In the current context of global communications, HF (High Frequency) NVIS (Near Vertical Incidence Skywave) data networks can be of strategic importance, providing short- and medium-range communication capabilities independent of terrestrial configuration and existing conventional communications infrastructure. They are essential in critical conditions, such as natural disasters or conflicts, when terrestrial networks are unavailable. This paper investigates the development of such systems for HF NVIS data communications by introducing a customized Orthogonal Frequency Division Multiplexing (OFDM) protocol with parameters adapted to HF ionospheric propagation, implemented on Software-Defined Radio (SDR) systems, which provide extensive configurability and high adaptability to varying HF channel conditions. This work presents an innovative approach to the application of OFDM narrow-channel aggregation in the HF spectrum, a technique that significantly enhances system performance. The aggregation enables a more efficient utilization of the available spectrum and an increase in the data transmission rate, which represents a substantial advancement in NVIS communications. The implementation was realized using an SDR system, which allows flexible integration of the new OFDM protocol and dynamic adaptation of resources. The work also includes the development of a messaging application capable of using this enhanced HF communication system, taking advantage of the new features of channel aggregation and SDR flexibility. This application demonstrates the applicability of the protocol in real-world scenarios and provides a robust platform for data transmission under conditions of limited access to other means of communication. Thus, this study contributes to the technological advancement of NVIS communications and opens new research and deployment directions in HF communications.

Abstract The development of a digital high‐frequency (HF) radio communication system requires an ionospheric channel model from the channel impulse response (CIR) measurement. Although the Watterson ionosphere channel model has been available and used for a long time, several CIR measurements have been conducted in all regions of the Earth in an attempt to validate or replace the Watterson channel model with a suitable model for their region. However, only a few CIR measurements were conducted in low‐latitude regions, especially over Indonesia. In this study, we develop the CIR measurement system for the near vertical incidence skywave (NVIS) propagation mode over Java Island based on the software defined radio platform to meet low‐cost and simple operation requirements. The specification of the system is based on the International Telecommunication Union ionospheric channel characteristic document and increased in order to be able to capture higher values. Results from a 1‐week campaign measurement period show the ability of the system to measure the root mean square of time delay within the range of 0.2–1.3 ms and the Doppler shift within the range of 0.7–1.1 Hz in the quiet conditions of the ionosphere. Further measurements will be conducted to obtain a comprehensive ionosphere CIR that is useful for designing the NVIS‐HF digital communication in Indonesia, which is located beneath the crest region of an equatorial ionospheric anomaly.

The Multi-Carrier Modulation (MCM) technique in the Near Vertical Incidence Skywave (NVIS) propagation mode poses a challenge due to the ionosphere’s dynamic behavior. The selection of the main bandwidth and transmission frequency in the MCM-NVIS system should consider the different ionospheric refraction indexes within each available frequency of the ionosphere channel. This ensures that all transmitted sub-carriers arrive at the receiver at an acceptable time. It is important to note that not all transmission frequencies may reach the receiver, and the propagation time of each received frequency may differ significantly. Using sub-carrier frequencies with varying propagation times in the MCM technique may negatively impact the system’s performance. Therefore, information on the NVIS propagation mode frequency window with homogeneous Time of Flight (ToF) values is crucial for successfully implementing the MCM technique. In this paper, we explore the frequency window with homogeneous ToF values for NVIS propagation based on ionograms. The width of the frequency window and its center frequency variation can be used to determine the main bandwidth and transmission frequency of the MCM-NVIS system. Our numeric simulation with the inputs of the widest frequency window based on the ionograms at Kupang in 2022 shows that the maximum value of ergodic capacity is up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{6}$ </tex-math></inline-formula> bps at 20 dB SNR during the daytime.

