Link adaptation in OFDM-based cognitive radio systems

Abstract

Radio spectrum is one of the most scarce and valuable resources in wireless communications. Given this fact, new insights into the use of spectrum have challenged the traditional static spectrum allocation approach to spectrum management. Actual measurements have shown that most of the allocated spectrum is largely underutilized [1]. The Spectrum-Policy Task Force appointed by the Federal Communications Commission (FCC) drew a similar conclusion. Specifically, FCC reported vast temporal and geographic variations in the usage of allocated spectrum, with utilization ranging from 15 to 85% [2]. Spectrum utilization can be significantly improved by giving opportunistic access to the frequency bands instead of employing static spectrum allocation. According to the opportunistic spectrum access policy, a group of potential users may use a frequency or spectrum band for wireless communications provided that the legacy users of this band are not deprived of their priority right to use the band. On the other hand, development of software-defined radio (SDR) technology [3] has enabled radio transceivers to perform baseband processing functionalities, e.g., modulation and demodulation, using software and digital logic. Software-defined radio, therefore, becomes the promising technology in developing versatile wireless transceivers that will have the capability of accessing different radio networks with different technologies. In order to facilitate opportunistic spectrum access, this versatile transceiver needs to be spectrally aware, which motivates the design of cognitive radio (CR) technology [4]. Cognitive radio technology is an innovative radio design philosophy that involves smartly sensing the swaths of spectrum and then determining the transmission characteristics (e.g., symbol rate, power, bandwidth, latency) of a group of secondary users (also referred to as CR users)1 based on the behavior of the users to whom the spectrum has been licensed [5, 6]. As such cognitive radio has been proposed as a way to improve spectrum utilization by exploiting unused spectrum in a dynamically changing environment. © 2007 Springer Science+Business Media, LLC.