Power allocation for multiuser precoded OFDM cognitive radio

The high sidelobe leakage from the transmitted signals in orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) has always been a subject of concern, particularly when the OFDM-based CR users and the licensed primary users (PUs) coexist in adjacent frequency bands. Orthogonal precoding in OFDM-based CR systems is an effective spectral shaping technique that can significantly reduce this sidelobe leakage in adjacent PU band(s). In this paper, we propose an optimal power allocation scheme which is augmented with orthogonal precoding-based spectral shaping for multiuser OFDM CR systems. The precoding is realized on a user basis such that the transmitted multiuser OFDM signal and its first few derivatives become time-continuous. The objective of the proposed (precoded OFDM) optimal power allocation scheme is to maximize the downlink sum data rate of all users in the OFDM-based CR system, while the interference introduced to PUs is maintained below predefined thresholds and the transmit power of CR base station transmitter is kept within its power budget. Through simulation results, we demonstrate that the proposed precoded OFDM optimal power allocation scheme can deliver much higher sum data rate of CR users in comparison to the conventional (uncoded OFDM) optimal power allocation scheme and the precoded OFDM uniform power allocation scheme. Further, whereas the computational complexity of the precoded OFDM uniform power allocation scheme is the least, the proposed precoded OFDM optimal power allocation scheme has computational advantage over the conventional uncoded OFDM optimal power allocation scheme.