Разработана методология низкоскоростной передачи информации по ионосферному радиоканалу при одновременном получении информации о его состоянии в результате использования сложного сигнала типа FMCW–FSK. Рассмотрены вопросы выбора параметров такого сигнала с учётом совмещения системы сенсорной диагностики ионосферного радиоканала и передачи информации. Разработан реализующий данную методологию макет программно-конфигурируемой радиосистемы передачи информации и проведена его апробация в лабораторных экспериментах при имитации ионосферного радиоканала многоотводной линией задержки. Introduction. For high-rate and reliable data transmission over the ionospheric channel, selecting the most appropriate operating frequency is crucial, which is achieved through spectral monitoring methods and sensor diagnostics across the entire frequency range. However, for scenarios involving small data volumes, employing a low-rate transmission mode alongside sensor diagnostics is reasonable. In this mode, a brief telegraph message is repetitively transmitted over all available frequencies, increasing the likelihood of finding the optimal frequency for transmission. Enhancing the dependability of message delivery over extremely long distances through HF communication systems represents a challenging scientific objective. It demands further research and the development of methods, algorithms, and hardware solutions to enable highly reliable, low-rate, and small-data volume transmission over ionospheric radio channels using spread spectrum signals. The aim of this research is to develop and experimentally validate a methodology and layout for sensor diagnostics and low-rate data transmission over an ionospheric radio channel, enhancing its reliability. Research methodology.To achieve low-rate data transmission across an ionospheric radio channel, we tackled the challenge of integrating Frequency-Modulated Continuous Wave (FMCW) diagnostic signals with Frequency-Shift Keying (FSK) communication. Signal parameters were chosen considering the peculiarities of processing echo signals for Near Vertical Incidence Skywave (NVIS) radio links. This methodology was then realized on a software-defined platform, USRP-N210, for low-rate data transfer. The methodology and prototype underwent testing in a laboratory experiment. We employed a multi-tap delay line and an adder to simulate a communication channel. The delay line enabled us to replicate various propagation modes and the bandwidth characteristics of the ionospheric radio channel. Findings. Following the experiment, we successfully obtained frequency-dependent power delay profiles for the simulated radio channel and the test telegraph message simultaneously. Our analysis of the results reveals that this method ensures reliable message transmission, even in cases where certain frequency ranges exhibit a high error rate within the channel's transparency band. We shall note, that 39% of the messages were received error-free, corresponding to the transparency band. Furthermore, in diagnostic mode, we can effectively identify the tracks of frequency-dependent signals reflected from the ionosphere, determining both the delay and energy losses. Conclusions. The results we've obtained have practical applications in the development of advanced HF communication systems designed for transmitting concise telegraph messages with exceptionally high reliability over extensive distances. These systems are crucial for sending control commands, brief emergency messages, and telemetry data in regions with limited or no continuous coverage by information and communication networks.

Antarctica is the land of science. Every year, many studies are carried out in diverse disciplines. Some of these studies collect relevant data for their research with sensors. However, Antarctica's lack of telecommunication technologies hardens the possibility of automatizing this data collection. In most cases, the collection is done manually, limiting research projects' time and space scopes. Over the last years, some alternatives have been studied to deploy remote wireless sensor networks in Antarctica. Near-Vertical Incidence Skywave (NVIS) communications are an example of these alternatives. However, NVIS presents problems that cannot guarantee persistent end-to-end connectivity. For this reason, this article assesses adapting a delay-tolerant network protocol, the Bundle Protocol, to deliver sensor data reliably through an NVIS network. The scenario is developed and tested in the Riverbed Modeler simulator, and performance is evaluated through a trustworthiness model. A practical testbed is also presented.

Radio communications in the High Frequency (HF) range are used to transmit information over long distances between one or more correspondents. Techniques and methodsto improve the capacity and reliability of HF communications become key objectives as they are generally limited to narrow bandwidths and low transfer rates. This work aims to implement an automated system for real-time measurement of ionospheric radio channel parameters under Near Vertical Incidence Skywave (NVIS) propagation conditions using Software Define Radio (SDR) platforms. Preliminary testing of the system was carried out both under laboratory conditions using signal generators and high-performance spectral vector analyzers and under real propagation conditions by installation at the established sites. The results showed that the system is capable of performing real-time measurements of signal power, noise power and calculating signal-to-noise ratio for bandwidths up to 24 kHz.

