  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 enhance
the transmission from the Access Point (AP) to stations (STAs). The downlink
Multi-User Multiple-Input Multiple-Output (MU-MIMO) technique, supported by the
latest IEEE amendment-802.11ac, is considered as one of the key enhancements
leading WLANs to the Gigabit era. However, as cloud uploading services,
Peer-to-Peer (P2P) and telepresence applications get popular, the need for a
higher uplink capacity becomes inevitable.
  In this paper, a unified down/up-link Medium Access Control (MAC) protocol
called Uni-MUMAC is proposed to enhance the performance of IEEE 802.11ac WLANs
by exploring the multi-user spatial multiplexing technique. Specifically, in
the downlink, we implement an IEEE 802.11ac-compliant MU-MIMO transmission
scheme to allow the AP to simultaneously send frames to a group of STAs. In the
uplink, we extend the traditional one round channel access contention to two
rounds, which coordinate multiple STAs to transmit frames to the AP
simultaneously. 2-nd round Contention Window (CW2nd), a parameter that makes
the length of the 2-nd contention round elastic according to the traffic
condition, is introduced. Uni-MUMAC is evaluated through simulations in
saturated and non-saturated conditions when both downlink and uplink traffic
are present in the system. We also propose an analytic saturation model to
validate the simulation results. By properly setting CW2nd and other
parameters, Uni-MUMAC is compared to a prominent multi-user transmission scheme
in the literature. The results exhibit that Uni-MUMAC not only performs well in
the downlink-dominant scenario, but it is also able to balance both the
downlink and uplink throughput in the emerging uplink bandwidth-hungry
scenario.
