On Peak versus Average Interference Power Constraints for Spectrum Sharing in Cognitive Radio Networks

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Abstract
This paper considers the spectrum sharing for mobile wireless communications, where a secondary user or cognitive radio (CR) communicates using the same bandwidth originally allocated to an existing primary user link. An effective means for the CR to use to protect the primary transmissions is by applying the so-called {\it interference-temperature} constraint, whereby the resultant interference power at each primary receiver is kept below some predefined value. For the fading primary and secondary user channels, the interference-power constraint at each primary receiver is usually one of the following two types: One is to regulate the {\it average-interference-power} (AIP) over all the fading states, while the other is to limit the {\it peak-interference-power} (PIP) at any of the fading states. From the secondary user's perspective, given the same (average or peak) power-constraint value, the AIP constraint is more favorable than the PIP because of the more flexibility in dynamically allocating transmit powers over the fading states. However, from the perspective of protecting the primary user transmissions, the more restrictive PIP constraint appears at a first glance to be a better option than the AIP. Surprisingly, in this paper it is shown that this seemingly apparent fact is usually untrue for the primary user transmission over the fading channel. This is due to an interesting {\it interference diversity} phenomenon, i.e., the randomized interference powers from the secondary transmitter in the AIP case can be in fact more advantageous for minimizing the resultant primary user capacity losses as compared to the more deterministic ones in the PIP case. Therefore, the AIP can be superior than the PIP for both the primary and secondary user transmissions.

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