Advanced HF Communications for Remote Sensors in Antarctica

*Joaquim Porté, Joan Lluis Pijoan, Josep Masó, David Badia, Agustín Zaballos and Rosa Maria Alsina-Pagès*

### **Abstract**

The Antarctica is a continent mainly devoted to science with a big amount of sensors located in remote places for biological and geophysical purposes. The data from these sensors need to be sent either to the Antarctic stations or directly to the home country. For the last 15 years, La Salle has been working in the application of HF communications (3–30 MHz) with ionospheric reflection for data collection of remote sensors in Antarctica. We have developed and tested the several types of modulations, the frame structure, the radio-modem, and the antennas for two different scenarios. First, a long-range transequatorial (approximately 12,800 km) and low-power communication system is used as an alternative to satellites, which are often not visible from the poles. This distance is covered with a minimum of four hops with oblique incidence in the ionosphere. Second, a low-power system using near vertical incidence skywave (NVIS) communications provides coverage in a surface of approximately 200–250 km radius, a coverage much longer than any other systems operating in either the VHF or UHF band without the need of line of sight.

**Keywords:** atmosphere science, ionospheric propagation, HF, NVIS, remote sensing, advanced modulations

### **1. Introduction**

There is a strong research activity in Antarctica in the fields of geophysics, meteorology, wildlife, flora, oceanography, and environment, among others. This research activity often involves the installation of sensors in remote places under severe/extreme weather conditions. Most of those sensors are operated with a data logger that stores the data between campaigns until it is recovered after several months.

If we need either to have access to the data throughout the year, or to install a sensor far away from the Antarctic station, we should install a radio transmission system. The standard VHF radios have a reach up to 50 km with line of sight. As the installation of a repeater station is not an option in Antarctica, it only remains to install either a satellite link or an HF link. The satellite services are expensive and are provided mostly by geostationary satellites. The geostationary orbits around the equator are not always easily visible from the poles, so the link is not fully reliable.

The HF band (3–30 MHz) is well known from the beginning of the age of the radio. The ionization of the upper layers of the atmosphere changes the direction of the radio waves in that band, so the ionosphere behaves as a mirror for a certain set

of frequencies. The reflection is strongly dependent on the solar activity, the solar radiation at any time, the terrestrial magnetic field, and the angle of incidence of the wave [1]. For oblique incidence, a range up to 3000 km for a single hop can be achieved, so we can establish a link all over the planet with a few hops. The antenna in those applications should have a maximum of radiation toward the horizon.

For near vertical incidence skywave (NVIS), we can achieve a circular coverage area with a radius up to 250 km without the need of line of sight. The most suitable frequency for vertical incidence is lower than in oblique incidence, so a larger size of the antenna is needed. The antenna should have a maximum of radiation upward, so horizontal dipoles, inverted V, and loops are the best options [2]. As the distance is much lower, the transmission power required can be reduced to tens of Watt, so the application to remote sensors with power restrictions is straightforward. In the world where an increasing number of devices are going to be interconnected in an Internet of Things paradigm, a system able to communicate sensors located hundreds of kilometers away without any additional infrastructure is really welcomed.

A first step when developing a new physical layer is the sounding and characterization of the channel. Apart from the classical model of Watterson [3] for narrowband communications and Mastrangelo [4] for wideband communications, a significant amount of research has been done in ionospheric channels for a single hop at high latitudes in the Arctic [5–7]. However, few works can be found about channel sounding for long-range links with multiple hops from the Antarctica.

For the last 15 years, our research group has been working in the application of HF communications (3–30 MHz) with ionospheric reflection for data collection of remote sensors in Antarctica. In particular, we have developed a system to communicate the Spanish Antarctic Station (SAS) Juan Carlos I at Livingston Island (62.6°S, 60.4°W) and the Ebre Observatory in Roquetes (Tarragona, Spain) (40.8°N, 0.5°E). It is a 12,700 km link with multiple hops and without the use of any repeater. First, we started to sound the channel and estimate the main parameters of the channel [8–12]. Then, we have developed and tested a wide range of modulations, with its frame structure, the radiomodem, and several antennas for both the long range with oblique incidence and the near vertical incidence scenario [13–17]. We are also developing a self-organized network of NVIS nodes that can handle the delays and the unavailability of the ionospheric channel. The NVIS nodes may behave as a hub able to collect data from other neighboring sensors and transmit the joint data to the central node.

