Phase tracking
Techniques for pilot-aided carrier frequency and phase synchronization may use a three-pass process. In a first pass, initial frequency offset may be addressed, and a frame start time may be established. In a second pass, a fine frequency correction may be performed. In a third pass, phase variation may be tracked and corrected using a minimum set of pilot symbols.
1. A method of phase tracking in a communication system, the method including:
performing, at a receiving apparatus of the communication system, an initial frequency correction based on received symbol samples, to output first corrected samples, wherein the initial frequency correction comprises performing at least one non-data-dependent frequency estimation and correction on a single packet-length's worth of the received symbol samples, wherein a number of the received symbol samples is greater than one packet length's worth of symbol samples, wherein the initial frequency correction further comprises iteratively stepping through packet-length sets of the received symbol samples to obtain the single packet-length's worth of received symbol samples, and wherein the iteratively stepping through packet-length sets of the received symbol samples comprises:
extracting a symbol block containing one packet-length's worth of received symbol samples;
performing non-data-dependent frequency estimation and correction on the symbol block to obtain frequency-adjusted symbol samples;
correlating the frequency-adjusted symbol samples with a packet sync pattern; and
identifying the packet start sample by the correlation value with maximum magnitude;
performing, at the receiving apparatus, a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing, at the receiving apparatus, pilot symbol-aided phase tracking/correction to the second corrected samples.
2. The method of claim 1 , wherein the iteratively stepping through packet-length sets of the received symbol samples further comprises:
storing at least one result derived from the correlating;
determining if there are further symbol samples available;
if further symbol samples are available, stepping by a length of one packet sync pattern to extract a further symbol block, performing the non-data-dependent frequency estimation, packet sync pattern correlation, identification of symbol timing instant with maximum correlation magnitude, and storing at least one set of these results; and
if no further symbol samples are available, selecting a symbol block, based on the at least one result, and performing the non-data-dependent frequency estimation and correction on the selected symbol block.
3. The method of claim 2 , wherein the at least one result comprises a correlation peak sample instant and a peak signal-to-noise ratio, and wherein selecting a symbol block comprises selecting a symbol block corresponding to a maximum correlation peak.
4. A method of phase tracking in a communication system, the method including:
performing, at a receiving apparatus of the communication system, an initial frequency correction based on received symbol samples, to output first corrected samples;
performing, at the receiving apparatus, a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing, at the receiving apparatus, pilot symbol-aided phase tracking/correction to the second corrected samples,
wherein performing the second frequency correction and initial phase correction comprises:
performing non-data-dependent frequency estimation and correction on the first corrected samples, to obtain a block of further corrected samples;
correlating the further corrected samples with a packet sync pattern;
using a peak correlation value to derive a packet timing instant and phase correction; and
applying the phase correction to a packet length of further corrected symbol samples beginning with the selected packet timing instant.
5. The method of claim 4 , wherein the initial frequency correction comprises performing at least one non-data-dependent frequency estimation and correction on a single packet-length's worth of the received symbol samples.
6. The method of claim 5 , wherein a number of the received symbol samples is greater than one packet length's worth of symbol samples, and wherein the initial frequency correction further comprises iteratively stepping through packet-length sets of the received symbol samples to obtain the single packet-length's worth of received symbol samples.
7. The method of claim 6 , wherein the iteratively stepping through packet-length sets of the received symbol samples comprises:
extracting a symbol block containing one packet-length's worth of received symbol samples;
performing non-data-dependent frequency estimation and correction on the symbol block to obtain frequency-adjusted symbol samples;
correlating the frequency-adjusted symbol samples with a packet sync pattern; and
identifying the packet start sample by the correlation value with maximum magnitude.
8. The method of claim 4 , wherein the phase correction corresponds to a negative value of a phase of the peak correlation value.
9. A method of phase tracking in a communication system, the method including:
performing, at a receiving apparatus of the communication system, an initial frequency correction based on received symbol samples, to output first corrected samples;
performing, at the receiving apparatus, a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing, at the receiving apparatus, pilot symbol-aided phase tracking/correction to the second corrected samples,
wherein the pilot symbol-aided phase tracking/correction comprises:
constructing an error phasor sequence based on the second corrected samples;
applying smoothing to the error phasor sequence to obtain a smoothed error phasor sequence;
deriving one or more phase errors from the smoothed error phasor sequence; and
correcting the second corrected samples based on the one or more phase errors.
10. The method of claim 9 , wherein the constructing the error phasor sequence comprises multiplying a subset of the second corrected samples corresponding to known values by complex conjugate phasors corresponding to those known values.
11. The method of claim 10 , wherein the known values are pilot symbols.
12. The method of claim 11 , wherein the pilot symbols are evenly spaced throughout a packet corresponding to the second corrected samples.
13. The method of claim 11 , wherein values of the pilot symbols are selected based on a pseudorandom sequence.
14. The method of claim 9 , wherein the applying smoothing comprises applying a low-pass filtering technique, and wherein an amount of smoothing is determined based on at least one item selected from the group consisting of: expected phase fluctuation rate due to phase noise; short-term frequency drift; and Doppler rate.
15. The method of claim 9 , wherein the deriving one or more phase errors comprises:
finding an angle corresponding to a respective smoothed error phasor location; and
interpolating between angles to obtain angles corresponding to second corrected samples for which error phasors were not obtained.
16. The method of claim 15 , wherein correcting the second corrected samples comprises applying complex phasors with negative values of the respective angles to the corresponding second corrected samples.
