I. Introduction
The TG 802.11ax has included multi-user (MU) transmissions in the 11ax Specification Framework Document (SFD) [1]. Uplink multi-user multiple-input multiple-output (UL MU-MIMO) scheme is a fundamental joint medium access control (MAC) and physical layer (PHY) protocol to improve the network throughput in ultra-dense wireless local area networks (WLANs) in usage cases where the networks are loaded with small form devices that are only capable of transmitting 1–2 spatial streams (SS) [2]. The 802.11ax draft 1.0 [3] was not approved in the group letter ballot that was carried out from December 2016 to January 2017. Therefore, the TG 802.11ax has started to work on resolutions of the remarks submitted on draft Dl.0.