, resulting in increased missed and false signal detection probability. An object of this invention is to provide a method and apparatus which can facilitate signal searching in communications systems such as CDMA systems.

Summary of the Invention

In accordance with one aspect of the invention, a signal searcher for a CDMA communications system generates at least one reference signal, and correlates the reference signal with a received signal in correlation intervals each sufficiently short that a phase of the received signal does not change appreciably during the correlation interval. The correlation results for the same code phase for a plurality of said correlation intervals are accumulated, and the presence and code phase of one or more desired signals in the received signal are determined in dependence upon the accumulated correlation results. The code phase of the reference signal is changed through all of N possible code phases in N successive correlation intervals constituting one signal scan cycle, the correlation results being stored for accumulation over a plurality of signal scan cycles. This aspect of the invention also provides means, for example a digital signal processor, for carrying out these functions.

Another aspect of the invention provides a method of signal searching in which a received signal is correlated with a reference signal and at least one parameter of the reference signal is changed to produce respective correlation resulrs for different ones of N possible offsets between the received signal and the reference signal, comprising the steps of: in a scan cycle, producing a respective correlation result for each of the N possible offsets from correlations between the received signal and the reference signal with a respective offset each during a correlation interval during which a phase of the received signal does not change appreciably, said at least one parameter being changed between consecutive ones of said correlations; and accumulating the correlation results for a plurality of said correlations having corresponding offsets in successive scan cycles to enable presence and offset of a desired signal to be determined from the accumulated correlation results.

In one embodiment of this method the correlation results are accumulated for all N possible offsets between the received signal and the reference signal. In another embodiment this method comprises the step of determining in a scan cycle largest correlation results for L of the N possible offsets, where L is an integer less than N, the correlation results being accumulated for only said L offsets. A further embodiment comprises the step of determining in a scan cycle a largest correlation result for each of a number L of groups each of a number J of the N possible offsets, where L = N/J, the correlation results being accumulated for only the largest correlation result for each of said L groups. In each of these last two embodiments, an identity of each respective offset, for example a position count, can be stored in association with the respective accumulated correlation result.

A further aspect f the invention provides a method of detecting the presence and PN code phase of a desired signal in a received signal of a CDMA communications system, comprising the steps of: producing a reference signal with different ones of N possible PN code phases in successive ones of N correlation intervals in a scan cycle; correlating the received signal with the reference signal during said correlation intervals to produce respective correlation results; accumulating at least some of the correlation results over successive scan cycles; and determining presence and code phase of a desired signal from the accumulated correlation results.

The correlation results can be accumulated for all N correlation intervals in each scan cycle. Alternatively, the correlation results can be accumulated for only L of the correlation intervals having greatest correlations in a scan cycle, where L is an integer less than N, the method including the steps of determining said greatest correlations and storing an identity of each of said L correlation intervals in association with the respective accumulated correlation results. As a further alternative, the correlation intervals can comprise L groups each of J correlation intervals in each scan cycle, where L and J are integers and L = N/J, and correlation results can be accumulated for only one of the correlation intervals in each group of J correlation intervals providing a greatest correlation in a scan cycle, the method further comprising the steps of determining said greatest correlations and storing an identity, of each correlation interval providing said greatest correlation out of the respective group of J correlation intervals, in association with the respective accumulated correlation results.

The invention also provides a signal searcher for a CDMA (code division multiple access) communications system, comprising: a control unit; a reference signal generator controlled by the control unit for generating a reference signal with different ones of N code phases in respective ones of N successive correlation intervals in a scan cycle; a correlator for correlating a received signal with the reference signal in the successive correlation intervals to produce respective correlation results, each correlation interval being sufficiently short that a phase of the received signal does not change appreciably during the correlation interval; and an accumulator responsive to the control unit for accumulating the correlation results from the correlator for each of a plurality of corresponding correlation intervals in a plurality of scan cycles to produce respective accumulated correlation results from which the presence and code phase of a desired signal in the received signal can be determined.

In a first embodiment of the searcher described below, the accumulator comprises a buffer for storing an accumulated correlation result for each of said N code phases. In a second embodiment, the signal searcher comprises a unit for determining in a scan cycle largest correlation results for L of the N code phases, and the accumulator comprises a buffer for storing an accumulated correlation result for each of said L code phases and an associated count identifying the respective code phase, where L is an integer less than N.

