Optimal Modulation for Cognitive Transmission over AWGN and Fading Channels

This paper proposes a new modulation method for an uncoded cognitive transmission (secondary user transmission) in presence of a Primary User (PU) for AWGN and time-varying flat-fading channels. Interference symbol of the PU is assumed to be known at the transmitter of the Cognitive User (CU) beforehand. Based on this knowledge and using a symbol by symbol approach, we design a CU modulation which can fulfill the coexistence conditions of the CU and the PU. The proposed method is a low-complexity modulation approach in a single (complex-valued) dimension rather than a high dimensional coding scheme, but still it achieves good performance. The robustness of the method is also investigated in case of having an imperfect knowledge about the PU transmitted symbols. An implementation algorithm for our modulation method is presented and its performance is evaluated by experiments.