By Doug Jones, CableLabs
The use of DOCSIS® 3.1 technology on a Radio Frequency over Glass (RFoG) network (SCTE 174) [1],requires special attention because the DOCSIS 3.1 upstream technology can increase the likelihood of optical beat interference (OBI) which can interrupt communication on the return path.
An RFoG network uses an RFoG optical network unit (R-ONU) with an upstream laser to transmit DOCSIS signals over the fiber network. OBI is caused on an RFoG network when two conditions aremet: 1) two or more R-ONUs operate at substantially the same wavelength, and 2) those R-ONUs simultaneously transmit. Preliminary testing and analysis completed at CableLabs shows that the highly efficient DOCSIS 3.1 upstream can increase the occurrence of OBI because of how many cable modems (CMs) (e.g., R-ONUs) can simultaneously transmit.
The DOCSIS 3.1 upstream introduces a new technology called orthogonal frequency division multiple access (OFDMA) which supports multiple CMs transmitting at once in order to make more efficient use of upstream spectrum. CableLabs has observed in laboratory testing that both usability and efficiency of the DOCSIS 3.1 upstream is directly impacted by OBI. Further, observations have confirmed that the DOCSIS 3.1 downstream configuration requires no modification because of how RFoG technology operates; there is a single laser transmitter in the downstream therefore OBI will not be generated.