PAPR improvement in opportunistic communication systems: A novel approach

Opportunistic usage of licensed frequency bands has been proposed as a solution to spectral crowding problem by using Cognitive Radio (CR) systems. Multicarrier techniques based on OFDMA (Orthogonal Frequency Division Multiple Access) have been the technique of choice for sharing the spectrum due to their flexibility in deactivating the sub-carriers to avoid interference to primary licensed users. However, OFDMA based techniques are associated with certain issues such as high PAPR, which is especially harmful at the user equipment (UE) end. Single Carrier FDMA (SC-FDMA) for uplink has been proposed as one of the solutions for reducing the PAPR (Peak-to-Average-Power-Ratio) at UE end. SC-FDMA may utilize localized mapping or the distributed mapping techniques to allocate the sub-carriers to the different users. In spectrum pooling, the availability of free sub-carriers depends upon spectral activity of the licensed user which is unpredictable and considered random. As a result, available carriers may be allocated to the rented or opportunistic users based on availability using a Non Contiguous OFDM (NC-OFDMA) scheme. NC-OFDMA makes efficient use of the spectrum at the cost of high PAPR at the User Equipment (UE). To be able for PAPR reduction using SC-FDMA requires certain structure to the available subcarriers; i.e. consecutive or equi-spaced, which may not be true inall cases. In this paper, we propose a spectrum allocation algorithm that takes into account system requirements for resource efficiency and proposes a DFT of appropriate size for spectrum spreading such that localized SC-FDMA may be implemented at the UE to reduce PAPR. Proposed algorithm seeks to balance the requirements of low PAPR and required spectral efficiency and provides a solution that satisfies the requirements.