Joint pre‐ and post‐equalization using optical multi‐level signaling

Abstract

A joint pre- and post-equalization scheme is proposed for an optical intensity modulation and direct detection (IM/DD) system. At the transmitter, a linear feed-forward pre-equalizer with just two, three, or four filter taps is suggested to limit the transmitter's complexity and to allow the use of multi-level modulation. Finally, this concept is combined with a zero-forcing post-equalizer at the receiver. By utilizing numerical optimization to design the multi-level signals, the intersymbol-interference originating from the channel is mitigated pre-equalizer taps, a reduction of 3 % in received power compared to a conventional zero-forcing design is achieved. The equalization scheme is tailored for an optical IM/DD system. Therefore, the specific power constraints of such a setup are taken into account in the equalizer design process. In contrast to existing research, the approach is tested for a multi-mode fiber system, which is affected by increased inter-symbol interference due to modal dispersion. The simulation results show that by shifting a small portion of the equalization complexity to the transmitter side, the required optical power to reach a bit-error rate of 10 − 3 $10^{-3}$ is reduced by 7.3% for zero-forcing and 6.8% for minimum mean square error post-equalizers. the noise increase by the post-equalizer is reduced by 6.9 % when including the pre-equalizer with just two filter taps. Compared to an equalization scheme that solely relies on a zero-forcing post-equalizer, the suggested joint pre- and post-equalization scheme is able to improve the bit-error rate performance by an average of 7.15 %. A testbed experiment with a 250 m multi-mode fiber channel and a data rate of 2.5 Gbps operating at 1550 nm confirms these simulation results.