A CSMA/CA MAC Protocol for Multi-User MIMO Wireless Lans

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings. A CSMA/CA MAC Protocol for Multi-User MIMO Wireless LANs Michelle X. Gong, Eldad Perahia, Robert Stacey, Roy Want Shiwen Mao Intel Corporation Dept. ECE, Auburn University Santa Clara, CA 95054-1549 Auburn, AL 36849-5201 Email: {michelle.x.gong, eldad.perahia, robert.j.stacey, roy.want}@intel.com Email: [email protected] Abstract—Multiple-input multiple-output (MIMO) is one form First, we propose a CSMA/CA based medium access protocol of the smart antenna technology that uses multiple antennas at with multiple response options for DL MU MIMO WLANs. both the transmitter and receiver to improve communication A dynamic MAC protection scheme is proposed to reduce the performance. In this paper, we investigate the problem of medium access control in wireless local area networks (WLANs) overhead of MAC protection. Secondly, we propose a novel with downlink multi-user MIMO (DL MU MIMO) capability. per-STA weighted queuing mechanism to mitigate the hidden We propose a CSMA/CA MAC protocol with three response node problem in the network. We derive the optimal satura- mechanisms for DL MU MIMO and compare the performance tion throughput with respect to the number of simultaneous of DL MU MIMO with the beam-forming (BF) based approach. contending devices. The proposed MAC protocol can fully A novel per-station weighted queuing mechanism is proposed to mitigate the hidden node problem in the network. Performance exploit spatial multiplexing gain and maximally reduce over- analysis and simulation study both show that the proposed head associate with the MAC protection mechanism and the DL MU MIMO mechanism incurs low overhead and provides response mechanisms. It can achieve better performance than significant throughput performance gain over BF based approach the 802.11n transmit beamforming (TxBF) mechanisms [1], in high SNR scenarios. as demonstrated in our simulation studies. The remainder of this paper is organized as follows. We I. INTRODUCTION discuss related work in Section II and introduce the system Multiple-input multiple-output (MIMO) is one form of model in Section III. The proposed DL MU MIMO MAC the smart antenna technology that uses multiple antennas at protocol is described in Section IV. We present an analysis both the transmitter and receiver to improve communication in Section V and our simulation evaluation in Section VI. performance. MIMO communications have been extensively Section VII concludes this paper. studied for next generation cellular networks and have been adopted for wireless local area networks (WLANs) as specified II. RELATED WORK in the IEEE 802.11n standard [1]. A MIMO system takes advantage of two types of gains, There have been several prior works that studied the benefit namely, spatial diversity gain and spatial multiplexing gain [2]. of DL MU MIMO techniques in WLANs [4]–[6]. Applying Spatial diversity can combat severe fading and improve the an Earliest Deadline First (EDF) scheduling algorithm, Choi, reliability of the wireless link by duplicating information Lee, and Bahk [4] demonstrated the performance benefit of across multiple antennas. Spatial multiplexing takes advantage DL MU MIMO over the single-user mechanism. This work of the multiple physical paths between the transmit and receive focused on the performance analysis of DL MU MIMO, but antennas to carry multiple data streams. It has been shown that did not consider MAC protocol design and MAC overhead in in a MIMO system with N transmit and M receive antennas, its analysis and simulations. the channel capacity grows linearly with min{N,M} [3]. A MIMO distributed coordination function (DCF) protocol Recent results show that similar capacity scaling applies was presented in [7], using modified request-to-send/clear- when an N-antenna access point (AP) communicates with M to-send (RTS/CTS) frames to exchange antenna selection users simultaneously [2]. A multi-user (MU) MIMO system information and exploiting diversity and multiplexing gains. A has the potential to combine the high capacity achievable modified acknowledgement (ACK) frame was also introduced with MIMO processing with the benefits of multi-user space- to indicate whether a packet is received successfully on per division multiple access. Such technology is being considered spatial stream basis. In contrast, our proposed protocol does for the next generation of 802.11 (802.11ac). not modify RTS/CTS/ACK frames. A distributed MIMO- Particularly, we’re interested in downlink (DL) MU MIMO aware MAC was proposed in [8], assuming a three element systems, where an AP can transmit to multiple users simulta- antenna array based MIMO system that allows two simultaneously. In this paper, we investigate the problem of medium neous transmissions in a single collision domain. As will be access control in WLANs with DL MU MIMO capability. The discussed in Section IV, our proposed solution can work for main contributions of this paper are summarized as follows: any antenna configuration. 