HF Link Research Articles

Aeronautical Communications

Aeronautical communications is a discipline covering the highly specialized and mission critical communications such as aircraft-to-ground, aircraft-to-aircraft, and aircraft-to-satellite. Recent changes in this field include: advanced satellite communications installations in aircraft, and very high frequency digital radio, and data link applications reporting aircraft position in transoceanic operations, as well as airline, air traffic, and passenger services. Because of such rapid changes in this field, it is critical to aviation safety that we have an expertise in satellite and data link communications, data link and aircraft surveillance applications, aeronautical communications network, HF and VHF data link, circuit mode and packet mode communications.

Fault Diagnosis Based on Machine Learning for the High Frequency Link of a Grid-Tied Photovoltaic Converter for a Wide Range of Irradiance Conditions

The objective of this work is to select a Machine Learning Technique (MLT) to develop a fault diagnosis scheme for the power switching devices of the High Frequency link (HF link) in a grid-tied Photovoltaic (PV) system, without increasing the total number of sensors, and being capable to operate online. Artificial Neural Network (ANN), Support Vector Machine (SVM), Probabilistic Neural Network (PNN) and Naive Bayes (NB) algorithms are considered to solve the problem of fault classification. These four MLTs are compared using the specificity and sensitivity indexes. The inputs of the models are obtained from the mean value of the signals given by the Discrete Wavelet Transform (DWT) of the dc link voltage and the power extracted from the PV panels. Support vector machine algorithm is chosen as the most suitable classifier to diagnose single and simultaneous open circuit faults with lower computational effort. Simulation and real-time hardware-based experimental tests demonstrate that the MLTs are suitable and reliable to diagnose open circuit faults in a wide range of irradiance levels, ranging from 200 W/m2 to 1000 W/m2, even under 6 % and 12 % measurement errors, without increasing the overall system cost.

Vertical and oblique ionospheric soundings performance comparison over the 12,760 km transequatorial HF link between Antarctica and Spain

AbstractThis paper presents the second part of the study of the oblique sounding of a 12,760 km transequatorial ionospheric link between Livingston Island (Antarctica) and Cambrils in Spain. The study consists of the comparison among the oblique sounding results using an inverted V as receiver antenna, the International Telecommunication Union Rec533 HF prediction model, and the vertical sounding results on the path of the long haul channel. The data were collected the last consecutive surveys, when the inverted V antenna was operative (2010–2011, 2011–2012, 2012–2013, and 2013–2014). The ionospheric channel is studied in terms of frequency availability depending on time using the measurements of an oblique incidence sounder (OIS) and also measurements of several vertical incidence sounding stations (VIS) on the path of the long haul channel. The results obtained show similarities between the VIS and the OIS measurements; following previous studies preliminary results, the experiments conducted led us to think that the frequency of large availability can be estimated depending on the VIS sounding results.

Polarization diversity in a long‐haul transequatorial HF link from Antarctica to Spain

AbstractThis paper describes a set of experiments which examine the feasibility of applying polarization diversity to a 12,760 km ionospheric link from Livingston Island (62.6°S, 60.4°W), South Shetlands, to Cambrils (41.0°N, 1.0°E), Spain. A transmitting monopole and both a monopole and a dipole (inverted‐V) at reception are the antennas used in this measurement campaign. Receiving antennas are orthogonal polarized to check whether wave polarization rotation causes differences between simultaneous signals. The results gathered in the paper indicate that there is a marked difference between simultaneous signals at vertical and horizontal polarized antennas, which depends on the hour of the day and the signal frequency. In this low‐throughput HF data link, polarization diversity could be used to increase SNR and performance through smart combination of receiving signals, due to the low cross‐correlation factor between them.

Physical Layer Definition for a Long-Haul HF Antarctica to Spain Radio Link

La Salle and the Observatori de l’Ebre (OE) have been involved in a remote sensing project in Antarctica for the last 11 years. The OE has been monitoring the geomagnetic activity for more than twenty years and also the ionospheric activity of the last ten years in the Spanish Antarctic Station Juan Carlos I (ASJI) (62.7 ° S, 299.6 ° E). La Salle is finishing the design and testing of a low-power communication system between the ASJI and Cambrils (41.0 ° N, 1.0 ° E) with a double goal: (i) the transmission of data from the sensors located at the ASJI and (ii) the performance of an oblique ionospheric sounding of a 12,760 km HF link. Previously, La Salle has already performed sounding and modulation tests to describe the channel performance in terms of availability, Signal-to-Noise Ratio (SNR), Doppler spread and delay spread. This paper closes the design of the physical layer, by means of the channel error study and the synchronization performance, and concludes with a new physical layer proposal for the Oblique Ionosphere Sounder. Narrowband and wideband frames have been defined to be used when the oblique sounder performs as an ionospheric sensor. Finally, two transmission modes have been defined for the modem performance: the High Robustness Mode (HRM) for low SNR hours and the High Throughput Mode (HTM) for the high SNR hours.

