Full-Duplex MIMO Radios: A Greener Networking Solution

Relative to half-duplex (HD) radios, in-band full-duplex (FD) radios have the potential to double a link's capacity. However, such gain may not necessarily extend to the network-wide throughput, which may actually degrade under FD radios due to excessive network interference. This paper identifies the unique advantages of FD radios and leverages multi-input multioutput (MIMO) communications to translate the FD spectral efficiency gain at the PHY level to the throughput and power efficiency gain at the network layer. We first derive sufficient conditions under which FD-MIMO radios can asymptotically double the throughput of the same network of HD-MIMO ones. Specifically, if a network of 2N HD radios (N links) can achieve a total throughput of dN bps (i.e., d bps per link), then an FD-capable network with the same number of links and network/channel realization can achieve 2N(d-1) bps [i.e., (d-1) bps per direction of a bidirectional link]. To leverage this theoretical gain, we exploit the “spatial signature” readily captured in the network interference to design an MAC protocol that allows multiple FD links to concurrently communicate while adapting their radiation patterns to minimize network interference. The protocol does not require any feedback nor coordination among nodes. Extensive simulations show that the proposed MAC design dramatically outperforms traditional CSMA-based and the nonorthogonal multiple access protocols with either HD or FD radios with respect to both throughput and energy efficiency. Note that in the literature, network interference is often treated as colored noise that then gets whiten during the signal detection process. However, through our MAC protocol, we emphasize that, unlike random noise, network interference has its own structure that can be “mined” for “intelligence” to better align the transceiver's signal.