In the mid 1990s, multiple system operators (MSOs) began replacing coaxial cable trunks with optical links, creating the first hybrid-fiber coaxial (HFC) networks. This new architecture reduced capital and operations costs, but required highly linear optical transceivers capable of transporting a full spectrum of analog RF signals while satisfying stringent electrical signal-to-noise ratio (SNR), composite triple beat (CTB), and composite second order (CSO) requirements. In the ensuing decades, wavelength division multiplexing (WDM) technology was applied to these linear optical networks to aggregate and route headend-to-hub traffic and to split fiber nodes (FNs) in the access plant, thereby decreasing the number of households served per FN and thus increasing the available bandwidth per subscriber. Nonlinear optical impairments, primarily four-wave mixing (FWM) and cross-phase modulation (CPM), limit WDM channel counts and optical launch powers in linear HFC networks, adding further constraints as compared to the digital optical links used for telecom and datacom networking.
The distributed access architecture (DAA), a widely accepted vision for the future evolution of HFC, includes remoting the digital-to-RF interface from the headend to the DAA node. In this scenario, the legacy analog optical links are replaced with digital links, opening a vast array of existing, high performance, digital optical technologies to MSO network designers. Here, we describe a spectrally efficient and flexible modulation technique that has been recently developed for digital optical transmission links, probabilistic constellation shaping (PCS), that has several potential applications in DAA networks:
PCS has been known for decades as an essential element of communications to approach the capacity of a Gaussian channel, known as the Shannon limit. It reached large scale adoption in the mid 90’s inside dial-up and fax modems, but has been omitted in subsequent higher rate technologies that adopted a paradigm shift from single-carrier to multi-carrier modulation. The invention, in 2015, of an efficient implementation of PCS for optical transmission systems has led to the rapid commercialization of this technology for core optical networks using coherent optics.
Although commercial optical coherent systems typically use square quadrature amplitude modulation (QAM) constellations with a uniform probability distribution, optimally performing constellations for a fixed average transmit power should follow a Gaussian probability distribution, yielding a “shaping gain” of up to ~1.5 dB in SNR compared to square QAM. New PCS-based coherent optical networks will benefit from this SNR improvement to increase the aggregate data rate to within a fraction of a dB of the Shannon capacity of optical fiber, and enable fully flexible control of transceiver rates, thereby realizing systems with optimized and consistent operating margins in any network configuration.
In this paper, we summarize the basics of PCS for high-speed optical links and describe how PCS-enabled coherent optics may be applied to aggregated digital links in DAA systems. We also report current progress toward transferring this technology to wired copper networks leveraging data over cable service interface specification (DOCSIS) and digital subscriber line (DSL) technologies. In the context of fiber deeper MSO networks, we explore the potential impact that PCS can have on the optical transport and access networks, including applications to future flexible-rate passive optical network (PON) systems for fiber-deeper architectures.