By Werner Coomans, Ph.D., Nokia Bell Labs; Rex Coldren, Nokia/Gainspeed
To enable support for symmetric multi-gigabit services over traditional coaxial cable plants, the new “Full Duplex (FDX) DOCSIS” specification was recently introduced, now morphed into the newly established DOCSIS 4.0 specification effort. This technology increases the upstream (US) capacity by allowing US signals to overlap with downstream (DS) signals in frequency and time, i.e. real Full Duplex operation. At its conception, Full Duplex DOCSIS was expected to only be deployed on fully passive cable plants (N+0),due to the challenges related to making a real full duplex amplifier that is able to simultaneously amplify signals that overlap in frequency in both the US and DS directions. Due to the significant deployment cost related to increasing the fiber penetration depth to reach the last amplifier location (N+0), several concepts have been proposed to deploy FDX DOCSIS technology over an active cable plant, i.e., N+X with X > 0.
In this paper, we will address the trade-offs of several options that allow deploying Full Duplex DOCSIS equipment on an active cable network containing amplifiers. We first briefly introduce traditional amplification methods and Full Duplex DOCSIS, and subsequently discuss three types of amplifiers in greater detail: diplexer-based amplifiers, ALI-cancelling or zero-guard-band amplifiers, and active FDX amplifiers. While the latter aims to support “real” FDX operation across the amplifier, the first two only support FDD operation across the amplifier.