US RE39540 E1 Abstract A searcher for a mobile station of a cellular telephony network. Pilot signal from nearby base stations are correlated with a pseudonoise sequence inside a search window, using a bank of correlators. Each correlator is assigned a different delay, from among a sequence of delays in the window. At each delay, correlation is performed initially for a first dwell time. If the resulting correlation value exceeds a threshold, the correlation is continued for a second dwell time. Otherwise, the correlator is set to the next delay in the sequence. Only the outputs of second dwell correlations are used to identify the nearest base station. Some correlators may perform first dwell correlations at new delays in the window at the same time that other correlators are still performing second dwell correlations at old delays in the window.
Claims(47) 1. A cellular telephony searcher comprising:
a plurality of correlators for correlating a received signal with a pseudonoise sequence;
an input mechanism for inputting said pseudonoise sequence into said correlators, each of said correlators receiving said pseudonoise sequence with a different delay; and
a delay management mechanism for initializing said delays and subsequently changing said delays, said changing being contingent, for each said correlator, only on an output of said each correlator.
2. The searcher of
3. The searcher of
4. The searcher of
a pseudonoise sequence generator for generating said pseudonoise sequence; and
a delay line for receiving said pseudonoise sequences and outputting a plurality of copies of said pseudonoise sequence, each said copy being outputted with a different said delay.
5. The searcher of
for each said correlator, an index register; and
a multiplexer for directing one of said copies of said pseudonoise sequence to each said correlator in accordance with an index value stored in said index register of said each correlator.
6. In a cellular telephony network including at least one base station and at least one mobile station, each of the at least one mobile station receiving a received signal from the at least one base station, the received signal including a plurality of received values, each said received value having a real part and an imaginary part, a method for each of the at least one mobile station to identify at least one multipath channel to use to communicate with one of the at least one base station, comprising:
generating a pseudonoise sequence;
simultaneously performing a plurality of initial correlations of the received signal with said pseudonoise sequence, each of said initial correlations being performed with a different initial delay of said pseudonoise sequence, said initial correlations being performed for a first dwell time to produce, for each of said initial correlations, an initial first dwell time correlation value; and
for each said initial correlation;
if an estimated absolute value of said initial first dwell time correlation value exceeds a threshold, continuing to perform said each initial correlation, otherwise, performing a first subsequent correlation of the received signal with said pseudonoise sequence at a first subsequent delay different from any of said initial delays;
wherein, if said performing of at least one of said initial correlations is continued and if at least one of said first subsequent correlations is performed, said continued performing of said at least one initial correlation and said performing of said at least one first subsequent correlation are effected simultaneously.
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
12. The method of
for each said first subsequent correlation:
if an estimated absolute value of said subsequent first dwell time correlation value exceeds a threshold, continuing to perform said each first subsequent correlation;
otherwise, performing a second subsequent correlation of the received signal with said pseudonoise sequence at a second subsequent delay different from any of said initial delays and from any of said first subsequent delays.
13. The method of
14. The method of
if, after said simultaneous initial correlations are completed up to said first dwell time, all of said delays, whereat said initial correlations are continued and whereat said first subsequent correlations are performed, exceed a shortest initial delay, pausing said generating of said pseudonoise sequence.
15. The method of
16. The method of
17. The method of
18. The method of
normalizing said correlations.
19. The method of
20. The method of
rotating said pseudonoise sequence by 45° prior to performing said correlations.
21. The method of
normalizing said correlations.
22. The method of
23. The method of
24. A wireless communication device comprising:
a searcher having at least first and second correlators to correlate a received signal with a pseudonoise sequence provided to the first and second correlators; a delay line operably coupled to a multiplexer to provide at least first and second delays to the pseudonoise sequence provided to the first and second correlators; and a next location unit to decide to change at least the first delay of the first correlator based, at least in part, on an output of the first correlator. 25. The wireless communication device of
26. The wireless communication device of
27. The wireless communication device of
28. A method comprising:
correlating a received signal with a pseudonoise sequence using two or more correlators; and changing a first delay applied to the pseudonoise sequence of at least one correlator of the two or more correlators independently from a second delay applied to the pseudonoise sequence at one other correlator of the two or more correlators based, at least in part, on an output of said at least one correlator. 29. The method of
correlating the received signal within a correlation time selected from a group consisting of a first dwell time and a sum of the first dwell time and a second dwell time. 30. The method of
31. The method of
correlating the received signal within the first dwell time to provide a correlation output; comparing the estimated absolute value of the correlation output to said threshold; and correlating the received signal at the second dwell time if said threshold is exceeded. 32. The method of
changing at least the first delay if said threshold is not exceeded. 33. The method of
identifying a base station based on the result of correlating the received signal during the second dwell time. 34. A cellular communication system comprising:
a mobile station including a searcher having at least first and second correlators, wherein at least the first correlator of the at least first and second correlators is able to correlate to a received signal by changing a first delay applied to a pseudonoise sequence of the received signal at the first correlator independently from a second delay applied to said pseudonoise sequence at the second correlator based, at least in part, on an output of said first correlator. 35. The cellular communication system of
