By Daniel Howard, Laura Hall, Keith Brawner, and Hans Hsu, Broadcom Corporation and Nick Hamilton-Piercy, Reynold Ramroop, and Sheng Liu, Rogers Cablesystems
Several methods to increase bandwidth utilization in DOCSIS 2.0 systems are presented, and results of a field trial that demonstrated several of the methods are reported. On the downstream where the majority of packets transported are large packets, the use of wider bandwidth RF channels of 12 or more MHz is discussed, where statistical multiplexing gains improve the channel capacity above and beyond the factor by which the bandwidth is increased. Similarly, going to higher order modulation on the downstream (1024 QAM) also increases the downstream capacity, and in particular can provide MSO's the ability to get more HDTV channels per 6 MHz of downstream bandwidth or 25% more data capacity in DOCSIS downstreams. On the upstream where small data and voice packets can be the majority of packets transported, bandwidth saving techniques such as dynamic header suppression and synchronous operation reduces the packet overhead that can account for a significant fraction of the minislots required to transport the packet. For medium and large packets on the upstream, the use of higher order modulation up to 256 QAM can be used to expand the capacity, and when combined with the ingress and impulse robustness features of DOCSIS 2.0 systems, 256 QAM upstreams can be supported on today's cable plants, even in bands which previously could not support more than QPSK. These facts are born out by the results of a field trial in which both 1024 QAM on the downstream and 256 QAM on the upstream were demonstrated, the latter in the presence of ingress noise on the cable upstream. The conclusion is that many cable plants are already capable of supporting higher orders of QAM modulation, and thus capacity increases of up to 33% above that already provided by DOCSIS 2.0 are possible.