In this paper we present the design of CHRoME, a downlink multi-user beamforming (MUBF) protocol that addresses the inherent sensitivity of multi-stream systems to mobility, inter-stream interference, and imperfect channel state information. Our contributions are: (i) a technique for accurately selecting the downlink bit rate in the presence of inter-stream interference via a custom multi-user probe and feedback signal, immediately preceding data transmission, and (ii) a fast retransmission scheme that exploits liberated antenna resources to increase the expected per-user signal-to-interference-plus-noise ratio (SINR) and retransmit without having to re-sound the channel. We implement each mechanism and evaluate via a combination of indoor over-the-air experiments and trace-driven emulation. We demonstrate that CHRoME increases the resilience of MUBF systems to inter-stream interference and achieves multi-fold throughput gains compared to IEEE 802.11ac.

Downlink Multi-User MIMO (MU-MIMO) enables the simultaneous spatial sharing of the channel by multiple users to achieve a capacity gain over Single-Input Single-Output (SISO) systems. Unfortunately, the overhead required to enable multi-user MIMO is much higher than the overhead required for single-stream systems. Namely, for K users, collection of channel state information requires K transmission exchanges (i.e., O(K)) between the AP and users. Likewise, the MU-MIMO acknowledgement process also requires the same amount of exchanges, thus reducing the performance gains attained via simultaneous downlink transmission. In this paper, we design, implement, and experimentally evaluate Concurrent Uplink Control Messages (CUiC) to scale the MU-MIMO control information exchange process and improve the efficiency of 802.11ac-based MU-MIMO networks. Our key technique is the design of new channel sounding and acknowledgement mechanisms that enable multiple users to transmit their reverse-direction control messages (i.e., beamforming reports and acknowledgments) concurrently to the AP, in O(1) transmission slots. We implement CUiC and perform an extensive set of experiments and demonstrate throughput gains of more than 100% compared to 802.11ac.

In an 802.11ac-based MU-MIMO network comprised of multiple cells1, inter-cell interference allows only a single AP to serve its clients at the same time, significantly limiting the network capacity. In this work, we overcome this limitation by letting the APs and clients in interfering cells coordinately cancel the inter-cell interference using their antennas for beamforming. To achieve such coordinated interference cancellation in a practical way, we propose a novel two-step optimization. First, without requiring any channel knowledge, each AP and client optimizes the use of its antennas for either data communication or inter-cell interference cancellation, in order to maximize the total number of deliverable streams in the MU-MIMO network. Second, with only partial channel knowledge, each AP and client optimizes their beamforming weights after the optimal antenna usage has been identified in the first step. Our solution, CoaCa, integrates this two-step optimization into 802.11ac with small modifications and negligible overhead, allowing each AP and client to locally perform the two-step optimization. Our experimental evaluation indicates that for a MU-MIMO network with two cells, by cancelling the inter-cell interference CoaCa can convert the majority of the expected number of streams increase (50%-67%) into network capacity improvement (41%-52%).

In this paper, we present the design, implementation, and evaluation of the novel downlink Multi-User MIMO sounding protocol called MUTE. Our protocol decouples the sounding set selection used to collect Channel State Information (CSI), from the transmission set selection in order to minimize or even eliminate the overhead associated with sounding, while maximizing user selection performance. To this end, MUTE exploits channel statistics to all the different users to predict whether a particular user's channel will remain sufficiently stable, thereby allowing the access point to preclude channel sounding before a MU-MIMO transmission. We show that in indoor WLANs, MUTE can reduce sounding overhead by close to 73% under certain conditions while minimizing rate performance losses due to inaccurate channel estimation.

Virtual Multiple-Input Single-Output (vMISO) systems distribute multi-antenna diversity capabilities between a sending and a cooperating node. vMISO has the potential to vastly improve wireless link reliability and bit error rates by exploiting spatial diversity. In this paper, we present the first design and experimental evaluation of vMISO triggers (when to invoke vMISO rather than traditional transmission) in Wi-Fi networking environments. We consider the joint effect of gains obtained at the physical layer with MAC and network-scale factors and show that 802.11 MAC mechanisms represent a major bottleneck to realizing gains that can be attained by a vMISO PHY. In contrast, we show how vMISO alters node interconnectivity and coordination and therefore can vastly transform the network throughput distribution in beneficial ways that are not described merely by vMISO link gains. Moreover, we show how to avoid triggering vMISO when the increased spatial footprint of the new cooperator would excessively hinder other flows' performance. In this paper, we build the first multi-flow vMISO testbed and explore the trigger criteria that are essential to attain substantial gains in a fully integrated vMISO system. We find that the largest gains are achieved by a largely isolated flow (gains of 110%) whereas cooperator interference and contention effects are pronounced in larger topologies, limiting typical gains to 14%.