Cognitive radio: An intelligent wireless communication system

The radio frequency spectrum is a scarce natural resource and its efficient use is of the utmost importance. The spectrum bands are usually licensed to certain services, such as mobile, fixed, broadcast, and satellite, to avoid harmful interference between different networks to affect users. Most spectrum bands are allocated to certain services but worldwide spectrum occupancy measurements show that only portions of the spectrum band are fully used. Moreover, there are large temporal and spatial variations in the spectrum occupancy. In the development of future wireless systems the spectrum utilization functionalities will play a key role due to the scarcity of unallocated spectrum. Moreover, the trend in wireless communication systems is going from fully centralized systems into the direction of self-organizing systems where individual nodes can instantaneously establish ad hoc networks whose structure is changing over time. Cognitive radios, with the capabilities to sense the operating environment, learn and adapt in real time according to environment creating a form of mesh network, are seen as a promising technology. This report collects the research work carried out in the CHESS and CHESSEXT projects on cognitive radios and networks in 2006-2008. CHESSEXT project included a one year research visit to Berkeley Wireless Research Center (BWRC) in Berkeley, California, which is a research center belonging to the University of California at Berkeley. The report presents an overview of cognitive radios and cognitive radio networks. The report lists enabling techniques for cognitive radios and describes the state-of-the-art in cognitive radio standards, regulation, products and research. Cognitive radio tasks are reviewed with a more detailed discussion on spectrum sensing, and frequency and power management functionalities. Mesh networks are reviewed as their selforganizing structure is appealing for the use of cognitive radios. Some measurements on current spectrum occupancy are described indicating that even with low overall spectrum occupancy figures, the spectrum band usage can still frequent and the temporal characteristics need to be identified to find spectrum opportunities. In addition to the literature review of cognitive radios, the results of the project include link budget calculations, evaluation of the performance of an energy detection scheme with and without cooperation between the nodes, transmitter power control, and intelligent frequency selection. The results include both analysis and computer simulations using Matlab. The availability of spectrum holes, i.e., frequency bands assigned to a primary user but that are vacant in a given place at a given time, can be estimated with spectrum sensing techniques, such as energy detection and feature detection. When little or no knowledge of the primary user signal is available, energy detection is useful while feature detection can exploit a priori information about the used waveforms. We have studied the performance of an energy detection scheme in terms of probability of detection and probability of false alarm without and with cooperation between the nodes. Cooperative detection by combining the observations of several cognitive radio nodes can be used to improve the performance of spectrum sensing. In addition to the estimation of the availability of spectrum holes, the predicted length of the spectrum holes is of interest in selecting suitable communication channels. Frequency and power management selects suitable frequency bands and transmission power levels for the cognitive radio system. We have studied intelligent channel selection for transmission of data and control information based on the spectrum sensing information, which should minimize harmful interference to other users. Cognitive radio can learn temporal characteristic of channels over time which can be exploited in intelligent channel selection to improve the performance. Transmitter power control needs to assure reliable communication in the changing environment without causing harmful interference to other users. A good candidate for cognitive radios is the inverse power control technique that allocates lower transmission power levels to good channel RESEARCH REPORT VTT-R-02219-08 5 (154) realizations and higher power levels to deeper fading, aiming at minimizing the interference and to allow more secondary users to share the spectrum. Moreover, truncation, i.e., cutting off transmission in poor channel realizations, leads to performance benefits. RESEARCH REPORT VTT-R-02219-08 6 (154)