On the combined effect of directional antennas and imperfect spectrum sensing upon ergodic capacity of cognitive radio systems

Abstract

We consider a cognitive radio system, consisting of a primary transmitter (PU tx ), a primary receiver (PU rx ), a secondary transmitter (SU tx ), and a secondary receiver (SU rx ). The secondary users (SUs) are equipped with steerable directional antennas. We assume the SUs and primary users (PUs) coexist. We find the ergodic capacity of channel between SU tx and SU rx , and study how spectrum sensing errors affect the capacity. In our system, the SUtx first senses the spectrum and then transmits data at two power levels, according to the result of sensing. The optimal SUtx transmit power levels and the optimal directions of SUtx transmit antenna and SUrx receive antenna are obtained by maximizing the ergodic capacity, subject to average transmit power and average interference power constraints. To study the effect of fading channel, we considered three scenarios: 1) when SU tx knows fading channels between SU tx and PU rx , PU tx and SU rx , SU tx and SU rx , 2) when SU tx knows only the channel between SU tx and SU rx , and statistics of the other two channels, and, 3) when SU tx only knows the statistics of these three fading channels. For each scenario, we explore the optimal SUtx transmit power levels and the optimal directions of SUtx and SUrx antennas.