This chapter is organized as follows. In Section 2, the evolution of the hardware of the radio-modem and the antennas is presented. The modem is prepared for both channel sounding and data transmission and has evolved to low-cost software radio platforms. In Section 3, the results of more than 15 years of experience in channel sounding are presented, for both the oblique and the vertical sounding. The variability of the channel as a function of time, season, and year is summarized. In Section 4, the physical layer of the communication system of the modem is introduced. The modulation, the frame structure, and the synchronization techniques for both the long-range modem and the NVIS modem are described. The performance in terms of bit rate and bit-error rate is presented. In Section 5, new routing strategies for NVIS networks are explained. The NVIS node has to collect the data from the sensors nearby and establish a delay tolerant network (DTN) with the rest of the nodes. Finally, Section 6 contains the conclusions and some other applications of HF communications.

#### **2. System**

From the beginning of the project, we pursued a software-defined radio (SDR) hardware platform which was able to both sound the ionospheric channel and

**23**

**Figure 1.**

*Block diagram of the NVIS communication system for remote sensors.*

*Advanced HF Communications for Remote Sensors in Antarctica*

referred to [11] for further details of that system.

detailed in the following section.

**2.1 The NVIS transceiver**

implement different modulation schemes without changing the hardware. This kind of platform was not available 15 years ago, so we had to develop our own platform with the FPGAs Virtex-IV and Spartan-II and the fastest ADC and DAC of that time. The control of the whole system and the rest of peripherals was performed by an embedded PC [10]. For the long-range link between the SAS and Spain, the system was able to transmit along the whole HF band (3–30 MHz), 24 hours a day, with two synchronized receivers connected to two wideband antennas: a monopole with antenna tuner and a wideband inverted V. The transmission power was 250 W and the system was designed to work, connected with the main power. The author is

NVIS communication between remote sensors and the SAS is a quite different challenge. The sensor will be often battery-powered so the transmission power should be as low as possible. Although we started developing our own platform for NVIS [18], we can now find different compact SDR platforms that include FPGA, embedded microprocessor, and AD/DA converters in a very cost-effective platform. The implementation of the low-cost transceiver for remote sensors using NVIS is

The hardware of the system (see **Figure 1**) is composed by three parts: a Red

sampling rate is 125 Ms/s at the RF plane, while it is 100 ks/s at the IQ plane. Internally, the RP contains a Zynq® 7010, based on the Xilinx System on Chip (SoC) architecture. These products integrate a feature-rich dual-core or single-core ARM® processing system (PS) and a Xilinx programmable logic (PL) in a single

The RP (FPGA board) is a low-cost SDR platform dedicated to the transmission and reception of the radiofrequency signals, converting them from analog to digital with an ADC of 14 bits, decreasing the frequency of the signal carrier, processing the signal, and sending the low-pass IQ components to the Rpi3 via Ethernet. Similarly, the Rpi3 can send the IQ components to the RP for the transmission. The

Pitaya (RP), a Raspberry Pi 3 (Rpi3), and different peripherals.

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

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

implement different modulation schemes without changing the hardware. This kind of platform was not available 15 years ago, so we had to develop our own platform with the FPGAs Virtex-IV and Spartan-II and the fastest ADC and DAC of that time. The control of the whole system and the rest of peripherals was performed by an embedded PC [10]. For the long-range link between the SAS and Spain, the system was able to transmit along the whole HF band (3–30 MHz), 24 hours a day, with two synchronized receivers connected to two wideband antennas: a monopole with antenna tuner and a wideband inverted V. The transmission power was 250 W and the system was designed to work, connected with the main power. The author is referred to [11] for further details of that system.

