# Cognitive Radio Networks for Internet of Things and Wireless Sensor Networks
## 2 5G and Beyond-5G IoT Based on Cognitive Radio Networks  
The limited availability of the communication spectrum is one of the challenges which hinder the massive deployment of the 5G and beyond-5G based IoT systems. The huge demand for the spectrum raises the difficulties in allocating the available frequency band. At the same time, due to various reasons, a large range of frequency band remain under-utilized. The 5G and beyond-5G communication networks are expected to leverage the under-utilized frequency band as well as increase the bandwidth by resolving the issue of spectrum scarcity for the billions of anticipated devices connected the Internet. The 5G and beyond-5G are intended to support the ultra-dense radio accessnetworks (UDRANETs) that will accommodate a vast number of devices operating in millimeter wave (mmWave) and THz frequency, which are currently unregulated except for certain proprietary cellular networks. Spectrum sharing is one of the strategies used to alleviate such difficulties. The concept of spectrum sharing is basically for multiple networks to use the spectrum in respect of a given temporal and spatial configuration. CRNs realize such spectrum sharing and effectively use the available bands, i.e., under-utilized spectrum. The cognitive radios (CRs), which are built on the principle of software-defined radios (SDRs), are capable of intelligent context-aware sharing. The goal of the CRs in 5G and beyond-5G is to alleviate spectrum crunch as well as spectrum under-utilization. The CR has two capabilities as follows:  
* _Cognitive ability_: The capability to sense the surrounding spectrum environments to identify the available frequency, i.e., under-utilized spectrum. The CR will decide whether the particular band is idle or occupied at a given time and location. * _Re-configurability_: This capability allows the radio to dynamically program the system in order to communicate with another set of frequencies and access technologies specified by the underlying hardware. The CRN allows dynamic spectrum access and can reconfigure the properties of spectrum utilization. In CRN topologies, there are two groups of users, grouped according to the priority of accessing the network. The principal users are called primary users (PUs), while the opportunistic users are called secondary users (SUs). The PUs have the priority by paying for band allocation, while the SUs are temporary spectrum consumers when the frequency is not used by the PU. The sharing of the spectrum between these two classes is the primary goal of the CRN while ensuring the performance for both the classes is maintained. One of the major tasks in this operation is to ensure that PU's priority is secured, while at the same time retaining the QoS for the SUs. To ensure a QoS-compliant efficiency, moreover, the interference between the PUs and the SUs should be minimized. Thus, the CRNs allow for the coexistence of unlicensed users with the licensed users amid challenges such as user intervention and optimization of the power and resource allocation of the SUs. Two major architectures, i.e., centralized and decentralized architectures, are adopted in the CRNs, based on the implementation of the IoT framework. A cooperative sharing of spectrum can be followed at the centralized CRN. Various cognitive radio units (CRUs) can be clustered together by adding a cluster head to reduce the overhead signal and computational expense. This method allows cooperative use of knowledge from the CRUs in the cluster. For non-cooperative sharing of bandwidth, on the other hand, the CRUs follow a greedy approach to maximize the individual capacity. This approach does not use the interference information from the other CRUs. In addition, the total ability of the network and the efficiency of the spectrum are adversely affected. There are four types of access methodology for the sharing of spectrum, namely interweave spectrum sharing, underlay spectrum sharing, overlay spectrum sharing, adaptive spectrum sharing.