KPad: Maximizing Channel Utilization for MU-MIMO Systems Using Knapsack Padding

In a Multi-User Multiple Input Multiple Output (MU-MIMO) system, an access point (AP) equipped with multiple antennas can serve multiple users simultaneously (i.e., support concurrent multi-streams) and thus achieves multi-fold through- put gain. In practice, however, the gain is significantly comprised by frame-size diversity, i.e., shorter frames need to wait for the finish of the longest frame, which leads to low channel utilization and thus throughput degradation. Frame padding (i.e., more than one short frames are grouped together to fill in the idle channel) has been proposed to solve the problem, but existing approaches are based on heuristic and cannot fully exploit the potential of padding. In this paper, we propose Knapsack Padding (KPad), a novel model-driven frame padding design to maximize MU- MIMO channel utilization. We first formally formulate the frame padding problem as a multi-stream knapsack model, and then design a stream decoupling mechanism to handle the unique and complicated inter-stream interference underlying the model, so as to derive the optimal padding schedule efficiently. We evaluate KPad using trace-driven emulation. Extensive evaluation results demonstrate remarkable throughput gain (up to 42%) compared with the state-of-the-art.