Background detection of primary user activity in Opportunistic Spectrum Access

In Opportunistic Spectrum Access (OSA), conventional secondary users´ (SU) radios can not transmit and detect primary user (PU) activity simultaneously, and therefore SU and PU transmissions may overlap, causing harmful interference at PU receivers. The extended approach of periodically interrupting SU transmission to perform spectrum sensing mitigates but does not completely solve the problem. Our proposal consists of a collision detection mechanism simultaneous to reception at the SU receiver (Background Detection), which aborts the ongoing secondary transmission if a collision is detected. While some previous works assumed the existence of a collision detection scheme, little work has been done in proposing a specific one for OSA systems, capable of operating during SU reception. Our proposal exploits local characterization of the PU activity, power levels and packet errors at each time slot. Based on these data, we develop a Maximum A Posteriori (MAP) estimator as the basis for the collision detection system. We describe the general methodology to build this estimator, and study the system´s robustness against parameter misestimations. The proposed scheme can be incorporated to existing OSA protocols. Our results show a notable throughput improvement for the SUs while assuring a lower collision probability with PU transmissions in most cases, even under severe estimation inaccuracies.