By Ron Hranac, SCTE Network Operations Subcommittee; Roger Fish, Broadcom; Tom Kolze, Broadcom; Satish Mudugere, MaxLinear; Alexander Podarevsky, Promptlink Communications; Jason Rupe, CableLabs; Foad Towfiq, Promptlink Communications; Sheldon Webster, CableLabs; Larry Wolcott, Comcast; Lei Zhou, Charter
Cable operators are extending the operating frequency range of their cable networks, including the upstream. Expanding the upstream operating bandwidth and adding more channels brings with it a variety of challenges, including an impact on cable modem (CM) upstream transmit power capability and headroom. In particular, the modem’s available transmit power spectral density (PSD) is reduced because it must be spread over a wider radio frequency (RF) bandwidth. Cable operators face new challenges for managing upstream power in the cable network, because of a complicated system of transmitted RF power, dynamic range window (DRW), long loop automatic level control (ALC), pre-equalization settings, and channel bandwidth. If a CM’s transmitted power is insufficient, then forward error correction (FEC) errors can result, and bit loading may need to be reduced. As well, modems can go into partial service, resulting in those modems being unable to achieve advertised upstream speeds. This paper and its companion presentation discuss the latest understanding of the dynamic system that is the CM’s upstream RF transmission operation and how it can impact service quality. Also included is guidance to create a spreadsheet for modeling available cable modem transmit headroom.