An Analytical Comparison of MU-MIMO and Single User Transmissions in IEEE 802.11ac

IEEE 802.11ac defines Multi-User MIMO (MU-MIMO) in the downlink that allows the Access Point (AP) to transmit multiple spatial streams to different receivers, simultaneously. However, MU-MIMO requires a periodic channel calibration which incurs a significant overhead. On the other hand, a Single User (SU) transmission may be used in both downlink and uplink. It allows the sender to transmit one (i.e. SISO) or multiple streams (i.e. SU-MIMO) to the same receiver. We note that SU transmissions do not require the channel calibration. In this paper, we compare the throughput of SISO, SU-MIMO and MU-MIMO, analytically, in an ideal condition (i.e. no frame losses) to understand which technique is more efficient and when. We measure the throughput for different channel widths, data rates, A-MPDU lengths, channel calibration rates and network sizes. Our results show that SU-MIMO is typically more efficient when the receiver supports the same number of spatial streams than the AP. But if the receivers support a single stream, MU-MIMO improves the network efficiency for a limited number of stations. Besides, the MU-MIMO scalability depends on the channel calibration rate and the number of receivers.