NVIS communication between remote sensors and the SAS is a quite different challenge. The sensor will be often battery-powered so the transmission power should be as low as possible. Although we started developing our own platform for NVIS [18], we can now find different compact SDR platforms that include FPGA, embedded microprocessor, and AD/DA converters in a very cost-effective platform. The implementation of the low-cost transceiver for remote sensors using NVIS is detailed in the following section.

## **2.1 The NVIS transceiver**

*Antarctica - A Key to Global Change*

of frequencies. The reflection is strongly dependent on the solar activity, the solar radiation at any time, the terrestrial magnetic field, and the angle of incidence of the wave [1]. For oblique incidence, a range up to 3000 km for a single hop can be achieved, so we can establish a link all over the planet with a few hops. The antenna in those applications should have a maximum of radiation toward the horizon.

For near vertical incidence skywave (NVIS), we can achieve a circular coverage area with a radius up to 250 km without the need of line of sight. The most suitable frequency for vertical incidence is lower than in oblique incidence, so a larger size of the antenna is needed. The antenna should have a maximum of radiation upward, so horizontal dipoles, inverted V, and loops are the best options [2]. As the distance is much lower, the transmission power required can be reduced to tens of Watt, so the application to remote sensors with power restrictions is straightforward. In the world where an increasing number of devices are going to be interconnected in an Internet of Things paradigm, a system able to communicate sensors located hundreds of kilometers away without any additional infrastructure is really welcomed. A first step when developing a new physical layer is the sounding and characterization of the channel. Apart from the classical model of Watterson [3] for narrowband communications and Mastrangelo [4] for wideband communications, a significant amount of research has been done in ionospheric channels for a single hop at high latitudes in the Arctic [5–7]. However, few works can be found about channel sounding for long-range links with multiple hops from the Antarctica. For the last 15 years, our research group has been working in the application of HF communications (3–30 MHz) with ionospheric reflection for data collection of remote sensors in Antarctica. In particular, we have developed a system to communicate the Spanish Antarctic Station (SAS) Juan Carlos I at Livingston Island (62.6°S, 60.4°W) and the Ebre Observatory in Roquetes (Tarragona, Spain) (40.8°N, 0.5°E). It is a 12,700 km link with multiple hops and without the use of any repeater. First, we started to sound the channel and estimate the main parameters of the channel [8–12]. Then, we have developed and tested a wide range of modulations, with its frame structure, the radiomodem, and several antennas for both the long range with oblique incidence and the near vertical incidence scenario [13–17]. We are also developing a self-organized network of NVIS nodes that can handle the delays and the unavailability of the ionospheric channel. The NVIS nodes may behave as a hub able to collect data from other neighbor-

ing sensors and transmit the joint data to the central node.

conclusions and some other applications of HF communications.

This chapter is organized as follows. In Section 2, the evolution of the hardware of the radio-modem and the antennas is presented. The modem is prepared for both channel sounding and data transmission and has evolved to low-cost software radio platforms. In Section 3, the results of more than 15 years of experience in channel sounding are presented, for both the oblique and the vertical sounding. The variability of the channel as a function of time, season, and year is summarized. In Section 4, the physical layer of the communication system of the modem is introduced. The modulation, the frame structure, and the synchronization techniques for both the long-range modem and the NVIS modem are described. The performance in terms of bit rate and bit-error rate is presented. In Section 5, new routing strategies for NVIS networks are explained. The NVIS node has to collect the data from the sensors nearby and establish a delay tolerant network (DTN) with the rest of the nodes. Finally, Section 6 contains the

From the beginning of the project, we pursued a software-defined radio (SDR)

hardware platform which was able to both sound the ionospheric channel and

**22**

**2. System**

The hardware of the system (see **Figure 1**) is composed by three parts: a Red Pitaya (RP), a Raspberry Pi 3 (Rpi3), and different peripherals.

