## 2 Related Works  
In recent years, several routing protocols have been proposed for WBANs. According to the different routing mechanisms, the energy efficient strategy of WBANs in the network layer is mainly divided into single hop routing, multi-hop routing and cooperative routing. Early research on WBANs mainly focused on the construction of end-to-end network structures, and energy consumption was not a primary concern, so the data was transmitted by using star topologies and single-hop communication modes. The multi-hop routing protocol was proposed and divided into cluster-based routing and cross-layer routing. AnyBody [17], and HIT [18; 19] are cluster-based routing protocols that after dividing the nodes into multiple clusters and selecting a cluster head, forward the data to the sink through the cluster head. These protocols aim to minimize the number of direct transmissions from nodes to the sink. However, the large amount of overhead required for cluster selection is the main drawback of these protocols. WASP [20], CICADA [21; 22] are cross-layer routing protocols, that by controlling the timing of sending data for every node, lower the energy consumption of WBANs, but require exchanging a lot of control information between nodes. After weighing the advantages and disadvantages of single hop routing and multi-hop routing, collaborative routing [23; 24; 25] became widely used because it can distribute the transmission load of the whole network. According to the works presented in [23; 24], the distance between the source node and sink node can be divided into high and low level. When a source node is located at a high level, multiple hops routing will be used. Otherwise, when it located at a low level, it will choose one hop routing, so the life-cycle of the whole network will be greatly improved. The authors in [25] proposed a decode-and-forward (DF) protocol which uses half duplex mode and builds two communication channels (S-D and S-R-D) between the source node and the sink node. Simulations show this protocol can effectively reduce the energy waste caused by packet retransmission. The pairs of different routing mechanisms are summarized in Table 1.  
Certainly, there are many other routing mechanisms. Opportununistic routing [26], PSR [27], PRPL [28], OBSFR [29] are cost-effective routing protocols which select the lowest cost path by monitoring the cost of energy consumption for each path, so the cost-effective relationships between all nodes need to be periodically updated and stored, and a large amount of transmissions and overhead is required to find routes, which also adds complexity to the system. LOCALMOR [30], DMQOS [31] are based on QoS [32] routing protocols, according to the data type collected by the node to select different path metrics to meet the node's demand for data transmission reliability, delay and energy consumption, but QoS routing protocols require too much information that leads to high energy consumption and huge overhead. TARA [33], LTR [34], LTRT [35] are temperature-based protocols, which are mainly designed to minimize the local or overall system temperature rise. The idea behind these protocols is to route data along different paths to avoid a dramatic temperature rise in some nodes leading to human tissue damage and depletion of the node. In fact, each classification of routing protocols only tries to satisfy a specific requirement in WBANs. Moreover, when choosing a path, the global network lifetime [36] is considered to be insufficient, and the greedy algorithm is used when selecting the path, which easily falls into a local optimum.