In a third embodiment, the correlation intervals comprise L groups each of J correlation intervals in each scan cycle, where L and J are integers and L = N/J, the signal searcher further comprising a detector for determining a greatest correlation result for each group of J correlation intervals in a scan cycle and for providing a count identifying a corresponding correlation interval in the respective group, wherein the accumulator comprises a buffer for storing the correlation result and the count associated therewith for each of the L groups, and a combiner for increasing the stored correlation result for each of the L groups in at least one subsequent scan cycle by the correlation result for the same code phase identified by said count. In this case the combiner can be arranged to increase the stored correlation result in the respective subsequent scan cycle only if the detector determines that the correlation result for the same code phase identified by said count is a greatest correlation result for the respective group of J correlation intervals.

The signal searcher can include a decision unit for determining a maximum one or more of the accumulated correlation results to determine the presence and code phase of one or more desired signals in the received signal.

Embodiments of the invention can provide significant advantages compared with the prior art discussed above. In particular, they are simple to implement and can provide substantial improvements in missed and false signal detection probability, and can facilitate the detection of multiple desired signals such as pilot signals from a plurality of base stations in a CDMA cellular communications system, due to their relative immunity to fading environments. The second and third embodiments described below also have reduced memory requirements, which is of particular importance for cases where the number N of code phases is very large.

Brief Description of the Drawings

The invention will be further understood from the following description by way of example with reference to the accompanying drawings, in which similar elements in different figures are denoted by the same reference numerals, and in which:

Fig.l is a block diagram of a known signal searcher;

Fig. 2 is a block diagram of a known correlator used in the signal searcher of Fig. 1;

Fig. 3 is a time diagram illustrating signal fading; Fig. 4 is a time diagram illustrating operation of the signal searcher of Fig. 1;

Fig. 5 is a block diagram of a signal searcher in accordance with a first embodiment of the invention;

Fig. 6 schematically illustrates a serial buffer of the signal searcher of Fig. 5;

Fig. 7 is a time diagram illustrating operation of the signal searcher of Fig. 5;

Fig. 8 is a block diagram of a signal searcher in accordance with a second embodiment of the invention;

Fig. 9 schematically illustrates a buffer memory of the signal searcher of Fig. 8;

Fig. 10 is a flow chart representing a data updating algorithm;

Figs. 11 and 12 are diagrams illustrating data updating in operation of the signal searcher of Fig. 8;

Fig. 13 is a graph comparing the performance of the signal searcher of Fig. 8 with that of Fig. 1;

Fig. 14 is a block diagram of a signal searcher in accordance with a third embodiment of the invention;

Fig. 15 schematically illustrates a serial buffer of the signal searcher of Fig. 14;

Fig. 16 schematically illustrates a maximum detector of the signal searcher of Fig. 14; Fig. 17 schematically illustrates a comparison and combining unit of the signal searcher of Fig. 14; and

Fig. 18 is a flow chart illustrating operations of the signal searcher of Fig. 14.

Detailed Description

Referring to the drawings, Fig. 1 illustrates a known wideband signal searcher for example for detecting signals in a CDMA cellular communications system. The signal searcher comprises a reference pseudo-noise (PN) signal generator 10, a correlator 12, an accumulator 14, a decision unit 16, and a timing control unit 18.

The signal searcher serves to detect a desired RF signal supplied to an input 20 of the correlator 12, the RF signal having a nominal carrier frequency fO and comprising a sum of two components I and Q in phase quadrature, these components being modulated by spectrum spreading PN sequences or PN codes PNI and PNQ respectively.

Typically, and as is assumed in the description below, a PN code phase is assumed to be a timing parameter with respect to which the signal search is performed. However, it can be appreciated that additional parameters, such as the input signal frequency f, may be involved. In any event, for carrying out a signal search a number of N offsets, positions, or states (referred to balance simply at positions) are provided each corresponding to a respective set of parameters within a search range or uncertainty area; in the case of a PN code phase search a respective PN code phase value corresponds to each position, and the distance between PN code phases of adjacent ones of the N positions is not more than one elementary symbol, or chip, of the communications system. The reference signal generator 10 produces and supplies to correlator 12 the PN code sequences PNI and PNQ and a reference frequency signal cos 20fOt, where t represents time in accordance with timing pulses supplied from the timing control unit 18.

