It demonstrates that sensor nodes capture energy from several types of ambient sources, like solar, thermal, vibrational, and RF energy. It also demonstrates the relevance of network actors (e.g., PLS for secure data transmission, cognitive radio techniques for managing a spectrum, and routing protocols) to energize harvesting modules. The integration of energy harvesting with CR-WSNs is especially important for overcoming the constraints of standard WSNs [41; 42]. CR-WSNs allow sensor nodes to dynamically access under-utilized spectrum bands, hence increasing spectrum efficiency. However, dynamic spectrum access adds additional energy overhead that must be carefully handled with efficient energy-harvesting strategies [43; 44]. CR is a dynamic and adaptive communication technology designed to overcome the limitations of conventional spectrum management by enabling radios to intelligently access an underutilized spectrum [45]. This concept hinges on the ability of CR-enabled devices to detect and exploit unused or lightly utilized frequency bands, which are otherwise wasted in traditional fixed spectrum allocation schemes. The primary advantage of CR lies in its ability to maximize spectrum efficiency by enabling opportunistic spectrum access [46], thus mitigating spectrum scarcity issues. In CR systems, users (secondary users) are allowed to access these unused bands (primary spectrum) without interfering with the primary users of the spectrum [47]. This dynamic and flexible spectrum management has the potential to greatly enhance the performance of wireless communication systems, especially in environments wherein spectrum resources are limited or heavily congested. In the context of (EH-WSNs), the integration of CR technology presents an opportunity to optimize both energy and spectrum usage [3; 12], two critical resources in the operation of WSNs. When considering the integration of CR with EH-WSNs [48], it is important to recognize how both systems can complement each other in various scenarios. Energy-harvesting technologies allow WSN nodes to autonomously collect ambient energy, which is typically unpredictable and variable. For example, solar-powered sensor nodes may experience fluctuations in energy harvesting due to environmental factors such as weather conditions, which can affect the available power. To ensure continuous operation under these fluctuating conditions, CR-enabled EH-WSNs can dynamically adapt their communication strategies by selecting the most appropriate spectrum bands based on the availability of both energy and spectrum. This synergy allows for more efficient and reliable communication, as CR can exploit the spectrum in real time, while EH ensures that sensor nodes have the energy required for such dynamic operations. Additionally, by using CR to access less congested frequency bands, EH-WSNs  
Figure 1: Conceptual architecture of energy-harvesting wireless sensor networks. can reduce interference, which in turn minimizes the energy consumed by retransmissions, thus further optimizing energy usage. A particularly relevant CR scenario for EH-WSNs is the opportunistic spectrum access [49; 50], which allows sensor nodes to temporarily switch to available frequencies when the primary user is inactive. In environments wherein sensor nodes harvest energy, the ability to dynamically adjust spectrum usage according to the availability of both energy and spectrum becomes crucial [12; 51]. For example, when the energy harvested by a sensor node is abundant, it can afford to transmit over a wider range of frequencies with higher data rates. However, during periods of low energy harvest, the system can switch to lower power modes or use spectrum bands that are more efficient for low-power transmissions. This adaptive approach not only ensures the sustainability of the network but also enhances the overall network performance by minimizing the risk of communication failure due to insufficient energy or spectrum.