By Richard A Primerano Principal Engineer Comcast Justin Riggert Senior Principal Engineer Comcast Larry Wolcott Engineering Fellow Comcast
DOCSIS® 3.1 technology introduced orthogonal frequency division multiplexing (OFDM) waveforms in both the upstream and downstream. These offer greater spectral efficiency over legacy single-carrier quadrature amplitude modulation (SC-QAM) waveforms and provide numerous “control knobs” allowing operators to tune performance to plant conditions. Two controllable parameters of the OFDM waveform that will be discussed in this study are the cyclic prefix (CP) and the roll-off period (RP). The CP counteracts inter-symbol interference (ISI) when operating on a channel that exhibits echoes. The RP controls spectral leakage from the OFDM waveform to signals in adjacent bands. The CP in the OFDM waveform is roughly equivalent to the 24-tap T-spaced equalizer of the DOCSIS 3.0 SC-QAM waveform. The RP of the OFDM waveform is equivalent to the root raised cosine (RRCF) pulse-shaping filter in SC-QAM. In the remainder of the paper, we use OFDM to refer to both the downstream (OFDM) and upstream (OFDMA) signals, unless otherwise noted.