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

Abstract

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.