Advanced modulation schemes for an Antarctic Long Haul HF Link

This paper presents the comparison of the BER performance between SC-FDE, orthogonal frequency division multiplexing (OFDMA) and single-carrier frequency division multiplexing access (SC-FDMA) in a long haul HF data link with low SNR. The goal of the comparison is to define two operation modes for two QoS; a robust mode and a high throughput mode, in order to transmit data from remote sensors in Antarctica through a 12760 km ionospheric link with a power amplifier of only 200 W. Clipping and amplification of the waveforms have been applied to SC-FDMA and OFDMA in order to compensate the PAPR and therefore maximize the SNR. The Antarctic HF link has been studied for 11 years, and we have described the link in terms of availability, SNR, delay and Doppler profile, and also spread spectrum techniques have been tested for data transmission. Based on those previous studies results we have developed a test bed to compare these transmission techniques, providing promising results in our low power ionospheric link from Antarctica.

Vertical and oblique ionospheric soundings over the long haul HF link between Antarctica and Spain

AbstractThis paper presents a comparative study between the oblique sounding results, the International Telecommunication Union Rec533 HF prediction model, and the vertical sounding results of a transequatorial long haul link. The long haul link is a 12,760 km link between the Spanish Antarctic Station, SAS, located in the Livingston Island and the Ebro Observatory (OE) in Spain. The data were collected during three consecutive surveys (2009/2010, 2010/2011, and 2011/2012). The ionospheric channel from the SAS to the OE is studied in terms of frequency availability as function of time using the measurements of an oblique incidence sounder (OIS) and measurements of several vertical incidence sounding stations (VIS) placed near the estimated radiopropagation path. The results obtained show promising correlations between VIS and OIS measurements and led us to think that the frequency of largest availability for this particular long haul radio link can be estimated from the VIS sounding measurements.

Digital transmission techniques for a long haul HF link: DSSS versus OFDM

This paper presents two digital transmission techniques for long haul ionospheric links. Since 2003 we have studied the HF link between the Antarctic Spanish Base, Juan Carlos I, and Spain; and we have described the link in terms of availability, signal-to-noise ratio, and delay and Doppler power profile. Based on these previous studies we have developed a test bed to investigate two digital transmission techniques, i.e., Direct-Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplexing (OFDM), which can provide a low power, low-rate ionospheric data link from Antarctica. Symbol length, bandwidth, and constellation are some of the features that are analyzed in this work. Data gathered from the link throughout the 2010/2011 and 2011/2012 Antarctic surveys show that the spread spectrum techniques can be used to transmit data at low rate when the channel forecast is poor, but when the channel forecast is good multicarrier techniques can be used to transmit sporadic bursts of data at higher rate.

Analysis, Design, Modeling, and Implementation of an Active Clamp HF Link Converter

This paper deals with the system oriented analysis, design, modeling, and implementation of active clamp HF link three phase converter. The main advantage of the topology is reduced size, weight, and cost of the isolation transformer. However, violation of basic power conversion rules due to presence of the leakage inductance in the HF transformer causes over voltage stresses across the cycloconverter devices. It makes use of the snubber circuit necessary in such topologies. The conventional RCD snubbers are dissipative in nature and hence inefficient. The efficiency of the system is greatly improved by using regenerative snubber or active clamp circuit. It consists of an active switching device with an anti-parallel diode and one capacitor to absorb the energy stored in the leakage inductance of the isolation transformer and to regenerate the same without affecting circuit performance. The turn on instant and duration of the active device are selected such that it requires simple commutation requirements. The time domain expressions for circuit dynamics, design criteria of the snubber capacitor with two conflicting constrains (over voltage stress across the devices and the resonating current duration), the simulation results based on generalized circuit model and the experimental results based on laboratory prototype are presented.

Multipath Separation-Direction of Arrival (MS-DOA) with Genetic Search Algorithm for HF channels

Direction-of-Arrival (DOA) defines the estimation of arrival angles of an electromagnetic wave impinging on a set of sensors. For dispersive and time-varying HF channels, where the propagating wave also suffers from the multipath phenomena, estimation of DOA is a very challenging problem. Multipath Separation-Direction of Arrival (MS-DOA), that is developed to estimate both the arrival angles in elevation and azimuth and the incoming signals at the output of the reference antenna with very high accuracy, proves itself as a strong alternative in DOA estimation for HF channels. In MS-DOA, a linear system of equations is formed using the coefficients of the basis vector for the array output vector, the incoming signal vector and the array manifold. The angles of arrival in elevation and azimuth are obtained as the maximizers of the sum of the magnitude squares of the projection of the signal coefficients on the column space of the array manifold. In this study, alternative Genetic Search Algorithms (GA) for the maximizers of the projection sum are investigated using simulated and experimental ionospheric channel data. It is observed that GA combined with MS-DOA is a powerful alternative in online DOA estimation and can be further developed according to the channel characteristics of a specific HF link.

