A Distributed Multi-User MIMO MAC Protocol for Wireless Local Area Networks Lin X. Cai 1 , Hangguan Shan 2 , Weihua Zhuang 1 , Xuemin (Sherman) Shen 1 , Jon W. Mark 1 , and Zongxin Wang 2 1 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada {lcai, wzhuang, xshen, jwmark}@bbcr.uwaterloo.ca 2 Communication Science and Engineering Department, Fudan University, Shanghai, 200433, P.R. China 061021021@fudan.edu.cn zxwang@fudan.ac.cn Abstract— Multi-user multiple-input multiple-output (MIMO) systems have been emerging and attracting considerable attention recently for its potential to substantially improve system capacity via space division multiple access. In this paper, we propose a distributed multi-user (MU) medium access control (MAC) protocol for wireless local area networks (WLANs) with MIMO capability, using a leakage-based precoding scheme. By exploiting the multi-user degree of freedom in a MIMO system to allow the access point (AP) to communicate with multiple users in the same frequency band simultaneously, the proposed MU MAC can effectively minimize the AP-bottleneck effect in legacy WLANs. We then develop an analytical model to study the performance of the proposed MU MAC, in terms of the maximum number of users that can be supported and the network throughput. The analysis and simulation results show that the proposed MU MAC significantly outperforms the single-user MAC. I. I NTRODUCTION IEEE 802.11 wireless local area networks (WLANs) have been widely deployed for wireless Internet access in the past decades. However, it is well known that the achievable throughput of a WLAN is less than half of the physical layer (PHY) raw data rate because of the protocol inefficiency, including packet header overheads, PHY preamble, inter-frame spaces (IFSs), and random access contentions. In addition, a majority of existing WLANs are set up in an infrastructure mode, where mobile users access the Internet through an access point (AP), and the AP usually carries a much higher downlink traffic load than other users and hence becomes the network bottleneck, which limits the achievable network throughput under the stable state [1]. To meet the demand for higher performance WLANs to support broadband multimedia services in both home and en- terprise environments, the next generation WLAN will employ promising multiple-input multiple-output (MIMO) technology in the PHY to improve the capacity and reliability of the wireless channel. Thus, it is important to study the impacts of MIMO technology on medium access control (MAC) of WLANs. Recently, MAC design for MIMO systems has been an interesting subject for many researchers. In [2], a unified MAC framework for ad hoc networks with smart antennas is proposed, assuming ideal interference cancellation in a closed-loop MIMO system. MAC design and routing issues This research was supported by a research grant from the Natural Science and Engineering Research Council (NSERC) of Canada and by a fund from China Scholarship Council (CSC). in mobile ad hoc networks are investigated in [3], focusing on exploiting spatial diversity. A MIMO-DCF protocol is presented in [4], using modified request-to-send/clear-to-send (RTS/CTS) frames to exchange antenna selection information and exploiting diversity and multiplexing gains. A distributed MIMO-aware MAC is proposed in [5]. This scheme is specifi- cally designed for a three element antenna array based MIMO system that allows two simultaneous transmissions in a single collision domain. However, due to the antenna size and high cost, it is more likely that the AP will be equipped with mul- tiple antennas while mobile users can only employ a limited number of antennas. On the other hand, it is well known that MIMO capacity increases linearly with the minimum number of employed antennas at the transmitter and receiver; some recent results indicate that similar capacity scaling also applies when an AP with multiple antennas communicates with multiple users [6]. Considering a homogeneous network where any user is equipped with N antennas, it is shown in [7] that multi-user transmission has higher link utilization than the single user case due to a large degree of multi-user diversity, using contention-free transmissions. Multi-user beamforming can provide a substantial gain in downlink throughput in centralized networks [8], where the channel conditions are usually available at the base station via a feedback channel. However, channel information is generally not available in a distributed random access network and thus it is difficult to exploit the multi-user diversity gain in the system. To the best of our knowledge, in the IEEE 802.11 MAC how to exploit multi-user degree of freedom of a MIMO system to minimize the AP-bottleneck effect remains an open issue. In addition, previous studies either focus on the design of MIMO techniques or the MAC performance analysis. There is little analytical work in the literature which addresses the performance of random access MAC in a MIMO system. The main contributions of this paper are as follows. First, we propose a distributed multi-user (MU) MIMO MAC protocol to exploit the multi-user and multi-antenna degrees of freedom in a MIMO system, using a leakage-based precoding scheme proposed in [9]. The proposed MU MAC is applicable for users with either one or multiple antenna elements. Second, we analytically study the performance of the proposed MU MAC protocol in terms of the maximum number of users that can be supported in a stable network and the corresponding network throughput, considering asymmetrical transmission rates of This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings. 978-1-4244-2324-8/08/$25.00 © 2008 IEEE.