In this paper, an underlay cognitive radio network that consists of two secondary users (SU) and one primary user (PU) is considered. The SUs employ best effort transmission, whereas the PU uses Type-I Hybrid ARQ. Exploiting the redundancy in PU retransmissions, each SU receiver applies forward interference cancelation to remove a successfully decoded PU message in the subsequent PU retransmissions. The knowledge of the PU message state (Known or Unknown) at the SU receivers in addition to the ACK/NACK message from PU receiver are sent back to the transmitters on the error free feedback channels. With this approach and using a Constrained Markov Decision Process (CMDP) model and Constrained Multi-agent MDP (CMMDP), centralized and decentralized optimum access policies for two SUs are proposed to maximize their average long term sum throughput under an average long term PU throughput constraint. In the decentralized case, we assume that the channel access decision of each SU is unknown to the other SU. Numerical results demonstrate the benefits of the proposed optimal policies in terms of sum throughput of SUs. The results also reveal that the centralized access policy design outperforms the decentralized design especially when the PU can tolerate a low average long term throughput. Finally, extensions to an arbitrary number of SUs as well as the difficulties in decentralized access policy design with partial state information are described.
['Roghayeh Joda', 'Michele Zorzi']
Access Policy Design for Two Cognitive Secondary Users under a Primary ARQ Process