Cycle slip compensation in 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 coherent detection based receiver, the method comprising:
receiving a sequence of frames, each frame comprising a data symbol block and a pilot symbol block, the pilot symbol block comprising one or more pilot symbols, the sequence of frames including at least a target frame and a subsequent frame;
estimating a first phase rotation of the target frame based on pilot symbols in the target frame and an expected sequence of pilot symbols;
estimating a second phase rotation of the subsequent frame based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols;
responsive to the first phase rotation not matching the second phase rotation, detecting a location of a cycle slip in the target frame; and
correcting the cycle slip based on the detected location and the first and second phase rotations.
2. 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.
3. The method of claim 1 , wherein correcting the cycle slip based on the detected location 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.
4. 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.
5. 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.
6. The method of claim 5 , 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.
7. The method of claim 6 , wherein determining the likelihood comprises computing a Euclidean distance between the pilot symbols in the target frame and the expected sequence of pilot symbols.
8. The method of claim 6 , 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.
9. A non-transitory computer-readable storage medium storing instructions for compensating for cycle slips in a coherent detection based receiver, the instructions when executed by a processor causing the processor to perform steps including:
receiving a sequence of frames, each frame comprising a data symbol block and a pilot symbol block, the pilot symbol block comprising one or more pilot symbols, the sequence of frames including at least a target frame and a subsequent frame;
estimating a first phase rotation of the target frame based on pilot symbols in the target frame and an expected sequence of pilot symbols;
estimating a second phase rotation of the subsequent frame based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols;
responsive to the first phase rotation not matching the second phase rotation, detecting a location of a cycle slip in the target frame; and
correcting the cycle slip based on the detected location and the first and second phase rotations.
10. The non-transitory computer-readable storage medium of claim 9 , the instructions when executed by the processor further causing the processor to perform steps including:
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.
11. The non-transitory computer-readable storage medium of claim 9 , wherein correcting the cycle slip based on the detected location comprises:
applying the first phase rotation to symbols of the target frame occurring prior to the detected location; and
applying the first phase rotation to symbols of the target frame occurring after the detected location.
12. The non-transitory computer-readable storage medium of claim 9 , 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.
13. The non-transitory computer-readable storage medium of claim 9 , 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.
14. The non-transitory computer-readable storage medium of claim 13 , 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.
15. The non-transitory computer-readable storage medium of claim 14 , wherein determining the likelihood comprises computing a Euclidean distance between the pilot symbols in the target frame and the expected sequence of pilot symbols.
16. The non-transitory computer-readable storage medium of claim 14 , 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.
17. A receiver comprising:
a carrier phase recovery block configured to output a sequence of frames based on a received signal, each frame comprising a data symbol block and a pilot symbol block, the pilot symbol block comprising one or more pilot symbols, the sequence of frames including at least a target frame and a subsequent frame;
a cycle slip detection block configured to estimate a first phase rotation of the target frame based on pilot symbols in the target frame and an expected sequence of pilot symbols, to estimate a second phase rotation of the subsequent frame based on pilot symbols in the subsequent frame and the expected sequence of pilot symbols, and responsive to the first phase rotation not matching the second phase rotation, to detect a location of a cycle slip in the target frame; and
a phase correction block configured to correct the cycle slip based on the detected location and the first and second phase rotations.
18. The receiver of claim 17 ,
wherein the cycle slip detection block is further configured to estimate 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
wherein the phase correction block further rotates the subsequent frame by the second phase rotation responsive to the cycle slip detection block determining that the third phase rotation matches the second phase rotation.
19. The receiver of claim 17 , wherein the phase correction block is configured to:
apply the first phase rotation to symbols of the target frame occurring prior to the detected location; and
apply the second phase rotation to symbols of the target frame occurring after the detected location.
20. The receiver of claim 17 , wherein the carrier phase recovery block comprises a carrier phase estimator configured to generate a phase estimation and the cycle slip detection block comprises a position estimator configured to estimate the location based on the phase estimation from the carrier phase estimator.