# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Antarctica Project
#### 3.5.5 Results and Conclusions  
This section highlights the main results obtained from the tests performed regarding the sounding of this long-haul trans-equatorial link between Antarctica and Spain. This information was essential for designing the appropriate physical layer in such a hostile channel.  
The campaign 2005/2006 is presented in Reference [60], where 22 consecutive days were sounding from January to February 2006. The frequencies tested were in the range from 4 to 18 MHz, and bandwidths up to 16.5 kHz. This work present the propagation losses, delay and Doppler spreads and signal to noise and interference ratio.  
The results of the campaign 2006/2007 is presented in Reference [61], corresponding to the soundings performed during 60 days from the 6 th December 2006 to the 5 th February 2007. The measurements include availability, multipath and Doppler spread and SNR. These measurements exhibited different results for daytime and nighttime. We can derive from this work that the maximum delay and the Doppler spread window values are \([-3.5,3.5]\) (ms) and \([-2.5,2.5]\) (Hz), respectively.  
The experiments conducted in the 2013/2014 campaign is presented in Reference [26]. These soundings were carried out for 25 days, between 25 January and 18 February. In this work, the authors measured SNR, availability, multipath delay spread and Doppler spread.  
In Reference [62], the frequency having the largest availability at a given time (FLA) measured in the Antarctica to Spain radio link has been correlated with the measurements of \(MUF(3000)\) of the vertical ionosonde (VIS) deployed along this radio link. The aim of this study is being able to estimate the best frequency of operation using the VIS information. This work has been extended in Reference [63] with more VIS and more campaigns (2009/2010, 2010/2011 and 2011/2012).  
A polarisation diversity work was conducted in Reference [64], where both a monopole and a dipole (inverted-V) at reception were the antennas used in this campaign. These receiving antennas are orthogonal polarized to check whether wave polarization rotation causes differences between simultaneous signals. Polarisation diversity could be used to increase SNR and performance through smart combination of receiving signals, due to the low cross-correlation factor between them.  
Throughout the sounding campaigns four different zones can be distinguished analyzing channel availability: day, night, dawn and dusk. These four time zones differ in the frequency that exhibits better performance in terms of availability, which is the condition for the change of zone. These differences are summarized in Table 5.  
The best reflective frequency during the nighttime are the lowest ones. And the best during daytime are the highest ones. During the day, from 10 to 17 UTC, 22-23 MHz is the optimum frequency in terms of availability, and during the night, from 21 to 06 UTC, two bands nearby 9 MHz and 12 MHz report the largest availability. As sunrise approaches, the Frequency of Optimum Transmission (FOT) increases from 9 to 23 MHz and the FOT decreases from 23 to 12 MHz in the sunset period.  
Finally, two different frame structures were defined to adapt the system to the ionosphere conditions [57]: (i) a frame aiming to high robustness data transmission (Figure 12(a) and (ii) a frame for high throughput data transmission (Figure 12(b)). The difference between these two last options is the data throughput and the chosen modulation, which has been designed based on the results of the soundings during several Antarctic campaigns [65].  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Zone** & **Interval (UTC)** & **Usable Frequencies [MHz]** \\ \hline Dawn & 07–09 & Increases from 6–15 to 20–30 \\ Day & 10–17 & 20–30 \\ Dusk & 18–20 & Falls from 20–30 to 10–20 \\ Night & 21–06 & 06–15 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of the results for the narrowband analysis in terms of availability by time zones