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 circuit configured to receive a plurality of samples of an optical signal having a pulse amplitude modulated (PAM) E-field; the circuit comprising:
a slicer configured to estimate a PAM level of a sample in the plurality of samples;
a subtractor configured to generate an error sample by subtracting the sample from the estimated PAM level;
a low-pass filter configured to filter 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 configured to produce an interference-mitigated sample by combining one of the estimates of MPI with the sample.
2. The circuit of claim 1 wherein the subtractor is a first subtractor configured to subtract the sample from the estimated PAM level to generate a corresponding error sample.
3. The circuit of claim 1 wherein:
the subtractor is a first subtractor; and
the combiner is a second subtractor that is configured to combine the one of the estimates of multi-path interference with the sample to produce the interference-mitigated sample by subtracting the one of the estimates of MPI from the sample to produce the interference-mitigated sample.
4. The circuit of claim 1 wherein:
the subtractor is a first subtractor; and
the combiner is second, level-dependent subtractor that, for a given sample, is configured to combine the one of the estimates of MPI with the sample to produce the interference-mitigated sample by producing a weighted estimate by multiplying the one of the estimates of MPI producing the weighted estimate by multiplying the one of the estimates of MH 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 interference mitigated sample.
5. The circuit of claim 4 wherein the first subtractor is configured to subtract the sample from the estimated PAM level to generate a corresponding error sample.
6. The circuit of claim 1 wherein the low-pass filter comprises:
a fixed block average component configured to determine an average of the error samples for a block of samples,
wherein the average is used as the one of the estimates of MPI that is combined with each sample in the block of samples.
7. The circuit of claim 6 wherein the block of samples comprises consecutive samples.
8. The circuit of claim 7 wherein a size of the block of samples comprising the consecutive samples is configured as a function of transmitter coherence.
9. The circuit of claim 1 wherein the low-pass filter comprises:
a moving window average component configured to determine, 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 one of the estimates MPI that is combined with the sample.
10. The circuit of claim 9 wherein the block of error samples comprises consecutive samples.
11. The circuit of claim 10 wherein a size of the block of error samples comprising the consecutive samples is configured as a function of transmitter coherence.
12. The circuit 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 circuit 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 circuit of claim 1 further comprising:
a delay element configured to delay the sample before combining to account for a delay in determining the one of the estimates of MPI.
15. The circuit of claim 1 further comprising:
a direct detection receiver configured to directly detect the optical signal to produce a direct detection output.
16. The circuit of claim 15 further comprising:
a converter configured to perform analog to digital conversion on the direct detection output to generate raw samples.
17. The circuit of claim 16 further comprising:
an equalizer configured to equalize the raw samples to produce the plurality of samples.
18. The circuit of claim 17 further comprising:
a PAM decision slicer configured to perform PAM decision slicing for each interference-mitigated sample.
19. The circuit of claim 1 further comprising:
an interference component estimator configured to estimate at least one interference component by estimating a respective delay and respective amplitude for each interference component;
wherein for each sample, the combiner is configured to combine the one of the estimates of MPI and the estimated at least one interference component with the sample; and
a PAM decision slicer configured to perform PAM decision slicing for each interference-mitigated sample.
20. The circuit of claim 1 further comprising:
a PAM decision slicer configured to perform PAM decision slicing for each interference-mitigated sample.