# An Energy-Efficient and Robust Multipath Routing Protocol for Cognitive Radio Ad Hoc Networks
###### Abstract  
Routing in cognitive radio ad hoc networks (CRAHNs) is a daunting task owing to dynamic topology, intermittent connectivity, spectrum heterogeneity, and energy constraints. Other prominent aspects such as channel stability, path reliability, and route discovery frequency should also be exploited. Several routing protocols have been proposed for CRAHNs in the literature. By stressing on one of the aspects more than any other, however, they do not satisfy all requirements of throughput, energy efficiency, and robustness. In this paper, we propose an energy-efficient and robust multipath routing (ERMR) protocol for CRAHNs by considering all prominent aspects including residual energy and channel stability in design. Even when the current routing path fails, the alternative routing path is immediately utilized. In establishing primary and alternative routing paths, both residual energy and channel stability are exploited simultaneously. Our simulation study shows that the proposed ERMR outperforms the conventional protocol in terms of network throughput, packet delivery ratio  
A cognitive radio network (CRN) is basically populated by two types of users: licensed primary users (PUs) and cognitive secondary users (SUs). The PUs have prioritized possession over the licensed bands, whereas the SUs can use only the vacant channels unused by PUs and, more importantly, without interfering with the ongoing communication of PUs [3]. One of the most attention-seeking issues in CRNs, therefore, the uncertainty of spectrum availability resulting from PU activity and the subsequent challenge of accomplishing SU-SU communication without impeding PU transmission and optimal spectrum usage. Cognitive radio ad hoc networks (CRAHNs) are an infrastructure-free form of CRNs. In fact, they are the enhanced form of conventional ad hoc networks, which are embedded with cognitive radio technology to ensure efficient and innocuous spectrum utilization [4]. The absence of a centralized network entity poses a number of challenges among them compared to their infrastructured counterparts. Without the network infrastructure, SUs must cooperate and communicate among themselves in an ad hoc fashion to exchange network-related information such as network topology, spectrum opportunities, and the presence of PUs [5]. Most existing CRAHN architectures assume that all SUs are equipped with cognitive radios and sense the radio environment. This cognitive functionality requires not only time and effort but also a significant amount of energy [6; 7]. Channel availability in CRAHNs is different from that in conventional multichannel multihop networks such that SUs have partially overlapping and nonoverlapping sets of available channels, and the channels vary with time and space [8]. Furthermore, the distinguishing features such as dynamic topology, self-configurability, scalability, limited energy, and multihop architecture represent inevitable hurdles in realizing CRAHNs. A great deal of research and relentless effort is needed to cope with this challenging environment. Routing in CRAHNs has emerged as one of the most sought-after topics for researchers worldwide. Because CRAHNs borrow attributes from both CRNs and ad hoc networks, the routing protocols must satisfy the requirements of both networks [9]. The unpredictable mobility of nodes, spectrum variation in time and space, energy constraints, and PU activities add burdens to the network layer protocols. A routing process in CRAHNs, therefore, must necessarily involve spectrum awareness, quality route setup based on some form of routing metric, and a route maintenance procedure to combat route failures, if any [10]. In addition, the interest of PUs must be protected, which can be best achieved by minimizing the interference experience due to imperfect spectrum sensing and spectrum allocation at SUs [11]. Furthermore, the sudden appearance of PUs forces SUs to vacate the current channel in use, whereas the mobility of SUs leads to potential route failures.