Referring to Fig. 2, the correlator 12 comprises multipliers 22, 24, 26, 28, 30, and 32, a quadrature phase shifter 34, low pass filters (LPFs) 36 and 38, an inverter 40, accumulating combiners 42 and 44, squaring units 46 and 48, and a combiner 50. The reference frequency signal cos 2Df0t is multiplied by the signal from the input 20 in the multiplier 22, and is phase shifted by the phase shifter 34 and the result multiplied by the signal from the input 20 in the multiplier 24, and the outputs of these multipliers are low pass filtered by the LPFs 36 and 38 respectively to produce I and Q demodulation channel signals respectively. These signals are supplied respectively to a first input of the multipliers 26 and 38 and to a first input of the multipliers 28 and 32; a second input of the multiplier 26 is supplied with the PN code sequence PNI, a second input of each of the multipliers 28 and 30 is supplied with the PN code sequence PNQ, and a second input of the multiplier 32 is supplied with the PN code sequence PNQ after inversion by the inverter 40.

The outputs of the multipliers 26 and 28 are combined and accumulated by the accumulating combiner 42, and the outputs of the multipliers 26 and 28 are combined and accumulated by the accumulating combiner 44, in each case over a signal coherence interval TCOH which is described below, to produce correlation values YI and YQ respectively. These correlation values are squared by the squaring units 46 and 48 respectively, and the squared values are added by the combiner 50 to produce a correlation estimate Y (Y = YI2 + YQ2 ) for the signal coherence interval TCOH.

The correlation estimates Y for a number m of consecutive coherence intervals are accumulated in the (non- coherent) accumulator 14 (Fig. 1), to produce an accumulated value Z which is supplied to the decision unit 16 to produce (for example by comparison with a threshold) at an output of the signal searcher a decision result as to whether or not a desired signal has been detected. If no desired signal has been detected, then via the timing control unit 18 the parameters of the reference signal generator 10 are changed to a next one of the positions, and the search process described above is repeated. This process continues for each of the N positions in turn to produce successive values Zl to ZN, and is subsequently repeated cyclically, until a desired signal has been detected.

Fig. 3 illustrates by a curve 52 the amplitude of a desired signal in a fading environment as a function of time, represented by successive signal coherence intervals TCOH which are small in relation to the fading rate. To the same time scale, Fig. 4 illustrates time intervals for determining the values Zl, Z2, D ZN, in N successive accumulation periods each of duration TA corresponding to m consecutive signal coherence intervals TCOH, for a total scan cycle of duration NTA. Fig. 4 illustrates this for the case of m = 6.

A disadvantage of the known signal searcher and method described above is that the value Z for each of the N positions is determined over m consecutive signal coherence intervals. In a fading environment such as for a CDMA cellular communications system, signal fading can seriously adversely affect determinations within such consecutive intervals, so that substantial random variations of the determined values Z (relative to a long term average for each respective position) are possible, with a resulting increased probability of missed signal detection and false signal detection. To reduce this, it becomes necessary to increase the value m, resulting in a proportional increase in the signal search time and scan cycle duration. This in turn results, in a CDMA cellular communications system, in increased intra-system interference, increased system access time and reduced system capacity.

A signal searcher in accordance with a first embodiment of the invention is illustrated in Fig. 5, and comprises a reference signal generator 10 and a correlator 12 which can be as described above, a recirculating accumulator 60 comprising a combiner 62 and a serial buffer 64, a decision unit 66, and a timing control unit 68. In contrast to the timing control unit 18 of Fig. 1 which produces an output pulse to change the parameters of the reference signal generator 10 once every m intervals TCOH, the timing control unit 68 produces an output pulse for each interval TCOH to change the parameters of the reference signal generator after every such signal coherence interval, so that a scan cycle through all of the N positions or states is completed every N intervals TCOH. The output pulses of the timing control unit 68 are also supplied to a clock input C of the serial buffer 64.

The correlation estimate Y produced at the output of the correlator 12 for each signal coherence interval TCOH is supplied to one input, and an output ZOUT of the serial buffer 64 is supplied to a second input, of the combiner 62, which adds these to produce at its output a result ZIN which it supplies to an input of the serial buffer 64. Fig. 6 illustrates the serial buffer 64, which comprises N buffer stages 701 to 70N each for storing a respective correlation sum, connected in series with one another in the manner of a shift register and all clocked by pulses supplied to the clock input C, at the periodicity of the intervals TCOH. An output of the final buffer stage 70N constitutes the output ZOUT of the serial buffer 64.

