The spectrum sharing system employing maximum ratio combining (MRC) is analyzed in Nakagami fading environment, for the case when the interference from the primary user is present at the input of the secondary user receiver. The closed-form expressions for the probability density function of the signal-to-interference-and-noise ratio, the outage probability and the ergodic capacity of the SU link are derived under both peak interference and maximal transmit power constraints. Asymptotical expressions are provided for the important region where peak interference power constraint dominates and the case when the interference from the primary user’s is dominant compared to the noise at the secondary user’s receiver. The obtained expressions are presented for both cases of outdated and mean-value based power allocation and verified by using Monte Carlo simulation method.

In this paper underlay cognitive radio system is analyzed in Nakagami fading environment, for the case when the available channel state information (CSI) of the link from the secondary transmitter to the primary receiver is outdated. We considered the case when both interference and transmit power constraints are applied. The impact of the co-channel interference that originates from the primary user is analyzed in details. The exact closed-form expressions for probability density function, outage probability, and the moments of signal-tointerference and noise ratio (SINR) at the secondary user's receiver are derived. The analytical expressions are verified by using independent Monte Carlo simulation method.

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