## 3 Results and Discussion
### Simulation Results  
The results obtained for the 3D radiation pattern of the cell's shell radiative structures at 868 MHz and 2.4 GHz at vertical and horizontal orientation towards the reflective ground plane are displayed in Figures 9 and 10, respectively. One primary difference between the two communication frequencies is that 868 MHz exhibits fewer influences from the ground plane in both orientations, with a vertical polarisation pattern approaching a torus, whereas 2.4 GHz exhibits a furrow lobe profile. The proximity to the ground plane is reflected in the effects on the RX station as interference and short-range transmission detection.  
Figure 8: Experimental measurements received signal strength indicator (RSSI), bit error rate (BER) and packet error rate (PER) for 868 MHz and 2.4 GHz communication over the 1 m up to 10 m separation between the two RF transceivers. The radiative cell’s shell is in **horizontal** orientation towards the ground plane.  
With the cell orientated horizontally compared with the ground plane, there is a higher coupling with the plane than the vertical configuration, both communication frequencies undergoing furrow lobe profiles while their horizontal maxima is rotated 90 degrees from the precedent set. The presence of the highly irregular profile at Tx i.e., multiple lobes and nulls, will manifest at Rx by inducing shadowing effects when the receiving station's orientation will align an area between the Tx lobes. As a consequence of the 3D radiative profile showing a more regular distribution of intensity in vertical orientation compared with horizontal, the first assumption is that vertical orientation will support better reception of signals. However, as confirmed by experimental and further three-ray propagation model, the signal variation is more dependent on the maxima-minima fields recorded for each simulation and the resulting lobe positions rather than the radiation profile uniformity or smoothness.  
Based on the Three-ray model for the two communication frequencies shown in Figure 11, the vertical and horizontal polarization simulated by the model are in agreement with other previous studies examining similar settings, such as experiments inside concrete tunnels [46] or buildings [47]. The FSPL line plot defines the simulation upper limit while the maxima and minima of the Three-ray model known as multipath fading define the lower bound. Since the 2.4 GHz frequency wavelength is approximatively 2.8 times smaller than for 868 MHz, the number of signal distortions due reflections is larger than for the latter case.  
Figure 10: MATLAB 3D radiation pattern simulation for the cell’s shell radiative structure integrating the SMA and the wired loop for 868 MHz and 2.4 GHz over the ground plane in a **horizontal** orientation.  
Figure 9: MATLAB 3D radiation pattern simulation for the cell’s shell radiative structure integrating the SMA and the wired loop for 868 MHz and 2.4 GHz over the ground plane in a **vertical** orientation.  
When plotting the experimental RSSI averaged values over the propagation simulation results, it can be observed that some values for both investigated frequencies exceed the FSPL line. For the above high values, since the simulation does not include any gain above units derived from antenna position alignment, the Tx and Rx positions should be assumed under optimised alignment. Although for the empirical measurements, the metric distance increased with a 1 m step did not account for the multipath fading, a possible Tx-Rx positioning optimisation can be achieved by following the Thee-ray pattern for which the spatial step is set to one tenth of the wavelength.