Multi-stage CDMA synchronization with parallel execution

• US 6,625,200 B1
• Filed: 01/25/1999
• Issued: 09/23/2003
• Est. Priority Date: 01/25/1999
• Status: Expired due to Term

• Current Assignee: Unwired Planet LLC (JP Morgan Chase & Co)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Dent, Paul W.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Ha, Dac V.
• Application Number:
  US09/236,083
• Time in Patent Office: 1,702 Days
• Field of Search: 375/142, 375/143, 375/145, 375/149, 375/150, 375/152, 375/343, 375/367, 375/368, 370/335, 370/342, 370/441, 370/479
• US Class Current: 375/142
• CPC Class Codes: H04B 1/70735 Code identification H04B1/7083 takes precedence
• CPC Class Codes: H04B 1/70752 Partial correlation
• CPC Class Codes: H04B 1/708 Parallel implementation

Abstract

Methods and systems for time synchronizing a receiver to a spread spectrum signal are described. Exemplary embodiments of the present invention provide for at least a two step process in which, at a first step, a number of candidate synchronization frequencies or timings are identified, followed by confirming one of the candidates as a correct synchronization state at a second or final step. According to the present invention, the confirmation step may be performed at the same time as the step of identifying further candidates by processing the same received signal samples in different ways.

Claims

1. A multi-stage synchronization method for synchronizing a receiver with a transmitted signal, comprising the steps of:

selecting a frequency channel and receiving a signal;

filtering, amplifying and sampling said signal to generate signal samples for processing;

performing a first stage of synchronization comprising correlating said signal samples with a first code at different timing offsets to generate a plurality of candidate signal timings;

performing a second stage of synchronization in parallel with said first stage of synchronization, said second stage comprising correlating said signal samples with at least one second code, using only said at least one candidate signal timing; and

processing said signal samples using a code related to said second code to decode said signal samples; and

testing for correct decoding of said signal samples to verify correct synchronization.

2. The method of claim 1, wherein said second stage of synchronization generates at least one correlation timing and associated group code, said method further comprising:

performing a third stage of synchronization in parallel with said first stage and said second stage, said third stage comprising correlating said signal samples with a plurality of codes identified by said group code.

3. The method of claim 1 wherein filtering, amplifying and sampling said signal to generate signal samples for processing comprises:

analog-to-digital converting said signal at a sampling rate of at least one sample per signaling symbol interval.

4. The method of claim 1 in which said first stage of synchronization comprises:

correlating a selected block of signal samples with corresponding symbols of said first code, selecting each signal sample in turn to be the start of said block.

5. The method of claim 4 in which said block correlations are performed using a sliding correlator.

6. The method of claim 4 in which said block correlations are performed using a matched filter.

7. The method of claim 4 wherein correlating said selected block of signal samples with corresponding symbols of said first code comprises:

coherently correlating said selected block of signal samples with corresponding symbols of said first code.

8. The method of claim 7 wherein correlating said selected block of signal samples with corresponding symbols of said first code further comprises:

non-coherently accumulating said coherently correlations in a plurality of bins, each bin corresponding to a candidate signal timing.

9. The method of claim 8 further comprising:

compensating said accumulation for timing drift.

10. The method of claim 9 wherein compensating said accumulation for timing drift further comprises:

periodically replacing a bin value by a largest of three adjacent bin values.

11. The method of claim 10, wherein said period is related to an accumulation time over which timing drift of the order of one signaling symbol is likely.

12. The method of claim 8 further comprising:

subtracting a minimum value over all of said bins, from all of said bins to prevent numerical growth and to form differential bin values.

13. The method of claim 12 wherein a candidate signal timing is determined when one of said differential bin values exceeds a predetermined threshold.

14. The method of claim 13 further comprising, when a bin value exceeds said predetermined threshold:

recording said candidate signal timing;

setting said bin value to zero; and

continuing said non-coherent accumulation in said bin.

15. The method of claim 1 wherein said transmitted signal includes a repetitive pattern with a fixed repetition period of a whole number of signaling symbols.

16. The method of claim 15 wherein performing a first stage of synchronization comprises:

forming a correlation value for each alignment of said first code with a symbol position within said repetition period; and

accumulating said correlation values having the same alignment in successive repetition periods in a bin for each alignment.

17. The method of claim 1 wherein performing said first stage of synchronization further comprises estimating a frequency error associated with said received signal.

18. The method of claim 17 wherein performing said second stage of synchronization further comprises using said frequency error estimate to compensate said signal for phase shift.

19. The method of claim 18 wherein correlating the same said samples with at least one second code in said second stage of synchronization further comprises:

performing a coherent correlation between the same said samples with at least one second code; and

using said phase shift compensation to perform said coherent correlation over a longer interval.