a delay line operably coupled to a multiplexer to provide at least first and second delays to the pseudonoise sequence provided to the first and second correlators, respectively; and a next location unit to change at least the first delay of the first correlator based, at least in part, on an output of the first correlator. 36. The cellular communication system of
37. The cellular communication system of
38. A communication device comprising:
an antenna to receive a signal having a pseudonoise sequence; and 39. The communication device of
40. The communication device of
41. The communication device of
42. An article comprising a storage medium, having stored thereon instructions, that when executed, result in:
correlating a received signal with a pseudonoise sequence using two or more correlators; and changing a first delay applied to the pseudonoise sequence of at least one correlator of the two or more correlators independently form a second delay applied to the pseudonoise sequence at one other correlator of the two or more correlators based, at least in part, on an output of said at least one correlator. 43. The article of
44. The article of
changing the first delay by comparing an estimated absolute value of the output of said at least one correlator to a threshold. 45. The article of
correlating the received signal within the first dwell time to provide a correlation output; comparing the estimated absolute value of the correlation output to said threshold; and correlating the received signal at the second dwell time if said threshold is exceeded. 46. The article of
changing at least the first delay if said threshold is not exceeded. 47. The article of
Description The present invention relates to cellular telephony and, more particularly, to a searcher for a DSSS cellular telephony system. In a DSSS cellular telephony system, the base stations identify themselves by transmitting pilot signals. Each pilot signal is a sequence of zero bits, modulated, according to the principles of DSSS encoding, by a pseudonoise (PN) sequence, or an extended pseudonoise sequence. For example, under the IS-95 interim standard, the PN sequence is 2 The same PN sequence is used by each of the base stations. The base stations are synchronized; and each base station uses the PN sequence with a different delay (also called “PN offset”) to produce the pilot signal. This enables the mobile units of the cellular telephony network to distinguish one base station from another. The total signal received by a mobile station, as a function of time t Each mobile unit of the cellular telephony network determines which base station to communicate with (typically, the nearest base station) by correlating this signal with the PN sequence at a set of trial delays. Because data D are modulated by sequences W, the correlation of the part of the signal that comes from other users is negligible. The correlation with the pilot signals also is negligible, except at trial delays that are equal to the PN offsets used by the base stations, as modified by multipath delays τ. Specifically, a pilot signal that arrives at a delay, that is equal to the sum of a base station offset and one of the multipath delays τ associated with transmissions from that base station, gives a significant contribution to the correlation at a matching trial delay; and all other pilot signals contribute negligibly to the correlation at that trial delay. This correlating is performed when the mobile station powers up, and continuously thereafter, to allow hand over from one base station to another when the mobile station crosses a call boundary. The delays of the various base stations are well separated, by more than the largest anticipated multipath delay, so in the absence of additive noise and in the absence of multipath delays, only a small number of correlations, equal to the number of potential nearest base stations, would have to be performed, to identify the base station whose delay gives the highest correlation as the nearest base station. According to the IS-95 standard, this separation is at least 256 chip duration T In order to ensure uninterrupted communication as a mobile station crosses from one cell to another, the correlations performed by searcher To make the correlations even more efficient, the dual dwell algorithm is used. At each delay in the window, the correlation is performed for a number M of chip durations that is less than N. Only if the correlation value after M chip durations exceeds a certain threshold is the correlation performed for the full N chip durations. The threshold, and the parameters N and M, are chosen to maximize the detection probability while minimizing both the false alarm probability and the time spent correlating. See, for example, M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt, Spread Spectrum Communication, Vol. III, Computer Science Press, 1989, chapter 1, particularly section 1.3, and D. M. Dicarlo and C. L. Weber, “Multiple dwell serial search: performance and application to direct sequence code acquisition”, IEEE Transactions on Communications vol. COM-31 no. 5 pp. 650-659, May 1983. In the prior art implementation of this algorithm, several correlators are used by searcher There is thus a widely recognized need for, and it would be highly advantageous to have, a configuration for a cellular telephony searcher that would allow the efficient use of many correlators. The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: The present invention is of a cellular telephony searcher which can be used by a mobile station to identify the several strongest multipath components of nearby base stations faster than presently known searchers. The principles and operation of a cellular telephony searcher according to the present invention may be better understood with reference to the drawings and the accompanying description. Referring now to the drawings, Also shown in Block Similarly, clock PN sequence generator Conceptually, once every T For example, let A=Re(RX Another method of avoiding actual multiplications exploits the fact that only the absolute values of the correlation values S are actually needed, to further reduce the number of calculations and achieve a further reduction in electrical current consumption by searcher Each delay unit Searcher Whenever a correlator In the special case of N=2M, K>M implies K=N, so the “no” branch of block Most preferably, the exact absolute value of S Recall that each memory Next location unit -
- (a) copied to index register
**22**of that correlator**20**and then - (b) changed to the index corresponding to the delay immediately following the delay to which that correlator
**20**has now been set.
- (a) copied to index register
Every yM chip intervals, while the interrupt service routine reads the output of searcher Preferably, the components illustrated in The following is an example of the functioning of searcher Following the IS-95 standard, the first correlation is performed at a delay of −30.
Now, an interrupt is generated. Hold unit
An interrupt is generated, but no hold is performed because the earliest correlator still is correlator
An interrupt is again generated by S The operations performed by searcher While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Patent Citations
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