Detection of access bursts in TDMA communications systems

• US 6,363,130 B1
• Filed: 02/26/1998
• Issued: 03/26/2002
• Est. Priority Date: 10/10/1997
• Status: Expired due to Term

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Bontu, Chandra Sekhar, Gu, Yonghai
• Primary Examiner(s): Pham, Chi
• Assistant Examiner(s): AL BESHRAWI, TONY
• Application Number:
  US09/030,551
• Time in Patent Office: 1,489 Days
• Field of Search: 375/366, 455/33.1, 370/442
• US Class Current: 375/366
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15

Abstract

The timing of an access burst received in a base station of a wireless communications system is determined by detecting one or more zero bit fields, constituting predetermined sequences of identical consecutive symbols in the access burst. For each possible timing of the access burst, whether a predetermined sequence is present in the respective position in the access burst is detected by predicting a sample of the received signal in the current symbol period from a sample of the received signal in the preceding symbol period assuming identity of symbols in the current and preceding symbol periods, producing a squared distance between predicted and received samples for the current symbol period, producing a threshold in dependence upon a power of the received signal for the preceding symbol period to accommodate fading, and comparing the squared distance with the threshold, continuing for successive current symbol periods until the end of the sequence or until the respective threshold is exceeded. The squared distances are accumulated for each possible timing, the least accumulated squared distance determining which of the possible timings for which the predetermined sequences are detected, if there is more than one, is the access burst timing.

Claims

1. A method of detecting an access burst in a TDMA communications system using differential modulation, the access burst including at least one predetermined sequence of N_T consecutive identical symbols in a predetermined position in the burst, where N_T is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising sampling a received signal and detecting the predetermined sequence in the received signal by the steps of, for each of N_T-L current symbol periods ending the predetermined sequence:

predicting at least one sample of the received signal in the current symbol period from at least one sample of the received signal in the preceding symbol period assuming identity of symbols represented by the received signal in the current and preceding symbol periods;

producing a squared distance between at least one sample of the received signal in the current symbol period and the predicted at least one sample; and

comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.

2. A method as claimed in claim 1 further including the step of, for each of the N_T-L current symbol periods, producing the threshold in dependence upon a power of the received signal in the preceding symbol period.

3. A method as claimed in claim 1 further including the step of accumulating the squared distance for a plurality of the N_T-L current symbol periods.

4. A method as claimed in claim 1 wherein said steps are performed for each of a plurality of predetermined sequences in the access burst.

5. A method as claimed in claim 4 wherein each of the predetermined sequences represents a zero bit field in the access burst.

6. A method as claimed in claim 1 wherein the at least one predetermined sequence comprises at least one of zero bit fields W, X, and Y of an access burst of an IS-54 system, together with any zero bit symbols at the end of a CDVCC (coded digital verification colour code) word preceding, and any zero bit symbols at the beginning of a sync word following, each of said fields W, X, and Y.

7. A method as claimed in claim 6 wherein said steps are carried out for each of a plurality of said fields W, X, and Y.

8. A method of determining timing of an access burst received in a TDMA wireless communications system using differential modulation, the access burst including at least one predetermined sequence of N_T consecutive identical symbols in a predetermined position in the burst, where N_T is an integer greater than an integer L representing a number of symbol periods over which the system has channel dispersion, comprising the steps of:

sampling a received signal;

for each of a plurality of possible timings of the access burst, detecting whether the predetermined sequence is present in the respective position in the access burst by performing the following steps (a) to (d) either for N_T-L current symbol periods which end the predetermined sequence if the access burst has the respective timing or until the squared distance in step (d) for the current symbol exceeds the respective threshold;

(a) predicting at least one sample of the received signal in the current symbol period from at least one sample of the received signal in the preceding symbol period assuming identity of symbols represented by the received signal in the current and preceding symbol periods;

(b) producing a squared distance between predicted and received samples for the current symbol period;

(c) producing a threshold in dependence upon a power of the received signal for the preceding symbol period; and

(d) comparing the squared distance with the threshold;

and determining the timing of the access burst in dependence upon results of the detection of the predetermined sequence for said possible timings of the access burst.