On the applications of the space time premise when exploiting location diversity of multiple antenna sites

This paper presents novel techniques to exploit the spatial dimension and the space time (ST) premise in a Large Scale Communication Networks LSCN(s). These techniques are based on an encapsulation framework aimed at enforcing an artificial none line of sight (NLOS) condition. The framework integrates the spatially diverse multiple antenna sites (MAS); composing parts of an LSCN into joint transmission/reception systems. The location diversity of these MAS provides for exploiting the spatial dimension whether for multiplexing gain, diversity gain and/or array gain. Whereas multiplexing gain is best exploited in multi input multi output (MIMO) architectures to boost spectrum efficiency; diversity and array gains lend themselves to equally useful applications e.g., in VSAT technology; array/diversity gain can improve the uplink efficiency to satellite, diversity gain may resolve uncertainty issues in HF link, etc. This paper develops a framework for exploiting MIMO in a network to network communications. It also outlines possible applications of location diversity of MAS in LSCNs. Selected scenarios include terrestrial satellite networks, VSAT network topologies, HF communications and broadband global area networks (BGAN).

Military messaging in IP networks using HF links

In the migration toward network-centric warfare, the NATO STANAG 4406 for Military Message Handling Systems (MMHS) may be used for direct information exchange between high-data-rate strategic users and low-data-rate tactical users by utilizing its new tactical protocol profile. In this article the protocol profiles are tested across a "worst case" tactical HF link providing IP services. There are considerable throughput gains available using the tactical profile. Among the various automated HF technologies tested, the new HDL+ data link protocol, suggested for standardization within NATO, shows the best throughput capabilities for short to medium-size messages on typical HF channels. 3G HF is the most robust technology at low signal-to-noise ratios. The 2G HF throughput performance suffers from inefficient linking procedures. There are optimization issues at different levels of the protocol stack, and implementation choices have great impact on overall performance.

Real-time output voltage control method of quasi-ZCS series resonant HF-linked DC-AC converter

This paper proposes a real-time control method of a series resonant high-frequency linked DC-AC converter employing quasi-zero current switching (quasi-ZCS) and a feedback control method for sinusoidal output voltage. An approximate analysis of the converter is performed, and then simplified equations and an equivalent circuit similar to the conventional PWM inverters/inverters are obtained. A real-time feedback control of the converter is realized using the equivalent circuit without detecting HF link current. The usefulness of the proposed control algorithm is confirmed by experimental results. >

A scheme for high performance real-time BER measurement

A statistically based method of bit error rate (BER) which does not necessitate the transmission of test patterns or the interruption of data for measurement purposes is presented. It is demonstrated that the method allows accurate estimation of BER several orders of magnitude faster than the Monte Carlo method. Design of an optimal BER estimator based on the maximum-likelihood principle is proposed. The performance of the technique is demonstrated by simulations for the case of QPSK modulation in AWGN and for a live Rayleigh-fading HF link. The ability of the method to accurately predict BER using relatively few samples means that it is particularly suitable for high-speed BER estimation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Trial results of novel bit error rate monitor scheme over skywave HF link

Details are given of an error monitoring technique applicable to any digital communications system. An example application of the technique to a live skywave HF link using parallel-tone modems is described. The performance of the scheme is presented and discussed for this channel.

Performance of adaptive frequency hopping modem on an HF link

The paper describes the implementation of a robust 75 baud adaptive frequency hopping (FH) model, and its evaluation on an 800 km HF link in which its performance is compared with fixed frequency signal formats. The aim has been to achieve effective data transmission, in very severe channel conditions, comprising interference from other users, noise, and frequency selective fading. The modem uses sixth order frequency diversity, with automatic excision of interference at the receiver, but without the use of a control link from the receiver to the transmitter. The open loop system approach has been used to avoid the difficulty of establishing the control link under difficult transmission conditions. The error structure resulting from the FH modem is such that forward error correcting codes are particularly effective, and their application is investigated extensively. Also, an interleaving technique which is especially relevant to the FH signal is described and used to advantage.

Evaluation of HF multipath propagation effects on a novel data transmission scheme

The performance of a novel bandlimited signal with independent AM and PM channels over an HF radio link is considered. A model HF link simulator consisting of two independent Rayleigh fading skywaves is described. Computer simulation is used to generate test signals, and some results associated with crosstalk effects are presented.

The Calculation of Ionospheric Doppler Spread on HF Communication Channels

Using some basic concepts of ionospheric propagation, and spread calculations are obtained for the HF ionospheric communication channel. Because of sensitivity to the solar control mechanism, the spread results are found to demonstrate a pronounced dependence on time of day and operating frequency. The practical consequences of these calculations are discussed in detail. This includes the definition and generation of the Doppler reduction factor, comparable to the multipath reduction factor, which can be used to determine the appropriate operating frequency on a given HF link.

Test results of a time-dispersed forward error control system

The purpose of this correspondence is to describe some results of a recent test of a forward error control system over operational HF radio circuits. This system employed simple algebraic block codes in conjunction with bit interlacing. In the system tested, symbol separation amounted to approximately one second, thereby allowing single bursts of two seconds to be corrected. Of course, many bursts of shorter duration are also correctable. Tests were conducted using two tone channels on a DCS HF link operating between NAVCOM centers located at Rtockton, Calif. and Cheltenham, Md. The basic test configuration is shown.