Full-duplex without strings: enabling full-duplex with half-duplex clients

Enabling wireless full-duplex (from an AP) with multiple half-duplex (HD) clients is key to widespread adoption of full-duplex (FD) in commercial networks. However, enabling FD in such networks is fundamentally challenged by a new form of uplink-downlink interference (UDI), arising between HD clients operating simultaneously in the uplink and downlink directions. In this context, we first show that spatial interference alignment (IA) between clients is an effective and scalable technique to address UDI and hence enable FD in these networks, especially in the presence of MIMO. We then present our solution and system FDoS: Full-Duplex without Stringsthat incorporates this notion. We build the theory of applying spatial IA to full-duplex networks in general and present elegant, implementation-friendly constructions for generating IA solutions, by leveraging the structure of interference specific to these networks. In the process, FDoS shows that only four HD clients are sufficient to eliminate UDI through IA and enable 2N streams at an N transceiver AP. FDoS also includes an efficient MAC design at the AP to handle clients with heterogeneous antenna capabilities, maximize the throughput of the enabled streams in the FD session as well as reduce the overhead incurred in FDoS by half by facilitating a distributed implementation. A prototype of FDoS on WARP radios showcases its ability to address UDI effectively, and hence enable 2N streams (for N=2,4) in varied settings with just four HD clients, and sustain rate gains of 1.75-2x over HD MU-MIMO systems.