Low Complexity Digital Resolution Enhancer for Clipping and Quantization Noise Suppression With Low-Resolution DAC

In this paper, we introduce a simplified digital resolution enhancer (DRE) achieved by reducing the survived state of the Viterbi algorithm for pre-equalization systems with low-resolution digital-to-analog converters (DACs). The clipping technique is employed based on the simplified DRE to mitigate the impact of quantization noise and improve the signal power. Both clipping and quantization noise are considered in the branch metric calculation of the simplified DRE aiming to improve the overall system performance. Simulation and experiment have been conducted to verify its effectiveness by transmitting 50 Gbaud pulse amplitude modulation (PAM)-4 and 40 Gbaud PAM-8 signals. The results show that for 50 Gbaud PAM-4 signal, 3-tap channel impulse response (CIR)-supported DRE with 3 soft-quantization possibilities designed for both clipping and quantization noise suppression can achieve 1 dB receiver optical power (ROP) improvement compared to the channel response dependent noise shaping (CRD-NS) technique at the 7% hard-decision forward error correction (HD-FEC) threshold. Increasing both CIR length and soft-quantization possibilities from 3 to 5 can yield an additional 0.4 dB ROP improvement. For 40 Gbaud PAM-8 signal transmission, only 5-tap CIR-supported DRE with 5 soft-quantization possibilities can guarantee the bit error rate (BER) of the signal below the HD-FEC threshold. DRE shaped for both clipping and quantization noise can obtain an additional 1 dB ROP improvement. Considering computational complexity and BER performance, simplified 3-tap CIR-supported DRE with 3 soft-quantization possibilities and 5-tap CIR-supported DRE with 5 soft-quantization possibilities have been experimentally studied for 50 Gbaud PAM-4 and 40 Gbaud PAM-8 signals, respectively. The results indicate that a computational complexity compression ratio of 55% and 95.2% can be realized for 50 Gbaud PAM-4 and 40 Gbaud PAM-8 signals in comparison with the traditional DRE, respectively, while only 0.2 dB ROP penalty is observed.