In this paper, the significance of the CSI feedback acquisition at the WLAN AP is highlighted. CSI is the channel state information obtained from the physical layer to support the throughput enhancement in the upper layer. However, this information should be optimized in order to mitigate the feedback overhead with the increasing number of users. Here, a survey of existing CSI feedback algorithms with the current 802.11 WLAN standards is done by considering factors like overhead, delay, and throughput performance. We also discussed the user selection and scheduling in MU-MIMO-based WLANs. The scheduling is usually performed with the help of feedback CSI from selected users. However, acquiring CSI from all user STAs results in large overhead and grows linearly with the channel sounding interval and with the increased number of users. The current literature is more focused on reducing CSI overhead. In order to fully realize the benefit of MU-MIMO and guarantee the required QoS, it is important to acquire updated CSI from all the users. Hence, there exists a trade-off between efficiency of scheduler and CSI overhead. Generally, the AP limits the number of users based on feedback CSI. The best user CSI and suitable channels are needed to be obtained before the user is scheduled.
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