On Study of Achievable Capacity with Hybrid Relay in Cognitive Radio Networks

This work studies the capacity limits of Cognitive Radio (CR) networks with a hybrid relay scheme in which PRimary (PR) nodes dominate the spectrum usage, while coordinating with secondary or CR nodes in forwarding packets. This is in contrast to previous efforts that are focused on the analysis with cooperative relay scheme where only secondary (CR) nodes participate in relay operation [1]-[3]. Intuitively, it is expected that the hybrid scheme can improve network capacity because primary nodes have higher power and transmission capacity compared to CR nodes and data transmission with hybrid relay scheme can be much faster and more reliable. Therefore, we aim to investigate such potential benefits with respect to cooperative relay. However, we find that given the total data rate W, number of usable channels ||C|| and the number of CR nodes n in a network, the achievable capacity of CR networks with hybrid relay is of the order of Θ(W/||C||n). This result is much lower than the capacity of cooperative relay, which is of the same order as the capacity of multi-channel wireless networks (e.g., multi-channel multi-radio (MC-MR) networks), Θ(W1/√n log n). It is plausible that PR nodes can potentially become bottlenecks when they are used as high-priority relay nodes in the hybrid scheme. Thus, our study suggests that for delay-sensitive applications such as realtime traffic, hybrid relay is a good option; otherwise, cooperative relay is preferred.