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Since cognitive radio technology can significantly boost spectrum utilization by exploiting radio spectrum unused by licensed users, it is rapidly gaining popularity and inspiring numerous applications. However, many technical issues still need to be addressed for successful deployment of CR networks, especially in the MAC layer. We focus on CR networks that have distributed architecture because they offer ease of deployment, self-organizing capability, and flexibility in design, and are believed to be more practical for future deployments compared to their centralized counterparts. The MAC protocols for distributed CR networks should consider the key features of these networks such as lack of a central unit to coordinate the communication, dynamic topology, requirements to keep interference to primary users minimal, and variation of spectrum availability with time and location. To clarify the relevant research challenges and issues, we provide a detailed study of the critical design issues and an overview of current state-of-the-art MAC protocols proposed for DCRNs. A classification of existing proposals is provided, and their salient features, advantages, and limitations are discussed. We then introduce and study a novel MAC protocol that addresses some of the research issues better than existing solutions. We also highlight important research challenges that could drive future research in this area.