Achievable Rates With Cooperative Spectrum Sharing in the Modulation-Based Dimension

Cooperative spectrum sharing (CSS) is a joint spectrum access technique that involves collaboration between primary user (PU) and secondary user (SU) systems. An instance of CSS occurs when the SU transmitter acts as a relay for the PU system. In forwarding the PU signal, the SU transmitter deliberately superimposes the signal intended for the SU receiver. This process unfortunately causes mutual interferences at both PU and SU receivers. Achievable rates for both systems are typically analyzed by assuming Gaussian signals and interferences. Unlike most of previous work, this paper studies the use of practical digital modulation schemes in CSS. The novelty of the proposed CSS approach lies on the smart alignment and superimposition of the SU's pulse amplitude modulated signal to the PU's phase-shift keying signal. It is shown that the SU signal does not interfere with the PU signal, it actually improves the PU signal strength. This paper then focuses on deriving the approximated theoretical transmission rates for both the PU and SU. Interestingly, the analysis shows superior transmission rates for both systems, compared to that achieved by conventional signal superimposition approaches. Simulation results then confirm the matching with the analytical results and validate the performance advantages of the proposed CSS approach.