The SHETLAND-NET research project aims to build an Internet of Things (IoT) telemetry service in Antarctica to automatize the data collection of permafrost research studies on interconnecting remote wireless sensor networks (WSNs) through near vertical incidence skywave (NVIS) long fat networks (LFN). The proposed architecture presents some properties from challenging networks that require the use of delay tolerant networking (DTN) opportunistic techniques that send the collected data during the night as a bulk data transfer whenever a link comes available. This process might result in network congestion and packet loss. This is a complex architecture that demands a thorough assessment of the solution’s viability and an analysis of the transport protocols in order to find the option which best suits the use case to achieve superior trustworthiness in network congestion situations. A heterogeneous layer-based model is used to measure and improve the trustworthiness of the service. The scenario and different transport protocols are modeled to be compared, and the system’s trustworthiness is assessed through simulations.

Despite high costs and lengthy deployments, satellite communications have traditionally been used to provide coverage in remote areas. However, given the fact that there is no radio infrastructure available in these areas, Near Vertical Incidence Skywave (NVIS) technology has positioned itself as an attractive alternative to communicate with low-power nodes in remote areas. This type of communication works in the HF frequency range complying with STANAG and MIL-STD standards, which define a physical layer for scenarios that differ from NVIS and low-power communication. The purpose of this paper was to present the definition of a new communication physical layer based on single-carrier frequency-domain equalization (SC-FDE) based on these standards but adapted to the ionospheric communication channel. This physical layer was compared to an OFDM-based layer from a previous study. The experiments performed show that this new approach achieves better results than OFDM in terms of a higher signal quality with a higher specific BER probability. Finally, this layer was also used in the theoretical design of an NVIS gateway to link sensor network devices spanning large-scale remote areas in a secure manner in the context of ubiquitous sensor networks (USN).

Sensor networks have become more popular in recent years, now featuring plenty of options and capabilities. Notwithstanding this, remote locations present many difficulties for their study and monitoring. High-frequency (HF) communications are presented as an alternative to satellite communications, being a low-cost and easy-to-deploy solution. Near vertical incidence skywave (NVIS) technology provides a coverage of approximately 250 km (depending on the frequency being used and the ionospheric conditions) without a line of sight using the ionosphere as a communication channel. This paper centers on the study of the ionosphere and its characteristic waves as two independent channels in order to improve any NVIS link, increasing its robustness or decreasing the size of the node antennas through the appliance of specific techniques. We studied the channel sounding of both the ordinary and extraordinary waves and their respective channels, analyzing parameters such as the delay spread and the channel’s availability for each wave. The frequency instability of the hardware used was also measured. Furthermore, the correlation coefficient of the impulse response between both signals was studied. Finally, we applied polarization diversity and two different combining techniques. These measurements were performed on a single frequency link, tuned to 5.4 MHz. An improvement on the mean bit energy-to-noise power spectral density (Eb/N0) was received and the bit error rate (BER) was achieved. The results obtained showed that the extraordinary mode had a higher availability throughout the day (15% more availability), but a delayed spread (approximately 0.3 ms mean value), similar to those of the ordinary wave. Furthermore, an improvement of up to 4 dB was achieved with the usage of polarization diversity, thus reducing transmission errors.

The number of Internet of Things (IoT) devices has experienced a large growth during the last decade, as well as the data volume gathered from remote sensors. Satellites are still a suitable communication method and may be preferable for a remote ubiquitous sensor network (USN), which sometimes are located in places without much communications infrastructure where coverage is the principal drawback. Alternatively, the proposed solution for this article aims at a near-vertical incidence skywave (NVIS) channel for high frequencies (HF) with a low-cost platform, allowing a low-power transmissions coverage area up to 250 km for USN. The HF standards are focused on generic communication channels not being robust for NVIS communications. In this article we study and test an alternative based on orthogonal frequency-division multiplexing (OFDM) modulations to make them more robust and less dependent on the channel NVIS communications. For that purpose, we test the HF standard modulations and a designed OFDM modulation to prove the robustness of each. This study has been tested between Barcelona and Tarragona, using different transmission power levels and modulation orders.

