# Towards Efficient Wireless Body Area Network Using Two-Way Relay Cooperation
## 5 Hybrid Cooperative Scheme for Energy Efficient WBAN
### Posture Changes  
In practical cases, WBAN channels greatly depend on body postures and geometry [32; 33]. To study the quasi-static behavior, various body postures are considered over the defined channel model and proposed hybrid scheme. As the described channel uses distance-based path loss and signal-to-noise ratio (SNR), the geometric distribution of channels associated with various postures should be considered to incorporate the effect of the quasi-static channel. A practical scenario is devised to account for posture changes, considering a subject of height 165 cm, trunk radius of 14 cm, arm length of 55 cm, arm radius of 4 cm and arm-body spacing of 2 cm. Three WBAN nodes are deployed on the body with \(S\) being on the wrist, \(R\) on the chest and \(D\) on the waist. The geometric distribution of channel for various postures and its effect on source-destination hop-length is elaborated in Figure 12.  
_Standing posture:_ The graphical interpretation of links between the nodes is shown in Figure12. It is clear that the hop-length \(d_{SD}\) of direct link (\(S-D\)) is smaller than the hop-length \(d_{SR}\) of relay link (\(S-R\)). Also for the proposed hybrid scheme, \(d_{SD}\) lies in the range of efficiently working direct link. Thus, in case of a still standing posture, direct link works and the efficiency over the link can be seen from EE curves of the proposed hybrid scheme in Figure 13 for a link length of 19 cm. Figure 12: Posture changes. (**a**) standing posture; (**b**) walking posture; (**c**) sleeping posture; (**d**) cycling posture. _Walking posture:_ In the case of walking, as the arm moves to and fro about the standstill position, the position of \(S\) node changes resulting in hop-length changes. The calculations show a transition in link length from about 19 cm to 23 cm for a direct link. The direct link (\(d_{SD}\)) is still smaller than relay link (\(d_{SR}\)). In addition, for the on-body network, the distance is within the range of efficient direct link and the efficiency over this distance variation can be studied from the EE curve of the hybrid scheme. The geometric interpretation of the posture is shown in Figure 12b, with the solid circle indicating the standing position of \(S\) node and hollow circles displaying to and fro motion. In the case of greater arm length or larger span of movement, the hop-length might increase further and exceed the effective range of direct communication. It can be noted here that, even after this threshold, the communication does not switch to relay link because \(d_{SR}>d_{SD}\). In such cases, JNC is an effective candidate for communicating over the link with extended efficient hop-length. The EE for walking posture is shown in Figure 13 for both uncoded and coded links. _Sleeping Posture:_ A sleeping posture where the hand is placed below the face is depicted in Figure 12c. The posture is considered temporarily stable for longer durations with little movement. The distance between source and destination becomes 40 cm, greater than the range of direct link as proposed by the hybrid scheme for both in-body and on-body networks except LOS. In addition, \(d_{SD}>d_{SR}\) so communication would occur over relay link and the efficiency can be seen from proposed hybrid curves with a little deterioration, given the asymmetrical relay link in accordance with Table 4. The EE for the specified posture including the effect of asymmetrical link is shown in Figure 13.  
_Cycling Posture:_ The body is bent a little forward with the hands held in front in this posture. The graphical representation along with hop-lengths between sensors is shown in Figure 12d and the EE of proposed hybrid scheme is shown in Figure 13. As the distance from direct link exceeds the effective range of the direct link except for the LOS scenario, the transmission occurs over the relay link.