802.11mc: Using Packet Collision as an Opportunity in Heterogeneous MIMO-Based Wi-Fi Networks

Multiple-input-multiple-output (MIMO) technology boosts 802.11 Wi-Fi system capacities by using concurrent transmission of multiple streams from multiple antennas. The MIMO system in 802.11 Wi-Fi, however, typically requires request-to-send/clear-to-send (RTS/CTS)-like control message exchanges to fully realize the advantages of MIMO, although they incur nontrivial overhead. Furthermore, uncontrolled packet collisions severely limit the concurrent transmission gain of the MIMO nodes and the throughput of legacy single-input-single-output (SISO) nodes. In this paper, we propose a new distributed medium access control (MAC) protocol called 802.11 MIMO-based collision resolution (802.11mc). The 802.11mc protocol not only resolves the packet collisions but actually extracts channel information from collided frames as well to use it for concurrent MIMO transmissions. In particular, 802.11mc attaches a postamble after an RTS frame such that the channel information can be obtained, even when RTS frames collide. This information is used for interference alignment (IA) and cancelation for the interpretations of simultaneous frames. To show the feasibility of our proposal, we prototyped the scheme on the Universal Software Radio Peripheral (USRP) N210 testbed. Through both USRP experiments and NS-2-based simulations, we prove that 802.11mc improves the throughput gain of both MIMO and SISO nodes significantly.