Low-Complexity Digital Signal Processing Techniques to Enable Coherent Optical Systems for Metro and Access networks

We summarize our recent research works on enabling coherent optical transmission systems for metro and access networks with low-complexity digital signal processing techniques, focusing on reduction of laser linewidth requirement with efficient carrier phase recovery.


I. Introduction
To date, the metro and access networks are constantly being extended to reach base stations for mobile applications, data centers, local exchanges and new enterprises, resulting in a continuous growth of data traffic at a significant increasing rate during the past five years [1].All this traffic is eventually routed through the metro and regional core networks, creating challenges to keep quality of services while maintaining a low-cost target for service providers.As a response, cost effective coherent technologies with reduced complexity and form factors are being introduced and developed.Currently, thousands of 100G+ coherent optical equipment ports are being deployed for the metro and access applications, thanks to the development of large-scale photonics integrated circuits (PICs) technologies [2] [3].One of the significant design aspects for PIC-based coherent transceivers to be suitable for metro and access applications is the laser, the linewidth of which correspondingly impacts the complexity of digital signal processing (DSP) module and the employed modulation formats.Monolithically integrated semiconductor lasers are generally more cost-effective, energy-efficient and easy to integrate, comparing to the external cavity lasers (ECLs) commonly used in coherent transceivers for longhaul networks.However, one drawback of using such semiconductor lasers is the higher linewidths, requiring potentially more complex DSP for carrier phase recovery (CPR) and coding/decoding, increasing ASIC power consumption and size, therefore the cost.Such a trade-off imposes challenges on the design and development of low-complexity and high-performance DSP algorithms.
In this abstract, we aim to take the initiative to discuss potential solutions to lower the complexity of the DSP algorithms for coherent transceivers, particularly targeting to lower the laser and CPR requirements.Mainly three approaches will be discussed: 1) lower the complexity of feedforward CPR structure for conventional square shaped QAM signals; 2) adapt the constellation shape of the QAM signals for lowcomplexity CPR algorithms that have difficulties to be applied to square QAM constellations; and 3) use self homodyne implementation to achieve a stable carrier phase performance.

II. Potential Coherent Technologies for Metro and Access Networks
From the perspective of DSP on conventional coherent system architecture, blind feedforward CPR methods including blind phase search (BPS) and, Viterbi & Viterbi (V&V) algorithms can be potentially applied on QAM signals, directly or indirectly.Furthermore, from a system architecture perspective, self-homodyne coherent system together with spatial division multiplexing (SDM) technologies may open a new way to address future metro and access applications and demand.

A. Low complexity CPR for square QAM
For square shaped QAM signals, BPS based CPR methods are preferred owning to its scalability for any constellation shapes and mo dulation orders.However, complexity induced with high phase search resolution should be considered and reduced for practical use.We have proposed to use a sliding average filter to mitigate the intrinsic high frequency noise enhancement induced by the limited discrete phase resolutions of the BPS algorithms [4] [5].The performance enhancement and complexity reduction can be observed in Fig. 1.

B. Circular shaped QAM constellation
The non-uniform phase distribution of square shaped QAM constellations limits the direct application of the low-complexity V&V algorithm and/or its multi-stage CPR implementations.We propose to use a circular shaped QAM constellation which transparently accommodates to the operational requirements of the algorithms [6][7].This is in line with the current trend of using flexible constellations with geometrical constellation shaping.Figure 2 shows the example of a multi-stage CPR structure for circular 64QAM.Such combination can greatly enhance phase noise tolerance comparing to that of the square QAM. C. Self-homodyne implementation and its synergy with SDM technology Self-homodyne coherent detection (SHCD) technique can offer stable carrier phase performance, thus reducing the requirements on both the DSP and the laser.A straight forward approach is to couple the SHCD with SDM technologies, in such a way the local oscillator (LO) can be forwarded with a separate spatial channel and shared by multiple data channels.Our recent experimental demonstration (Fig. 3) shows that this approach can potentially support high data rate with multilevel modulation formats, e.g.64QAM [8].

III. Conclusions
We summarized our recent research initiatives on investigating possibilities of using low-cost coherent transceivers with low DSP complexity.The main effort up to now focuses on the carrier phase recovery for large linewidth integrated semiconductor lasers.Considering both the signal and the system architecture, different approaches can be utilized to achieve improved performance with lower transceiver complexity.Additionally, the envisioned target should also include future efforts on lowering the complexity of equalizer and forward error correction coding schemes, etc.
Fig. 1. a) Frequency noise spectrum of different BPS phase noise tracked sequences with different p with and without LPF at 37.5 dB o f OSNR and 2.2 MHz of linewidth.b) BER vs OSNR for three different linewidths and four CPR algorithms.c) BER vs number of test phases for three different linewidths at 31.8 dB OSNR.[4]