Protocol Design and Experimental Evaluation for Efficient Multi-User MIMO Wireless Networks
Bejarano Chavez, Oscar
Knightly, Edward W.
Doctor of Philosophy
Information theoretic results on Multi-User MIMO (MU-MIMO) have demonstrated a many-fold increase in capacity compared to Single-Input Single-Output. By leveraging multiple antennas at the Access Point (AP) and beamforming techniques, MU-MIMO enables simultaneous transmissions of multiple independent streams on the downlink. Ideally, with sufficient antennas at the AP, MU-MIMO can attain capacity gains proportional to the number of streams. However, the cost required to enable efficient and robust multi-stream transmissions is much higher than that for the single-stream case and worsens with increasing number of streams. More specifically, two key factors hinder the potential gains that can be attained via MU-MIMO: (i) To serve multiple users simultaneously, the AP needs to collect Channel State Information (CSI) from all users to be served (i.e., sounding). Sounding overhead reduces the effective data airtime utilization of the overall system. (ii) Multi- stream transmissions are highly susceptible to inter-stream interference originated due to inaccurate or outdated CSI, thereby reducing packet reception performance. I demonstrate that in practice, the costs of MU-MIMO not only decrease the gains demonstrated by theory but can completely outweigh the benefits. I identify those adverse situations and propose several techniques that alleviate the negative impact caused by sounding overhead and CSI inaccuracies. First, I design CUiC and MUTE, two protocols that address MU-MIMO sounding overhead by performing overhead compression along spatial and temporal domains, respectively. CUiC exploits the available Degrees-of-Freedom (DoF) at the AP to allow multiple users to reply with their control messages (e.g., channel estimates and acknowledgements) simultaneously, therefore reducing the time required for users to reply, to a constant. MUTE exploits epochs characterized by slowly moving channels to reduce the frequency of channel sounding. Second, I design CHRoME, a protocol that addresses interference-leakage caused by outdated and inaccurate CSI as well as out-of-cell interference. CHRoME proposes a bit rate selection strategy that re-tunes the selection according to current channel and interference conditions. Additionally, if necessary, CHRoME realizes a fast soundless retransmission that exploits liberated DoF at the AP to minimize retransmission overhead. I implement and evaluate all three schemes using a combination of WARP FPGA-based transceivers, and custom emulation platforms.