Memory-based pulse amplitude modulation for short-reach fiber communications with intensity modulation and direct detection.

The low cost of an intensity modulation and direct detection system has made it an attractive choice for short-reach fiber communications. A longer fiber transmission length can be supported with the help of a combination of high-ary pulse amplitude modulation (PAM) and maximum likelihood sequence estimation (MLSE). For a further improvement on the system performance under a fiber dispersive channel, this paper proposes to add memory into PAM for spectrum compression. The memory-based PAM can be implemented with a simple pre-coding at the transmitter side while no extra requirement is necessary on the receiver side. Both theoretical and simulation results show that the memory-based PAM has a smaller mainlobe bandwidth and thus has a higher tolerance to the accumulated fiber dispersion. Compared with conventional memory-less PAM-4, the memory-based PAM-4 can effectively extend the maximum achievable fiber transmission length from 60km to 70km with the same number of MLSE states at the receiver. Moreover, this paper also proposes an optimization on the memory-based PAM and a further extension on the allowed fiber transmission length is achieved.