# Field-Based Optimal Placement of Antennas for Body-Worn Wireless Sensors
## 4 Conclusions  
In this paper we present original results of our investigation of an evolutionary optimization of a simple body-worn wireless sensor network comprising three on-body nodes. This case was motivated by the practical need of improving performance of a WBAN designed to monitor firefighters' vital signs and environmental conditions. A number of design assumptions and constraints were imposed by our application, for example the fixed position of the head and arm nodes predetermined by the types of sensors they use. One degree of freedom available to us was the placement of the chest node in the front-left part of the jacket. In this paper we show that even with such strong design constraints it is possible to considerably decrease the path loss between nodes using an automated optimization procedure based on evolutionary computing (EStra) combined with a computational electromagnetics code (XFdtd) to evaluate the value of the objective function. Depending on the starting point, the improvement will differ but in our experiments it reached about 20 dB. This improvement would result in reduced power consumption of the WBAN node transmitters.  
The three-variable optimization that allowed the change of both the antenna position and the antenna distance to the human body provided better results than the two-variable optimization with a fixed distance. However, this result may be difficult to apply in practice due to the difficulty of controlling the distance e.g., by adding dielectric spacers into the jacket.  
We show that the proposed methodology of sensor network optimization provides satisfactory results both in the 2.4 GHz band and in the 5.8 GHz band. Since the simplified human body model was positively verified for a broad range of frequencies [20], probably the same optimization routine could be successfully applied in other frequency bands.  
We also show that the path loss values obtained at 2.4 GHz with the simplified homogeneous model are comparable to the results achieved with a much more detailed heterogeneous anthropomorphic multi-tissue model. In our study, one advantage of using the simplified cylindrical model is the ease of defining and controlling the antenna displacement with the rotation angle and height only, while keeping a constant antenna distance to the body. Another advantage is reduced computation time resulting from sparser gridding because the complexity of inner organs does not have to be represented. Our approach to the optimization problem is computationally effective. A single iteration lasted less than one minute and the overall simulation time was well below one hour.  
Further investigation is planned for ultra wideband WBAN optimization.  
Figure 11: Comparison of results obtained with simplified model and heterogeneous model.  
**Author Contributions:** This paper was prepared through a collective effort of all the authors. In particular:  
_[PERSON]_ has contributed: Concept of the research, objective function definition, constraints definition, XEDTD script with a simplified human body model, Octave script for objective function calculation, performing the simulations, analysis of the results, preparation of diagrams and figures, preparation of the text;  
_[PERSON]_ has contributed: XFdtd script for geometry generation, simulation setup and gridding control, research into the state-of-the-art in WBAN, compilation of references, interpretation of the simulation results, preparation of the text and its revision;  
_[PERSON]_ has contributed: state-of-the-art and references, optimization algorithm concept, description of the optimization algorithm, Octave scripts encoding the optimization algorithm, formal definition of the optimization problem, statement of objective function and design variables.  
**Conflicts of Interest:** The authors declare no conflict of interest.