A Cooperative MAC for Distributed Space-Time Coding in an IEEE 802.16 Network Pei Liu, Chun Nie, Thanasis Korakis, Shivendra Panwar Department of Electrical and Computer Engineering, Polytechnic University, Brooklyn, NY 11201, USA Abstract— In the next-generation WiMAX system, cooperative communication is being considered as an advanced technique to increase the throughput and improve the signal quality. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Unlike conventional space-time coding (STC) mechanisms used by a IEEE 802.16e antenna array, distributed space-time coding (DSTC) is employed across the cooperating stations to achieve a higher spatial diversity gain. In this paper, we present the framework for DSTC in the emerging relay-assisted WiMAX network, and develop a cooperative MAC layer protocol, called CoopMAX, for DSTC deployment in a WiMAX system. Through extensive simulations, we evaluate the performance of CoopMAX and show that DSTC can yield capacity gains of up to about 50% for the uplink of an IEEE 802.16 network. I. I NTRODUCTION As an advanced broadband wireless access technology, WiMAX has attracted a lot of research attention. While the current IEEE 802.16d/e [1], [2] have been specified for the current single-hop WiMAX network, relay-assisted WiMAX has become the focus for the future evolution of WiMAX standards, and is being actively investigated [3]. Recently, the 802.16j Relay Task group was formed to standardize a WiMAX mobile multi-hop relay (MMR) system. An MMR system enables a subscriber station (SS) to route through intermediate relay stations (RSs) in order to reach the BS. In the MMR scenario, the IEEE 802.16j baseline mainly focuses on the relay operation that allows a single intermediate station to forward the received signal to the next hop. Such a technique seems promising, but the participation of a single relay in the forwarding process may limit the benefits of multi- hop transmission, since such a data communication over a pair of links may undergo severe fading, and consequently packet corruption. Cooperative wireless communication provides an efficient solution that provides robust forwarding by recruiting multiple intermediate stations on the fly to collaboratively transmit the source signal to the destination. These intermediate stations are called helpers and form a virtual multi-input multi-output (MIMO) infrastructure where the helpers act as distributed antenna array elements. Since MIMO systems allow multiple antennas to transmit together in order to achieve high diversity This work is supported by the NSF Grant CNS-0435303, and also by the New York State Center for Advanced Technology in Telecommunications (CATT), and the Wireless Internet Center for Advanced Technology (WICAT), an NSF Industry/University Cooperative Research Center. gains using space-time coding (STC), it is natural to apply the same functionality to a cooperative environment in a dis- tributed fashion. STC that employs geographically distributed stations is commonly known as distributed space-time coding (DSTC). The basic idea of DSTC is to coordinate and synchronize the helpers so that each of them acts as one antenna element of a conventional STC. In a typical DSTC system, each helper participating in a DSTC is numbered in order to emulate the antenna it will mimic in the underlying STC [4], [5]. Recently, DSTC is being considered by the IEEE 802.16j/m standard task groups. Several contributions [6]–[8] are proposing to incorporate DSTC into the framework of the next-generation WiMAX standards. These contributions present the concept of cooperation and discuss the challenges that arise in a potential WiMAX cooperative system. Although the research and standardization efforts afore- mentioned are devoted to the DSTC physical layer (PHY) studies, limited attention has been given to the Medium Access Control (MAC) layer for the deployment of DSTC in WiMAX system. An efficient MAC layer protocol should cope with the discovery of helpers, channel estimation, management mes- sage handshaking and rate adaptation, among other functions. The main contribution of this paper is to develop the MAC layer protocol in order to support the deployment of DSTC in an IEEE 802.16 WiMAX network. The proposed protocol is called CoopMAX in this paper. To the best of our knowledge, CoopMAX is the first compatible framework that facilitates the implementation of DSTC in the next-generation IEEE 802.16 network. The remainder of this paper is outlined as follows. Section II introduces the fundamentals of the IEEE 802.16 MMR system and examines the physical layer of a DSTC system. In section III, we describe the MAC layer framework that supports DSTC in the IEEE 802.16 system. Extensive simulation results are presented in section IV that shows the significant performance gains of the new scheme. In section V, we present conclusions and future work. II. WI MAX MMR SYSTEM AND DSTC PHY A. WiMAX MMR Ststem Overview The mobile multi-hop relay (MMR) architecture is being considered by the IEEE 802.16j baseline in order to extend the cell coverage and enhance the transmission rate of a conventional WiMAX system. While the multi-hop WiMAX