The RP (FPGA board) is a low-cost SDR platform dedicated to the transmission and reception of the radiofrequency signals, converting them from analog to digital with an ADC of 14 bits, decreasing the frequency of the signal carrier, processing the signal, and sending the low-pass IQ components to the Rpi3 via Ethernet. Similarly, the Rpi3 can send the IQ components to the RP for the transmission. The sampling rate is 125 Ms/s at the RF plane, while it is 100 ks/s at the IQ plane.

Internally, the RP contains a Zynq® 7010, based on the Xilinx System on Chip (SoC) architecture. These products integrate a feature-rich dual-core or single-core ARM® processing system (PS) and a Xilinx programmable logic (PL) in a single

**Figure 1.** *Block diagram of the NVIS communication system for remote sensors.*

device. On the PS, there is a Linux operating system for controlling the PL, where there are all the hardware configurations that allow the different transmission modes.

Although there is a true microprocessor inside the RP, it is not able to handle all the control functions, since the biggest part of the RP is dedicated to the real-time processing in the PL part. That is the reason for adding the Rpi3, a single-board computer that controls the overall system operation. Rpi3 is able to collect data from many wireless sensors connected via Zigbee and encapsulate them on a single frame to be sent through the NVIS channel. In our case, we have used the solution of Libelium [19], with a maximum range between 1 and 8 km, depending on the chosen solution. When Rpi3 either stores 1000 bytes of data or a defined time lapse runs out, it sends the baseband IQ components of the frame to the RP to be transited to the SAS. On the other side, the RP is waiting for reception. When the preamble of the transmission is found, the RP sends the received IQ components to the Rp3, which demodulates all the data frames and extracts the data from all the sensors.

As mentioned before, the transmission power has to be less than 10 W to ensure a proper operation in a battery-powered scenario. As the maximum output power of the DAC is 0 dBm, we need a linear minimum amplification of 40 dB. The power amplifier is controlled by an electronic circuit that switches the supply voltages of the power stages in the proper order. This hardware measures the forward and reverse power, so the Rpi3 could switch the transmission off in case of mismatch. Finally, a band pass filter (BPF) attenuates the out-of-band emissions.

At the receiver site, another BPF from 2 to 7 MHz is placed close to the antenna to filter all the unwanted noise and interferences typical of the HF band, such as AM broadcast stations. As the received signal may be around −90 dBm, an amplifier of 30 dB is needed in order to take the maximum advantage of the dynamic range of the ADC, without saturating the converter. Finally, the received data are stored in a solid-state disk.

The total measured power consumption of the system is about 7.2 W in sleep mode, which is the dominant mode. Once every hour, the transceiver transmits and receives a few seconds with a power consumption of 96 W. Under these conditions, the transceiver will operate for 2 weeks with a battery of 110 Ah.

#### **2.2 The radiating system**

When working in the HF band, we have to expect large-size antennas if we want to achieve dimensions close to half wavelength. If we are going to install the antennas in Antarctica, we will have strict environmental conditions, so no complex installations can be built. Moreover, if the antennas are for remote sensors powered by batteries, we should try to maximize the gain of the antennas in order to reduce the transmission power of the amplifier at minimum. Taking all this into consideration, the best choice are monopoles or wire antennas that need only one elevated point at most.

There is a great difference between the long range with oblique incidence and the near vertical incidence scenario. In the first case, we need a maximum of radiation toward the horizon, so a vertical monopole is the simplest option. In the second case, we need a radiation lobe with elevation angles between 70 and 90**°**, so the horizontal dipole or the inverted V should work better.

The monopole is the simplest antenna for long-range HF communications. It is easy to carry and install and is very robust against adverse weather conditions. As a wideband antenna, we need an antenna tuner that has to be tuned a low power before every frequency change. As the conductivity of the soil gets worse, the angle of maximum radiation is not 0**°** any more, but it rises up to 20–30**°** with respect to the horizon [10]. To prevent this, we have to install a set of radials above the soil to

