Hierarchical modulation based spectrum sharing in an overlay cognitive radio network

In this paper, we present a two phase cooperative relaying scheme for spectrum sharing in a 2 × 2 overlay cognitive radio. Secondary transmitter acts as decode and forward relay to the primary and employs hierarchical modulation to enhance primary SNR reception. Primary receiver employs cooperative maximal ratio combining to improve SNR and secondary receiver recovers its own data by successive interference cancellation. We have calculated symbol error rate (SER) analytically for such system and optimal power allocation factor between primary and secondary data in hierarchical modulation have been found out. The analytical result of SER is found close agreement with Monte-Carlo simulation.