CHAPTER Defining cognitive radio Przemyslaw Pawelczak 1 and Rangarao Venkatesha Prasad 2 1 University of California, Los Angeles, United States 2 Delft University of Technology, The Netherlands 13 13.1 INTRODUCTION Since the early 1990s we observed increasing interest around the world in cognitive radio (CR) technology. For example in Figure 13.1, we present the result of a simple web crawl, which shows a monotonic increase in the amount of Internet web pages containing phrases like cognitive radio, dynamic spectrum access,