Performance Analysis of CSMA/CA Based MU-MIMO WLANs Md. Moin Uddin Chowdhury, Md. Asif Tanveer and Md. Forkan Uddin * Dept. of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology Dhaka, Bangladesh * mforkanuddin@eee.buet.ac.bd Abstract—In this paper, the performance of carrier sense multiple access with collision avoidance (CSMA/CA) based multi- user MIMO (MU-MIMO) uplink wireless local area networks (WLANs) is analysed. Analytical models are provided to compute the throughput of each user and the total energy consumption in a WLAN with a single user MIMO (SU-MIMO) technique as well as a MU-MIMO technique. By means of the analytical models, the throughput, fairness and energy consumption performances are studied for several WLAN scenarios. The numerical results show that the MU-MIMO technique provides higher through- put than the SU-MIMO technique only when the number of MIMO communication antennas is high and the medium access parameters of the users are configured properly. Further, the SU-MIMO technique is found to be more energy efficient than the MU-MIMO technique. However, the MU-MIMO technique provides better fairness over the SU-MIMO technique. Index Terms—Medium access control, wireless local area network, multi-user MIMO, interference, throughput, fairness. I. I NTRODUCTION Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple antennas at transmitter and receiver to transfer multiple data streams at the same time. In the recent years, MIMO transmission schemes have been adopted by many systems to achieve high spectral efficiency [1]-[2]. In these schemes, aggregate data rate can be increased by transmitting several independent data streams simultaneously through different antennas, which is called spatial multiplexing. If multiple data streams are originated from a single transmitter and transmitted to a single receiver then it is called single-user MIMO (SU-MIMO) technique. If a transmitter is allowed to transmit only one data stream by one of its antennas, a set of transmitters can transmit to a particular receiver simultaneously such that the receiver can receive multiple streams using its antennas. Such a technique is known as multi-user (MU-MIMO) [3], [4]. Presently, the researchers in wireless communications are showing tremendous interest to the MU-MIMO technique due to its advantages over the SU-MIMO technique. Wireless local area networks (WLANs) are widely deployed in most of the counties to provide high speed Internet access to the users. The current WLANs are based on IEEE 802.11 standard and use carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) proto- col [5]. Like the others recent standards, the working group of IEEE 802.11 is also preparing MU-MIMO based physical layer standard IEEE 802.11ac to provide data rate up to 7 Gbps [3]. Throughput, fairness and energy consumption are the most important performance metrics in any wireless network. Throughput is a measure of traffic carrying capacity while fairness measures the fair share of system resources among the users. In a WLAN, a user near the access point (AP) may prevent a user located far away from the AP from achieving sufficient throughput. Moreover, with the rapid and radical evolution of information and communication technol- ogy (ICT), the total energy consumption in wireless networks is also growing at a staggering rate. So it is necessary to study the throughput, fairness and energy consumption performances in MU-MIMO based WLANs. In [6], an analytical model is provided by G. Bianchi to calculate the total throughput in a single-input single- output (SISO) based WLAN. In [3], analytical model of total throughput is provided for a MU-MIMO WLAN and it is shown that the MU-MIMO scheme over performs the SU- MIMO scheme in term of total throughput. To the best of our knowledge, fairness and energy consumption performances of CSMA/CA based MU-MIMO WLANs are not yet studied. To study the fairness among the users in a WLAN, analytical model to compute the throughput of each user of the WLAN is necessary. In [9], Libin and Jean propose a quasi-reversible distributed CSMA model and provide an explicit throughput formula of each user in SISO based multi-hop wireless net- works. To study energy consumption in wireless networks, an analytical model is presented in [11] to predict energy consumption in saturated IEEE 802.11 single-hop ad hoc networks under ideal channel conditions. Analytical models of throughput of each user and the total energy consumption for MU-MIMO based WLANs are necessary to study fairness and energy consumption performances. In this paper, a CSMA/CA based multi-rate uplink MU- MIMO WLAN with zero forcing (ZF) [7] MIMO decoding is considered. The physical interference model based on signal to interference plus noise ratio (SINR) is assumed for the study since it is a more realistic interference model for wireless interference [8]. The contributions in this paper are as follows. • Analytical models are provided to compute the through- put of each user and the total energy consumption in a CSMA/CA based MU-MIMO WLAN. • By means of the proposed analytical models, numerical results on throughput, fairness and energy consumption are provided for several WLAN scenarios. From the numerical results, the following engineering insights are obtained: (i) the MU-MIMO technique is beneficial in term of throughput only when the number of MIMO communication antennas is high, (ii) the MU-MIMO technique provides better throughput fairness than the SU-MIMO technique, and (iii) to achieve the same total throughput, the energy consumption in the SU-MIMO technique is significantly lower than that in the MU- MIMO technique.