**1. Introduction**

The idea of an intelligent wireless communication framework cognitive radio (CR) network has been proposed as a solution to deal with the disparity between the increasing demand of wireless radio spectrum and the spectrum underutilisation by licenced users [1]. Unlike conventional spectrum policy in which designated parts of the spectrum are allocated specifically for

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exclusive use to licenced users (usually referred to as primary users), CR technology permits unlicenced users (usually referred to as CR users) to utilise idle bands as long as they do not cause harmful interference to primary users [2].

The operation of a CR network is more complicated than other wireless networks because the CR nodes dynamically access the available channels. Detecting the presence of primary users and further determining the availability of certain channels are regarded as a major technical challenge in CR networks [3]. Hence, spectrum sensing is considered as an important issue of CR networks that aim to find the vacant frequency bands in order to allow CR users access to licenced bands in an opportunistic manner [4].

According to the deployment scenario, CR networks can be classified into two basic types of networks: one is the infrastructure-based CR networks, and the second is the infrastructureless CR networks [5]. In the infrastructure-based CR networks, all CR nodes directly communicate with the central network entity, which is responsible for managing the network operations, for instance, spectrum sensing and spectrum assignment [6]. On the other hand, in the infrastructure-less CR networks, also known as CR ad hoc network, no central entity is present. Therefore, CR nodes have to rely on themselves for spectrum sensing, assignment and management. The application of CR technology in distributed scenarios remains underdeveloped due to a lack empirical research [7].

Broadcasting is considered a fundamental operation in wireless and cognitive radio networks (CRNs). The operation of most network protocols in the ad hoc network depends on broadcasting control information among neighbouring nodes, such as spectrum sensing and routing information.

In traditional single-channel or multichannel ad hoc networks, due to uniform channel availability, broadcasting is easily implemented as nodes are tuned to a single common channel. On the contrary, broadcasting in CR ad hoc networks is a challenging task and much more complicated. The complexity emerges from the fact that different CR users might acquire different channels at different times. Consequently, this partitions the network into different clusters. Cognitive radio (CR) ad hoc networks rely on extensive exchange of control messages among neighbouring nodes to coordinate critical network functions such as cooperative sensing, routing, medium access, etc. To reliably broadcast these messages, a preassigned common control channel is needed. However, assigning a static control channel contradicts the opportunistic access nature of cognitive radio networks (CRNs).

In this chapter, the problem of broadcast in ad hoc CR networks is discussed, current solutions for the problem are reviewed and an intelligent solution for broadcasting based on graph theory to connect different local topologies is developed, which is a unique feature in CRNs. The remainder of this chapter is organised as follows: Section 2 describes the related work in this area. Then the broadcast problem is presented with the system model in Section 3. The proposed broadcast protocol for multi-hop CR ad hoc networks is presented in Section 4. Performance evaluation is conducted in Section 5, followed by conclusions in Section 6.
