Cognitive radio (CR) has been considered as a promising technology to enhancespectrum efficiency via opportunistic transmission at link level. Basic CRfeatures allow SUs to transmit only when the licensed primary channel is notoccupied by PUs. However, waiting for idle time slot may include large packetdelay and high energy consumption. We further consider that the SU may decide,at any moment, to use another dedicated way of communication (3G) in order totransmit its packets. Thus, we consider an Opportunistic Spectrum Access (OSA)mechanism that takes into account packet delay and energy consumption. Weformulate the OSA problem as a Partially Observable Markov Decision Process(POMDP) by explicitly considering the energy consumption as well as packets'delay, which are often ignored in existing OSA solutions. Specifically, weconsider a POMDP with an average reward criterion. We derive structuralproperties of the value function and we show the existence of optimalstrategies in the class of the threshold strategies. For implementationpurposes, we propose online learning mechanisms that estimate the PU activityand determine the appropriate threshold strategy on the fly. In particular,numerical illustrations validate our theoretical findings.
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