"Once we finished the DOCSIS (3.1) specs, we started thinking about what is next ... what could we do to fix the upstream which (has been) limited by spectrum," said Dan Rice, CableLabs' SVP of network technology. "We applied a nascent technology in the wireless space to cable .... We were able to build the electronics that are associated with this and try it out. It looks like it works really great."
Unlike existing technology solutions, like frequency division duplexing (FDD) or time division duplexing (TDD), with Full Duplex, the upstream and downstream utilize the same spectrum at the same time, which doubles the efficiency and capacity.
Rice and colleague Belal Hamzeh, CableLabs' VP of research & development for wireless technologies, used the analogy of a conversation between two people. With TDD, the same spectrum is used for upstream and downstream, but the speakers take turns. One person talks, the other listens. But with full duplex, two people are talking at the same time, but still understanding each other even if one is yelling and the other whispering.
Typically with a communication system, when signals come out of the system, they are high power, but as they go across the network the power level drops. By the time upstream signals reach the CMTS, the power can be much lower than the signal power coming out of the CMTS. The CMTS transmits at around 60 dBmV, but receives closer to 0 dBmV, Rice said.
"The CMTS is yelling at (its) loudest, and somebody is trying to whisper back at the CMTS. It can still hear the whisper and understand it," Hamzeh said.
Full Duplex utilizes the self-interference cancellation and intelligent scheduling of DOCSIS 3.1. Remember that the CMTS is transmitting at high power and receiving signals from modems at low power. To hear the modem, the CMTS needs to be able to subtract its own transmission from what it is receiving.
"If I am talking, I can hear what I am saying. If you are whispering back to me, I am able to subtract what I am saying. The brain is (capable) and knows what you said," Hamzeh said.
Putting this cancellation ability into the CMTS is straightforward, but putting it into every single cable modem could be challenging and expensive, Rice said. The good news is that modems very close to each other on the HFC network don't transmit and receive in the same spectrum at the same time, but one three houses over, for example, that is naturally separated can.
"Implementing this just in the CMTS is more cost-effective. That said, (cancellation) might be implemented in modems in the future for more benefits, but I think it won't have to be in the modems, only in the CMTS," Rice said.
All this said, Full Duplex is a feature update of DOCSIS 3.1, not a complete rework of the specification. It is still in the "innovation funnel," and CableLabs will continue work on proving feasibility over the next couple of months. The next step, which is likely to occur this summer, is research and development where all vendors in the community will be invited to work on the project to ready it for deployment.