Fig. 7, which can be compared with Fig. 4 described above, illustrates the resulting operation of the signal searcher of Fig. 5. In each of m scan cycles, each of duration NTCOH and comprising N signal coherence intervals, the reference signal generator 10 is changed through all of its N positions and the contents of the stages 70 of the serial buffer 64 are shifted cyclically through the buffer, in each case being supplemented by having the current correlation estimate Y, for the respective reference signal generator position, added by the combiner 62. The contents of the buffer stages 70 of the serial buffer 64 can be initially set to zero, and after m scan cycles are accumulated for m signal coherence intervals. However, as shown in Fig. 7 this accumulation to determine a corresponding value Zi (i = 1 to N) takes place over non-adjacent signal coherence intervals, so that the effects of signal fading as shown in Fig. 3 are substantially reduced by averaging. These accumulated correlation values Zi are conveniently supplied sequentially from the serial buffer 64 as its output ZOUT, a maximum one, or maximum ones, of these values being determined in the decision unit 66 and compared with a threshold to detect the presence and offset (code phase) of one or more desired input signals.

It can be appreciated that instead of being supplied serially from the buffer 64 to the decision unit 66, the values Zi can (for suitable values of N) be supplied in parallel or in a combined serial and parallel manner. Furthermore, it can be appreciated that instead of the search cycle being carried out in discrete groups of m scan cycles each of N signal coherence intervals, the values Zi can be updated iteratively in an ongoing manner with each scan cycle of N signal coherence intervals. In this case the signal ZOUT fed back to the combiner 62 can be reduced by weighting in known manner. In any event, it can be appreciated that the averaging of the accumulated correlation values Zi provided by the searcher of Fig. 5 reduces the disadvantages due to signal fading discussed above .

A signal searcher in accordance with a second embodiment of the invention is illustrated in Fig. 8, and comprises the reference signal generator 10, correlator 12, combiner 62, and timing control unit 68 which can be as described above, a counter 72, a buffer memory 74, a data updating unit 76, and a decision unit 78. As described above, the timing control unit 68 produces an output pulse for each interval TCOH to change the parameters of the reference signal generator after every such signal coherence interval, so that a scan cycle through all of the N positions is completed every N intervals TCOH. The output pulses of the timing control unit 68 are also supplied to a clock input C of the data updating unit 76, and to an input of the counter 72, which is a modulo-N counter which counts these pulses to provide via its output, connected to the data updating unit 76, a position count i representing the current position in a scan cycle.

The buffer memory 74 is shown in greater detail in Fig. 9. In contrast to the serial buffer 64 having N stages as described above, the buffer memory 74 contains a typically much smaller number L of stages 801 to 80L, each of which has two memory fields, for a respective one of L correlation values Zl to ZL and an associated position count il to iL. The respective memory fields of the stages 80 are connected in series with one another between inputs for respective signals ZIN and ilN and outputs for respective signals ZOUT and iOUT, but are individually clocked by clock signals Cl to CL supplied from the data updating unit 76, and supply the respective signals Zl to ZL to the data updating unit 76. The data updating unit 76 is also supplied with the signals ZOUT and iOUT from the buffer memory 74, and produces on a line 82 a signal which is either zero or the correlation value ZOUT as described below. The signal on the line 82 is supplied to the second input of the combiner 62, the first input of which is supplied with the current correlation estimate Y as described above, and the output of which constitutes the signal ZIN to the buffer memory 74. The signal ilN to the buffer memory 74 is constituted by the current position count i from the counter 72. The current correlation estimate Y is also supplied to the data updating unit 76.

As described below, the number L may be of the order of one-fifth or one-tenth the number of positions N, so that the capacity of the buffer memory 74 can be much smaller than that of the serial buffer 64. This is especially significant for large values of N.

