## 4 Ionospheric Channels Analysis  
This section presents the detailed results obtained with the channel sounding performed. The availability of the ordinary and extraordinary NVIS Channels, their cross-correlation coefficient, and the delay spread are exhibited. Furthermore, the frequency offset caused by the SDR's frequency difference was also computed. The first three parameters (availability, correlation, and delay spread) were studied and computed using the second data frame (Figure 6), as they focus on data frame detection and PN sequence correlations. The remaining frequency instability was analyzed using the first data frame (Figure 5) as it was computed packet-wise. Finally, we analyzed the usage of polarizations diversity (the combining of the ordinary (O) and extraordinary (X) channels) and its improvement to the robustness of the NVIS link. Two different combining methods were used and studied: selection combining and equal-gain combining. All the results of this study are the product of observing the data sent over the 12 days. The data of all the days was put together in different graphs to analyze the behavior of our link in the described period. First of all, the availability of the ordinary and extraordinary wave's channels was evaluated. Figure 9 displays the percentage of data frames detected at 5.4 MHz at the reception point in Cambrils. This data frame detection was based on the PN sequences received and their correlation with our known sequence. The availability was defined as follows: the total number of data frames received with respect to the total number of sent data frames. The maximum availability (number of transmitted data frames) was defined as the peak performance (corresponding to the 100% in our graph). The number of data frames detected for every hour was based on this factor and then displayed in the graph. Figure 9 states that the ionospheric channel is not active up until 7 Coordinated Universal Time (UTC) (8 a.m. Central European Time (CET)) and stops being active at 17 UTC time (6 p.m. CET). This result matches with the sunrise and the sunset in the month when the tests were performed (November/December), as it corresponds approximately to the hours of the activation and deactivation of the ionospheric channels. The best availability was between 7 UTC and 16 UTC (8 a.m. and 5 p.m. CET). This high availability corresponds to the day's highest amount of solar activity. Comparing the ordinary and extraordinary channels, it can be affirmed that the extraordinary channel clearly performed better. The extraordinary wave received reached a peak performance of data frames detected at 15 UTC. Two exceptional intervals (7 UTC and 16 UTC) can be identified in Figure 9, in which almost only the extraordinary wave propagated and right-hand circular polarization (RHCP) was received. The ordinary wave (LHCP) rarely propagated, resulting in availabilities between 30% and 40%. These intervals are known as \"happy hours\" [13]. At sunrise the ionization showed a steep gradient and, accordingly, the morning happy hour was short (typically 30 min at mid-latitudes in winter, our scenario). The evening happy hour often lasted more than an hour due to the slower ion recombination processes [13]. Consequently, the highest differences in availability between the two channels coincided with the happy hours mentioned. Figure 9 also exhibits the performance (percentage of data frames detected) \begin{table}
\begin{tabular}{c c} \hline \hline
**Transmitting Power** & **Min** \\ \hline  & 05, 06, 07, 08, 09 \\  & 15, 16, 17, 18, 19 \\  & 25, 26, 27, 28, 29 \\
50 W & 35, 36, 37, 38, 39 \\  & 45, 46, 47, 48, 49 \\  & 55, 56, 57, 58, 59 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experiment for the second data frame (see Figure 6). of both the ionospheric channels. The legend of the graph is defined as follows: OR is the performance of the ordinary wave, XOR is the performance of the extraordinary wave received, and OR and XOR refers to the total performance between both ordinary and extraordinary modes.