Digital Coherent Transmission for NextGeneration Cable Operators’ Optical Access Networks (2017)

By Zhensheng (Steve) Jia, Ph.D., L. Alberto Campos, Ph.D., Chris Stengrim, Jing Wang Ph.D. & Curtis Knittle, Ph.D., CableLabs

Cable operators’ residential offerings of Gigabit per second service are occurring on a regular basis now, and access bandwidth requirements are expected to grow to multi-Gigabit per second speeds driven by increasing 4K/8K video streaming, proliferation of cloud computing, big data, social media, Internet of Things, and mobile data delivery. Existing Hybrid Fiber Coax (HFC) networks have typically been designed with 6 to 8 fibers connecting the hub to the fiber node; however, many of these fibers have been repurposed for business services, node splits and backhaul services. In many instances, only the two primary fibers remain available for access network transport. This fiber shortage will only intensify as fiber demand for business and wireless backhaul increases and fiber deep architectures become prevalent.

Efficient use of optical fiber infrastructure and adoption of innovative technology becomes critical in the evolution towards next-generation cable access networks.

The current analog or direct detection optical schemes face huge challenges because of their low receiver sensitivity and limited options for long-term upgrading, especially in the legacy fiber environment, where operators continue to take advantage of the existing infrastructure to avoid costly fiber re-trenching.

Coherent technologies have been recently considered as the most effective future-proof approach for both brown and green field optical access deployments. Thanks to the advancements in digital signal processing (DSP), digital coherent detection enables superior receiver sensitivity that allows an extended power budget and high frequency selectivity enabling dense wave division multiplexing (DWDM) without the need of narrow-band optical filters. Moreover, the multi-dimensional recovered optical signal provides additional benefits to compensate linear transmission impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). In the cable access environment, coherent optics allows operators to best leverage the existing fiber infrastructure to withstand the exponential growth in capacity and services. However, there are several engineering challenges of introducing digital coherent technologies into optical access networks. To reduce the power consumption and thereby meet the size and cost requirements for access applications, development of both low-complexity application-specific integrated circuits (ASICs) and optics is essential. In particular, co-design of a DSP ASIC and optics to trade performance against complexity, cost and power consumption is imperative.

In this paper, use cases are explored for near-term and long-term applications, including the deployment for aggregation points in distributed HFC architecture (Remote PHY or Remote MAC and PHY, abbreviated R-PHY/ R-MAC-PHY), remote Passive Optical Network (PON) systems, and eventually coherent optics to the premises. The corresponding economic model for near-term aggregation transmission system will be presented for the comparison with WDM direct detection system. This paper provides an in-depth analysis describing a typical digital coherent optical system, including basic elements of multi-dimensional modulation scheme and a digital coherent receiver structure with fundamental DSP building blocks for both optical transmitter and receiver. The current evolution of coherent optical modules is also introduced.

This paper highlights the motivation for coherent optics in access and potential approaches to re-design and re-engineer the digital coherent concept from long-haul and metro solutions to the access network, leveraging reduction in complexity and cost as well as the benefits of capacity increases and operational improvements. Proof-of-concept experimental results demonstrating multi-wavelength multi-terabit per second within an access environment and the evaluation of coexistence between legacy analog and coherent system are also shown here.

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