## 7 Evaluation
Figure 18 shows the occupancy of the 2.4 GHz bandwidth over time and the amplitude of the interference noise as received by the gateway. Figure 19 represents the channel selection for different metrics and hopping techniques. In order to simplify the visualization, only four hopping techniques (HGFH, WRFH, AFH and SAFH) and two metrics (mean value and standard deviation of RSSI) have been represented, but the analysis has been done for all the eight hopping techniques in Section 5 and all the five metrics in Section 4, as will be summarized at the end of this section. The configuration parameters of some of the metrics and hopping techniques (when needed), are presented in Table 4. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**OBSERVATION** & & 0.1 s & \\
**TIME** & & & \\
**OPERATION** & & & \\
**TIME** & & & \\
**ANALYSIS** & & 1 s & \\
**TIME** & & & \\
**TIME****STEP** & & 1 ms & \\ \hline \hline  & \multicolumn{4}{c}{**INTERFERENCE**} & **WSN** \\ \hline  & **WLAN** & **Bluetooth** & **LR\_WPAN A** & **LR\_WPAN B** & **802.15.4** \\ \hline
**SLOT TIME** & 3 ms & 3 ms & 3 ms & 3 ms \\
**WAIT TIME** & 2 ms & 2 ms & 2 ms & 2 ms \\
**HOP TIME** & - & 5 ms & - & - & 5 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Time parameters employed in the analysis. Figure 18: Interference noise at the gateway location: **(a)** Frequency pattern. The WLAN network transmits in the 6 th channel of the 802.11 standard, centred at 2437 MHz with a bandwidth of 22 MHz (red). The LR-WPAN A network transmits in the 4 th channel of the 802.15.4 standard, centred at 2420 MHz with a bandwidth of 3 MHz (yellow). The LR-WPAN B network transmits in the 13 th channel, centred at 2465 MHz (green). The Bluetooth network transmits over seven frequency channels with a bandwidth of 1 MHz each, hopping from channel to channel in a random way (blue); **(b)** Signal strength. The signal strength of each interfering network at the gateway position is calculated according to the free-space path loss model. The four interfering networks are at the same distance from the gateway, but have different signal strengths as they have different transmission powers. For the reduced-hop-set techniques (HGFH and AFH), \(M=10\) channels have been selected. The HGFH technique selects the 10 best channels directly from the channel gain, while the AFH technique selects the 10 best channels from the cumulative sum of the modified power metric (Equation (11)). With HGFH, both the mean and standard deviation metrics result in the same subset of channels, whereas with AFH, the resulting subset of channels are slightly different. The WRFH and the SAFH use all the available channels, but the best channels are used more often than the worst ones. The channel quality is determined from the cumulative metric: the higher the slope of the cumulative metric, the higher the probability for the corresponding channel to be selected. The resulting hopping sequence is represented in Figure 20. The interference noise from all the interfering networks is represented in red and the signal of interest is represented in green. The overlapping of the interference noise and signal is represented in black, but as we have mentioned before, aside from the overlapping, the signal to noise ratio has to be higher than the receiving sensitivity for the communications to fail. The bad channel avoidance is best observed here. The HGFH, for instance, totally avoids the 22 MHz band of the WLAN network. However, it does not manage to avoid the 3 MHz bands of the LR-WPAN A (channel 4) and LR_WPAN B (channel 13) networks when choosing 10 channels. The AFH tries to avoid the occupied channels while maintaining certain level of frequency dispersion and, thus, the WLAN is not always avoided. WRFH and SAFH do not avoid any of the occupied channels, but channels within the 22 MHz band of the WLAN network are used less often than good channels. The amplitude gains of the signal of interest and noise are represented in Figure 21 for the specific connection between the first node and the gateway.