# Medium Access Control Protocols for Cognitive Radio Ad Hoc Networks: A Survey
## 1 Introduction  
The current wireless networking paradigm consists of the connectivity of many heterogeneous devices through links operating under dynamic environments. These devices will demand an increasing usage of communication resources such as spectrum, and it will not be possible to handle the increasing demand unless automatic administration and protocols without human intervention are used. Moreover, with the new trends in wireless devices and services, the spectrum scarcity problem has become one of the main research focuses [1; 2; 3]. Therefore, today's wireless communication networks encounter difficulties on the efficient management of this increasing complexity. The answer for an efficient use of scarce communication resources is the use of multiple access protocols that maximize the number of users and throughput. Although protocols at the MAC layer have been studied for many years, their use in new communication paradigms, e.g., Cognitive Radio (CR), requires modifications that need to be identified. This paper provides a survey in detail of the state-of-the-art in MAC protocols especially for CR networks. In 2000, the Cognitive Radio Network (CRN) paradigm was proposed by [PERSON] [4], which uses cognitive processes that can observe the current state of the network and then decide, act and planbased on those observations. CR became a favorable concept to overcome inefficiency in spectrum usage, both in unlicensed and licensed bands [5; 6; 7; 8; 9]. Meanwhile, static spectrum allocation in wireless services causes temporal and geographical holes in the spectrum in authorized bands [10; 11]. To utilize the unoccupied band and to resolve the spectrum scarcity problem, the Federal Communications Commission (FCC) has decided to allow the coexistence of unlicensed users defined as Secondary Users (SUs) and licensed users named Primary Users (PUs), through the opportunistic use of spectrum holes without hindering the performance of the PUs [12; 13]. Currently, there are several ongoing CR-related standardization efforts by the IEEE 802 community like the IEEE 802.22 [14], IEEE 802.11 af [15] and also IEEE 1900 [16]. In cognitive radio networks, SUs are equipped with one or more Cognitive Radios (CRs) to enable them to sense the presence of PUs and opportunistically tune to the spectrum band, which is not in use by PUs. As soon as a particular PU starts its activity in a certain channel, the SUs have to vacate that channel and move between unused spectra to avoid interference with PUs. CRN can be divided into two categories based on the network architecture, namely the infrastructure-based CR network and Cognitive Radio Ad Hoc Networks (CRAHNs) [17]. The first category uses a central entity, such as a base station (similar to a cellular network) in order to manage and control the network [18; 19]. Whereas CRAHNs do not rely on any predefined infrastructure backbone, instead, the communication between CR users takes place in an ad hoc manner [20]. As CRAHN has a lower cost of implementation, lower system complexity and faster align positioning compared to infrastructure-based CRN, it is a more suitable option. In recent years, several MAC protocols for CR have been proposed. To assess and review the existing work, a few surveys have been published. In [21], MAC protocols are divided based on control information exchange into four groups and later compared with each other using an extended framework proposed in [22]. [PERSON] et al. [23] categorized multi-channel MAC protocols based on the channel negotiation mechanism and discussed their characteristics, as well as highlighted their necessary modifications to operate in an opportunistic manner. [PERSON] et al., in 2008 [24], discussed the differences between MAC protocols for CR and traditional multi-channel protocols. A survey on Opportunistic Spectrum Access (OSA) networks presented in [25] classifies the network into distributed and centralized, and several CR MAC protocols were analyzed and reviewed. In 2009, [PERSON] et al.