Method and system for applying Viterbi type PSK demodulation for optimum correlation of GPS signals

1. In a receiver, a method of improving sensitivity in the demodulation of a received signal by said receiver over an arbitrary measurement time epoch, said method comprising the steps of:

correlating said received signal with a local replica of pseudo noise code in a coherent fashion over a plurality of time intervals in said time epoch creating a correlation signal;

creating a trellis of evenly distributed phase state nodes at each time interval, said creating step comprising:

defining a plurality of phase states representing phases evenly quantized over 0 to 360 degrees;

defining possible state transitions from and to each phase state node;

creating paths between phase state nodes in one time interval and phase state nodes in another time interval according to said possible state transitions;

assigning a transition probability to each path; and

creating a likelihood metric for each path based on a measured phase of the correlation signal and the transition probability for the path, said measured phase of the correlation signal having a random process approximated utilizing a discrete Markov process; and

utilizing a Viterbi algorithm on said trellis to perform a maximum likelihood estimation of a phase trajectory of the correlation signal with said quantized resolution of phase states over 0 to 360° throughout the measurement time epoch.

2. The method of claim 1 , wherein the Markov process is a first order Markov process.

3. The method of claim 1 , wherein the possible state transitions and the probability of the paths are assigned to reflect properties of said receiver.

4. The method of claim 3 wherein the step of creating possible state transitions for each node is performed based on a known phase slew rate limitation of said receiver.

5. The method of claim 1 , wherein the received signal is a direct sequence spread spectrum signal.

6. The method of claim 1 , wherein the received signal is a global positioning system (GPS) coarse/acquisition LI signal generated by a space vehicle (SV).

7. The method of claim 4 , wherein the received signal is a global positioning system (GPS) coarse/acquisition LI signal generated by a space vehicle (SV).

8. The method of claim 1 wherein the received signal is a code-division multiple access (CDMA) pilot signal.

9. The method of claim 1 , wherein the likelihood metric is created based on an approximation of a probability distribution function of the phase of said correlation signal.

10. The method of claim 9 , wherein the approximation is to model the probability distribution function of the phase as a periodic gaussian pulse on top of a constant function.

11. The method of claim 1 wherein said receiver is a mobile receiver.

12. A receiver for improving the sensitivity in the demodulation of a received direct sequence spread spectrum signal, said receiver comprising:

an antenna for receiving the direct sequence spread spectrum signal;

a downconverter for downconverting the received signal, producing a downconverted signal;

an analog to digital converter to convert the downconverted signal to a digital signal;

a despreader for despreading and coherently correlating the digital signal to a known signal, creating a despread signal; and

a processor for applying a Viterbi algorithm to a trellis created for the despread signal, the processor:

breaking the despread signal into time intervals;

creating the trellis of evenly distributed phase state nodes at each time interval by:

defining a plurality of phase states representing the phases evenly quantized over 0 to 360 degrees;

defining possible state transitions from and to each phase state node;

creating paths between phase state nodes in one time interval and phase state nodes in another time interval according to said possible state transitions;

assigning a transition probability to each path; and

creating a likelihood metric for each path based on a measured phase of said despread signal and the transition probability for the path, the measured phase of the despread signal having a random process approximated by a Markov process; and

utilizing the Viterbi algorithm on said trellis to perform a maximum likelihood estimation of a phase trajectory of the despread signal with said quantized resolution of phase states over 0 to 360° throughout the time interval.

13. The receiver of claim 12 , wherein the Markov process is a first order Markov process.

14. The receiver of claim 12 , wherein the received signal is a global positioning system coarse/acquisition (C/A) LI signal generated by a space vehicle (SV).

15. The receiver of claim 12 , wherein the received signal is a CDMA pilot signal.

16. The receiver of claim 12 , wherein the receiver is a mobile receiver.

17. The receiver of claim 12 , wherein the known signal is a GPS C/A L1 signal.

18. The receiver of claim 17 , wherein the possible state transitions and the probability of the paths are assigned to reflect properties of said receiver.

19. The receiver of claim 18 , wherein the possible state transitions for each node are based on a known phase slew rate limitation of said receiver.

20. The receiver of claim 14 , wherein the possible state transitions for each node are based on a known phase slew rate limitation of said receiver.

21. The receiver of claim 12 , wherein the likelihood metric is created based on an approximation of a probability distribution function of the phase of said despread signal.

22. The receiver of claim 21 , wherein said approximation is to model the probability distribution function of the phase as a periodic gaussian pulse on top of a constant function.

23. A wireless mobile device comprising the receiver of claim 12 .