XXXV SIMPÓSIO BRASILEIRO DE TELECOMUNICAÇÕES E PROCESSSAMENTO DE SINAIS– SBrT2017, 3-6 DE SETEMBRO DE 2017, SÃO PEDRO, SP IEEE 802.11ax: On Time Synchronization in Asynchronous OFDM Uplink Multi-User MIMO Physical Layer Roger Pierre Fabris Hoefel Abstract—The High Efficiency WLANs (HEW) Task Group (TG) has been developing since 2014 the IEEE 802.11ax amendment, the 6 th generation of wireless local area networks (WLANs). One major novel feature of IEEE 802.11ax standard is the specification of uplink multi-user multiple-input multiple-output (UL MU- MIMO) techniques to cope with the increase of UL data/video traffic from small-form devices in ultra-dense networks. In this paper, we first show that the UL MU-MIMO physical layer performance can be dramatically impaired due to the asynchronous reception over the uplink channel. Next, we propose and evaluate the performance of a time advance mechanism designed for IEEE 802.11ax WLANs that mitigates adequately the performance loss due to asynchronous UL MU-MIMO channels. Keywords—802.11ax; Uplink Multi-User MIMO; Synchronization. I. INTRODUCTION A first draft elaborated by the IEEE Task Group (TG) 802.11ax was proposed in March 2016 [1]. The TG 802.11ax has included multi-user (MU) transmissions in the 11ax Specification Framework Document (SFD) [2]. The uplink multi-user multiple-input multiple-output (UL MU-MIMO) scheme is an essential 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) [3]. A simulation study on the susceptibility of orthogonal frequency division multiplexing (OFDM) UL MU-MIMO PHY performance with relation to (w.r.t.) differences in power, timing and carrier frequency offset (CFO) between the uplink clients was presented in [4] during the standardization of the IEEE 802.11ac amendment. Although, the 2013 IEEE 802.11ac spec has only defined a downlink (DL) MU-MIMO scheme. We also have been analyzing the effects of hardware impairments on the performance of UL MU-MIMO 802.11ax WLANs. In [5] we show that a frequency domain CFO estimation scheme has a superior performance in relation to the time domain CFO estimation algorithm in the context of 802.11ax UL MU-MIMO PHY. The reference [5] aims at complementing the references [6] (where a three-stage CFO mitigation scheme was proposed) and [7] (where it was shown that the performance of the clients without CFO is not impaired due to the residual CFO that impinges the other clients). In [8] we show that, besides on-chip calibration, additional counterbalance algorithms should be implemented to reduce substantially the non-linear effects of in-phase and quadrature (IQ) imbalance on the IEEE 802.11ax UL MU-MIMO PHY when realistic channel estimation schemes are considered. ____________________________ Roger Pierre Fabris Hoefel, Department of Electrical Engineering, Federal University of Rio Grande do Sul (UFRGS), Porto Alegre, Rio Grande do Sul (RS), Brazil; E-mail: roger.hoefel@ufrgs.br. The 802.11ax Project Authorization Request (PAR) aims at four times network throughput gain in relation to the 802.11ac spec. Simulation results presented by the industry in the 802.11ax TG meetings have indicated that four times throughput gain can be obtained by the UL MU-MIMO PHY [9]. Although, the effects of non- ideal aspects (e.g., CFO, time synchronization) on the system throughput were not investigated in [9]. Qualitatively remarks on the effects of time and frequency synchronization on UL MU-MIMO PHY performance were presented in [10] based on the simulation results shown in [4] during the specification of the IEEE 802.11ac amendment. Fundamentally, it was proposed in [10] that the TG 802.11ax align their time/frequency synchronization requirements with those ones specified by 3GPPP for Long Term Evolution (LTE) cellular systems. This paper is organized as follows: Section II succinctly describes an IEEE 802.11ax PHY simulator. To the best of our knowledge, this is the first peer-review paper that investigates thoroughly the performance of the asynchronous OFDM UL MU-MIMO 802.11ax PHY over different scenarios, as we have carried out in Section III. Section IV proposes and evaluates a simple protocol designed to mitigate the effects of asynchronous UL MU-MIMO channel on the performance of IEEE 802.11ax WLANs. Finally, we present our conclusions in Section V. II. IEEE 802.11AX SIMULATOR We shall evaluate in this paper the system performance of an asynchronous UL MU-MIMO PHY using an IEEE 802.11ax simulator that we have been developing, as summarized in Tab. I [5]. Results that allow a first order validation of our simulator can be found in [5,8]. A description of the MIMO Least Squares (LS) channel estimation scheme can be found in [11, pp. 98]. Table I– Parameters of IEEE 802.11ax simulator. Acronyms: GI (Guard Interval); MCS (Modulation and Code Scheme). Parameter Value Parameter Value Carrier Frequency 5.25 GHz MCS 0-9 Bandwidth 20 MHz, 40 MHz, 80 MHz Number of Spatial Streams 1 to 8 GI Length 800 ns Synchronization Auto-Correlation Modulation BPSK, QPSK, 16-QAM, 64QAM, 256-QAM MIMO Channel Estimation Least Squares (LS) Binary Convolutional Code (BCC) Code rate: r=1/2, r=2/3, r=3/4, r=5/6 Channel Decoder Hard and Soft- Decision Viterbi Decoding In this paper, we investigate the performance of UL MU- MIMO 802.11ax PHY for the modulation and code schemes (MCS) shown in Tab. II. Observe that six clients, transmitting simultaneously one SS per client, produces a total PHY rate of 1.755 Gbps when the MCS7 is used in a system with a bandwidth (BW) of 80 MHz. In the present research, we implement an interference cancellation minimum mean squared error (IC-MMSE) MU-MIMO detector [5,8]. 527