Operating Legacy Cable Modems in an FDX Environment (2019)

By Shaul Shulman, Intel Corporation

The Full Duplex (FDX) capability of the DOCSIS® 4.0 specification allows significantly increasing the upstream capacity of the HFC network, without sacrificing the downstream capacity and without extending the upper band edge of the downstream band. This is achieved by means of echo cancelation on the remote PHY node, enabling it to transmit downstream signals and receive upstream signals simultaneously in the band that is assigned to downstream in a Legacy Frequency Division Duplex (FDD) partitioning scheme. The Legacy Cable Modems (CMs) that are connected to the same network will experience upstream transmissions from FDX capable modems in the frequency region intended only for downstream at the time when these CM were designed and manufactured.

This can potentially cause service disruption if not handled properly, thus raising several questions.

Can Legacy CMs function properly at all in an FDX environment? Should all CMs be replaced when even a single FDX capable modem is connected to the node? What are the methods to mitigate the impact? Are there methods to guarantee robust operation of Legacy CMs, perhaps even at the expense of downgrading their throughput capabilities or other attributes? What can be done with and without firmware upgrade to Legacy CM? All these questions must be addressed and understood before starting FDX CM deployment. In this paper we suggest a framework to address these questions. We first describe the different use cases of Legacy CMs in an FDX plant. Then, we address the interference and its impact on the CM’s receiver for different receiver architectures. Lastly, we suggest several methods that can be employed to mitigate, monitor, and control the impact on Legacy CMs.

The analysis proposed in this paper allows understanding the considerations behind Legacy CM capabilities with respect to dealing with FDX interference, with the purpose of helping MSOs to focus on the necessary parameters that need attention and to enable estimating the expected performance.

Eventually, a careful lab characterization and certification is required for each specific Cable Modem or set-top box model to characterize its specific behavior under the FDX interference.

Note that while FDX CMs will create an environment of signals on the plant which on some parameters may go beyond what the Legacy CMs were designed for originally (primarily the upstream interference power), other signal conditions of an FDX-ready plant (N+0 or N+1) are by design better. Due to a small number or a lack of amplifiers on the node, the network SNR is expected to be much better than what these CM were designed to. Other impairments, such as nonlinear distortion products associated with analog optical to electrical conversion are not present either. These and other differences enable the necessary wiggle room for making adjustments that enable the Legacy CM to operate in an FDX plant.

The considerations presented in the paper also apply to a more general case where not all CMs in the plant adhere to the same frequency split, thus creating a situation where some CMs may transmit upstream in the frequency range designed to be for downstream for other CMs. Such scenarios occur when there is an “ultra-high split”, for example, when 10G CMs use all or part of the FDX band for a static upstream without actually employing FDX methods of operation

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