Circuit for multi-path interference mitigation in an optical communication system
A circuit and method for mitigating multi-path interference in direct detection optical systems is provided. Samples of an optical signal having a pulse amplitude modulated (PAM) E-field are processed by generating a PAM level for each sample. For each sample, the sample is subtracted from the respective PAM level to generate a corresponding error sample. The error samples are lowpass filtered to produce estimates of multi-path interference (MPI). For each sample, one of the estimates of MPI is combined with the sample to produce an interference-mitigated sample.
1. A method comprising:
receiving a plurality of samples of an optical signal having a pulse amplitude modulated (PAM) E-field;
estimating a PAM level of a sample in the plurality of samples;
a subtractor generating an error sample by subtracting the sample from the estimated PAM level;
a low-pass filter filtering the error sample for the sample and error samples for the other samples in the plurality of samples to produce estimates of multi-path interference (MPI); and
a combiner producing an interference-mitigated sample by combining one of the estimates of MPI with the sample.
2. The method of claim 1 wherein the subtractor is a first subtractor subtracting the sample from the estimated PAM level to generate a corresponding error sample.
3. The method of claim 1 wherein:
the subtractor is a first subtractor; and
the combiner is a second subtractor that combines one of the estimates of multi-path interference with the sample to produce an interference-mitigated sample by subtracting the estimate from the sample to produce the interference-mitigated sample.
4. The method of claim 1 :
the subtractor is a first subtractor; and
the combiner is second, level-dependent subtractor that, for a given sample, combines one of the estimates of MPI with the sample to produce an interference-mitigated sample by producing a weighted estimate by multiplying the one of the estimates of MPI producing a weighted estimate by multiplying the one of the estimates of MPI with a value proportional to a respective PAM level modulating the E-field estimated from the sample, and subtracting the weighted estimate from the sample to produce the WI-mitigated sample.
5. The method of claim 4 wherein the first subtractor subtracts the sample from the estimated PAM level to generate a corresponding error sample.
6. The method of claim 1 wherein the low-pass filter comprises:
a fixed block average component that determines an average of the error samples for a block of samples;
wherein the average is used as the estimate of MPI that is combined with each sample in the block of samples.
7. The method of claim 6 wherein the block of samples comprises consecutive samples.
8. The method of claim 7 further comprising:
a size of the block of consecutive samples is configured as a function of transmitter coherence.
9. The method of claim 1 wherein the low-pass filter comprises:
a moving window average component that determines, for each sample, an average of the error samples for a respective block of error samples defined by a moving window, wherein the average is used as the estimate of MPI that is combined with the sample.
10. The method of claim 9 wherein the block of samples comprises consecutive samples.
11. The method of claim 10 further comprising:
a size of the block of consecutive samples is configured as a function of transmitter coherence.
12. The method of claim 1 operating at a baud rate of greater than 25 GBaud, in which the low-pass filter operates to filter frequencies below 100 MHz.
13. The method of claim 1 operating at a baud rate of greater than 25 GBaud, in which the low-pass filter operates to filter frequencies below 10 MHz.
14. The method of claim 1 further comprising:
a delay element delaying the sample before combining to account for a delay in determining the one of the estimates of MPI.
15. The method of claim 1 further comprising:
a direct detection receiver directly detecting the optical signal to produce a direct detection output.
16. The method of claim 15 further comprising;
performing analog to digital conversion on the direct detection output to generate raw samples.
17. The method of claim 16 further comprising:
equalizing the raw samples to produce the plurality of samples.
18. The method of claim 17 further comprising:
a PAM decision slicer performing PAM decision slicing for each interference-mitigated sample.
19. The method of claim 1 further comprising:
an interference component estimator estimating at least one interference component by estimating a respective delay and respective amplitude for each interference component;
wherein for each sample, the combiner combines the estimate of MPI and the estimated at least one electrical interference component with the sample;
a PAM decision slicer that performs PAM decision slicing for each interference-mitigated sample.
20. The method of claim 1 further comprising:
a PAM decision slicer performing PAM decision slicing for each interference-mitigated sample.