Impact of Access Environment in Cable’s Digital Coherent System – Coexistence and Full Duplex Coherent Optics (2018)

By Zhensheng (Steve) Jia, Ph.D., L. Alberto Campos, Ph.D., Mu Xu, Ph.D., Haipeng Zhang, Ph.D., Jing Wang, Ph.D. & Curtis Knittle, Ph.D., CableLabs

Cable access networks have been undergoing significant technology and architecture changes driven by the ever-increasing residential data service growth rate and an increasing number of services types being supported, such as business services and cellular connectivity. Digital fiber technologies and distributed access architecture for fiber deep strategies offer an infrastructure foundation for cable operators to deliver the best service quality to the end users in the years ahead. The combination of the natural evolution of coherent optics technology, along with this increasing demand for capacity and the unique features of a cable-specific fiber access environment with only a few fibers available for a 500-household passed serving area, prompted the evaluation of coherent optics as an alternative for a long-term fiber access connectivity solution in next-generation cable access networks.

During its 2017 Winter Conference, CableLabs® announced the launch of the point-to-point (P2P) Coherent Optics specification project. The project looks into the evolution of cable’s optical access network, addresses its fiber shortage challenge, and re-designs digital coherent system from long-haul and metro solutions to the access network applications. This specification allows operators to best leverage the existing fiber infrastructure to withstand the exponential growth in capacity and services for residential and business subscribers while keeping cost down as much as possible.

When cable operators look to deploy coherent optics into their access networks, they are typically faced with two options: deploy coherent optics on the existing 10G system or build a new coherent-only connection. The ideal network for deploying such coherent systems would be a green field deployment on fibers without any compensation devices such as dispersion compensation modules (DCM) and other wavelength channels. However, in practice, to make the upgrade cost-effective, only one or a few channels may be upgraded in many brown field installations, depending on capacity demand. That means many of these networks that are deployed already with WDM analog DOCSIS technology and/or 10G on-off keying (OOK) services will coexist with a coherent system to support a hybrid scenario over the same fiber transmission. Such a hybrid configuration needs to be studied, especially the cross-phase modulation (XPM) impairment in the fiber nonlinear regime, to provide this option for operators to effectively support 100G on their existing networks. In this work, we fill the gap by presenting extensive experimental verifications under various coexistence scenarios and provide operational and deployment guidance for such use cases.

Additionally, according to a recent operators’ survey, 20 percent of existing cable access networks use a single-fiber topology. This means that downstream and upstream transmission to nodes takes place on a single strand of fiber. This number is expected to grow further in the near future. Therefore, bidirectional transmission is needed for coherent signals to support single-fiber topologies and to facilitate the business use and redundancy of optical links. CableLabs’ Full Duplex Coherent Optics (FDCO) proposal and the experiments that demonstrate simultaneous bi-directional transmission over single fiber and single wavelength are described. This paper shows how FDCO effectively doubles fiber capacity in a coherent optics-based fiber distribution network. The major impairment in the FDCO system is optical return loss (ORL) or optical reflections including all discrete reflections (Fresnel) and continuous reflections (backscatter). In this paper, the impact of ORL for FDCO is also analyzed and quantified for various configurations.

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