Measuring parameters of the scattering function of near vertical incidence skywave (NVIS) radio links employed for data transmission is topical and fosters better understanding signal propagation as well as developing state-of-the-art cognitive radio systems. In this regard, the paper presents the findings of the research into the application of a noise-like phase-shift keying (PSK) signal for vertical sounding of NVIS radio links. There are presented the developed algorithms for processing the PSK signal to measure the scattering function and its parameters (signal-to-noise ratio, delay spread and Doppler spread) for multiple communication channels in the high frequency (HF) band. To carry out experiments on measuring scattering function of radio channels with bandwidth of 24 kHz and different mid-band frequencies, we employed developed ionosonde based on the universal software radio peripherals (USRP) platform.

We present a systematic method for designing dual-polarized, platform-mounted, high-frequency (HF) antennas for near vertical incidence skywave (NVIS) applications. To overcome the bandwidth (BW) and efficiency limitations of electrically small antennas, the characteristic mode (CM) theory is used to exploit the metallic platform as the main radiator. Low-profile coupling elements are mounted on different locations of the platform to excite two orthogonal, horizontally polarized CMs. This way, dual-polarized operation is achieved with a high isolation and sufficiently wide BW. In the design process, several key practical issues are taken into account and their impacts on the performance of the antenna are examined carefully. These include the type and design of the coupling elements, the presence and type of the earth, and the non-idealities and power handling capability in the impedance matching network components. The simulation results of both a full-scale and a 1:50 scaled model of the proposed structure are presented. The scaled-model prototype was fabricated and characterized in the presence of a ground plane emulating the properties of real earth at the scaled frequency of operation. The measurement results show a good agreement with the simulations, demonstrating the efficacy of the proposed approach in designing dual-polarized, platform-based HF antennas.

Every year, the number of ubiquitous sensor networks (USN) is increasing and the need for remote USN communications is emerging in some scenarios. As an alternative to satellite communications, more interests are focused on high frequencies (HF) communications as a low-cost option to reach links of more than 250 km without a line of sight. The HF standards are designed for generic communication channels being not robust for near vertical incidence skywave (NVIS) USN. In this article, we propose a new protocol for USN in remote places based on NVIS communications. For that purpose, we study the main characteristics of the NVIS channel with the presence of groundwaves, particularly in Antarctica. We analyze the availability of the channel, the height of the layers, the delay spread, and the Doppler spread. On the basis of the results obtained, we propose two protocols based on an OFDM (orthogonal frequency division multiplexing) modulation depending on the presence of the groundwave at the receiver. Finally, we make a simulation of the two OFDM configurations and we compare it with real tested standard modulations. The results show a better performance of the new protocol compared to the current HF standards.

Every year more interest is focused on high frequencies (HF) communications for remote sensing platforms due to their capacity to establish links of more than 250 km without a line of sight and due to them being a low-cost alternative to satellite communications. In this article, we study the ionospheric ordinary and extraordinary waves to improve the applications of near vertical incidence skywave (NVIS) on a single input multiple output (SIMO) configuration. To obtain the results, we established a link of 95 km to test the diversity combining of ordinary and extraordinary waves by using selection combining (SC) and equal-gain combining (EGC) on a remote sensing platform. The testbench is based on digital modulation transmissions with power transmission between 3 and 100 W. The results show us the main energy per bit to noise spectral density ratio (Eb/N0) and the bit error rate (BER) differences between ordinary and extraordinary waves, SC, and EGC. To conclude, diversity techniques show us a decrease of the power transmission need, allowing for the use of compact antennas and increasing battery autonomy. Furthermore, we present three different improvement options for NVIS SIMO remote sensing platforms depending on the requirements of bitrate, power consumption, and efficiency of communication.

Near-vertical-incidence skywave (NVIS) propagation is defined as providing continuous coverage from nearly 0 km (just beyond the line of sight) to a couple hundred kilometers from the transmitter with no skip or dead zones. NVIS communications are especially effective during disaster-relief operations when infrastructure is severely damaged. The ability to accurately determine the performance limiters of NVIS propagation can help in the planning of high-frequency (HF) (3-30 MHz) emergency communication links. In the literature, widely varying radial distances (from as few as 50 to up to 160 or even 320 km) for the coverage attainable by NVIS propagation have been reported. It is very difficult to plan an NVIS link for homeland security or disaster relief when the published guidelines vary to such a degree. In this study, a scientific approach was utilized to determine the NVIS performance limiters for varying solar conditions, times of day, and geophysical locations.

