Best Paper Award

A Novel Low-Overhead Channel Sounding Protocol for Downlink@Multi-User MIMO in IEEE 802.11ax WLAN

Toshihisa NABETANI, Narendar MADHAVAN, Hiroki MORI, Tsuguhide AOKI


  Recently, there has been an enormous increase in the demand of wireless LANs (WLANs) in a variety of fields. The latest WLAN standard IEEE 802.11ac has adopted the Multi-User MIMO (MU-MIMO) technique which enables a plurality of users to communicate simultaneously in the spatial domain to improve system throughput. However, Access Points (APs) need to obtain the channel matrices between the AP and each user for pre-coding before MU-MIMO transmissions. Hence, the sounding mechanism for obtaining channel state information is specified in the 802.11ac standard. CB (Compressed Beamforming) frames that include the channel state information are fed back sequentially from each user in the conventional sounding method of 802.11ac. Therefore, the time required for sounding increases with the number of sounding users. This leads to an increase in overhead associated with the sounding protocol in 802.11ac.
  This paper proposes a new, highly-efficient sounding method for WLAN systems focusing on overhead reduction. In the proposed method, the CB frames from users are simultaneously fed back using an up-link (UL) multi-user multiplexing scheme solicited by a control frame, that indicates the necessary information to each user for orthogonal multiplexing of the CB frames and transmission timing alignment. Thereby, the proposed method can be realized at a relatively constant amount of time required for CB frame feedback regardless of the number of sounding users. Therefore, the advantages of the proposed method are expected to increase with the number of sounding users as compared with the 802.11ac conventional sounding method. This paper clarifies the effectiveness of overhead reduction in the proposed method using computer simulations. Furthermore, this paper evaluates the effect of sounding intervals on system throughput performance and provides the optimum sounding interval which is an important factor in the actual operation of MU-MIMO.
  In summary, this paper proposes a new method for sounding which is essential for MU-MIMO and shows the proposed method is academically effective by comparing it with the conventional method adopted in the current WLAN standard. In addition, the proposed method has been adopted as a new sounding method for MU-MIMO in IEEE 802.11ax, which is the next-generation WLAN currently under the process of standardization, having a very large industrial impact. Accordingly, this paper deserves the IEICE Best Paper Award.