Cognitive radio system (CRS) is a radio system which is aware of its operational and geographical environment, established\r\npolicies, and its internal state. It is able to dynamically and autonomously adapt its operational parameters and protocols and to\r\nlearn fromits previous experience. Based on software-defined radio (SDR), CRS provides additional flexibility and offers improved\r\nefficiency to overall spectrum use. CRS is a disruptive technology targeting very high spectral efficiency. This paper presents an\r\noverview and challenges of CRS with focus on radio frequency (RF) section.We summarize the status of the related regulation and\r\nstandardization activities which are very important for the success of any emerging technology. We point out some key research\r\nchallenges, especially implementation challenges of cognitive radio (CR). A particular focus is on RF front-end, transceiver, and\r\nanalog-to-digital and digital-to-analog interfaces which are still a key bottleneck in CRS development.
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