9. A method as claimed in claim 8 wherein the step of determining the timing of the access burst in dependence upon results of the detection of the predetermined sequence for said possible timings of the access burst comprises, for each of said possible timings for which steps (a) to (d) are carried out for N_T-L current symbol periods without the squared distance in step (d) exceeding the respective threshold, accumulating the squared distance produced in each step (b), and determining as the timing of the access burst one of said possible timings having a least accumulated squared distance.

10. A method as claimed in claim 8 wherein said steps (a) to (d) are performed, for each of the plurality of possible timings of the access burst, for each of a plurality of predetermined sequences in respective predetermined positions in the access burst.

11. A method as claimed in claim 8 wherein each of the predetermined sequences represents a zero bit field in the access burst.

12. A method as claimed in claim 8 wherein the at least one predetermined sequence comprises at least one of zero bit fields W, X, and Y of an access burst of an IS-54 system, together with any zero bit symbols at the end of a CDVCC (coded digital verification colour code) word preceding, and any zero bit symbols at the beginning of a sync word following each of said fields W, X, and Y.

13. A method as claimed in claim 12 wherein said steps (a) to (d) are performed, for each of the plurality of possible timings of the access burst, for each of a plurality of said fields W, X, and Y.

14. A method of detecting an access burst in a TDMA communications system not using differential modulation, the access burst including at least one predetermined sequence of N_T consecutive identical symbols in a predetermined position in the burst, where N_T is an integer greater than L+1 and L is an integer representing a number of symbol periods over which the system has channel dispersion, comprising sampling a received signal and detecting the predetermined sequence in the received signal by the steps of, for each of N_T-(L+1) current symbol periods ending the predetermined sequence, producing a squared distance between samples of the received signal in the current and preceding symbol periods, and comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.

15. A method as claimed in claim 14 further including the step of, for each of the N_T-(L+1) current symbol periods, producing the threshold in dependence upon a power of the received signal in the preceding symbol period.

16. A method as claimed in claim 15 further including the step of accumulating the squared distance for a plurality of current symbol periods.

17. A method as claimed in claim 16 wherein said steps are performed for each of a plurality of predetermined sequences in the access burst.

18. A method of detecting an access burst in a TDMA communications system, the access burst including a predetermined sequence of consecutive identical symbols in a predetermined position in the burst, comprising detecting the predetermined sequence by the steps of, for each of a plurality of current symbol periods of a subset of the consecutive identical symbols ending said predetermined sequence and for which a received signal waveform can be predicted from a received signal waveform in a preceding symbol period:

producing a prediction of the received signal waveform for the current symbol period from the received signal waveform for the preceding symbol period assuming identity of symbols in the current and preceding symbol periods;

producing a squared distance between the prediction and the received signal waveform for the current symbol period; and

comparing the squared distance with a threshold to determine said identity if the threshold is not exceeded.

19. A method as claimed in claim 18 further including the step of, for each current symbol period, producing the threshold in dependence upon a power of the received signal win the preceding symbol period.

20. A method as claimed In claim 18 further including the step of accumulating the squared distance for said plurality of current symbol periods.

21. A method as claimed in claim 18 wherein said steps are performed for each of a plurality of predetermined sequences in the access burst.

22. Apparatus for detecting an access burst received in a TDMA communications system, comprising:

a buffer for samples of a received signal;

means for producing a squared distance between samples of the received signal in a current symbol period and a preceding symbol period;

means for producing a threshold dependent upon a power of the received signal in the preceding symbol period;

means for comparing the squared distance with the threshold thereby to determine identity of symbols in the current and preceding symbol periods when the squared distance is less than the threshold; and

control means for controlling selection of samples supplied from the buffer as the samples of the received signal in the current and preceding symbol periods, the control means being responsive to an output of the means for comparing for identifying samples of the received signal representing a predetermined sequence of consecutive identical symbols in the access burst.

23. Apparatus as claimed in claim 22 wherein the TDMA communications system uses differential modulation and the means for producing the squared distance includes means for predicting at least one sample for the current symbol period from at least one sample for the preceding symbol period in accordance with the differential modulation assuming identity of symbols in the current and preceding symbol periods.

24. Apparatus as claimed in claim 23 further including means for accumulating the squared distance for a plurality of consecutive current symbol periods.

25. Apparatus as claimed in claim 24 wherein the control means is arranged for controlling said selection of samples supplied from the buffer for identifying samples of the received signal representing each of a plurality of predetermined sequences of consecutive identical symbols in the access burst.