Resume and Starting-Over-Again Retransmission Strategies in Cognitive Radio Networks

Abstract

Cognitive radio has emerged as a promising technology to realize dynamic spectrum access and increase the efficiency of a largely under utilized spectrum (Haykin, 2005). In a cognitive radio network, a cognitive or secondary user (SU) opportunistically makes use of temporary vacant licensed frequency bands (channels) to set up communication links with other devices. The SUs are capable of detecting channels that are unused by the primary users (PUs) and then making use of the idle channels. With respect to the licensed or PUs, such kind of spectrum access is unlicensed and secondary. To avoid interference to the PUs, SUs are forced to vacate the primary channels as soon as PUs return. Those prematurely terminated secondary sessions degrade quality of service. To reduce this adverse impact, interrupted SUs may be allowed to move to other vacant channels. This process is called spectrum handoff (Zhu et al., 2007). Additionally, to further reduce the impact of service interruption, for delay tolerant services, interrupted SUs can be queued in a buffer to wait for the releasing of an occupied channel. When a SU detects or is informed of an arrival of a PU call/session in its current channel, it immediately leaves the channel and switches to an idle channel, if one is available, to continue its call. These unfinished cognitive transmissions may be simply discarded (Zhu et al., 2007; Zhang, 2008; Ahmed et al., 2008; Pacheco-Paramo et al., 2009). Nonetheless, prematurely terminated secondary sessions degrade quality of service. Alternatively, if at that time all the channels are occupied, the secondary call is queued in a buffer and the call waits until a channel becomes available. Queued secondary calls are served in first-come first-served (FCFS) order. That is, the secondary call at the head of the queue is reconnected to the system when a channel becomes available and transmits its information according to a given retransmission strategy. In this Chapter, the performance of cognitive radio networks for two different retransmission strategies for interrupted secondary user’s calls is mathematically analyzed and evaluated. Resume retransmission and Start Over Again retransmission strategies are considered.