17. A storage device containing executable instructions for implementing operations for phase tracking in a communication system, the operations including:
performing an initial frequency correction based on received symbol samples, to output first corrected samples, wherein the initial frequency correction comprises performing at least one non-data-dependent frequency estimation and correction on a single packet-length's worth of the received symbol samples, wherein a number of the received symbol samples is greater than one packet length's worth of symbol samples, wherein the initial frequency correction further comprises iteratively stepping through packet-length sets of the received symbol samples to obtain the single packet-length's worth of received symbol samples, and wherein the iteratively stepping through packet-length sets of the received symbol samples comprises:
extracting a symbol block containing one packet-length's worth of received symbol samples;
performing non-data-dependent frequency estimation and correction on the symbol block to obtain frequency-adjusted symbol samples; and
correlating the frequency-adjusted symbol samples with a packet sync pattern; and
identifying the packet start sample by the correlation value with maximum magnitude;
performing a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing pilot symbol-aided phase tracking/correction to the second corrected samples.
18. The storage device of claim 17 , wherein the iteratively stepping through packet-length sets of the received symbol samples further comprises:
storing at least one result derived from the correlating;
determining if there are further symbol samples available;
if further symbol samples are available, stepping by a length of one packet sync pattern to extract a further symbol block, performing the non-data-dependent frequency estimation, packet sync pattern correlation, identification of symbol timing instant with maximum correlation magnitude, and storing at least one set of these results; and
if no further symbol samples are available, selecting a symbol block, based on the at least one result, and performing the non-data-dependent frequency estimation and correction on the selected symbol block.
19. The storage device of claim 18 , wherein the at least one result comprises a correlation peak sample instant and a peak signal-to-noise ratio, and wherein selecting a symbol block comprises selecting a symbol block corresponding to a maximum correlation peak.
20. A communication apparatus including:
at least one processor; and
the storage device of claim 17 , wherein the storage device is communicatively coupled to the at least one processor, and wherein the at least one processor is configured to execute the executable instructions.
21. A storage device containing executable instructions for implementing operations for phase tracking in a communication system, the operations including:
performing an initial frequency correction based on received symbol samples, to output first corrected samples;
performing a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing pilot symbol-aided phase tracking/correction to the second corrected samples,
wherein performing the second frequency correction and initial phase correction comprises:
performing non-data-dependent frequency estimation and correction on the first corrected samples, to obtain a block of further corrected samples;
correlating the further corrected samples with a packet sync pattern;
using a peak correlation value to derive a packet timing instant and phase correction; and
applying the phase correction a packet length of further corrected symbol samples beginning with the selected packet timing instant.
22. The storage device of claim 21 , wherein the initial frequency correction comprises performing at least one non-data-dependent frequency estimation and correction on a single packet-length's worth of the received symbol samples.
23. The storage device of claim 22 , wherein a number of the received symbol samples is greater than one packet length's worth of symbol samples, and wherein the initial frequency correction further comprises iteratively stepping through packet-length sets of the received symbol samples to obtain the single packet-length's worth of received symbol samples.
24. The storage device of claim 23 , wherein the iteratively stepping through packet-length sets of the received symbol samples comprises:
extracting a symbol block containing one packet-length's worth of received symbol samples;
performing non-data-dependent frequency estimation and correction on the symbol block to obtain frequency-adjusted symbol samples; and
correlating the frequency-adjusted symbol samples with a packet sync pattern; and
identifying the packet start sample by the correlation value with maximum magnitude.
25. The storage device of claim 21 , wherein the phase correction corresponds to a negative value of a phase of the peak correlation value.
26. A storage device containing executable instructions for implementing operations for phase tracking in a communication system, the operations including:
performing an initial frequency correction based on received symbol samples, to output first corrected samples;
performing a second frequency correction and initial phase correction on the first corrected samples to output second corrected samples; and
performing pilot symbol-aided phase tracking/correction to the second corrected samples,
wherein the pilot symbol-aided phase tracking/correction comprises:
constructing an error phasor sequence based on the second corrected samples;
applying smoothing to the error phasor sequence to obtain a smoothed error phasor sequence;
deriving one or more phase errors from the smoothed error phasor sequence; and
correcting the second corrected samples based on the one or more phase errors.
27. The storage device of claim 26 , wherein the constructing the error phasor sequence comprises multiplying a subset of the second corrected samples corresponding to known values by complex conjugate phasors corresponding to those known values.
28. The storage device of claim 27 , wherein the known values are pilot symbols.
29. The storage device of claim 28 , wherein the pilot symbols are evenly spaced throughout a packet corresponding to the second corrected samples.
30. The storage device of claim 28 , wherein values of the pilot symbols are selected based on a pseudorandom sequence.
31. The storage device of claim 26 , wherein the applying smoothing comprises applying a low-pass filtering technique, and wherein an amount of smoothing is determined based on at least one item selected from the group consisting of: expected phase fluctuation rate due to phase noise; short-term frequency drift; and Doppler rate.
32. The storage device of claim 26 , wherein the deriving one or more phase errors comprises:
finding an angle corresponding to a respective smoothed error phasor location; and
interpolating between angles to obtain angles corresponding to second corrected samples for which error phasors were not obtained.
33. The storage device of claim 32 , wherein correcting the second corrected samples comprises applying complex phasors with negative values of the respective angles to the corresponding second corrected samples.