SDM provides up to 7 times the digital capacity for the
same comparable silicon cost of today's 64/256QAM

SDM will achieve data signaling rates up to 12 bits per second per Hz in a normal, well-maintained downstream cable plant. Data capacity examples presented later in this article will show digital capacity rates for 5SDM (10 bits per second per Hz) and 6SDM (12 bits per second per Hz), which is equivalent to 4096QAM.

All capacities presented in this article are based on the Information Rate of the channel for the particular modulation. Statistical multiplexing gains are not considered as these techniques can be applied above any modulation.

To compare SDM to QAM, the following two services are considered: SD and HD television. In one case, streaming SD and HD are considered; in another case, bulk “DVD movie to disk” is considered. For streaming, the network load in megabits per second is considered. For movie download, the average DVD movie-per-hour download rate is considered.

Cable operators have the option at tuning the quality of the SD TV image by running the MPEG2 encoding at various rates from 1.5 to 6.0 Mbps. The value of 4.5 was chosen as an example rate for these capacity comparisons.

The following table compares QAM vs SDM for various channel sizes (6MHz, 8MHz, and 12MHz) and modulation rates of 64QAM, 256QAM, and 1024QAM. The sizes were chosen to illustrate capacities for both U.S. and European channel standards bandwidths, as well as the 12MHz 1024QAM bandwidth that was presented by Broadcom at the NCTA 2003 conference.

The following table illustrates the capacities obtainable using SDM for channel bandwidths from 6MHz to 18MHz. The modulation rates shown are 5SDM (equivalent to 10 bits/sec/Hz) and 6SDM (equivalent to 12bits/sec/Hz). The capacities shown assume all sub-bands are running at the same modulation rate.