Cognitive radio (CR) has been considered as a promising technology to enhance spectrum efficiency via opportunistic transmission at link level. We consider Opportunistic Spectrum Access (OSA) mechanism that takes into account packet delay and energy consumption. We formulate the OSA problem as a Partially Observable Markov Decision Process (POMDP) by explicitly considering the energy constraint as well as, the delay constraint, which are often ignored in existing OSA solutions. Specifically, we consider a POMDP with an average reward criterion. We further consider that the secondary user (SU) may decide, at any moment, to use another dedicated way (3G) of communication in order to transmit its packets. We derive structural properties of the value function and we show the existence of optimal strategies in the class of the threshold strategies. In particular, numerical illustrations validate our theoretical findings. It is shown that optimal policy has a threshold structure.
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