The operation of the signal searcher of Fig. 8 is described below with additional reference to the flow chart of Fig. 10, comprising steps 81 to 87, and the data updating diagrams of Figs. 11 and 12, in each of which upper and lower parts indicate contents of the buffer memory stages respectively before and after the data updating. In a step 81, the data updating unit 76 determines a minimum (but non-zero) one Zn of the correlation values currently stored in the buffer memory 74, and determines the position in of this. If there is more than one equal minimum value, the value and position of any of the minima is used. It can be appreciated that this determination can be carried out during the interval TCOH for which the current correlation estimate Y is being produced, because it only involves stored data.

In a subsequent step 82, the unit 76 determines whether the current position count i is equal to the position iOUT (= iL) from the output of the buffer memory 74. If it is not, as will be the case during a first scan cycle through the N positions, in a step 83 the unit 76 determines whether the current correlation estimate Y is less than or equal to the determined minimum value Zn stored in the buffer memory 74. If so, there is no updating of data for the current position count i and an exit from the flow chart of Fig. 10 is reached.

If in the step 83 it is determined that Y > Zn, then steps 84 and 85 are executed to update data in the buffer memory 74 in the manner represented in Fig. 11, for which it is assumed for example that n = 4, i.e. that Z4 is the minimum value determined in the step 81. In this case the unit 76 produces a zero signal on the line 82, so that as indicated by step 84 the combiner 62 outputs the current correlation estimate Y as the correlation value ZIN. As shown by step 85, this value and its position count ilN = i are stored in the first stage 801 of the buffer memory, and the contents of the buffer memory stages 801 to 80n-l are shifted into the stages 802 to 80n respectively by clock pulses Cl to Cn. The previous contents of the buffer memory stage 80n = 804 are overwritten, and the contents of the buffer memory stages 80n+l to 80L are unchanged because clock pulses Cn+1 to CL are not produced. In this manner, in the first scan cycle of N signal coherence intervals TCOH the L largest correlation estimates Y and their respective positions i are stored in the L stages of the buffer memory 74.

If it is determined at the step 82 that the current position count i is equal to the position iOUT (= iL) from the output of the buffer memory 74, then steps 86 and 87 are executed to update data in the buffer memory 74 in the manner represented in Fig. 12. In this case the unit 76 produces the correlation value ZOUT on the line 82, so that as indicated by step 86 the combiner 62 outputs the sum of the current correlation estimate Y and the correlation value ZOUT as the correlation value ZIN. As shown by the step 87, all of the stages of the buffer memory are then updated by shifting by the production of clock pulses Cl to CL. In this case the arrangement operates substantially as a recirculating accumulator for the correlation values Zl to ZL and their positions il to iL stored in the buffer memory 74.

After the desired number m of scan cycles, during each of which the parameters of the reference signal generator 10 are changed after every interval TCOH to provide the same advantages with respect to fading as described above for the first embodiment, the buffer memory 74 contains accumulated substantially maximum correlation values for L positions, and the locations i of these positions, out of the total N positions. These accumulated correlation values Zl to ZL and their positions il to iL are then supplied serially from the buffer memory 74 as shown in Fig. 8, or in parallel or series- parallel in a similar manner to that discussed above, to the decision unit 78. The decision unit 78 determines a maximum one, or maximum ones, of these values for example by comparison with a threshold to detect the presence and offset (code phase) of one or more desired input signals, and supplies the resulting correlation values (if desired) and position information i to its output.

As indicated above, the fact that successive correlation estimates which are accumulated and stored for respective positions are separated in each case by N signal coherence intervals TCOH reduces the effects of fading on the operation of the signal searcher of Fig. 8. However, a missed detection probability is increased because the first scan cycle is used to determine the positions for which correlation estimates are accumulated, this being dependent upon the ratio between the number of positions N and the buffer memory capacity L. Fig. 13 illustrates computer simulation results in this respect, showing a missed detection probability as a function of m for a Rayleigh fading channel with N = 675. An upper curve 88 represents performance of the known signal searcher of Fig. 1, and three lower curves 90 represent the substantially improved performance for the signal searcher of Fig. 8 for different ratios of L to N. As can be appreciated from these curves, the signal searcher of Fig. 8 provides a substantial improvement in missed signal detection probability, which is not significantly degraded for values of L above about N/10, so that there can also be a substantial reduction in buffer memory capacity.

