By Richard S. Prodan, Ph.D., Broadcom Limited
DOCSIS 3.1 has significantly increased the bandwidth utilization and capacity flexibility of cable data service through the use of new physical layer modulation based on OFDM/OFDMA, as well as extending bandwidth in both upstream and downstream transmission bands. DOCSIS 3.1 and prior versions were concerned with forward and reverse path loss and minimum power levels to achieve acceptable signal-to-noise-ratio (SNR) for good spectral efficiency. Only unidirectional frequency response limits need be considered and reflected signals back toward the source can be reasonably ignored.
The addition of full duplex (FDX) simultaneous transmission and reception within the same spectrum introduces numerous additional considerations that can be neglected in prior DOCSIS frequency division duplex (FDD) versions. Such concurrent full duplex transmit/receive operation in the same spectrum introduces interference into and reflections back toward the transmission source that must be effectively canceled for simultaneous reception of signals traveling in the opposite direction within the same frequency band.
This paper analyzes the additional considerations for full duplex operation within a fiber deep (passive node plus zero amplifier) required cable system architecture and the resulting expected SNR and spectral efficiency impacts both at the node and the cable modem. These include:
- Signal levels and path loss over the fiber deep coax plant and within the node and modem
- Tap signal conditioning (equalization) over wide bandwidths (up to 1218 MHz)
- Noise and interference sources and levels both across different taps and across ports on the same tap including co-channel interference, adjacent channel interference, and adjacent channel leakage interference from transmitter noise and spurious emissions
- Transmitted signal reflection levels impacting received signals within the same or adjacent frequency bands
- Interference due to limited isolation between transmitted and received signals both at the fibernode and the cable modem
- The segmentation of cable modems into “interference groups”
- SNR and spectral efficiency (bit-loading) achievable within such cable modem groupings.