DCF/USDMA: Enhanced DCF for Uplink SDMA Transmissions in WLANs Ruizhi Liao, Boris Bellalta and Miquel Oliver NeTS Research Group Department of Information and Communication Technologies Universitat Pompeu Fabra, Barcelona, Spain {ruizhi.liao, boris.bellalta, miquel.oliver}@upf.edu Abstract—The traditional IEEE 802.11 DCF allows only one node to access the channel each time. In order to take the benefit of multiple simultaneous uplink transmissions when the AP is equipped with multiple antennas, a new MAC protocol called DCF/USDMA is presented in this paper. DCF/USDMA extends the IEEE 802.11 Distributed Coordination Function (DCF) by introducing a second contention round, which is used to accommodate the multiple simultaneous transmissions from multiple stations. Results show that the DCF/USDMA protocol is able to achieve a significant performance gain compared to that of the conventional DCF, the analysis of which also provides some insights about how the different parameters that control the DCF/USDMA operation can be optimized. Index Terms—MU-MIMO, MAC, SDMA, uplink, WLANs, DCF, 802.11 I. I NTRODUCTION The IEEE 802.11 Wireless Local Area Networks (WLANs) are widely deployed and have been hugely successful in the last decade. However WLANs suffer from different problems, such as high link-layer protocol overheads and collisions, which lead to an effective throughput sometimes is less than a half of the raw data rate that the Physical (PHY) layer can achieve [1][2]. Multiple-Input Multiple-Output (MIMO) techniques are a good option to significantly improve the performance of WLANs by operating either in the Spatial Diversity mode (the sender transmits several duplicate data frames at the same time) or the Spatial Multiplexing mode (the sender transmits several independent data frames at the same time). With respect to Spatial Multiplexing, the single-user MIMO (SU-MIMO) configuration has already been implemented in the IEEE amendment 802.11n-2009 [3]. Multi-user MIMO (MU-MIMO) has not been standardized yet, while it is now taken into the consideration of the ongoing IEEE 802.11ac [4] amendment, which is due to be finalized in late 2012, and has a target to provide up to 1 Gbps throughput. In this paper, we present a new MAC protocol for WLANs, called DCF/USDMA (DCF with Uplink Spatial Division Multiple Access), which allows simultaneous transmissions in the uplink, from different single-antenna nodes to an AP equipped with multiple-antennas (Figure 1). Results show that the DCF/USDMA is able to significantly improve the uplink Node Node Access Point N antennas Fig. 1. Multi-Packet Reception at the AP performance of WLANs and it can be a suitable candidate for the future amendments of IEEE 802.11 standards. The rest of the paper is organized as follows: Section II gives a brief review of the related work. The uplink SDMA MAC protocol is presented in Section III. After that, the analysis and evaluation of the protocol are given in Section IV. Finally, Section V concludes the paper. II. RELATED WORK The authors in [5] present a simple Multi-Packet Reception (MPR) MAC protocol for WLANs. A limitation of the pro- posed scheme is that it follows the conventional IEEE 802.11 DCF mechanism, which means the probability of more than a single station choosing the same random back-off is low, thus not using efficiently the MPR capability of the AP. To address this concern, an enhancement called Two-Round RTS Contention (TRRC) is also presented in [5], which introduces a second-round Request to Send (RTS) contention period to allow more nodes to schedule their transmissions. The protocol in [6] also follows the original IEEE 802.11 standard. The differences compared to [5] are that the authors adopt the DCF basic access instead of using the RTS/CTS handshake mechanism, and an orthogonal preamble is assumed to be possessed by each node. Therefore, the performance limitation that exists in [5] also remains here, as both proposals rely on the original IEEE DCF mechanism. The authors in [7] propose a back-to-back MU-MIMO downlink-uplink transmission scheme to circumvent the syn- chronizing problem among distributed wireless nodes. The idea is that the AP sends out a special frame, called Ready to Receive (RTR), immediately after a successful downlink transmission to notify the wireless nodes about the readiness 978-1-4577-1379-8/12/$26.00 c  2012 IEEE