A Differentiated Cooperative MAC for QoS Enhancement in Wireless LANs Tao Guo, Rolando Carrasco, and Wai Lok Woo School of Electrical, Electronic and Computer Engineering Newcastle University, NEI 7RU, United Kingdom E-mail: {tao.guo.r.carrasco.w.l.woo}@ncl.ac.uk Abstract-The Quality of Service (QoS) support in Wireless Local Area Networks (WLANs) faces a big challenge due to time- correlated fading channel. Conventional Automatic Repeat reQuest (ARQ) schemes based on time diversity may result in consecutive retransmission failures degrading QoS severely. This paper proposes a novel Differentiated Cooperative Medium Access Control (MAC) protocol, called DC-MAC, to enhance QoS in WLANs based on the IEEE 802.lle architecture. The retransmission is initiated from an appropriate transmission queue of an appropriate relay node instead of the original source to exploit spatial diversity. A novel Negative AcKnowledgement (NAK) feedback mechanism is introduced for loss distinguishing and channel estimation such that cooperative retransmission will be employed only when necessary and only by competent nodes. Simulations conducted on the OPNET platform show that the proposed scheme significantly improves the performances of both multimedia applications and data applications in terms of throughput and delay while supporting service differentiation. Keywords-Quality of Service; cooperative communication; wireless LAN I. INTRODUCTION With the increasing popularity of multimedia applications over wireless links, Quality of Service (QoS) support has become a critical demand for next generation Wireless Local Area Networks (WLANs). The recent IEEE 802.11e Enhanced Distributed Channel Access (EDCA) [1] provides service differentiation with a priority-based medium access control (MAC) mechanism. A link layer Automatic Repeat reQuest (ARQ) scheme is used to retransmit the corrupted packets due to channel errors or collisions. Retries will continue until either the corresponding acknowledgement is received or the retry count reaches a predefined limit. However, this scheme may not be effective under a fading channel with correlated errors, where there is a high probability that bad channel condition continues for a certain period when a transmission error occurs. In that case, the channel may stay in a deep fade on the order of multiple frame transmission durations. The frame drop rate and delay will considerably increase due to consecutive retransmission failures and the bandwidth is wasted from a system point of view as useless messages occupy the channel. Cooperative communication is becoming a promising technology for wireless networks [2] by exploiting multipath fading instead of mitigating its impact. Single antenna devices can share their antennas in a cooperative manner to emulate a Multi-Input Multi-Output (MIMO) system and exploit the 978-1-4244-2644-7/08/$25.00 ©2008 IEEE spatial diversity benefits traditionally realized by an antenna array hosted on a single device. Inspired by this idea, cooperative retransmission has been shown to be an effective technology to improve the reliability of wireless links [3][4][5]. A retransmission could be initiated from a relay node that overheard the information packet instead of the original source. Since signals from different locations undergo independent fading gains, the retransmission success probability can be greatly increased by exploiting this spatial diversity. In [3], a simple but effective ARQ scheme called Node-Cooperative Stop and Wait (NCSW) is proposed to reduce the average duration of retransmission trials. Significant gain has been shown for a single sender-receiver pair by an analytical model. A cross-layer relaying protocol based on hybrid-ARQ with incremental redundancy, termed Hybrid-ARQ-Based INtra- cluster GEographic Relaying (HARBINGER), is proposed in [4]. The nodes decoding the data use Global Positioning System (GPS) to identify their positions relative to the destination and the one closest to the destination will relay the frame. In [5], a Cooperative communication MAC (CMAC) is proposed to improve link reliability for WLANs. Two transmission queues are maintained in each node. One is the data queue for its own outgoing data and another is the partner queue to buffer the copies of the overheard frames. A higher priority is given to the partner queue such that the cooperative retransmission can occur before the original source retransmits. To the best of our knowledge, there is no scheme considering the QoS support with differentiated services. Furthermore, a practical and effective protocol and a system-level investigation are lack in the literature. From a system perspective, retransmissions from relays may equivalently increase the number of competing nodes in a system and thus may degrade the system performance if inappropriately employed. In addition, a distributed coordination mechanism is needed to solve the relay collision problem since there may be several nodes that can serve as relays. In this paper we propose a novel Differentiated Cooperative MAC, called DC-MAC, for QoS enhancement in WLANs based on the IEEE 802.11 e architecture. Retransmission priority is introduced at relays for different traffic classes. A channel-aware feedback mechanism is proposed to make cooperative retransmission employed only when necessary and only by competent nodes. The rest of this paper is organized as follows: The details of our proposed protocol are presented in Section II. In Section III, simulation model and performance evaluation are discussed. Conclusions are given in Section IV. 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