## 1 Introduction  
Cognitive radio (CR) is an intelligent technology to improve the spectrum utilization in mobile ad-hoc networks (MANETs) by allowing cognitive users flexibly use the spectrum allocated to licensed primary users. In this approach, each cognitive user can sense status of channels and immediately switch to vacant channels to avoid interfering with primary users or other users [1; 2]. Hence, CR has been considered as a promising technology to improve the spectrum utilization in CR-MANETs. There are three main cognitive radio network paradigms: underlay, overlay and interweave. In underlay systems, cognitive (secondary) user knows the channel strengths to primary users and can transmit simultaneously with primary user as long as interference caused is below an acceptable limit. Moreover, the transmit power of cognitive user is limited by the interference constraint. In overlay systems, cognitive users know channel gains, codebooks and the message of the primary users. Each cognitive user can transmit simultaneously with primary user. The interference to primary user can be offset by using part of the power of cognitive user to relay the primary user'smessage. Moreover, cognitive user can transmit at any power, the interference to primary user can be offset by relaying the primary user's message. In interweave systems, cognitive user knows the spectral holes in space, time, or frequency when primary user is not using these holes. Each cognitive user can transmit simultaneously with a primary user only in the event of false spectral hole detection. Moreover, the transmit power of cognitive user is limited by the range of its spectral hole sensing. In our paper, we considered the system model as a overlay system. There are many challenges for clustering and routing in MANET such as no fixed infrastructure, decentralized management, dynamic topology, limited bandwidth, limited resources, limited physical security, poor transmission quality, dynamic connection and disconnection of the links and updating information of dynamic links among nodes. In this paper, we focus on clustering and routing issues in network layer. However, there exist important issues of lower layers which can be occured at network layer such as throughput, energy efficiency, traffic optimization, packet loss, security, delay, QoS, admission control, mobility, routing and data gathering. These issues significantly impact to the routing process in network layer. However, we mainly focus on establishing the best route in network layer to improve packet delivery ratio (PDR), control overheads, delay and energy consumption. Thus, we assume that these above issues were fully supported by lower layers, that is, physical and data link layers. Since MANET is a mobile environment which does not depend on any fixed infrastructure or centralized management, clustering approach is an efficient approach to design virtual infrastructures and make routing process stable [3]. In this paper, we focus on clustering and routing problems with multi-channel to improve network performance in CR-MANETs. In routing process, routes established at the network layer will affect significantly the licensed channel of primary users, arising a challenging problem in guaranteeing quality of service in MANETs. CR technology is adopted for routing process by enabling channel selection and transmission direction to solve such a problem. However, the impacts of clustering and routing problems with channel selection and transmission direction on multi-channel cognitive MANETs, which cover a lot of attractive research problems, have received less attention in the literature. In this section, we discuss the recent works on clustering and routing algorithms in MANETs.