# Physical Layer Definition for a Long-Haul HF Antarctica to Spain Radio Link
## 4 Physical Layer Tests Analysis
### Previous Work  
Before detailing the novelty of the work in this paper, we detail part of the previous studies performed over the 12,760 km long-haul ionospheric channel. Soundings of the channel were conducted since 2004 campaign, and their first results were interpreted in [6], with a new digital platform consisting on an FPGA, an Application Specific Integrated Circuit (ASIC) and a high speed D/A converter; it showed results about both narrowband and wideband soundings. Just after that, we improved the sounding measurement in [11] with a new hardware platform (already described in Section 3) and obtained encouraging results in terms of Frequency of Largest Availability (FLA) and availability for all time of the day and frequency sweep. In this moment of the project, the sounding was already obtaining the required data in detail to define the best transmission frequency table once analyzed the FLA for each campaign [23] (see Figure 8 [24]). This led the system to settle a table of transmitting frequencies to test the performance of several modulation techniques. See Table 2 for an example of the transmission frequency proposals during the 2014 campaign; they were decided after the availability study averaging the results for several days. Figure 9 compares the selected transmission frequencies to the availability measured _a posteriori_ on a certain day, showing that the best FLA using the 10-day averaging may not always be the best \begin{table}
\begin{tabular}{l l l} \hline \hline
**Campaign** & **Starting Date** & **Ending Date** \\ \hline
2009/2010 & 1 January 2010 & 2 February 2010 \\
2010/2011 & 22 January 2011 & 26 February 2011 \\
2011/2012 & 30 December 2011 & 24 February 2012 \\
2012/2013 & 6 January 2013 & 22 February 2013 \\
2013/2014 & 25 January 2014 & 18 February 2014 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detail of the last 5 Antarctic campaigns studied. Figure 8: Comparison of the inter-day variation of the FLA, MUF Rec533 (obtained from VOACAP [24]) and the channel availability during 2011/2012 survey. choice at some frequencies for determined day. The transmission frequencies vary depending on the campaign, and even for weeks or days, and have to be redefined while the tests are being conducted in order to adjust their value to the maximum propagation frequency. As it can be observed in Figures 8 and 9, two propagation zones can be defined, separated by the sunrise and the sunset, which present variable propagation performance [7]. From 00 UTC to 07 UTC, propagation is good at low frequencies, and from 18 UTC to 23 UTC. From 08 UTC to 17 UTC, during daylight hours, the propagation is good at high frequencies. The sunrise (around 07 UTC) and the sunset (around 17 UTC) present unstable performance in terms of propagation, as they are classified as change points from daytime to nighttime. More information about this classification can be found in [6; 7; 11]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Hour** & **Frequency** & **Hour** & **Frequency** & **Hour** & **Frequency** \\
**(UTC)** & **(MHz)** & **(UTC)** & **(MHz)** & **(UTC)** & **(MHz)** \\ \hline
00 & 13 & 08 & 12.5 & 16 & 25 \\
01 & 12.5 & 09 & 13 & 17 & 23 \\
02 & 13 & 10 & 19 & 18 & 24 \\
03 & 12.5 & 11 & 24 & 19 & 13 \\
04 & 13 & 12 & 24 & 20 & 13.5 \\
05 & 8.5 & 13 & 24 & 21 & 13 \\
06 & 5 & 14 & 24 & 22 & 13.5 \\
07 & 13.5 & 15 & 24 & 23 & 13 \\ \hline \end{tabular}
\end{table} 
Table 2: Table of the FLA corresponding to 1 February 2014 measured using the average of the previous 10 days of propagation. Hence, the transmission frequencies are continuously being adapted to the channel performance. Figure 9: Channel availability measured on 10 February during 2014 survey. The asterisks show the chosen transmission frequency for each hour depending on the maximum availability of the previous measured days. It is stated that not all the measures of punctual availability match the chosen averaged frequencies.