Abstract
Multi-User Multiple Input Multiple Output (MU-MIMO) enables a multi-antenna access point (AP) to serve multiple users simultaneously, and has been adopted as the IEEE 802.11ac standard. While several PHY-MAC designs have recently been proposed to improve the throughput performance of a MU-MIMO WLAN, they, however, usually assume that all the concurrent streams are of roughly equal length. In reality, users usually have frames with heterogeneous lengths even after aggregation, leading to different lengths of transmission time. Hence, the concurrent transmission opportunities might not always be fully utilized when some streams finish earlier than the others in a transmission opportunity (TXOP). To resolve this inefficiency, this thesis presents acPad, a PHY-MAC design that adds additional frames to fill up the idle channel time and better utilize the spatial multiplexing gain. acPad includes three schemes to better utilize the channel time and further increase the throughput of padding users. The first scheme chooses padding users with higher SINR based on the original precoder. The second scheme re-calculates the precoder to further improve the bit-rate of padding users. The third one combines the two schemes to simultaneously achieve high utilization and bit-rate. Our acPad identifies proper users as the padding so as to improve the padding gain, while preventing this padding from harming all the ongoing streams. Our evaluation via large-scale trace-driven simulations demonstrates that acPad improves the throughput by up to 283%, or by 136% on average, as compared to the conventional 802.11ac.