By Daniel Howard, Hal Roberts, Broadcom Corporation and ADC
The use of advanced receiver processing and system adaptation in the cable modem termination system (CMTS) can easily quadruple the upstream capacity by opening up new RF spectrum and by more efficiently using existing RF spectrum. The advent of DOCSIS 2.0 provides a ‘toolkit’ of physical layer features that allow this potential. However, since the upstream spectrum in previously unused portions of the band is highly dynamic in the level and type of interference present, it is critical that the CMTS be able to dynamically sense and adapt to changing channel conditions. Such dynamic adaptation ensures that the channel remains active, even in the presence of strong interference, and (as importantly) ensures that as the channel conditions improve, the capacity is restored to higher levels. The DOCSIS 2.0 ‘toolkit’ provides many more options for the CMTS to handle ingress while maintaining high bandwidth, but without well designed adaptive algorithms the toolkit will be unused or worse, ill-used. In this paper, an intelligent CMTS with advanced receiver processing and advanced system algorithms for dynamic adaptation is shown to provide significant benefits to existing deployments of DOCSIS 1.0 and 1.1 cable modems, as well as set the stage for future improvements using DOCSIS 2.0 technology. The performance improvements will be demonstrated in the presence of all of the most common upstream plant impairments: additive white Gaussian noise (AWGN), ingress, common path distortion (CPD), and impulse/burst noise. Mitigation of these impairments will be shown to open up spectrum below 20 MHz that may previously have been considered unusable. Further, the reliability of interactive services is increased by such dynamic adaptation, improving the market appeal of applications such as voice over IP over cable. Since the CMTS cost per subscriber is far less than either the cost of the cable modem itself and/or further plant upgrades, the solution described in this paper provides the lowest cost and fastest time to market approach for quadrupling the upstream capacity of existing cable modem networks.