# A Lifetime Maximization Relay Selection Scheme in Wireless Body Area Networks
## 2 Related Work  
To extend network lifetime, there are already a number of routing protocols and relay selection algorithm for WSNs in the literature [10; 11; 12; 13]. In [10], [PERSON] et al. proposed an energy-aware routing algorithm that uses a minimum number of hops for transmission of data. By varying the transmission distance, the interconnections between the nodes can be changed and different network topologies can be obtained. An energy balanced robust scheme based on swarm intelligence that chooses the next node based on node's local information was suggested by [PERSON] and [PERSON] [11]. This method balances load evenly among the nodes and is able to achieve longer lifetime. Another approach proposed in [12] reduces the total consumed energy based on two optimization objectives, i.e., path selection and bit allocation. Packets with the optimum size are relayed to the fusion node from sensor nodes in the best intermediate hops. In [13], a relay selection algorithm was proposed to formulate an optimization problem to maximize user data rates and minimize the total transmission power of the network.  
However, porting these solutions from WSNs to WBANs is problematic due to the different network architectures and operating conditions. In WSNs, hundreds to thousands of sensor-nodes cover large areas, offering a considerable degree of redundancy, and use multi-hop communications. On the contrary, WBANs cover an area limited to the human body and offer no redundancy, only involving two hops. Data must be collected reliably under unique characteristics such as frequent varying channel conditions and transmission power restriction that is not presented in WSNs. In a word, an efficient relay selection algorithm should be designed specifically for WBANs.  
In the literature [14; 15; 16; 17], the authors proposed some relay-aided communication process for WBANs to enhance energy-efficiency. These works put an emphasis on the interaction between transmitter sensor and relay node with the consideration of link establishment procedure and overhead costs. However, how to select a suitable relay for a transmitter sensor is still not addressed in these works.  
Routing protocols were also considered in the literature [18; 19; 20] to extend WBAN lifetime based on multi-hop topology. Nevertheless, these works violate the topology restriction in IEEE 802.15.6.  
In [21], [PERSON] proposed an energy-aware WBAN topology model, which optimizes the number and the location of relays to be deployed by an integer linear programming model to minimize the total energy cost of the network. A Ultra Wide Band(UWB)-based WBAN relay selection algorithm was proposed in [22]. The proposed algorithm utilizes an energy-efficient selection criterion to make relay selection for total power consumption minimization. Another cooperative transmission strategy for UWB-based WBAN was proposed in [23] using single-relay to improve the network lifetime. In the paper, an energy efficiency optimization problem is formulated on two practical on-body transmission scenarios. In [24], authors proposed a mechanism for WBANs to prolong network lifetime through formulating the network lifetime as a function of node transmission mode, cooperative node, transmission power and time slot, and maximizing the network lifetime subject to resource allocation constraints, then obtaining an optimal joint relay selection strategy. The authors in [25] presented a game-theoretic relay selection and power control method to investigate the problem of relay selection and power control with quality of service constraints in WBANs. The proposed method focused on energy efficiency, and its performances are examined in various scenarios. However, References [21; 22; 23; 24; 25] only concentrate on the energy consumption rate when selecting relays without considering sensor nodes' other conditions like residual energy. When residual energy of each node is not the same, the lifetime enhancement of these algorithms will be degraded.