**25**

**Figure 3.**

**Figure 2.**

*Advanced HF Communications for Remote Sensors in Antarctica*

*Diagram of radiation of a 7.5 m monopole with radials over permafrost.*

*Installation of the monopole with radials in the SAS.*

improve its conductivity. In the simulation, for a 7.5 m monopole over a permafrost soil, we demonstrate that 32 radials of 15 m each, we can expect a 2 dB improvement by using radials (see **Figure 2**). In **Figure 3**, we can see the monopole installed in a

For NVIS applications, the horizontal dipole would be one of the best antennas,

hill nearby the SAS and a more detailed explanation can be found in [13].

but it needs one mast at either end. The inverted V, however, achieves a similar performance with only one mast at the center. For the V of **Figure 4**, we have optimized the height of the antenna (Mast h), the heights of the end (Min h), and the distance from the center to the end (Yf). The length of the V changes accordingly. In **Table 1**, we can see the results for three types of soil: ideal (infinite σ, εr = 6), rural (σ = 0.01, εr = 15), and permafrost (σ = 0.00005, εr = 3), where σ and εr stand for the conductivity and the dielectric constant, respectively, and j is the imaginary unit. The conductivity of the permafrost causes a drop up to 5.5 dB in the gain with respect to the ideal ground. We need a mast height of 13 m to achieve a gain of 1.3 dB. As we want to have a minimum power consumption, we will operate

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

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

*Antarctica - A Key to Global Change*

stored in a solid-state disk.

**2.2 The radiating system**

point at most.

device. On the PS, there is a Linux operating system for controlling the PL, where there are all the hardware configurations that allow the different transmission modes. Although there is a true microprocessor inside the RP, it is not able to handle all the control functions, since the biggest part of the RP is dedicated to the real-time processing in the PL part. That is the reason for adding the Rpi3, a single-board computer that controls the overall system operation. Rpi3 is able to collect data from many wireless sensors connected via Zigbee and encapsulate them on a single frame to be sent through the NVIS channel. In our case, we have used the solution of Libelium [19], with a maximum range between 1 and 8 km, depending on the chosen solution. When Rpi3 either stores 1000 bytes of data or a defined time lapse runs out, it sends the baseband IQ components of the frame to the RP to be transited to the SAS. On the other side, the RP is waiting for reception. When the preamble of the transmission is found, the RP sends the received IQ components to the Rp3, which

demodulates all the data frames and extracts the data from all the sensors.

Finally, a band pass filter (BPF) attenuates the out-of-band emissions.

the transceiver will operate for 2 weeks with a battery of 110 Ah.

horizontal dipole or the inverted V should work better.

As mentioned before, the transmission power has to be less than 10 W to ensure a proper operation in a battery-powered scenario. As the maximum output power of the DAC is 0 dBm, we need a linear minimum amplification of 40 dB. The power amplifier is controlled by an electronic circuit that switches the supply voltages of the power stages in the proper order. This hardware measures the forward and reverse power, so the Rpi3 could switch the transmission off in case of mismatch.

At the receiver site, another BPF from 2 to 7 MHz is placed close to the antenna to filter all the unwanted noise and interferences typical of the HF band, such as AM broadcast stations. As the received signal may be around −90 dBm, an amplifier of 30 dB is needed in order to take the maximum advantage of the dynamic range of the ADC, without saturating the converter. Finally, the received data are

The total measured power consumption of the system is about 7.2 W in sleep mode, which is the dominant mode. Once every hour, the transceiver transmits and receives a few seconds with a power consumption of 96 W. Under these conditions,

When working in the HF band, we have to expect large-size antennas if we want to achieve dimensions close to half wavelength. If we are going to install the antennas in Antarctica, we will have strict environmental conditions, so no complex installations can be built. Moreover, if the antennas are for remote sensors powered by batteries, we should try to maximize the gain of the antennas in order to reduce the transmission power of the amplifier at minimum. Taking all this into consideration, the best choice are monopoles or wire antennas that need only one elevated

There is a great difference between the long range with oblique incidence and the near vertical incidence scenario. In the first case, we need a maximum of radiation toward the horizon, so a vertical monopole is the simplest option. In the second case, we need a radiation lobe with elevation angles between 70 and 90**°**, so the

