Digital Coherent Optical Receivers: Algorithms and Subsystems

TLDR

Digital coherent receivers have revolutionized optical transmission system design by embedding advanced subsystems and algorithms. The paper discusses future challenges for coherent optical transmission systems. The authors review the optical front end, ADC, DSP algorithms, and detail dynamic‑channel equalization—including a dual‑polarization constant‑modulus algorithm and various equalizers for PDM‑QPSK and PDM‑16‑QAM—along with timing and carrier phase synchronization techniques.

Abstract

Digital coherent receivers have caused a revolution in the design of optical transmission systems, due to the subsystems and algorithms embedded within such a receiver. After giving a high-level overview of the subsystems, the optical front end, the analog-to-digital converter (ADC) and the digital signal processing (DSP) algorithms, which relax the tolerances on these subsystems are discussed. Attention is then turned to the compensation of transmission impairments, both static and dynamic. The discussion of dynamic-channel equalization, which forms a significant part of the paper, includes a theoretical analysis of the dual-polarization constant modulus algorithm, where the control surfaces several different equalizer algorithms are derived, including the constant modulus, decision-directed, trained, and the radially directed equalizer for both polarization division multiplexed quadriphase shift keyed (PDM-QPSK) and 16 level quadrature amplitude modulation (PDM-16-QAM). Synchronization algorithms employed to recover the timing and carrier phase information are then examined, after which the data may be recovered. The paper concludes with a discussion of the challenges for future coherent optical transmission systems.