20. The method of claim 1 wherein selecting a frequency channel comprises:

selecting said frequency channel from a prioritized channel list.

21. The method of claim 20 herein said prioritized channel list is prioritized based on memorized historical information.

22. The method of claim 20 wherein said prioritized channel list is prioritized in order of received signal strength on each channel.

23. The method of claim 20 wherein said prioritized channel list is prioritized based on the channels on which synchronization was achieved most recently.

24. The method of claim 20 wherein said prioritized channel list is prioritized such that channels on which synchronization has not historically been achieved are given a low priority.

25. The method of claim 20 in which said channel list is prioritized based on measured received signal strength on each channel and on historical information.

26. The method of claim 25 in which channels with signal strength above a threshold at which synchronization has previously been achieved are given a high priority and channels with signal strength below a threshold at which synchronization has previously not been achieved are given a low priority.

27. A method for synchronizing a receiver with a transmitted signal comprising the steps of:

sequentially tuning said receiver to successive frequency channels and measuring received signals strength on said channels;

forming said frequency channels into a first list for testing in a prioritized order;

tuning said receiver to the channel in said first list with highest priority and receiving a signal;

starting a timer to record time spent tuned to said channel and to compare the recorded time with a time-out value;

filtering, amplifying and sampling the received signal to generate signal samples for processing;

processing said signal samples using a first code to detect correlation with said first code;

upon detecting correlation with said first code, recording in a second list the timing position within said signal samples of each such detected correlation;

if said second list contains one or more entries, processing said signal samples to detect correlation with at least one second code at the corresponding recorded timing position, said processing concurrent with said processing said signal samples using a first code to detect correlation with said first code; and

if said timer reaches said time-out value, retuning said receiver to the channel of next highest priority in said first list and resetting said timer.

28. A receiver for detection of synchronization with a transmitter, comprising:

a filtering, amplifying and conversion circuit for receiving signals and producing signal samples for processing;

a correlator for correlating said signal samples with a first code using time-shifts between said received signal samples and said first code and for correlating said same signal samples with at least one second code at specified time shifts, simultaneously with correlating said signal samples with said first code;

a first detector for detecting time shifts at which correlation with said first code exceeds a first threshold and recording said time-shifts in a first memory;

a controller for specifying the time-shifts to said correlation means for performing correlations with said at least one second code based on the time-shifts recorded in said first memory;

a second detector for detecting when a correlation with said at least one second code specified by said control means exceeds a second threshold and for recording the associated at least one second code and time-shift in a second memory;

a confirmation unit for processing said same signal samples using a code based on said second code to decode information and performing a decoding error check; and

a deletion unit for deleting records from said second memory when said confirmation unit indicates decoding error.

29. A method of synchronizing a receiver with a transmitted signal, said transmitted signal comprising a first and second repeated coded pattern having a determined repetition period, comprising the steps of:

selecting a frequency channel and receiving a signal;

filtering, amplifying and sampling the signal to generate signal samples for processing;

correlating said signal samples with said first coded pattern using different timing shifts in said repetition period between said first coded pattern and said signal samples to produce a first correlation value for each time shift;

accumulating correlation values that correspond to the same time shift in successive repetition periods in corresponding timing bins;

detecting when the cumulative value in one of said timing bins exceeds the value in another of said timing bins by more than a first threshold and recording the tuning bin number in a first memory;

when said memory contains at least one timing bin number, correlating said same signal samples with said second repeated code pattern using timing shifts corresponding to said recorded timing bin numbers, simultaneously with correlating said signal samples with said first coded pattern, and producing corresponding second correlation values; and

accumulating said second correlation values in a number of second bins, each corresponding to an entry recorded in said first memory while continuing to accumulate said first correlation values in said timing bins.

30. A method for time-synchronizing a receiver with a transmitted signal and determining a coarse frequency error estimate, comprising the steps of:

computing correlations between received signal samples and known symbols included in said transmitted signal at periodic intervals, using several time shifts between said received signal samples and said known symbols corresponding to early and late timing postulates;

combining successive correlations corresponding to the same timing postulate using a combining method with no frequency error compensation to obtain first cumulative correlations;

combining correlations made with successively later timing postulates using a combining method compensated for a receiver frequency that is relatively high compared to the transmitted signal frequency to obtain second cumulative correlations;

combining correlations made with successively earlier timing postulates using a combining method compensated for a receiver frequency that is relatively low compared to the transmitted signal frequency to obtain third cumulative correlations; and

determining the largest of said first, second and third correlations to determine a timing and a coarse frequency error estimate.