On the Transmission Rate Strategies in Cognitive Radios

We investigate instantaneous transmission rate strategies for secondary users in cognitive radio networks by analyzing their effective capacity performance in different signal-to-noise ratio regimes with different quality-of-service constraints and transmission block sizes. Describing a channel model with one secondary transmitter and one secondary receiver with the potential presence of primary users, we present an interference power constraint that limits the transmission power of secondary users not only when a channel is sensed as busy but also when a channel is sensed as idle. Calling the existing transmission rate strategy Optimistic Policy, we introduce two other strategies, particularly Conservative Policy and Greedy Policy. Secondary users in Optimistic Policy set the instantaneous transmission rate to the instantaneous mutual information assuming the correctness of channel sensing results, whereas they set the instantaneous transmission rate to the instantaneous mutual information regarding possible transmission outages in Conservative Policy and disregarding possible transmission outages in Greedy Policy. We construct a state transition diagram and formulate the effective capacity employing these policies. We calculate the minimum energy-per-bit requirements and the high signal-to-noise ratio slope in order to explore performance variations in low and high signal-to-noise ratio regimes, respectively. Correspondingly, we show that Optimistic Policy is, in general, more favorable in secondary users when the quality-of-service constraints are loose, the transmission blocks are shorter, and the signal-to-noise ratio is low. On the other hand, Conservative Policy is better when the quality-of-service constraints are strict, the transmission blocks are longer, and the signal-to-noise ratio is high.