IEEE 802.11ac multi-user MIMO capacity and impact of antenna array geometry based on indoor measurements

Based on channel measurements conducted at 5 GHz, this paper examines the impact of transmitting antennas on the Block Diagonalization (BD) capacity gain for IEEE 802.11ac Multi-User Multiple Input Multiple Output (MU-MIMO) in Home Networks. We study in details a system with two users with two antennas each by evaluating multiple numbers as well as various geometries of transmitting antennas. The experiments reveal that Crossed Circular Array (CCA) is recommended for small sized transmitter with 8 antennas (70% of MU-MIMO capacity gain over Single User MIMO (SU-MIMO) is achieved for a 20 dB of Signal-to-Noise Ratio (SNR)). In the context of a less congested system, it has been shown that using 6 transmitting antennas arranged in Uniform Linear Array (ULA) gives a gain close to that obtained with 8 antennas. We have also shown, using measured path loss values, that the capacity gain of MU-MIMO to SU-MIMO goes beyond the double when the difference between the received power of each user is high. This is obtained in comparison with the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) as a channel access method, 130% of gain is achieved when the gap between the received powers of each user is around 40 dB.