## 2 Computer Simulations  
The simulations of the indoor BAN channel were carried out with the use of Remcom XFdtd(r) [14] package that implements the finite-difference time-domain (FDTD) method for electromagnetic analysis. With the BioPro(r)[14] module the simulation can use the NMR Hershey heterogeneous model of the human body. The model is available with different degrees of discretization of tissues allowing the selection of voxel size from 1 to 10 mm. For further analysis the 10 mm model was used, as it combines a reasonable accuracy of human body representation with an acceptable amount of computer memory occupancy that is required to run simulations on the server available to the authors for relatively large simulation domain including office room walls. The simulations were performed on a PC computer equipped with two nVIDIA Tesla C2075 GPU cards. To benefit from massively multi-thread calculations that can be performed on Tesla cards, the model size had to be limited in such a way that it can fit into the RAM memory available on the cards. The model used for further analysis fits the cuboid (simulation domain) of the following dimensions: 5 m \(\times\) 5.5 m \(\times\) 2.52 m which resulted in a ten-fold reduction of the simulation time compared to the simulations on a 4-core CPU. In Figure 1 the body model is presented along with the location of the transmitter and receiver antennas. Both are half wave dipole antennas shortened for resonance at 2.45 GHz. The transmitter was located on the human body (phantom) chest (position 1 in Figure 1) and the receiver was placed on the back of the torso (position 2) and in the second case on the arm (position 3). Radio link path loss (signal attenuation between the antennas) for 2.2-2.8 GHz frequency range was determined for all simulation scenarios. The simulations were performed for three environments in which human body model was located. In Figure 2 the environment that consists of a conducting ground plane (modelling a floor reinforced with a steel mesh) is presented (Figure 2a) as well as an environment with a ground plane with walls (Figure 2b). Figure 2: Human body model in two environments: (**a**) with a ground plane only; (**b**) with a ground plane and four walls. The third case with a ceiling is not shown but was also implemented. As a compromise between maximum FDTD simulation domain size (limited by memory available to the nVIDIA Tesla cards) and the accessibility of office rooms that could be used for experimental verification of the model, the size of the environment was set up as the cuboid of dimensions: 5 m \(\times\) 5.5 m \(\times\) 2.52 m. The thickness of the wall is in all simulations is 10 cm, typical of partition walls. The parameters of gypsum walls at 2.4 GHz were taken after [15]: \(\varepsilon=1.5\), loss tangent \(\mathrm{tg}\ \delta=0.01\), width \(w=0.01\) m, while the floor and ceiling are represented by a homogenous imperfect conductor with conductivity \(\sigma=1.39\cdot 10^{6}\) (construction steel). In Figure 3 signal attenuation _vs._ frequency plots are shown for antennas located on the chest and the back of the human body for different propagation environments (presented in Figure 2). It can be observed that in the free space model there is one strong, \(-115\) dB null at 2.55 GHz. It is likely that it corresponds to the destructive superposition of the components that get across and around the body (direct and diffracted rays). Adding a conductive floor causes multiple nulls of 10 dB depth. When walls are added the average level of attenuation is reduced significantly, most probably by the dominating multiple reflections that provide a more effective coupling path. The minimum average attenuation is obtained where all four walls, the conductive ceiling, and the floor are present (a complete cuboid). The conductive ceiling introduced multiple nulls of significant depth which is very close to the results obtained from our experimental study.