urrent wireless networks are based on a fixed spectrum assignment policy that is regulated by governmental agencies. Although spectrum is licensed on a long-term basis over vast geographical regions, recent research has shown that significant portions of the assigned spectrum are utilized, leading to waste of valuable frequency resources [1]. To address this critical problem, the FCC recently approved the use of unlicensed devices in licensed bands. Toward this end, cognitive radio (CR) technology is envisaged that enables the identification and use of vacant spectrum, known as spectrum hole or white space [1]. In this article we focus on the challenges faced in CR ad hoc networks (CRAHNs), which do not have infrastructure support and must rely on local coordination for different CR functionalities.Since most of the spectrum is already assigned, a key challenge is to share the licensed spectrum without interfering with the transmission of other licensed users (also known as primary users or PUs). If this band is found to be occupied by a licensed user, the CR user moves to another spectrum hole to avoid interference. In CRAHNs the distributed multihop architecture, dynamic network topology, diverse quality of service (QoS) requirements, and time and location varying spectrum availability are some of the key factors that must be considered in network design. These challenges necessitate novel design techniques that simultaneously address a wide range of communication problems spanning several layers of the protocol stack.In CRAHNs CR users are mobile and can communicate with each other in a multihop manner on both licensed and unlicensed spectrum bands, as shown in Fig. 1a. Furthermore, due to the lack of central network entities, CRAHNs necessitate each CR user having all the spectrum related CR capabilities, and determining its actions based on local observation, leading to distributed operation [2]. In order to adapt to the dynamic spectrum environment, the CRAHN requires spectrum-aware operations, which form a cognitive cycle [1]. As shown in Fig. 1b, the steps of the cognitive cycle consist of four spectrum management functions: spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. To implement CRAHNs, each function needs to be incorporated into the classical layering protocols, as shown in Fig. 2. The following are the main features of spectrum management functions:• Spectrum sensing: A CR user should monitor the available spectrum bands, capture their information, and then detect spectrum holes. Spectrum sensing is a basic functionality in CR networks, and hence closely related to other spectrum management functions as well as layering protocols to provide information on spectrum availability. • Spectrum decision: Once the available spectra are identified, it is essential that CR users select the best available band according to their QoS requirements [2]. Especially in CRAHNs, spectrum decision involves jointly undertaking spectrum selection and route formation.• Spectrum sharing: ...