978-1-4244-5638-3/10/$26.00 ©2010 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings. In [5], the authors proposed a distributed DL MU MIMO used instead. To describe this approach, we first present the MAC protocol that is based on the IEEE 802.11 MAC and entire system model including all STAs as follows. provided an analysis of the proposed MU MAC protocol ⎡ ⎤ ⎡ ⎤ ⎡ ⎤T ⎡ ⎤ ⎡ ⎤ Y1 H1 W1 X1 Z1 in terms of the maximum number of supported users and ⎢ ⎥ ρ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎣ . ⎦ = ⎣ . ⎦ ⎣ . ⎦ ⎣ . ⎦ + ⎣ . ⎦ . network throughput. The MAC protocol proposed in [5] is . M . similar to one of our proposed protocols, i.e. the scheduled YM HM WM XM XM response mechanism (see Section IV). However, the protocol That is, we have in a tighter form proposed in [5] requires multi-user RTS (MU-RTS) and MU CTS exchange before every DL MU MIMO transmission, ρ Y = HWX + Z. which incurs large overhead. We propose and evaluate mul- M (2) tiple different response mechanisms for DL MU MIMO in The MMSE precoding weights are then given as follows. this paper. Neither response mechanism requires an extra −1 frame exchange before a DL MU MIMO transmission. We ρ † ρ † W = H HH +Φz , (3) also propose enhancement mechanisms that work with DL M M MU MIMO, such as dynamic MAC protection and per-STA † where Φz is the noise covariance matrix and H is the weighted queuing. Hermitian of H. Interference cancellation techniques can be implemented III. SYSTEM MODEL in the receiver to further reduce degradation from multiple We consider an enhancement to an IEEE 802.11n system access interference. When the receiving STA has more receive where the AP has N transmit and receive antennas.. Assume antennas than the number of spatial streams it intends to the AP transmits simultaneously to different stations (STAs) in received, the extra antennas can be used to cancel out the the same basic service set (BSS). With N transmit antennas, spatial streams intended for other STAs. If channel state the AP can transmit a total of N spatial streams. These N information (CSI) is known for the channel dimensions of streams can be distributed across a maximum of N STAs. the interference streams (i.e., HiWj), the CSI can be used to When the AP transmits different streams to multiple STAs, null interference in an MMSE receiver. This type of equalizer streams intended for one STA will cause interference to the structure is given by GiYi, where other STAs. This is represented by the following equation. −1 M ρ H H ρ H H ρ ρ Gi = W H HiWkW H +Φz . (4) Y = H W X + ···+ H W X + M i i M k i i M i 1 1 M i i i k=1 ρ To compare DL MU MIMO to single user 802.11n TxBF, ···+ H W X + Z M i M M i we assume that the transmitter weights are generated using ⎡ ⎤ the eigenvectors from singular value decomposition (SVD). X1 ρ ⎢ ⎥ Though a specific weighting scheme is not defined in 802.11n, = [W , ··· ,W ] ⎣ . ⎦ + z , M 1 M . i (1) SVD yields maximum likelihood performance with a simple XM linear receiver [10]. The system equation with single user TxBF is expressed as, where Yi is the received signal at the ith STA (with dimensions NRx × 1), Xi is the transmitted streams to the ith STA (with Y = ρHV X + Z. (5) dimensions Nss × 1), Nss is the number of spatial stream where the SVD of H is UΣV . When the AP has more for each STA, Hi is the channel between the AP and the ith antennas than transmitted spatial streams, the TxBF gain can STA (with dimensions NRx ×NTx), Wi’s are weights applied be substantial even when the receiver has the same number of at the transmitter (with dimensions NTx × Nss), ρ is the receive antennas as spatial streams. received power, M is the number of STAs, Zi is addition white Gaussian noise at the ith STA (with dimensions NRx×1), NRx IV. CSMA/CA BASED DL MU MIMO PROTOCOL N is the number of receiving antennas at a STA, and Tx is the In this section, we describe a DL MU MIMO MAC protocol number of transmitting antennas at the AP. based on CSMA/CA. Three different response mechanisms are The signal HiWjXj received by Yi causes interference proposed in the following, as well as a novel weighted queuing when decoding its streams Xi for i =j. The AP can mitigate mechanism to mitigate the fairness problem. this interference with intelligent beamforming techniques [9].

Load more