## 4 Link Reliability Evaluation  
The PDR and RSSI are the main metrics used to characterize the reliability of wireless links in the indoor environment. In this section, we demonstrate whether PDR and RSSI could be improved using the quasi-isotropic antenna compared to the Tx and Rx pair of antennas with omni-directional patterns. The performance evaluation setup based on IEEE 802.15.4 is as follows: The RE-Mote from Zolertia [32] was used as an evaluation board together with Contiki-NG software [33]. The transmit power was 5 dBm and the receiver sensitivity was \(-95\) dBm. The inter-packet transmission interval was 50 ms and each experimental point ran for 10 min. That is, there were 12,000 packet transmissions. The comparison group used the commercial antenna with an omni-directional radiation pattern for both the transmitter and the receiver as a reference. We replaced the receiver with the built prototype of the quasi-isotropic antenna to evaluate the PDR and RSSI performance. The photos of the antenna are shown in Figure 11. The PDR and RSSI values are all measured at 2.47 GHz. The measurement under the LOS environment was carried out in a typical corridor with a concrete wall and glass windows as shown in Figure 12a. The distance (d) between the transmitter and receiver was varied from 3.6 to 36 m, with a step size of 5.4 m. For the NLOS environment, the transmitter and receiver were blocked by a thick concrete wall and metal door, as shown in Figure 12c. Its associated  
Figure 11: Antennas for the performance evaluation setup: (**a**) Zolertia board with default omni-directional antenna and (**b**) Zolertia board with quasi-isotropic antenna. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & **Frequency** & **Height** & \multirow{2}{*}{_ka_} & **Gain Deviation** \\  & **(GHz)** & **(\(\Omega_{\text{g}}\))** & & **(dB)** \\ \hline [14] & 2.465 & 0.116 & 1.05 & 5.6 \\ \hline [20] & 2.45 & 0.0063 & 0.73 & 3.14 \\ \hline [21] & 2.465 & 0.0065 & 1.63  
photo is shown in Figure 12b. We also evaluated the link reliability when the transmitter and receiver were located at different floor levels of the building. The measured PDR and RSSI values are given in Figure 13. In Figure 13a, where the PDR across the distance under LOS is plotted, there is comparable performance between the different pairs of the antennae. Next, RSSI across the distance was measured under LOS, and is plotted in Figure 13b. There still did not appear to be a big difference between the two pairs, but the one with a quasi-isotropic antenna at the receiver generally performed better. This is because there were still scattered fields from the ground and walls despite the transmitter and receiver directly seeing each other. The measured values under NLOS are shown in Figure 13c,d. The position of the receiver was moved from position 1 to 6 in Figure 12c. Note that LOS was still achieved when the receiver is at position 1. In Figure 13c, PDR degradation is observed when the omni-directional antenna is at position 4 due to the metal obstacle and weak multipath signal. Note that the PDR was better at position 4 when the designed quasi-isotropic antenna was used. It generally provided robust performance for various NLOS  
Figure 12: Measurement setup. (**a**) Photo for the LOS environment. (**b**) Photo for the NLOS environment. (**c**) Floor map for the NLOS environment. positions. The same situation can be observed in the case of measuring the RSSI again in Figure 13d. The RSSI was again degraded for the omni-directional antenna at position 4, which was due to the metal obstacle and weak multi-path signal, whereas the antenna pair with the quasi-isotropic antenna generally provided robust performance for different NLOS positions. Consequently, we demonstrate that the quasi-isotropic radiation pattern would be useful in improving communication performance under NLOS. Figure 13: Measured PDR and RSSI. **(a)** PDR under LOS, **(b)** RSSI under LOS, **(c)** PDR under NLOS, and **(d)** RSSI under NLOS.