The downstream signal
is a continuous stream of modulation symbols, so it can be received and demodulated
with relatively low-cost hardware.
The upstream physical layer in the DOCSIS 1.0 and 1.1 specifications uses a combination
of quadrature-phase shift keying (QPSK) and 16-QAM with Reed-Solomon FEC.
Because the upstream consists of a series of transmission bursts from various CMs,
each burst begins with a well-known preamble in order to aid acquisition by the burst
demodulator at the CMTS. The upstream channel width is configurable from 200 kHz
to 3.2 MHz in power-of-two increments.
In the DOCSIS 2.0 and 3.0 specifications, the upstream physical layer is extended in
two ways. The first is by adding more choices for modulation, stronger FEC, and wider
channels. The second is by adding a second, operator-selectable, physical layer technology
based on synchronous code-division multiple access (S-CDMA) technology.
The upstream modulation choices in the DOCSIS 2.0 and 3.0 specifications are
QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM, and 128-QAM. The upstream FEC is enhanced
by allowing a greater level of Reed-Solomon error correcting capability (up to 16
correctable symbols per codeword), as well as by allowing the inclusion of interleaving
and TCM.
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