Cross-Layer-Optimized User Grouping Strategy in Downlink Multiuser MIMO Systems

This paper proposes a cross-layer-optimized user selection and packet transmission technique for the downlink multi-user (MU) multiple-input multiple-output (MIMO) system in IEEE 802.11 wireless LAN (WLAN). In IEEE 802.11 WLAN, packet duration is decided by a data size and a selected transmission mode (modulation and coding scheme, MCS), and therefore, in general, the packet duration of each user varies according to the channel conditions and the applications. If multiple users whose packet durations are considerably different are selected as the target of the downlink MU-MIMO transmission, then the efficiency of spatial multiplexing degrades and the network throughput cannot be enhanced. When selecting users as the target of the downlink MU-MIMO transmission, the degree of spatial channel correlation among users (from the viewpoint of the physical layer) and the packet waiting times in a queue (from the viewpoint of the MAC layer) have to be considered. In addition to these criteria, in IEEE 802.11 WLAN, uniformity of the packet durations among users has to be considered as a criterion. In the proposed method, data size is adjusted in order to make the packet durations of the users uniform after selecting the users based on the spatial compatibility and the packet waiting times. This means that the proposed downlink MU- MIMO transmission is optimized in 3 dimensions by the cross- layer approach: the spatial channel correlations, packet waiting times and the uniformity of the packet durations. Computer simulations are performed and the results show the effectiveness of the proposed method.