**2. Related work for broadcasting in CR network**

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

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

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 under-

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

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

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 conclu-

cause harmful interference to primary users [2].

licenced bands in an opportunistic manner [4].

developed due to a lack empirical research [7].

information.

8 Cognitive Radio

networks (CRNs).

sions in Section 6.

In the literature, several works have extensively studied the broadcasting issue in traditional ad hoc networks, Mobile Ad Hoc Networks (MANETs), Wireless Mesh Networks (WMNs), vehicular ad hoc networks (VANETs) and wireless sensor networks (WSNs). Nevertheless, there are a few studies that investigate the problem of broadcasting in CR ad hoc networks. These works propose numerous performance goals, for example, optimisation of throughput, delay and data delivery. However, most of these techniques cannot be used in practical scenarios due to their limitations and impractical assumptions.

In the recent literature, many protocols have been presented for exchanging messages in CR networks. One of the simplest suggestions is broadcasting over the unlicenced bands such as ultra-wide band (UWB) or industrial, scientific and medical (ISM) [8]. This proposal cannot guarantee the reliability because these unlicenced bands are already overcrowded. Since many wireless devices communicate in the same band, harmful interference may significantly degrade the performance of broadcasting.

The authors in Ref. [9] propose a new strategy for broadcasting. They classify the channels based on the primary radio (PR) vacancy and CR occupancy. This strategy transmits on a single channel; therefore, CR nodes within the transmission range of the sender may not be able to receive the transmitted data if they tune onto a different channel. In Ref. [10], the authors proposed that the secondary network composed of a set of single-antenna secondary receivers (SRs) and one multi-antenna secondary transmitter (ST). The main responsibility of ST is to broadcast the message to the SRs without interfering the primary communication. Since the secondary users use orthogonal beamforming techniques, they can access the licenced spectrum without causing an interference to primary transmission.

The use of a dedicated control channel has been proposed to enable control message exchanging in multichannel networks [11, 12]. To transmit or receive messages, the CR node must tune onto the common control channel (CCC). In CR networks, it is very difficult to find an idle common channel for all nodes. Hence, this technique is not considered to be feasible. Different schemes have been proposed for establishing a local common control channel for exchanging messages [13, 14]. However, most of these schemes require prior information about the set of idle channels across all the CR nodes in the network.

In Ref. [15], the authors assume that the same idle channels between CR nodes are a must to successfully broadcast data. The proposed approaches in Ref. [16] assume that the CR node hops across the channels based on a random channel-hopping sequence to transmit broadcast data. This scheme cannot guarantee reliable dissemination even if there is a common channel between nodes. In Ref. [17], the authors study the issue of broadcasting using multiple transceivers. It is assumed that the number of transceivers of each CR node is equal to the number of channels. This will raise the operational cost and the complexity of the CR device; therefore it is considered an impractical choice.

Many algorithms assume prior knowledge of the channel availability information and the global network topology [18, 19]. A time-efficient broadcast algorithm is presented in Ref. [18], where a set of channels and nodes are selected to convey a message from the source node to its neighbours. The authors in Ref. [19] propose a simple heuristic algorithm to transmit the messages between CR nodes in CR ad hoc networks. In this work, CR nodes are assumed to be equipped with multiple transceivers to broadcast to multiple channels.
