Comparison of MIMO schemes in IEEE 802.11ac under time-varying channels: analytical approach

In the recent wireless local area network (WLAN) IEEE 802.11ac standard, various multiple antenna technologies are adopted to improve the throughput without additional bandwidth. These multiple input multiple output (MIMO) techniques include open-loop and closed-loop approaches depending on the existence of channel feedback. The space-time block code (STBC) is included in the open-loop manner while a transmit beamforming (BF) methods is in the closed-loop one. The single-user BF (SU-BF) mode in which a single station (STA) is supported by an access point (AP) and multi-user BF (MU-BF) mode in which multiple STAs are simultaneously served by an AP are categorized by a closed-loop method. If a STA has no capability to support closed-loop BF scheme, i.e., it cannot feedback its CSI with a compressed BF manner defined in IEEE 802.11ac, the AP should transmit data by using a STBC scheme. In cases of SU-BF and MU-BF, channel variation over time is a critical impairment. Under a severe condition, a STBC scheme can achieve higher throughput than the BF schemes even without channel feedback. As another example, the open-loop method is preferable to the close-loop one since the feedback can be an overhead when the amount of transmit data is not large. In this paper, we derive the rate loss due to channel variation and calculate the throughput by taking the frame structures and feedback overhead into consideration. Based on these analysis, we can give a guideline for system design of IEEE 802.11ac.