ΔSNR-MAC: A priority-based multi-round contention scheme for MU-MIMO WLANs

The performance of uplink multiuser MIMO (MU-MIMO) transmissions heavily depends on which users to transmit together. InWLANs where each user independently determines when to transmit by random access, the performance degradation occurs when a set of users for concurrent transmissions are not chosen properly. In this paper, we address this problem and propose SNR-MAC protocol to enhance the uplink throughput in MU-MIMO WLANs. In SNR-MAC, a set of users transmitting together are determined one after another through a multi-round contentionwhere the number of rounds equals the number of antennas at the AP. In each round, given winning users that are already transmitting, each user calculates its SNR reduction amount due to the winning users. SNR-MAC gives a higher priority to users with less SNR reduction amounts. To achieve this, each round consists of multiple stages where earlier stages are reserved for users with less SNR reduction amounts. In this way, users with the strong channel orthogonality can transmit together in a fully distributed manner. We theoretically analyze the throughput of SNR-MAC and propose a parameter selectionmethod to maximize the throughput. Our evaluation results confirm that SNR-MAC improves the uplink throughput over existing schemes both in twoand three-antenna AP cases and achieves temporal fairness in mobile environments. © 2015 Elsevier B.V. All rights reserved.