**3.1 Oblique sounding results**

When we try to characterize a channel, we have to distinguish between the narrowband analysis and the wideband analysis. The narrowband analysis allows us to

**Figure 5.** *Evolution of the sunspot number (SSN) [24] from 1995 to 2017.*

determine the channel availability and the signal-to-noise ratio (SNR). This sounding is performed with tones that range from 2 to 30 MHz with steps of 500 Hz and are measured with a 10 Hz bandwidth during a time interval of 10 s. We concluded that a good quality of service is achieved for a SNR larger than 6 dB in a 10 Hz of bandwidth [8]. The SNR is measured after filtering the signal with some different windows (Hanning, Blackman, Flattop, Kaiser) defined in [11]. In **Figure 6**, we can see the evolution of the signal envelope during the 10 s transmission interval for each window. The SNR is measured as the ratio between the average of the signal power in case of absence of interference and the noise power measured in the seconds where there is no transmission. The Kaiser window turned out to be the best filtering technique.

The SNR and the channel availability (defined in [8]) are studied as a function of frequency and the time of day (see **Figure 7**). The big differences between day and night and between sunrise and sunset are explained in detail in [11].

The wideband analysis allows the characterization of the rest of the channel parameters such as time and frequency dispersion and wideband SNR. This analysis is performed by sending a pseudorandom noise (PN) sequence that allows us to obtain an estimation of the channel response as a function of time. The channel matrix, that is, the evolution of the channel impulse response over time, is calculated correlating the received signal with the original PN sequence. Then, the scattering function is calculated as the fast Fourier transform (FFT) of the channel matrix. A detailed analysis of the scattering function is key to determine modulation, frame length, separation between data block and data corrector, and other issues of the physical layer [9].

In **Figure 8**, we can see the averaged channel parameters for the campaign 2013–2014. During daytime, high frequencies (20–30 MHz) show the highest delay spread (up to 4 ms) and Doppler spread (up to 1.5 Hz). That means a bigger amount of intersymbolic interference and a higher degree of variability. Sunset and sunrise are the most unstable moments because the ionosphere is changing due to the ion formation or ion recombination. They are, therefore, the least suitable periods for the transmissions. Finally, nighttime is the most stable moment from 19 to 06 UTC.

**29**

capabilities.

**3.2 NVIS sounding results**

*(b) mean of Doppler spread (in Hz).*

**Figure 7.**

**Figure 8.**

For the NVIS channel, there are a few factors to be taken into account. For the narrowband analysis, we only have to check the nearby ionograms and choose the optimum working frequency as the 0.85 × foF2, being foF2 the critical frequency of the upper layer of the ionosphere [27]. For the wideband analysis, we have to notice that the NVIS channel is the same channel that affects the ionosonde when it is measuring the height of the different layers to build the ionogram. The ionosonde temporarily stores a file with the IQ components that is used to calculate the critical frequency, virtual height, and total electron content of each layer. If we have access to this raw data file, we can have an initial estimation of the wideband channel parameters [28]. Of course, an ad hoc channel sounding will yield to better results. In **Table 2**, we can see the studies performed with the raw data of the ionosonde of Ebre Observatory. As expected, both the delay spread and the Doppler spread are lower than in the oblique sounding, so the modulation, the length of the frame, and many other parameters will allow a physical layer with higher transmission

*Wideband channel measurements during the campaign 2013–2014: (a) mean of delay spread (in ms);*

*SNR as a function of frequency and time of day (February 17, 2014) during the campaign 2013–2014.*

*Advanced HF Communications for Remote Sensors in Antarctica*

*DOI: http://dx.doi.org/10.5772/intechopen.81108*

**Figure 6.** *Time evolution of the signal envelope for narrowband analysis.*

*Advanced HF Communications for Remote Sensors in Antarctica DOI: http://dx.doi.org/10.5772/intechopen.81108*

#### **Figure 7.**

*Antarctica - A Key to Global Change*

best filtering technique.

issues of the physical layer [9].

determine the channel availability and the signal-to-noise ratio (SNR). This sounding is performed with tones that range from 2 to 30 MHz with steps of 500 Hz and are measured with a 10 Hz bandwidth during a time interval of 10 s. We concluded that a good quality of service is achieved for a SNR larger than 6 dB in a 10 Hz of bandwidth [8]. The SNR is measured after filtering the signal with some different windows (Hanning, Blackman, Flattop, Kaiser) defined in [11]. In **Figure 6**,

we can see the evolution of the signal envelope during the 10 s transmission interval for each window. The SNR is measured as the ratio between the average of the signal power in case of absence of interference and the noise power measured in the seconds where there is no transmission. The Kaiser window turned out to be the

The SNR and the channel availability (defined in [8]) are studied as a function of frequency and the time of day (see **Figure 7**). The big differences between day

The wideband analysis allows the characterization of the rest of the channel parameters such as time and frequency dispersion and wideband SNR. This analysis is performed by sending a pseudorandom noise (PN) sequence that allows us to obtain an estimation of the channel response as a function of time. The channel matrix, that is, the evolution of the channel impulse response over time, is calculated correlating the received signal with the original PN sequence. Then, the scattering function is calculated as the fast Fourier transform (FFT) of the channel matrix. A detailed analysis of the scattering function is key to determine modulation, frame length, separation between data block and data corrector, and other

In **Figure 8**, we can see the averaged channel parameters for the campaign 2013–2014. During daytime, high frequencies (20–30 MHz) show the highest delay spread (up to 4 ms) and Doppler spread (up to 1.5 Hz). That means a bigger amount of intersymbolic interference and a higher degree of variability. Sunset and sunrise are the most unstable moments because the ionosphere is changing due to the ion formation or ion recombination. They are, therefore, the least suitable periods for the transmissions. Finally, nighttime is the most stable moment from 19 to 06 UTC.

and night and between sunrise and sunset are explained in detail in [11].

**28**

**Figure 6.**

*Time evolution of the signal envelope for narrowband analysis.*

*SNR as a function of frequency and time of day (February 17, 2014) during the campaign 2013–2014.*

#### **Figure 8.**

*Wideband channel measurements during the campaign 2013–2014: (a) mean of delay spread (in ms); (b) mean of Doppler spread (in Hz).*

#### **3.2 NVIS sounding results**

For the NVIS channel, there are a few factors to be taken into account. For the narrowband analysis, we only have to check the nearby ionograms and choose the optimum working frequency as the 0.85 × foF2, being foF2 the critical frequency of the upper layer of the ionosphere [27]. For the wideband analysis, we have to notice that the NVIS channel is the same channel that affects the ionosonde when it is measuring the height of the different layers to build the ionogram. The ionosonde temporarily stores a file with the IQ components that is used to calculate the critical frequency, virtual height, and total electron content of each layer. If we have access to this raw data file, we can have an initial estimation of the wideband channel parameters [28]. Of course, an ad hoc channel sounding will yield to better results. In **Table 2**, we can see the studies performed with the raw data of the ionosonde of Ebre Observatory. As expected, both the delay spread and the Doppler spread are lower than in the oblique sounding, so the modulation, the length of the frame, and many other parameters will allow a physical layer with higher transmission capabilities.


**Table 2.**

*NVIS sounding results from data of the ionosonde at Ebre Observatory.*
