OFDM-IM-aided cooperative relaying protocol for cognitive radio networks

In this paper, a novel two-phase orthogonal frequency division multiplexing-index modulation (OFDM-IM)-aided cooperative relaying protocol is proposed for overlay cognitive radio (CR) networks, where the primary and secondary networks work concurrently over the same spectrum band. In the proposed scheme, OFDM-IM technique is used at the secondary user (SU) to split the transmission space into the signal constellation domain and the index domain. Specifically, the secondary transmitter (ST) of SU acts as a decode-and-forward (DF) relay to transmit the information of the primary user (PU) in the signal constellation domain, while the information bits of SU are carried by the index domain. Through this design concept, the mutual interference between the PU and SU can be partly avoided. Upper bounds on the bit error probability (BEP) of the PU and SU are analytically derived. We also analyse the achievable rates of this protocol with various finite-size constellations. Several numerical results and discussions are provided to substantiate the theoretical analysis, and to show that the proposed protocol is a viable candidate for OFDM-based overlay CR networks since it can enhance the performances of both PU and SU.