# An Energy-Efficient and Robust Multipath Routing Protocol for Cognitive Radio Ad Hoc Networks
###### Abstract
Multipath routing in CRAHNs allows SUs to switch dynamically among multiple paths whenever the normal routing is obstructed by either the appearance of PUs into the current channel or the movement of the node itself into the region of the PU. Multipath routing provides a set of primary and alternate paths to cope with route failures. The dynamic switching from the primary path to the alternate path can significantly reduce the frequency of the route discovery process and contribute to energy-efficient and reliable routing. An energy-efficient routing scheme is highly needed for CRAHNs powered by energy-constrained batteries. In such a routing scheme, channel switching and rerouting should be minimized. When any PU appears, SUs should vacate the current channel in use, so a route that survives for a long period must be discovered. As a result, many factors including the residual energy of each participating node, the energy consumption along the path, the stability of the channel selected, and the backup path to counteract the sudden appearance of PUs must be considered. Such considerations can aid in achieving energy efficiency in routing. Even though there are many routing schemes in the literature, most of the routing approaches handle node selection, energy awareness, spectrum decision or multipath routing separately. In this paper, the combined form of residual energy and route stability is exploited as the integrated route selection metric to discover energy-efficient and robust multiple paths while keeping in record the spectrum heterogeneity and primary user activity. That is, we propose an energy-efficient and robust multipath routing (ERMR) protocol that promises not only to conserve valuable energy with balanced energy consumption throughout the network but also to improve the robustness of routing paths. The channel selection algorithm integrated with route formation sets up a strong route with a long lifetime in terms of residual mode energy and link stability, resulting in improved energy efficiency and robustness. According to our extensive simulation results, the proposed ERMR outperforms the conventional protocol in terms of network throughput, packet delivery ratio, energy consumption, and end-to-end delay. The rest of this paper is organized as follows: In the following section, related works are reviewed in brief. Section 3 depicts the system model for CRAHNs used in our study. In Section 4, the proposed ERMR protocol is presented along with its routing processes, and a qualitative comparison of various routing protocols is also addressed. In Section 5, the performance of the proposed ERMR is evaluated via computer simulation and compared with the conventional scheme. Finally, the paper is concluded in Section 6. In the paper, some symbolic notations are used as follows: \(S_{t}(n)\) denotes the state of a PU \(n\) at time \(t\). \(P_{on}\) and \(P_{off}\) denote the probability of a PU being in ON and OFF state, respectively. Likewise, \(u\) and \(v\) represent the periods for which a PU is expected to be in ON and OFF states, respectively. \(E_{res}\) and \(S\) are indicators for residual energy measure and route stability measure, respectively. \(E_{init}(n)\) is the initial energy of node \(n\), \(E_{con}(n)\) is the energy consumed by node \(n\), and \(E_{res}(n)\) denotes the remaining energy of node \(n\).