Space-Time Metrics for Spectrum Sensing

Cognitive radio systems must robustly sense spectrum holes if they want to use spectrum opportunistically. A traditional sensitivity based time-domain perspective on this sensing problem is relatively straightforward and such sensitivity based sensors are easy to certify. However, this problem is more subtle than it first appears. To really understand the question of {\em what should be the right level of sensitivity for a sensor?}, especially for multi-user sensing algorithms, one is forced to think more deeply about the spatial dimension of sensing and the role of fading. In this paper we propose a framework to model the joint space-time dimension of spectrum sensing. This framework naturally gives us reasonable approximate metrics that capture the two desirable features of a spectrum sensor: safety to primary users and performance for the cognitive radios. It is the tradeoff between these two that is fundamental. This framework helps us to quantify the tradeoff between space and time. By simulating the space-time performance of a single-radio energy detector we see that there is tension between the performance in time and the performance in space for a fixed value of protection to the primary user.