Cycle slip compensation in a coherent receiver
A receiver architecture and method recovers data received over an optical fiber channel in the presence of cycle slips. In a first cycle slip recovery architecture, a receiver detects and corrects cycle slips based on pilot symbols inserted in the transmitted data. In a second cycle slip recovery architecture, a coarse cycle slip detection is performed based on pilot symbols and a cycle slip position estimation is then performed based on carrier phase noise. The receiver compensates for cycle slips based on the position estimation.
1. A method for compensating for cycle slips in a receiver, the method comprising:
converting, by an optical front end circuit, an optical signal to an analog electrical signal;
sampling, by an analog front end circuit, the analog electrical signal based on a sampling clock to generate an input digital signal, the input digital signal comprising a sequence of frames;
processing the sequence of frames to correct for phase noise smaller than a rotational distance between constellation points to generate a phase-corrected sequence of frames;
estimating a first phase rotation of a target frame in the phase-corrected sequence of frames based on pilot symbols in the target frame and an expected sequence of pilot symbols;
estimating a second phase rotation of a subsequent frame in the phase-corrected sequence of frames based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols, the subsequent frame following the target frame in the phase-corrected sequence of frames;
responsive to the first phase rotation not matching the second phase rotation, detecting a location of a cycle slip in the target frame;
adjusting, based on the first and second phase rotations, a phase of data symbol blocks in the phase-corrected sequence of frames to generate a corrected target frame;
demodulating a corrected sequence of frames including the corrected target frame by a demodulator; and
decoding the corrected sequence of frames by a decoder to generate a digital output signal.
2. The method of claim 1 , further comprising:
prior to demodulating the corrected sequence of frames, removing pilot symbol blocks from the corrected sequence of frames.
3. The method of claim 1 , further comprising:
estimating a third phase rotation of a third frame following the subsequent frame based on pilot symbols in the third frame and the expected sequence of pilot symbols; and
responsive to the third phase rotation matching the second phase rotation, rotating the subsequent frame by the second phase rotation.
4. The method of claim 1 , wherein adjusting the phase of the data symbol blocks comprises:
applying the first phase rotation to symbols of the target frame occurring prior to the detected location; and
applying the second phase rotation to symbols of the target frame occurring after the detected location.
5. The method of claim 1 , wherein detecting the location of the cycle slip in the target frame comprises:
estimating the location based on a phase estimation from a carrier phase estimator.
6. The method of claim 1 , wherein estimating the first phase rotation comprises selecting from a predefined set of possible phase rotations, the predefined set of possible phase rotations corresponding to phases of a symbol space of pilot symbols in the expected sequence of pilot symbols.
7. The method of claim 6 , wherein selecting from the predefined set of possible phase rotations comprises:
determining a likelihood of each of the possible phase rotations based on the pilot symbols in the target frame and the expected sequence of pilot symbols; and
selecting the phase rotation having a maximum likelihood.
8. The method of claim 7 , wherein determining the likelihood comprises computing a Euclidean distance between the pilot symbols in the target frame and the expected sequence of pilot symbols.
9. The method of claim 7 , wherein determining the likelihood comprises:
slicing the pilot symbols of the target frame to map each of the pilot symbols of the target frame to a nearest constellation point; and
computing a Euclidean distance between the sliced pilot symbols of the target frame and the expected sequence of pilot symbols.
10. A receiver system comprising:
an optical front end circuit configured to convert an optical signal to an analog electrical signal;
an analog front end circuit configured to sample the analog electrical signal based on a sampling clock to generate an input digital signal, the input digital signal comprising a sequence of frames;
a digital signal processor; and
a non-transitory computer-readable storage medium storing instructions that when executed by the digital signal processor perform steps including:
processing the sequence of frames to correct for phase noise smaller than a rotational distance between constellation points to generate a phase-corrected sequence of frames;
estimating a first phase rotation of a target frame in the phase-corrected sequence of frames based on pilot symbols in the target frame and an expected sequence of pilot symbols;
estimating a second phase rotation of a subsequent frame in the phase-corrected sequence of frames based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols, the subsequent frame following the target frame in the phase-corrected sequence of frames;
responsive to the first phase rotation not matching the second phase rotation, detecting a location of a cycle slip in the target frame;
adjusting, based on the first and second phase rotations, a phase of data symbol blocks in the phase-corrected sequence of frames to generate a corrected target frame;
demodulating a corrected sequence of frames including the corrected target frame by a demodulator; and
decoding the corrected sequence of frames by a decoder to generate a digital output signal.
11. The receiver system of claim 10 , wherein the instructions when executed further cause the digital signal processor to remove pilot symbol blocks from the corrected sequence of frames.
12. The receiver system of claim 10 , wherein adjusting the phase of the data symbol blocks comprises:
applying the first phase rotation to symbols of the target frame occurring prior to the detected location; and
applying the second phase rotation to symbols of the target frame occurring after the detected location.
13. The receiver system of claim 10 , wherein detecting the location of the cycle slip in the target frame comprises:
estimating the location based on a phase estimation from a carrier phase estimator.
14. The receiver system of claim 10 , wherein estimating the first phase rotation comprises selecting from a predefined set of possible phase rotations, the predefined set of possible phase rotations corresponding to phases of a symbol space of pilot symbols in the expected sequence of pilot symbols.
15. The receiver system of claim 14 , wherein selecting from the predefined set of possible phase rotations comprises:
determining a likelihood of each of the possible phase rotations based on the pilot symbols in the target frame and the expected sequence of pilot symbols; and
selecting the phase rotation having a maximum likelihood.
16. A non-transitory computer-readable storage medium storing instructions that when executed by a processor cause the processor to perform steps including:
receiving an input digital signal comprising a sequence of frames;
processing the sequence of frames to correct for phase noise smaller than a rotational distance between constellation points to generate a phase-corrected sequence of frames;
estimating a first phase rotation of a target frame in the phase-corrected sequence of frames based on pilot symbols in the target frame and an expected sequence of pilot symbols;
estimating a second phase rotation of a subsequent frame in the phase-corrected sequence of frames based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols, the subsequent frame following the target frame in the phase-corrected sequence of frames;
responsive to the first phase rotation not matching the second phase rotation, detecting a location of a cycle slip in the target frame;
adjusting, based on the first and second phase rotations, a phase of data symbol blocks in the phase-corrected sequence of frames to generate a corrected target frame; and
outputting a corrected sequence of frames including the corrected target frame.
17. The non-transitory computer-readable storage medium of claim 16 , wherein adjusting the phase of the data symbol blocks comprises:
applying the first phase rotation to symbols of the target frame occurring prior to the detected location; and
applying the second phase rotation to symbols of the target frame occurring after the detected location.
18. The non-transitory computer-readable storage medium of claim 16 , wherein detecting the location of the cycle slip in the target frame comprises:
estimating the location based on a phase estimation from a carrier phase estimator.
19. The non-transitory computer-readable storage medium of claim 16 , wherein estimating the first phase rotation comprises selecting from a predefined set of possible phase rotations, the predefined set of possible phase rotations corresponding to phases of a symbol space of pilot symbols in the expected sequence of pilot symbols.
20. The non-transitory computer-readable storage medium of claim 19 , wherein selecting from the predefined set of possible phase rotations comprises:
determining a likelihood of each of the possible phase rotations based on the pilot symbols in the target frame and the expected sequence of pilot symbols; and
selecting the phase rotation having a maximum likelihood.