Closed-form Outage and Diversity Analysis of Opportunistic Nulling for Secondary Spectrum Sharing

In this paper, we study a cognitive radio (CR) communication system in which multiple antennas assist the secondary user to achieve transmission diversity, while the constraint of zero interference to the primary user is satisfied. In particular, by assuming perfect channel state information between a secondary transmitter (ST) and a primary receiver (PR) at ST, a closed form expression for a opportunistic nulling scheme, which guarantees to meet zero interference between ST and PR, is obtained. Further, ns antennas of ST and np antennas of PR provide ns - np diversity order for one primary user. In addition, all primary users use the orthogonal spectrum. Finally, if there are M primary users, we can achieve sum M m=1(ns - nmp) diversity order. Numerical results regarding the effects of the number of the primary users, ST's antennas and PR's antennas are provided to evaluate the performances of the proposed scheme.