Optimum sample spacing in over-sampled maximum likelihood sequence estimation receivers
In an over-sampled maximum-likelihood sequence estimation (MLSE) receiver system, the optimal sample spacing is determined for a variety of conditions. In an illustrative implementation, the system includes an optical filter for tightly filtering an incoming optical data signal with an on-off-keying (OOK) non-return-to-zero (NRZ) format, followed by an optical-to-electrical converter, an electrical filter, a sampler, and a MLSE receiver. The sampler samples the filtered electrical data signal twice each bit period with unequal sample spacings. For wide optical filtering bandwidths, the optimal sample spacing occurs at less than 50% of a bit period. For narrow bandwidths, the optimal sample instances occur closer to the maximum eye opening.
1. An optical data communications receiving system comprising:
a converter, the converter converting an optical data signal to an electrical data signal;
a sampler, the sampler sampling the electrical data signal and generating at least two samples for each bit of the electrical data signal, wherein a sampling instant of each of the samples is optimized and displaced from a maximum eye opening of the electrical data signal by a portion of a bit period greater than 0% and less than 25%; and
a receiver, the receiver generating a recreated data signal based on the samples of the electrical data signal, wherein the receiver includes a maximum likelihood sequence estimation (MLSE) receiver.
2. The system of claim 1 , wherein the sampling instants of the samples are selected to reduce the optical signal to noise ratio (OSNR) required for a predetermined bit error rate.
3. The system of claim 1 , comprising an electrical filter for filtering the electrical data signal.
4. The system of claim 1 , wherein the sampling instants of the samples are arranged symmetrically about a maximum eye opening of the electrical data signal.
5. The system of claim 1 , wherein the sampling instant of each of the samples is displaced from the maximum eye opening of the electrical data signal by a portion of the bit period which is substantially 20%.
6. The system of claim 1 comprising an optical filter for filtering the optical data signal and wherein the displacement of the sampling instants from the maximum eye opening is selected in accordance with a bandwidth of the optical filter.
7. The system of claim 6 , wherein the bandwidth of the optical filter is between 0.5 and 2.5 times the bit rate of the optical data signal.
8. The system of claim 1 , wherein the optical data signal has an on-off-keying, non return-to-zero format.
9. The system of claim 1 , wherein the converter includes a square-law photodetector and the samples represent a magnitude of the electrical data signal.
10. The system of claim 1 , wherein the MLSE receiver is correlation-insensitive.
11. The system of claim 1 , wherein the MLSE receiver is correlation-sensitive.
12. An optical data communications receiving method comprising steps of:
receiving an optical data signal;
converting the optical data signal to an electrical data signal;
sampling the electrical data signal and generating at least two samples for each bit of the electrical data signal, wherein a sampling instant of each of the samples is optimized and displaced from a maximum eye opening of the electrical data signal by a portion of a bit period greater than 0% and less than 25%; and
generating a recreated data signal based on the samples of the electrical data signal, wherein the step of generating the recreated data signal includes performing a maximum likelihood sequence estimation (MLSE).
13. The method of claim 12 , wherein the sampling instants of the samples are selected to reduce the optical signal to noise ratio (OSNR) required for a predetermined bit error rate.
14. The method of claim 12 , comprising electrically filtering the electrical data signal.
15. The method of claim 12 , wherein the sampling instants of the samples are arranged symmetrically about a maximum eye opening of the electrical data signal.
16. The method of claim 12 , wherein the sampling instant of each of the samples is displaced from the maximum eye opening of the electrical data signal by a portion of the bit period which is substantially 20%.
17. The method of claim 12 , comprising optically filtering the optical data signal, wherein the displacement of the sampling instants from the maximum eye opening is selected in accordance with a bandwidth of the optical filtering.
18. The method of claim 17 , wherein the bandwidth of the optical filtering is between 0.5 and 2.5 times the bit rate of the optical data signal.
19. The method of claim 12 , wherein the optical data signal has an on-off-keying, non return-to-zero format.
20. The method of claim 12 , wherein the converting step includes performing a square-law photodetection and the samples represent a magnitude of the electrical data signal.
21. The method of claim 12 , wherein the MLSE is correlation-insensitive.
22. The method of claim 12 , wherein the MLSE is correlation-sensitive.