Xin Haiyun, Zhang Kuo, Fu Yan, Hussain Syed Baqar, Guo Zidong, Sultan Amber, He Hao, Hu Weisheng
Opt Express. 2018 Oct 29;26(22):28206-28215. doi: 10.1364/OE.26.028206.
Directly modulated lasers (DMLs) and electro-absorption modulated lasers (EMLs) are key transmitter options in future short-haul networks. However, both of them suffer from frequency chirp, which incurs nonlinear distortions, especially to high order modulation signals. In this paper, we investigate their application in PAM4-based digital mobile fronthaul and propose a scheme to remarkably improve the fidelity of radio signal. We first give a detailed study of the BER distribution of DML/EML based PAM4 signals and find that the BER of the second bit is much higher than that of the first bit in both systems. Accordingly, we propose to adopt sample bit interleaving to reduce the radio signal distortions caused by sample bit errors. Experimental results of 56Gbps I/Q data transmission reveal that, in a DML-based transmission system, the proposed scheme respectively leads to up to 8dB and 13dB EVM reduction to accommodate 33 × 100MHz 1024QAM OFDM signals and 64QAM OFDM signals in 10km and 20km cases. As well as in an EML-based transmission system, 14dB EVM reduction is achieved in 10km to finally accommodate 33 × 100MHz 256QAM OFDM signal.
直接调制激光器（DML）和电吸收调制激光器（EML）是未来短距离网络中的关键发射机选项。然而，它们都存在频率啁啾问题，这会导致非线性失真，尤其是对高阶调制信号。在本文中，我们研究了它们在基于PAM4的数字移动前传中的应用，并提出了一种显著提高无线电信号保真度的方案。我们首先详细研究了基于DML/EML的PAM4信号的误码率分布，发现两个系统中第二位的误码率都远高于第一位。因此，我们建议采用采样比特交织来减少由采样比特错误引起的无线电信号失真。56Gbps I/Q数据传输的实验结果表明，在基于DML的传输系统中，所提出的方案在10km和20km情况下分别使EVM降低高达8dB和13dB，以适应33×100MHz 1024QAM OFDM信号和64QAM OFDM信号。同样，在基于EML的传输系统中，在10km时EVM降低了14dB，最终适应了33×100MHz 256QAM OFDM信号。
