Downlink MIMO in IEEE 802.11ac-based Infrastructure Networks

The new IEEE 802.11ac standard offers improvements like the use of large bandwidth and advanced Multiple-Input and Multiple-Output (MIMO) signal processing. However, the use of wide channels in a dense 802.11 network increases co-channel interference and contention. Fortunately, MIMO signal processing enables interference management where Access Points (AP) on the same channel can cancel their interference at each other's Stations (STA), while beamforming their signal to their own STA. We focus on the Downlink (DL) of 802.11ac-based infrastructure networks with unequal network load at the APs and show that a combination of the two physical layer MIMO techniques, namely DL multi-user MIMO and interference nulling, can significantly improve the DL performance. The proposed algorithm performs in two steps. First, it identifies in each cell the spatial compatible STA groups which can be served by MU-MIMO in the DL. Second, the unused degree of freedom of lightly loaded APs is utilized to perform interference management by means of null steering towards STAs in highly loaded adjacent cells. The algorithm takes the overhead due to 802.11ac compliant channel sounding into account when computing the set of STAs to be nulled. Our simulation results from an indoor hotspot environment conclude that the proposed scheme outperforms state-of-the-art protocols especially in high density AP deployments. Moreover, the proposed method is fully compatible with commodity 802.11ac network cards that implement MU-MIMO and requires only slight modifications to the 802.11 MAC layer.