MAC enhancements for downlink multi-user MIMO transmission in next generation WLAN

For the first time in the WLAN technology, multi-user downlink transmission will be made possible by the upcoming IEEE standard, 802.11ac. In this paper, we will report its current status, focusing on the MAC modifications and enhancements that are necessary to support the unique feature of downlink multi-user MIMO transmission. In particular, the technique of enhancing Transmit Opportunity (TXOP) for downlink MU-MIMO transmission and the revised backoff procedures for secondary ACs will be introduced and discussed. The performance of the proposed mechanisms is evaluated using simulations. The proposed TXOP Sharing mechanism has been accepted in the 802.11ac draft specification and is currently under letter ballot in the IEEE 802.11 WG.