A signal searcher in accordance with a third embodiment of the invention is illustrated in Fig. 14, and comprises a reference signal generator 10, correlator 12, timing control unit 68, position counter 72, and decision unit substantially as described above, except that the position counter 72 in this case is a modulo-J counter where J is an integer as described below, and the position determination of the decision unit 78 is modified as described below. As described above, the timing control unit 68 produces an output pulse for each interval TCOH to change the parameters of the reference signal generator 10 after every such signal coherence interval. The position counter 72 counts these pulses on a clock pulse line C and, after every J pulses, produces an output pulse on a clock pulse line Cl . The pulses on this line Cl could alternatively be provided directly from the timing control unit 68.

The signal searcher of Fig. 14 also comprises a maximum detector 92 and a recirculating accumulator 94 comprising a comparison and combining unit 94 and a serial buffer 96. As shown in Fig. 15, the serial buffer 98 comprises L stages 1001 to 100L, each of which has two memory fields, for a respective one of L correlation values Zl to ZL and an associated position count j 1 to jL. The respective memory fields of the stages 100 are connected in series with one another between inputs for respective signals ZIN and j IN and outputs for respective signals ZOUT and jOUT, and are all commonly clocked by clock signals on the clock pulse line Cl.

The signal searcher of Fig. 14 is particularly advantageous when N is very large, for example N = 32768, and for searching in cases where a plurality of signals may need to be detected, for example for detecting pilot signals of several base stations. In the latter case the PN code phases of different pilot signals are separated by J of the N positions; for example J = 64 in the case of PN code chips for the forward pilot channel of an IS-95 cellular communications system. The integers J, L, and N are related by the equation J = N / L, the value of N being increased if necessary to ensure that J and L are integers.

In the signal searcher of Fig. 14, the maximum detector 92 is supplied with the correlation estimate Y from the correlator 12 for each signal coherence interval TCOH, the clock pulses C from the timing control unit 68, and a position count j from the counter 72, and determines a maximum one YMAX of the correlation estimates Y within each subset of J of the N positions being searched, providing at its outputs this maximum correlation estimate YMAX and a value jMAX of the position count j for this maximum.

To this end, as shown in Fig. 16 the maximum detector comprises a comparator 102, a memory cell 104 for storing a correlation estimate Y, and a memory cell 106 for storing an associated position count j . It should be noted that the memory cell 106 is required to store numbers only up to J, requiring relatively few bits compared with the number of bits which would be required for storing position counts up to N (e.g. 6 bits for J = 64 compared with 15 bits for N = 32768). The same applies to the number fields jl to jL of the L stages 100 of the serial buffer 98, resulting in a substantial reduction in the memory capacity required for storing position counts .

The comparator 102 has inputs coupled to the input and output of the memory cell 104, so that it compares each input correlation estimate Y with the current contents YMAX of the memory cell 104. In the event that Y > YMAX, the comparator 102 supplies an output to enable inputs E of the memory cells 104 and 106, to write inputs W of which the clock pulses C are supplied so that the larger correlation estimate Y is stored in the memory cell 104 and its position count j is stored in the memory cell 106. After J signal coherence intervals TCOH defining the respective position subset, the stored maximum YMAX and its position jMAX are supplied to the unit 96 of the recirculating accumulator 94, if necessary via clocked buffers which are not shown.

The signals ZOUT and jOUT from the output of the serial buffer 98 and the signals YMAX and jMAX from the maximum detector 92 are supplied to the unit 96, one form of which is illustrated in Fig. 17. As shown in Fig. 17, the unit 96 comprises comparators 108 and 110, a switch 112, a combiner 114, and a selector 116 having outputs for the signals ZIN and jIN to the inputs of the serial buffer 98. Fig. 18 shows a flow chart of the operation of the unit 96, comprising steps 121 to 125 which are referred to below.

The comparator 108 is supplied with and compares the position counts jMAX and jOUT and, if they are equal as determined at step 121, closes the switch 112 to supply the correlation estimate YMAX to one input of the combiner 114, to another input of which the correlation value ZOUT is supplied. The combiner 114 sums its inputs to produce an output Zl. The selector 116 supplies Zl or YMAX as the signal ZIN, and jOUT or jMAX as the signal jIN, under the control of the comparator 110 which compares YMAX and ZIN. The selector 116 has the switch positions shown in Fig. 17 except when the comparator 116 determines that YMAX > Zl. Thus in this case, as shown by block 122 in Fig. 18, the selector 116 has the switch positions shown to provide outputs ZIN = Zl = ZOUT + YMAX and j IN = jOUT (which is also equal to jMAX) . Consequently, when the maximum correlation estimate YMAX occurs with the same position count jMAX in the same position subset in repeated scan cycles, these correlation estimates are accumulated in a similar manner to that described above.

