## 3 MAC Protocols for CRBANs
### HCV: Hybrid Cognitive Validation Platform for WBANs  
Because of their complexity and energy inefficiency, field-programmable gate arrays (FPGAs) are not reliable for wireless networks, like WBANs. Instead, a hybrid cognitive validation platform (HCV) [26] was proposed by integrating software and hardware devices. An adaptive CR-MAC algorithm was implemented to minimize the impact of interference. HCV is based on programmable system-on-chip (SoC) processors and provides a simulation environment close to the practical scenario.  
Network simulation tools, such as network simulator version 2 (NS-2), optimized network engineering tool (OPNET), _etc._, are widely used and are convenient to develop, deploy, debug and modify different algorithms and parameters. However, the simulation software faces several challenges, such as modifying the PHY layer, which is very difficult to accurately implement. The HCV is composed of two parts, the lower part and the upper part. The lower part consists of the PHY and MAC layers, and the upper part comprises the CR algorithm and the network layer. Figure 7 shows the system architecture of HCV. The interference controller generates traffic distribution for the evaluation. In HCV, the workflow of a CRBAN includes the WBAN control process and the CR process.  
To mitigate coexisting interference, an adaptive CR MAC is implemented, as shown in Figure 8. The coordinator node is equipped with a high-capacity battery. The nodes in a CRBAN form a star topology with the central coordinator node. The coordinator node assumes the responsibility of managing the entire CRBAN, as well as CR functions. The CR algorithm analyzes the channel parameters according to sensing results, estimates the idle time and passes the result on to the network controller. The network coordinator broadcasts a beacon with the information regarding each superframe. Having received a beacon, member nodes attempt to access the channel during a superframe.  
For the implementation of HCV, a programmable wireless SoC CC2510 was used. CC2510 is a high-performance, low-cost, low power consumption 2.4-GHz system SoC designed for power-critical wireless systems. This chip is integrated with a microcontroller, memory, RF transceiver and other common interferences, such as the universal synchronous/asynchronous receiver/transmitter (USART). In this system, the MAC layer is bound to the firmware of SoC, whereas the PHY layer corresponds to the RF module. To evaluate the performance of the adaptive MAC algorithm, two different traffic models are used. The exponential distribution model is applied to generate same-frequency interference. Four categorized priority levels of the traffic model are established based on real applications, such as mobile healthcare. Without CR functions, the collision rate increases linearly according to the increase in interference traffic.  
The HCVP platform effectively integrates the hardware and software with the PHY and MAC layers working on practical hardware devices and the CR algorithm. That is, HCVP is based on programmable SoC processors, in which a simulation environment close to the practical scenario is provided, as well. The network throughput is greatly improved by implementing the CR algorithm, because the collision rate gradually decreases when the CR algorithm is introduced. The channel estimation is effectively carried out on the basis of the RSSI measured during the sensing period and the packet collision rate during the WBAN accessing period. HCVP achieves excellent performance on energy efficiency and is suitable for the validation of power-critical applications.  
Figure 8: Adaptive CR-MAC diagram.  
Figure 7: System architecture of hybrid cognitive validation platform (HCVP).