Performance Analysis of a Spectrum Sharing System in the Modulation-Based Dimension

This paper studies the exploitation of the modulation-based dimension in cognitive radio systems to perform cooperative spectrum sharing (CSS). Unlike conventional opportunistic spectrum sharing techniques where a secondary user (SU) system attempts to utilize the spectrum hole in either time, frequency, or spatial dimension created by the primary user (PU) system, this study focuses on finding accessible spectrum spaces created by the structure of modulation constellations. In this novel CSS scheme, by taking advantage of the knowledge on the PU's modulation scheme, the SU superimposes its own modulated signal on the PU's signal. We examine two CSS scenarios: 1) phase-shift-keying (PSK) is used at both the PU and SU and 2) PSK is used at the PU while pulse-amplitude modulation is used at the SU. Interestingly, the proposed superposition approach in CSS Scenario 2 enables a CSS mechanism without either impairing the performance of the primary receiver or requiring a change in its demodulation rule. At the same time, a much better symbol-error-rate (SER) performance is achievable for the SU's receiver in Scenario 2 than in Scenario 1. Theoretical SER performances of both the PU and SU are then thoroughly analyzed and confirmed by numerical simulations.