A Spectrum Adaptive NC-CI/OFDM System

Carrier Interferometry Orthogonal Frequency Division Multiplexing (CI/OFDM) can mitigate the problem of high peak-to-average power ratio (PAPR) of OFDM signal. In practice, some subcarriers have to be deactivated in order to avoid harmful interference to licensed system in cognitive radio (CR) context. For these cases, Non-Continuous CI/OFDM (NC-CI/OFDM) has recently been proposed, for which some subcarriers are unused intentionally. Unfortunately, the PAPR performance of current NC-CI/OFDM schemes is sensitive to the distribution of unused subcarriers. In this paper, we propose a new spectrum adaptive NC-CI/OFDM system with improved CI spreading scheme to achieve desired PAPR performance. A novel iterative method is proposed herein to design pilot symbols so that desirable balance between the channel estimation performance and the PAPR can be obtained. We further propose a minimum mean square error (MMSE) based soft signal detection and its complexity-reduced implementation. Simulation results show that our proposed new NC-CI/OFDM scheme can achieve significantly better PAPR performance than that of existing schemes, while its bit-error-rate (BER) performance close approaches that of the existing schemes with negligible performance gap.