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Distributed space-time coding (DSTC) is a key physical (PHY) layer technique to enable cooperative relaying in wireless networks. Compared with direct transmission and single-relay cooperation, DSTC exploits spatial diversity gain and achieves a higher end-to-end throughput performance. In particular, a novel variation of DSTC, which is called randomized DSTC (R-DSTC), further enhances the performance of conventional DSTC in a mobile environment due to its decentralized relay recruitment. This paper presents a medium access control (MAC) protocol, which is called CoopMAX, for Worldwide Interoperability for Microwave Access (WiMAX) systems to provide cooperative relaying with DSTC and R-DSTC techniques by the use of fixed and mobile relays. CoopMAX explores a joint PHY layer and MAC layer optimization for rate adaptation. The efficiency of CoopMAX is investigated, showing pronounced advantages, such as a doubling of throughput as compared with conventional WiMAX protocols in some typical scenarios.