FDX & D3.1 Capacity Scenarios (2018)

By Karthik Sundaresan, CableLabs

Full Duplex DOCSIS 3.1 (FDX) represents the next evolution of DOCSIS 3.1 (D3.1) technology, significantly increasing upstream capacity and enabling multi-Gbps symmetric service tiers over HFC networks.

Over the last many years, service offerings and IP capacity needs have continued to grow at a rapid pace.

As a result, operators will require cost effective means of adding capacity to their HFC networks in order to provide the services that their customers will expect, eventually reaching gigabit and multi-gigabit speeds. D3.1 was designed to be a cost-effective way of meeting these performance targets, especially for typical consumption patterns where downstream consumption is much higher than upstream consumption. FDX is an evolution of D3.1 that increases the upstream capacity to similar levels as the downstream.

FDX fundamentally changes the nature of information delivery across the cable plant, and how it will be maintained and managed. FDX significantly increases the upstream capacity by enabling upstream and downstream channels to concurrently exist over the same spectrum without the need to time share the use of the spectrum. The upstream and downstream channels each fully access the same spectrum at the same time, practically doubling the use of the spectrum. Using D3.1 as a foundation, FDX accomplishes this by using a combination of interference cancellation and intelligent scheduling at the CMTS through enhancements to the existing D3.1 technology. This allows a migration path that allows operators to cost effectively migrate to FDX, while still maintaining and leveraging their existing installed base.

New FDX procedures such as sounding and echo cancellation happen at FDX initialization as well as periodically. The FDX channel may not be available for data transmission during this for some amount of time. These processes place some overhead on the raw bandwidth available. D3.1 allows for multiple OFDM/OFDMA profiles each tuned to account for plant conditions experienced by a set of CMs, using different modulation orders on the same channel. Aggregate channel capacity varies with the CMs and the profiles in use at a time. In FDX operation interference from other CMs, and Echo Cancellation training at the CM and at the FDX Node, the MER signature at the Node & CM will be different, the question is how the full duplex operation will affect the capacity of the channel and the network.

The FDX band is divided into sub-bands and the CMTS assigns sub-band(s) for upstream or downstream operation. Now this Resource Block Assignment (RBA), where a sub-band can change direction at a given time, directly impacts the available DS and US bandwidth seen by the CM in the FDX Band.

Different CMs will have different bandwidth demand for both the upstream and downstream directions which can change over time, and FDX allows for the RBA to be changed dynamically to match. FDXCMs are grouped into Interference groups and Transmission groups each with a unique RBA.

This paper helps understanding how an operator can estimate US and DS capacity for FDX channels indifferent scenarios. The channel capacity affects how many subscribers can be assigned to use the same set of channels and affects traffic engineering and operational decisions (such as when an operator would need to split the node to increase available capacity). This paper presents a framework to understand the FDX/D3.1 downstream and upstream channel capacities across a range of MSO operational scenarios.

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