Fiber QAM vs RF QAM: Demystifying Coherent Optics (2023)

By Amarildo Vieira, Ph.D., Comcast Connect; Venk Mutalik, Comcast; Dan Rice, Comcast; John Chrostowski, Comcast; Saif Rahman, Comcast

Quadrature Amplitude Modulation (QAM) transmission has been the mainstay of Radio Frequency (RF) networks for the better part of two decades. During that period, we have witnessed modulation complexity rise from 64-QAM to 4096-QAM. With each increase in complexity, we have adapted network configurations to higher SNR and spectrum to deliver higher and higher capacity. Similar is the case for optical transmission as operators move to higher modulation order Coherent Optics to keep up with the ever-increasing capacity needs in the transport, metro, and access networks. There is a need to get beyond contemplating the many similarities and challenges coherent optics are currently facing, however, and use the lessons learned in the RF domain for the journey with Fiber QAM. 100Gbps QPSK optics have been routinely used in different segments of Comcast’s network and 800Gbps 32-QAM and beyond are not far on the horizon. In the access network, 100Gbps bi-directional optics have been instrumental in supporting different initiatives beyond capacity increases [1]. This includes fiber conservation, pushing Distributed Access Architecture (DAA) further down close to the node, and faster and lower-cost deployment and upgrades. In this paper we first share our experience in preparing for higher order m-QAM modulation in terms of Coherent Optics terminology and tracing parallels with the steps the industry took during their m-QAM RF transition. Concerns about the fiber infrastructure, managing fiber non-linearity, and network architecture are then addressed with our recommendation to operators on how to get ready for Fiber QAM evolution. Lastly, as part of the envisioned evolution, the concept of Orthogonal Frequency-Division Multiplexing (OFDM) and Orthogonal Frequency-Division Multiplexing Access (OFDMA) in the optical domain leveraging the current cable operator experiences are explored. This includes a review of current industry initiatives and some predictions based on the uncovered parallels.

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