In this study, the authors propose a new idea to employ mantle cloaking in order to reduce the mutual coupling between electrically small antennas (ESAs) placed on man-pack radio systems. They determined and numerically optimised the parameters of the mantle cloak on loop and monopole antennas; these antennas are to provide near vertical incidence skywave (NVIS) and ground wave high frequency (HF) communication in man-pack radio systems. The authors’ designed cloak can effectively minimise the mutual coupling between the ESAs – in the man-pack radios – for only a very small impact on the antenna's radiation characteristics. The authors believe that these improvements pave the way toward effectively applying these compact radio structures in emergency HF communication in areas with limited access to conventional communication links.

A multiple-input multiple-output (MIMO) system is studied to improve the channel capacity in high frequency (HF) communication systems. There has not been any theoretical explanation for the channel capacity improvement by invoking the orthogonal circularly polarized ordinary (O) and extra-ordinary (X) modes inherent in HF skywave propagation, especially when a pair of horizontal antennas with orthogonal linear polarization are used at both transmit and receive sides. This letter reports the formulation of $2{\times }2$ MIMO HF near vertical incidence skywave (NVIS) channel matrix elements that can be used to explain capacity improvement in systems employing horizontal linear antennas over that of single-input single-output (SISO) systems. The result suggests that the capacity improvement relies on the diversity phenomenon due to the presence of the O and X waves, which is observable in experiment and simulation of a 7 MHz NVIS link.

SummaryAlthough many physical layer solutions have appeared for remote sensors and Internet of things during the recent years, none of them is suited to very remote sensors in areas away from any mobile operator coverage. In that case, a solution on the basis of near vertical incidence skywave (NVIS) with reflection in the ionosphere may be very attractive. Using NVIS, no line of sight is needed and the coverage is much bigger than any other system operating in either the very high frequency (VHF) or ultra high frequency (UHF) band. In this paper, we present a new transmission scheme for very remote sensors using the NVIS transmission technique.

This article describes the design and validation of deployable low-power probes and sensors to investigate the influence of the ionosphere and the Earth’s magnetic field on radio wave propagation below the plasma frequency of the ionosphere, known as Near Vertical Incidence Skywave (NVIS) propagation. The propagation of waves that are bent downward by the ionosphere is dominated by a bi-refractive mechanism called ‘magneto-ionic propagation’. The polarization of both downward waves depends on the spatial angle between the Earth’s magnetic field and the direction of propagation of the radio wave. The probes and sensors described in this article are needed to simultaneously investigate signal fading and polarization dynamics on six radio wave propagation paths. The 1-Watt probes realize a 57 dB signal-to-noise ratio. The probe polarization is controlled using direct digital synthesis and the cross-polarization is 25–35 dB. The intermodulation-free dynamic range of the sensor exceeds 100 dB. Measurement speed is 3000 samples/second. This publication covers design, practical realization and deployment issues. Research performed with these devices will be shared in subsequent publications.

Design of an electrically small, low-profile vehicular on-the-move (OTM) wideband high frequency antenna with 24 kHz instantaneous-tuned bandwidth is discussed for near vertical incidence skywave (NVIS) communications. Improvement in performance is obtained by applying multiarming and offset feed antenna modifications. These allow for attaining the desired gain >−20 dBi at 3 MHz with a lower profile compared to traditional OTM 3 kHz bandwidth antennas. Specifically, the addition of the second arm improves the bandwidth, whereas NVIS gain is enhanced by offset feeding. The electrical dimensions of the proposed two-arm design over the platform are $\lambda /13 ~(\text {L}) \times \lambda /30 ~(\text {W}) \times \lambda /400$ (H) at the lowest frequency of operation. Antenna performance and achieved benefits under the condition of mechanical breakdown are also discussed. The scaled prototype of the proposed antenna mounted on a selected vehicular platform is built using additive manufacturing. Measurement results agree well with simulations thereby verifying the design procedure and all the discussed findings.