1AirSync: Enabling Distributed Multiuser MIMO with Full Spatial Multiplexing

Abstract—The enormous success of advanced wireless devices is pushing the demand for higher wireless data rates. Denser spectrum reuse through the deployment of more Access Points (APs) per square mile has the potential to successfully meet the increasing demand for more bandwidth. In principle, distributed multiuser MIMO (MU-MIMO) provides the best approach to infrastructure density increase, since several access points are connected to a central server and operate as a large distributed multi-antenna access point. This ensures that all transmitted signal power serves the purpose of data transmission, rather than creating interference. In practice, however, a number of imple-mentation difficulties must be addressed, the most significant of which is aligning the phases of all jointly coordinated APs. In this paper we propose AirSync, a novel scheme which provides timing and phase synchronization accurate enough to enable distributed MU-MIMO. AirSync detects the slot boundary such that all APs are time-synchronous within a cyclic prefix (CP) of the OFDM modulation, and predicts the instantaneous carrier phase correction along the transmit slot such that all transmitters maintain their coherence, which is necessary for multiuser beam-forming. We have implemented AirSync as a digital circuit in the FPGA of the WARP radio platform. Our experimental testbed, comprised of four APs and four clients, shows that AirSync is able to achieve timing synchronization within the OFDM CP and carrier phase coherence (after the correction) within a few degrees. For the purpose of demonstration, we have implemented two MU-MIMO precoding schemes, Zero-Forcing Beamforming (ZFBF) and Tomlinson-Harashima Precoding (THP). In both cases our system approaches the theoretical optimal multiplexing gains. We also discuss aspects related to the MAC and multiuser scheduling design, in relation to the distributed MU-MIMO architecture. To the best of our knowledge, AirSync offers the first ever realization of the full distributed MU-MIMO multiplexing gain, namely the ability to increase the number of active wireless clients per time-frequency slot linearly with the number of jointly coordinated APs, without reducing the per client rate