If in the step 121 it is determined that jMAX is not equal to jOUT, then the switch 112 remains open and the combiner 114 produces the signal Zl = ZOUT. In this case the comparator 110 determines at the step 123 whether YMAX > Zl and, if so, controls the selector 116 to adopt its alternative switch position in which it passes the signals YMAX and jMAX to its outputs ZIN and j IN respectively, as shown by the step 124. Otherwise, the comparator 112 controls the selector 116 to adopt the switch position shown in which it passes the signals Zl = ZOUT and jOUT to its outputs ZIN and j IN respectively, as shown by the step 125 in Fig. 18.

After m scan cycles each of N signal coherence intervals TCOH as described above, the contents of the serial buffer 98 are supplied serially to the decision unit 78, and one or more maximum values ZI, where I is an integer from 1 to L denoting a respective one of the position subsets and corresponding buffer stages 100, are determined and compared with a threshold to determine the presence or absence of one or more signals to be detected. The code phase or position of each such signal is J(I - 1) + jl, where jl is the position count or value jOUT associated with the determined maximum.

Although as described above each correlation result stored in the serial buffer 98 is only increased when the respective position count jMAX is the same as the stored position count jOUT, this need not necessarily be the case. Instead, in the first signal scan cycle the maximum correlation result YMAX and its position jMAX could be determined by the maximum detector 96 as described above, and in subsequent ones of the m signal scan cycles the correlation result Y for the respective position count jMAX in the respective group could be accumulated regardless of whether or not it is the maximum correlation result for that group in the respective scan cycle. However, it is expected that significant performance degradation will occur to the detection probability since the decision for the maximum position in each subset is based on the first scan cycle only and no accumulation is utilized.

Although embodiments of the invention have been described above in terms of specific physical devices such as counters and comparators, it can be appreciated that these may be replaced, and the embodiments of the invention more easily implemented, by one or more digital signals processors or application-specific integrated circuits.

It will also be appreciated that numerous other changes, variations, and modifications may be made to the specific embodiments of the invention described by way of example above without departing from the scope of the claims.

Claims

CLAIMS :

1. A method of signal searching in which a received signal is correlated with a reference signal and at least one parameter of the reference signal is changed to produce respective correlation results for different ones of N possible offsets between the received signal and the reference signal, comprising the steps of:

in a scan cycle, producing a respective correlation result for each of the N possible offsets from correlations between the received signal and the reference signal with a respective offset each during a correlation interval during which a phase of the received signal does not change appreciably, said at least one parameter being changed between consecutive ones of said correlations; and

accumulating the correlation results for a plurality of said correlations having corresponding offsets in successive scan cycles to enable presence and offset of a desired signal to be determined from the accumulated correlation results.

2. A method as claimed in claim 1 wherein the correlation results are accumulated for all N possible offsets between the received signal and the reference signal.

3. A method as claimed in claim 1 and comprising the step of determining in a scan cycle largest correlation results for L of the N possible offsets, where L is an integer less than N, wherein the correlation results are accumulated for only said L offsets.

3AMEIMIOIHHH JIHCT (ϋPABHJIO 26)

4. A method as claimed in claim 3 and including the step of storing an identity of each of said L offsets in association with the respective accumulated correlation results .

5. A method as claimed in claim 3 wherein N/L is of the order of about 5 to 10.

6. A method as claimed in claim 1 and comprising the step of determining in a scan cycle a largest correlation result for each of a number L of groups each of a number J of the N possible offsets, where L = N/J, wherein the correlation results are accumulated for only the largest correlation result for each of said L groups.

7. A method as claimed in claim 6 and including the step of storing an identity of each of said largest correlation results in association with the respective accumulated correlation results.

8. A method as claimed in claim 1 and including the step of determining at least a maximum one of the accumulated correlation results to determine the presence and offset of a desired signal.

