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Demand for efficient operation of radio frequency (RF) devices with limited resources, such as energy and frequency spectrum, is increasing as a variety of wireless applications quickly become more popular. Cognitive radio (CR), which can sense a radio environment and use an unoccupied spectrum, is thought to be a drastic solution for problems such as battery life shortage and/ or spectrum scarcity. This article presents a brief overview of CR systems and describes design challenges for implementing RF integrated circuits (RFICs). A design example of a wideband CMOS receiver for such systems is also discussed.