Maximizing the viterbi winning path metric to estimate carrier phase offset in continuous phase modulated signals
View Patent ↗A system and method for estimating carrier frequency offset Δf and carrier phase offset φ 0 inherent in a received CPM signal. Samples of a continuous phase modulated (CPM) signal are received. A maximum of an objective function J is determined over a two-dimensional region parameterized by frequency offset v and phase offset w. The coordinates v max and w max of a maximizing point in the region represent estimates of the carrier frequency offset Δf and the carrier phase offset φ 0 . To evaluate the objective function J at a point (v,w), apply a frequency shift of amount −v and a phase shift of amount −w to the received samples to obtain modified samples, and perform Viterbi demodulation on the modified samples to obtain a winning path metric value at a final time. The winning path metric value is the objective function value J(v,w).
1. A method comprising:
receiving a block of samples of a continuous phase modulated (CPM) signal from a receiver;
estimating a carrier phase offset φ 0 inherent in the CPM signal based on the received block of samples, wherein said estimating comprises computing a maximum of an objective function J as a function of phase offset w, wherein a maximizing value w max of the phase offset w represents an estimate of the carrier phase offset φ 0 , wherein said computing the maximum includes computing a plurality of values J(w) of the objective function J at a respective plurality of values of the phase offset w, wherein said computing the objective function value J(w) at any given one of the values of the phase offset w comprises:
phase shifting the received block of samples by −w to obtain a phase shifted block of samples;
performing Viterbi demodulation on the phase shifted block of samples to obtain a winning path metric value at a final time, wherein the winning path metric value is the objective function value J(w);
storing the estimate of the carrier phase offset in a memory.
2. The method of claim 1 , wherein the CPM signal does not include a preamble or pilot.
3. The method of claim 1 , wherein the CPM signal is a single-h CPM signal.
4. The method of claim 1 , wherein the CPM signal is a multi-h CPM signal.
5. The method of claim 1 , further comprising:
performing a traceback process to recover information bits from a winning path of the Viterbi demodulation corresponding to the maximum value of the objective function.
6. The method of claim 1 , further comprising:
phase shifting a second block of samples of the CPM signal by amount −w max ;
recovering information from the phase-shifted second block by performing Viterbi demodulation on the phase-shifted second block.
7. The method of claim 1 , further comprising:
phase shifting a local oscillator of the receiver by amount −w max .
8. A non-transitory computer-accessible memory medium storing program instructions that are executable by a computer system, wherein the program instructions, when executed by the computer system, cause the computer system to:
receive a block of samples of a continuous phase modulated (CPM) signal from a receiver;
estimate a carrier phase offset φ 0 inherent in the CPM signal based on the received block of samples, wherein said estimating comprises computing a maximum of an objective function J as a function of phase offset w, wherein a maximizing value w max of the phase offset w represents an estimate of the carrier phase offset φ 0 , wherein said computing the maximum includes computing a plurality of values J(w) of the objective function J at a respective plurality of values of the phase offset w, wherein said computing the objective function value J(w) at any given one of the values of the phase offset w comprises:
phase shifting the received block of samples by −w to obtain a phase shifted block of samples;
performing Viterbi demodulation on the phase shifted block of samples to obtain a winning path metric value at a final time, wherein the winning path metric value is the objective function value J(w); and
store the estimate of the carrier phase offset in a memory.
9. The memory medium of claim 8 , wherein the CPM signal does not include a preamble or pilot.
10. The memory medium of claim 8 , wherein the CPM signal is a single-h CPM signal.
11. The memory medium of claim 8 , wherein the CPM signal is a multi-h CPM signal.
12. The memory medium of claim 8 , wherein the program instructions, when executed by the computer system, further cause the computer system to:
perform a traceback process to recover information bits from a winning path of the Viterbi demodulation corresponding to the maximum value of the objective function.
13. The memory medium of claim 8 , wherein the program instructions, when executed by the computer system, further cause the computer system to:
phase shift a second block of samples of the CPM signal by amount −w max ; and
recover information from the phase-shifted second block by performing Viterbi demodulation on the phase-shifted second block.
14. The memory medium of claim 8 , wherein the program instructions, when executed by the computer system, further cause the computer system to:
phase shift a local oscillator of the receiver by amount −w max .
15. A computer system comprising:
a processor; and
a non-transitory memory medium storing program instructions that are executable by the processor, wherein the program instructions, when executed by the processor, cause the processor to:
receive a block of samples of a continuous phase modulated (CPM) signal from a receiver;
estimate a carrier phase offset φ 0 inherent in the CPM signal based on the received block of samples, wherein said estimating comprises computing a maximum of an objective function J as a function of phase offset w, wherein a maximizing value w max of the phase offset w represents an estimate of the carrier phase offset φ 0 , wherein said computing the maximum includes computing a plurality of values J(w) of the objective function J at a respective plurality of values of the phase offset w, wherein said computing the objective function value J(w) at any given one of the values of the phase offset w comprises:
phase shifting the received block of samples by −w to obtain a phase shifted block of samples;
performing Viterbi demodulation on the phase shifted block of samples to obtain a winning path metric value at a final time, wherein the winning path metric value is the objective function value J(w); and
store the estimate of the carrier phase offset in a memory.
16. The computer system of claim 15 , wherein the CPM signal does not include a preamble or pilot.
17. The computer system of claim 15 , wherein the CPM signal is a single-h CPM signal.
18. The computer system of claim 15 , wherein the CPM signal is a multi-h CPM signal.
19. The computer system of claim 15 , wherein the program instructions, when executed by the processor, further cause the processor to:
perform a traceback process to recover information bits from a winning path of the Viterbi demodulation corresponding to the maximum value of the objective function.
20. The computer system of claim 15 , wherein the program instructions, when executed by the processor, further cause the processor to:
phase shift a second block of samples of the CPM signal by amount −w max ; and
recover information from the phase-shifted second block by performing Viterbi demodulation on the phase-shifted second block.
21. The computer system of claim 15 , wherein the program instructions, when executed by the processor, further cause the processor to:
phase shift a local oscillator of the receiver by amount −w max .