SCTE 258 2020

DOCSIS 3.1 Downstream OFDM Power Definition, Calculation, and Measurement Techniques

Cable operators are familiar with characterizing and measuring downstream per-channel RF power, or signal level, for 6 MHz- and 8 MHz-wide analog TV signals and single carrier quadrature amplitude modulation (SC-QAM) signals. Analog TV channel signal level in most cases refers to the visual carrier’s peak envelope power (PEP).[1] An SC-QAM channel’s signal level is its average power, also known as digital channel power or digital signal power.[2]

DOCSIS 3.1 uses orthogonal frequency division multiplexing (OFDM) in the downstream. The nature of OFDM is such that transmission is not limited to legacy 6 MHz and 8 MHz channel bandwidths. Indeed, DOCSIS 3.1 supports downstream OFDM signals with a minimum modulated spectrum of 22 MHz to a maximum encompassed spectrum of 190 MHz, which occupy at least 24 MHz and 192 MHz respectively, including a portion of the OFDM band-edge spectral skirts (taper regions).

Since the introduction of DOCSIS 3.1, there have been many questions about how to quantify downstream OFDM channel power. The DOCSIS 3.1 Physical Layer Specification [1] (“DOCSIS 3.1 PHY spec”) includes details about CMTS downstream transmit signal fidelity and power. This Operational Practice explains the channel power-related parameters included in the specification, how OFDM power is measured, and includes examples to illustrate how to calculate these parameters. The reader is urged to review Sections 7.5.9 and 7.5.10 of the DOCSIS 3.1 PHY spec for more information.

[1] Peak envelope power is the average power of one cycle during the modulation crest. The modulation crest of an analog TV signal’s modulated visual carrier occurs during synchronizing pulses.

[2] Digital channel power or digital signal power is the average power of the entire SC-QAM signal, across its occupied bandwidth (i.e., 6 MHz or 8 MHz).