# A Cooperative Transmission Scheme in Radio Frequency Energy-Harvesting WBANs
## 2 Related Works  
Unlike the flexible network topology of traditional wireless networks, WBANs typically use a single-hop or two-hop star network. When designing a relay selection scheme, it is necessary to select the appropriate relay node for the source node to forward the message to the coordinator. We can classify the existing routing protocols into five categories according to the following metrics: network structure, location, temperature, layer, and QoS [14]. A fitness function is proposed in [15] which can determine the placement of relay nodes, and a cuckoo-based scheme is presented using an adaptive step size proportion combined with a fitness function; energy efficiency can be improved and network load can also be balanced. A clustering-base routing scheme is proposed in [16]; a fuzzy function is used and reducing the radiation absorbed by the human body is considered. Both node temperature and the wireless link quality are considered when designing the routing protocol in [17]; potential routing paths are first discovered and then optimized based on the cost for data uploading using a prim's algorithm. To improve the energy consumption and network reliability, a routing scheme is designed aiming to optimize relay node selection in [18]; the distance estimated based on RSSI and the direction calculated according to the MUSIC algorithm are the main criteria considered when choosing the relay. Various network typologies are considered in [19]; different transmission powers are evaluated based on the energy efficiency and network lifetime performances, while two routing schemes are presented considering different strategies keeping SAR in consideration. To take into account more decision factors, a flexible relay candidate scheme is proposed in [20] which adopts an analytical hierarchy process; network performances can be improved under various situations. A clustering process is added in addition to relay selection in [21]; both cluster head and relay node can collect data from sensors and further forward these data to the sink. A routing tree is maintained to balance the energy consumption. Both the energy consumption rate and residual energy are considered when selecting relays in [22]; a heuristic solution is proposed to solve the optimization problem. Virtual MIMO is used to improve the network performances and different cooperative schemes are designed in [23]; network reliability can be effectively improved and fault tolerance can be reduced. A new mobile agent-based DA scheme for WBANs is presented in [24]; nodes in a WBAN form clusters and required data are collected by the base station through a generated mobile agent. The above-mentioned protocols are specifically designed for battery-powered WBANs; potential relay nodes are selected from sensors according to the proposed algorithms, which consider different constraints, and no dedicated relay nodes are needed. By selecting appropriate relay nodes or routing paths, the network's lifetime and transmission performance can be effectively improved. The advantage of a battery-powered network is that, before the energy is exhausted, nodes are not heavily constrained by energy when transmitting data and can transmit larger packets when needed. However, once the battery energy is depleted, nodes are unable to transmit data, which leads to a rapid decline in network performance or even network failure. Battery reset operations can also reduce the quality of user experience. Energy harvesting technology is a promising solution for prolonging the lifetime of communication networks by introducing self-sustainability to energy-constrained devices [25]. Considering the situation where sensors can harvest energy at any time, path cost is calculated in [26], which can be used for the relay selection; efficient data transmission and multi-hop routing can be achieved since multiple parameters are considered when designing the network.