Fundamental BER performance trade-off in cooperative cognitive radio systems with random number of secondary users

Abstract

A cooperative underlay cognitive radio system with an instantaneous interference constraint at the primary receiver is considered. Secondary users (SU) whose interference above this threshold will relay primary user's (PU) signal, and the rest will fulfill their own underlay transmissions. Hence the numbers of relay and underlay SUs are random at a time instant. As the mean number of relay SUs increases, the PU's performance will be improved which causes degradation of underlay SUs' performance concurrently. We will quantify this fundamental performance trade-off by deriving the optimal ratio between the mean number of relay and underlay SUs in closed form.