Towards 5G

16Full-duplex Radios

Dinesh Bharadia and Sachin Katti

Stanford University, Stanford, CA, USA

A long-held assumption in wireless is that radios have to operate in half-duplex mode; that is, either transmit or receive but not both simultaneously on the same channel. Recent work has attempted to invalidate this assumption. Researchers at Stanford [1, 2], Rice [3, 4], and several other groups in industry and academia [5, 6] have proposed designs to build in-band full-duplex radios. Full duplex, if possible, has tremendous implications for network design, not least of which is the fact that cellular networks could cut their spectrum needs by half. For example, LTE uses equal-width separate uplink and downlink channels to enable radios to achieve full duplex. With an in-band full-duplex system, we could use a single channel to get the same performance. Consequently, the problem has attracted significant attention, both from industry and academia and has spurred significant follow-up work.

To achieve full duplex, a radio has to completely cancel the significant self-interference that results from its own transmission on the received signal. For example, in WiFi, since signals are transmitted at 20 dBm (100 mW) average power, and the noise floor is around dBm, the transmit self-interference has to be canceled by 20 dBm dBm dB to reduce it to the same level as the noise floor and render ...