# Combined Fusion Rules in Cognitive Radio Networks Using Different Threshold Strategies
## 1 Introduction  
The available spectrum resources are limited and insufficient to satisfy the spectrum requirements, i.e., high sampling rates and large bandwidth, for the newly emerging communication technologies, such as Internet of Things (IoT) and 5G. Moreover, the regulatory bodies in every country impose rigid spectrum assignments to the licensed users, i.e., Primary Users (PUs), to use designated portions of spectrum with exclusive rights to own them over the whole country. Those spectrum assignments resulted in exacerbating the problem, since the PUs do not allow other unlicensed users, i.e., Secondary Users (SUs), to share their bands during their inactivity periods. Furthermore, the spectrum shortage increases by the ever increase in the number of users of those technologies demands more spectrum resources. Dynamic Spectrum Access (DSA) principle was proposed as a solution to relieve the spectrum shortage [1] by allowing SUs to opportunistically utilise PUs' bands during their inactivity period and leave those bands once the PUs recommence their activities without causing an interference to the PUs. The DSA principle is implemented by using Cognitive Radio (CR) technology which operates through four functional components, namely, spectrum sensing, spectrum sharing, spectrum management and spectrum mobility [2]. Spectrum sensing is the key functional component by which a SU detects the existence of a PU over a designated spectrum band. PU detection can be performed using different spectrum sensing approaches, such as Energy Detector (ED), matched filter, feature detection techniques, eigenvalue-based techniques, etc. The simple hardware and low computational requirements made EDas the most widely used approach to perform spectrum sensing. However, the detection performance of ED significantly deteriorates in low signal to noise ratio (SNR) scenarios, noise uncertainty situations, and severe radio conditions, such as multipath fading and shadowing [3]. In such cases, collaboration between SUs significantly improves the detection performance of individual SU [4]. The collaboration process is called Cooperative Spectrum Sensing (CSS) and requires to establish common control channel for information exchange between SUs in a Cognitive Radio Network (CRN) which results in increasing the bandwidth requirement, energy consumption and time delay. Moreover, the attaining improvement in detection results also in reducing the spectrum efficiency, i.e., throughput, due to increasing the global probability of false alarm [4; 5]. Therefore, it is mandatory to trade off between the attaining detection improvement, i.e., gain of CSS, and the incurred overhead, i.e., cost of CSS. Note that the global probability of detection of a CRN is generally used to assess its detection performance, where higher probability of detection reflects more PU protection from CRs' interference which is the essential condition for DSA. Clustering is a promising technique to balance between the cooperation gain, and cooperation cost. Therefore, Cluster-Based CSS (CBCSS) has been extensively adopted in CRN research works. The survey in [6] classified CBCCSS techniques into three classes which are:  
* Detection performance gain oriented schemes. These schemes mainly focus on improving the detection performance, i.e., probability of detection and/or throughput. For instance, optimizing the fusion rule employed in a CRN [7; 8; 9], optimizing the sensing time [10; 11] fall in this class. * Overhead reduction-oriented schemes which focus on reducing the incurred overhead including energy consumption, time cost and bandwidth occupation. Reducing the overhead can be implemented using different approaches, such as saving sensing time technique [12], reducing the required bandwidth technique [13], and saving the consumed energy in reporting observations technique [14]. * Combined metric-based schemes which balance the trade-off between the detection performance and the incurred overhead simultaneously.