The monopole is the simplest antenna for long-range HF communications. It is easy to carry and install and is very robust against adverse weather conditions. As a wideband antenna, we need an antenna tuner that has to be tuned a low power before every frequency change. As the conductivity of the soil gets worse, the angle of maximum radiation is not 0**°** any more, but it rises up to 20–30**°** with respect to the horizon [10]. To prevent this, we have to install a set of radials above the soil to

**24**

improve its conductivity. In the simulation, for a 7.5 m monopole over a permafrost soil, we demonstrate that 32 radials of 15 m each, we can expect a 2 dB improvement by using radials (see **Figure 2**). In **Figure 3**, we can see the monopole installed in a hill nearby the SAS and a more detailed explanation can be found in [13].

For NVIS applications, the horizontal dipole would be one of the best antennas, but it needs one mast at either end. The inverted V, however, achieves a similar performance with only one mast at the center. For the V of **Figure 4**, we have optimized the height of the antenna (Mast h), the heights of the end (Min h), and the distance from the center to the end (Yf). The length of the V changes accordingly. In **Table 1**, we can see the results for three types of soil: ideal (infinite σ, εr = 6), rural (σ = 0.01, εr = 15), and permafrost (σ = 0.00005, εr = 3), where σ and εr stand for the conductivity and the dielectric constant, respectively, and j is the imaginary unit. The conductivity of the permafrost causes a drop up to 5.5 dB in the gain with respect to the ideal ground. We need a mast height of 13 m to achieve a gain of 1.3 dB. As we want to have a minimum power consumption, we will operate

**Figure 2.** *Diagram of radiation of a 7.5 m monopole with radials over permafrost.*

**Figure 3.** *Installation of the monopole with radials in the SAS.*

**Figure 4.** *Simulation parameters of the inverted V.*


#### **Table 1.**

*Optimization of the inverted V antenna.*

at a single frequency and try to maximize the gain at that frequency. So, we do not intend to have a wideband antenna in this scenario. Checking the reports of ionograms of the last decade from Ebre Observatory [20] and Lowell Digisonde International [21], we came to the conclusion that the best frequency for maximum availability would be 4.5 MHz.

It is important to note that the optimization of the gain is a key factor for the transmission, while the radiation diagram with a maximum around 90° and a minimum at the rest of elevation angles is the most important issue in reception, because we minimize the noise and interference from the nondesired directions [22].

As far as the horizontal dipole is concerned, there is a 3 dB gain improvement, bearing in mind that two mast installations are needed. The optimum height, as discussed in [23], is between 0.16 and 0.22λ depending on the type of soil.

#### **3. Channel sounding**

The ionosphere is one of the layers of the upper atmosphere situated between about 90 and 400 km above the surface. Thanks to its atomic composition, the ionic charge allows radiofrequency signals to rebound and go back to the terrestrial surface, thus creating a communication channel. The ionization of this layer is caused by solar radiation producing the apparition of free electrons that change the refraction index of the medium. As more radiation there is, more ions are exited and the maximum reflected frequency increases. As the sun radiation is the major cause of frequency variation, daily, monthly, seasonal, and yearly variations have to be taken into account. Apart from the variation between day and night, one of

**27**

**Figure 5.**

*Advanced HF Communications for Remote Sensors in Antarctica*

than in previous years (between 4.5 and 6.5 MHz during the day).

the ionosphere behavior along the day in quasi-real time.

noise, interferences, and channel availability [26].

of the sounding of both types of communications.