9. A method of detecting the presence and PN code phase of a desired signal in a received signal of a CDMA communications system, comprising the steps of:

producing a reference signal with different ones of N possible PN code phases in successive ones of N correlation intervals in a scan cycle;

correlating the received signal with the reference signal during said correlation intervals to produce respective correlation results;

3AMEHflK⁾mHH ΛHCT (ΪIPABHJIO 26) accumulating at least some of the correlation results over successive scan cycles; and

determining presence and offset of a desired signal from the accumulated correlation results.

10. A method as claimed in claim 9 wherein the correlation results are accumulated for all N correlation intervals in each scan cycle.

11. A method as claimed in claim 9 wherein the correlation results are accumulated for only L of the correlation intervals having greatest correlations in a scan cycle, where L is an integer less than N, the method including the steps of determining said greatest correlations and storing an identity of each of said L correlation intervals in association with the respective accumulated correlation results.

12. A method as claimed in claim 9 wherein the correlation intervals comprise L groups each of J correlation intervals in each scan cycle, where L and J are integers and L = N/J, and wherein correlation results are accumulated for only one of the correlation intervals in each group of J correlation intervals providing a greatest correlation in a scan cycle, the method further comprising the steps of determining said greatest correlations and storing an identity, of each correlation interval providing said greatest correlation out of the respective group of J correlation intervals, in association with the respective accumulated correlation results.

13. A signal searcher for a CDMA (code division multiple access) communications system, comprising:

a control unit;

3AMEUJΠOIHHH JIHCT (ΠPABHJIO 26) a reference signal generator controlled by the control unit for generating a reference signal with different ones of N code phases in respective ones of N successive correlation intervals in a scan cycle;

a correlator for correlating a received signal with the reference signal in the successive correlation intervals to produce respective correlation results, each correlation interval being sufficiently short that a phase of the received signal does not change appreciably during the correlation interval; and

an accumulator responsive to the control unit for accumulating the correlation results from the correlator for each of a plurality of corresponding correlation intervals in a plurality of scan cycles to produce respective accumulated correlation results from which the presence and code phase of a desired signal in the received signal can be determined.

14. A signal searcher as claimed in claim 13 wherein the accumulator comprises a buffer for storing an accumulated correlation result for each of said N code phases.

15. A signal searcher as claimed in claim 13 and comprising a unit for determining in a scan cycle largest correlation results for L of the N code phases, wherein the accumulator comprises a buffer for storing an accumulated correlation result for each of said L code phases and an associated count identifying the respective code phase, where L is an integer less than N.

16. A signal searcher as claimed in claim 15 wherein N/L is of the order of about 5 to 10.

17. A signal searcher as claimed in claim 13 wherein the correlation intervals comprise L groups each of J correlation

3AMEHJnOIHHH JIHCT (ΠPABHJIO 26) intervals in each scan cycle, where L and J are integers and L = N/J, the signal searcher further comprising a detector for determining a greatest correlation result for each group of J correlation intervals in a scan cycle and for providing a count identifying a corresponding correlation interval in the respective group, wherein the accumulator comprises a buffer for storing the correlation result and the count associated therewith for each of the L groups, and a combiner for increasing the stored correlation result for each of the L groups in at least one subsequent scan cycle by the correlation result for the same code phase identified by said count.

18. A signal searcher as claimed in claim 17 wherein the combiner is arranged to increase the stored correlation result in the respective subsequent scan cycle only if the detector determines that the correlation result for the same code phase identified by said count is a greatest correlation result for the respective group of J correlation intervals.

19. A signal searcher as claimed in claim 17 wherein N = 32768 and J = 64.

20. A signal searcher as claimed in claim 13 and including a decision unit for determining a maximum one or more of the accumulated correlation results to determine the presence and code phase of one or more desired signals in the received signal.

21. A signal searcher for a CDMA (code division multiple access) communications system comprising means for generating at least one reference signal, means for correlating the reference signal with a received signal in correlation intervals each sufficiently short that a phase of the received signal does not change appreciably during the correlation interval, means for accumulating the correlation results for

3AMEH OIII,HH JIHCT (IIPABHJIO 26) the same code phase for a plurality of said correlation intervals, and means for determining ^'presence and code phase of one or more desired signals in the received signal in dependence upon the accumulated correlation results, characterized by control means for changing the code phase of the reference signal through all of N possible code phases in N successive correlation intervals constituting one signal scan cycle, the correlation results being stored for accumulation over a plurality of signal scan cycles.

3AMEHiHOU_lHH JIHCT (IIPABHJIO 26)