Cognitive relay-sharing by using hierarchical modulation under interference constraints

In this paper, we investigate relay-sharing in cognitive radio (CR) networks to improve the secondary user's (SU's) performance in the presence of interference. It is assumed that the relay is shared simultaneously between both the primary user (PU) and the SU by using hierarchical modulation at the relay node while mapping the PU's and the SU's symbols onto two different layers of the hierarchical constellation. The proposed scheme relies on power adjusting, where the SU power adjustment is based on channel side information (CSI) and tolerable interference limit at the PU. Theoretical analyses along with simulation results are provided to evaluate the performance of the proposed dynamic spectrum sharing scheme. The results demonstrate that the proposed relay-sharing scheme may result in about 7dB and 15dB signal to noise ratio (SNR) improvement for the SU in low-SNR and high-SNR regimes, respectively.