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

**3.1 Oblique sounding results**

the most significant variations is caused by the solar cycle reaching maximum and minimum levels approximately about every 11 years. The sunspot number (SSN) measures the number of spots visible on the face of the sun. The higher the number, the more radiation ionizes this layer of the atmosphere. **Figure 5** shows the variation of the solar cycle radiation from 1995 to 2017, and the estimation until 2019. We can point out that in the current year (2018), solar radiation is in minimum levels. This minimum level causes that the frequency used for the transmissions is much lower

The ionosphere is observed throughout the world by a set of observatories. Most of them have an ionosonde developed by Lowell Digisonde International [21] and publish the ionogram in a common webpage [25], enabling the parameterization of

As the ionosphere is divided into several layers (D, E, F1, and F2), which are always moving, the ionospheric channel behaves as a slow fading multipath channel, similar to the wireless channels for mobile communications. Hence, the ionosphere can be characterized by the following parameters: time dispersion (multipath, delay spread), frequency dispersion (Doppler shift, Doppler spread),

All these factors will allow us to determine the best modulations, size of the frame, and occupied bandwidth to optimize the transmission in both the long-range and the NVIS case. In the next two following points, we aim to describe the 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

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

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

*Antarctica - A Key to Global Change*

availability would be 4.5 MHz.

*Optimization of the inverted V antenna.*

**Soil type Optimization** 

*Simulation parameters of the inverted V.*

**Table 1.**

**Figure 4.**

**algorithm**

**3. Channel sounding**

at a single frequency and try to maximize the gain at that frequency. So, we do not intend to have a wideband antenna in this scenario. Checking the reports of ionograms of the last decade from Ebre Observatory [20] and Lowell Digisonde International [21], we came to the conclusion that the best frequency for maximum

**Gain (dBi)** **SWR Impedance (Ω)**

Ideal Evolve 6.8 1.96 25.6 + 3.2j 11.01 2.00 12.39 Rural Evolve 3.8 1.05 47.7 + 0.4j 10.81 1.87 12.39 Permafrost Evolve 1.3 1.27 63.3 – 1.0j 13.08 2.00 11.51

**Mast h. (m)** **Min h. (m)**

**Yf (m)**

It is important to note that the optimization of the gain is a key factor for the transmission, while the radiation diagram with a maximum around 90° and a minimum at the rest of elevation angles is the most important issue in reception, because

As far as the horizontal dipole is concerned, there is a 3 dB gain improvement, bearing in mind that two mast installations are needed. The optimum height, as discussed in [23], is between 0.16 and 0.22λ depending on the type of soil.

The ionosphere is one of the layers of the upper atmosphere situated between about 90 and 400 km above the surface. Thanks to its atomic composition, the ionic charge allows radiofrequency signals to rebound and go back to the terrestrial surface, thus creating a communication channel. The ionization of this layer is caused by solar radiation producing the apparition of free electrons that change the refraction index of the medium. As more radiation there is, more ions are exited and the maximum reflected frequency increases. As the sun radiation is the major cause of frequency variation, daily, monthly, seasonal, and yearly variations have to be taken into account. Apart from the variation between day and night, one of

we minimize the noise and interference from the nondesired directions [22].

**26**

the most significant variations is caused by the solar cycle reaching maximum and minimum levels approximately about every 11 years. The sunspot number (SSN) measures the number of spots visible on the face of the sun. The higher the number, the more radiation ionizes this layer of the atmosphere. **Figure 5** shows the variation of the solar cycle radiation from 1995 to 2017, and the estimation until 2019. We can point out that in the current year (2018), solar radiation is in minimum levels. This minimum level causes that the frequency used for the transmissions is much lower than in previous years (between 4.5 and 6.5 MHz during the day).

The ionosphere is observed throughout the world by a set of observatories. Most of them have an ionosonde developed by Lowell Digisonde International [21] and publish the ionogram in a common webpage [25], enabling the parameterization of the ionosphere behavior along the day in quasi-real time.

As the ionosphere is divided into several layers (D, E, F1, and F2), which are always moving, the ionospheric channel behaves as a slow fading multipath channel, similar to the wireless channels for mobile communications. Hence, the ionosphere can be characterized by the following parameters: time dispersion (multipath, delay spread), frequency dispersion (Doppler shift, Doppler spread), noise, interferences, and channel availability [26].

All these factors will allow us to determine the best modulations, size of the frame, and occupied bandwidth to optimize the transmission in both the long-range and the NVIS case. In the next two following points, we aim to describe the results of the sounding of both types of communications.
