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In recent years, cognitive radio (CR) has been introduced as a new paradigm for enabling much higher spectrum utilization by dynamically accessing and sharing the spectrum with incumbent radio devices. This paper proposes an innovative and practical spectrum-sharing approach and evaluates its performance. The goal is to minimize CR's harmful interference to incumbent primary users and to maximize the utility of spectrum-sharing networks by exploiting the proposed radio environment map (REM). REM-enabled CR adaptation algorithms are developed for various operational environments, namely, the open area and the dense urban area. This paper also compares the performance gain when using the Global REM and the Local REM, respectively. The impact of imperfect REM information due to node mobility and REM dissemination delay is simulated. By exploiting the REM information, CR can make …