# A Cooperative Transmission Scheme in Radio Frequency Energy-Harvesting WBANs
## 1 Introduction  
Chronic diseases like cancer and cardiovascular disease are the top health killers around the world. Studies demonstrate that these severe illnesses can be cured if diagnosed in the early stage [1]. Therefore, new medical concepts place greater emphasis on proactive prevention and early detection of diseases. Meanwhile, building an efficient and reliable e-health system is an important issue for ensuring sustainable development of humankind. For example, the spread of COVID-19 has greatly impacted the sustainable development of humanity. A well-functioning healthcare system and technology serve as the foundation for ensuring normal human life and work. Thanks to the development of the miniaturization of the integrated circuit, sensing technologies, and wireless communication, patient management such as remote monitoring, disease prevention, and disease diagnostics can be realized by the integrated e-health system. The wireless body area network (WBAN) is a key part of the e-health system, which enables efficient and long-term health monitoring and management. The development of the e-health system has played a positive role in promoting smart healthcare and sustainable development of humanity. WBAN, as a key technology of the e-health system, plays an important role in the sustainable development of the intelligent medical system. A typical WBAN is a heterogeneous network consisting of a group of low-power, miniature, and lightweight wireless sensors designed to monitor various physiologicalparameters. It is important to improve the energy efficiency of WBANs, in addition to ensuring the quality of the service [2]. To enhance network performance and improve energy efficiency, several techniques, such as media access control (MAC) protocol [3; 4; 5], power adjustment scheme [6; 7; 8], sleep schedule [9], and the cooperative relay method [10; 11] are commonly optimized and designed. However, most of the research studies in this area are focused on battery-powered WBANs, which may not be practical to replace or recharge batteries for sensor nodes in some cases [12]. Radio frequency (RF) energy harvesting, as a potential technology that can prolong the lifetime and usage lifespan of sensors, has drawn significant interest recently. The use of RF energy harvesting technology, which can recharge sensors without relying on external environments, provides a suitable solution for certain long-term, low-power medical monitoring applications. Consequently, the MAC protocol must be carefully designed to cater to the requirements of both energy harvesting and data transmission. Simultaneously, it is crucial to enhance energy efficiency since the available energy harvested from the received RF signal is restricted and limited. Researchers deploy dedicated relay nodes to forward packets generated by the sensors [13], where these nodes utilize all the harvested energy to forward packets generated by common sensors. For RF energy harvesting-based WBANs, sensors located far from the coordinator harvest less energy but require more energy for data uploading, leading to poor transmission performance. Although the network performance can be improved when dedicated relays are deployed, the network size and complexity also increased, and it is also a complex issue to consider how many dedicated relays need to be deployed and where to deploy them. Improving network performance based on collaborative transmission among sensors is a potential solution. In an energy-heterogeneous network, nodes with sufficient energy can act as relays to forward packets generated by other nodes to reduce the packet loss rate. To solve the above problems, in this paper, we present a novel cooperative-based relay scheme for RF energy harvesting-based WBANs. We provide a classification scheme which divides the sensors into two categories, relay nodes and common nodes, based on their energy harvesting efficiency and optimized transmission power.