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Due to the dominance of the downlink traffic in Wireless Local Area Networks(WLANs), a large number of previous research efforts have been put to enhancethe transmission from the Access Point (AP) to stations (STAs). The downlinkMulti-User Multiple-Input Multiple-Output (MU-MIMO) technique, supported by thelatest IEEE amendment-802.11ac, is considered as one of the key enhancementsleading WLANs to the Gigabit era. However, as cloud uploading services,Peer-to-Peer (P2P) and telepresence applications get popular, the need for ahigher uplink capacity becomes inevitable.  In this paper, a unified down/up-link Medium Access Control (MAC) protocolcalled Uni-MUMAC is proposed to enhance the performance of IEEE 802.11ac WLANsby exploring the multi-user spatial multiplexing technique. Specifically, inthe downlink, we implement an IEEE 802.11ac-compliant MU-MIMO transmissionscheme to allow the AP to simultaneously send frames to a group of STAs. In theuplink, we extend the traditional one round channel access contention to tworounds, which coordinate multiple STAs to transmit frames to the APsimultaneously. 2-nd round Contention Window (CW2nd), a parameter that makesthe length of the 2-nd contention round elastic according to the trafficcondition, is introduced. Uni-MUMAC is evaluated through simulations insaturated and non-saturated conditions when both downlink and uplink trafficare present in the system. We also propose an analytic saturation model tovalidate the simulation results. By properly setting CW2nd and otherparameters, Uni-MUMAC is compared to a prominent multi-user transmission schemein the literature. The results exhibit that Uni-MUMAC not only performs well inthe downlink-dominant scenario, but it is also able to balance both thedownlink and uplink throughput in the emerging uplink bandwidth-hungryscenario.

Uni-MUMAC: A Unified Down/Up-link MU-MIMO MAC Protocol for IEEE 802.11ac WLANs