## 1 Introduction  
In recent times, advancements in wireless sensor networks have enabled them to support a wide range of applications, including medical and healthcare systems. A wireless body area network (WBAN) is a special-purpose sensor network designed to operate autonomously to connect medical sensors and appliances located inside and outside the human body and is used for long-term health monitoring within a hospital or remotely. A WBAN consists of biomedical sensor nodes used to monitor physiological data, such as temperature, blood pressure, electrocardiogram (ECG), electroencephalography (EEG), heart rate, _etc._[1]. The key requirements of WBANs are low power consumption, negligible electromagnetic interface with the body, low delay, high reliability and effective communication. With regard to the location of the sensors that collect physiological data, the areas inside and around the patient's body pose a challenging environment for the design of adaptable, dynamic and flexible protocols in WBANs. For the successful implementation of WBANs in medical, as well as consumer products, a standard protocol is required. The IEEE 802.15.6 standard [2] defines the physical (PHY) and medium access control (MAC) layers to address both medical/healthcare applications and other non-medical applications with varying requirements. The MAC layer in the standard is used to define short-range wireless communication in and around the human body. The standard aims to support a low-complexity, low-cost, ultra-low power and highly reliable wireless communication for use in close proximity to or inside the human body (but not limited to humans) in order to satisfy an evolutionary set of entertainment and healthcare products and services. It is notable that many parts of the MAC protocol in the IEEE 802.15.6 standard are derived from the IEEE 802.15.4 standard [3], because of the similarities between wireless personal area networks (WPANs) and WBANs. Because of the spectrum scarcity problem recently encountered in wireless communication domains, cognitive radio (CR) technology has emerged as a key technique in wireless networks, including fifth-generation mobile networks. CR technology is a promising technique for unlicensed users to access underutilized licensed (or white space) spectra. It enables unlicensed users (or secondary users (SUs)) to opportunistically access underutilized licensed spectra whenever the licensed users (or primary users (PUs)) are idle [4]. The MAC layer plays a key role in several cognitive radio functions, such as channel sensing, resource allocation, spectrum mobility and spectrum sharing. Channel sensing is the ability of a cognitive user (an SU) to estimate the channel state information and to analyze the radio environment. Resource allocation is the opportunistic assignment of available channels to cognitive users (SUs) according to quality of service (QoS) requirements. Spectrum mobility allows an SU to hand off its assigned channel when a PU is detected and to access a vacant channel to re-establish communication. Spectrum access can help avoid conflicts between primary and secondary users to avoid harmful interference. CR technology can be implemented in WBANs to reduce interference caused by conventional wireless medical applications. Electromagnetic interference (EMI) may cause malfunction (e.g., wave distortion, shutdown, restart, _etc._) in the network, which critically affects the operation and communication of various sensitive devices [5]. In cognitive radio body area networks (CRBANs), the transmission parameters of SUs can be adjusted based on EMI constraints to overcome the interference with PUs. CR technology can also improve the QoS of wireless communication between medicaldevices by defining the priority levels of each device [6]. Cognitive tasks are performed by different radio modules, most of which are directly controlled by the MAC layer. In [7], the key design features, standard radio technologies and challenges in WBANs are extensively addressed.