# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 1 Ionospheric Historical Context  
The first steps in wireless communications led us back to the Treatise on Electricity and Magnetism [1], by [PERSON], presented in 1873. It somehow founded the current modern electromagnetism propagation theories; nevertheless, it was not until 1888 when the first radiowaves were effectively detected by [PERSON], who demonstrated [PERSON]'s theory by means of the perturbation over a spark coil. This work later inspired [PERSON], who started experimenting with wireless telegraphy; he sent messages in Morse code using a radiolink in 1896. It was not only until 1901, when a 3000-km transatlantic telegraphic signal was detected, breaking the theory that any radiolink required a direct line-of-sight (LOS) to propagate [2]. It was in the 1920s when [PERSON] [3] managed to demonstrate that signals could be detected without LOS anywhere in the planet by means of the reflection over an electrically-charged region of the atmosphere [4]. It was around 1950 when the Institute of Radio Engineers (IRE) defined the ionosphere as the _part of the outer atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves_[5]. This region of the Earth's outer atmosphere begins around 60 km high and includes the thermosphere, as well as parts of the mesosphere and the exosphere [6, 7]. Communication researches have been widely interested in the study of the ionosphere, due to the fact that it is a time and frequency dispersive channel in the HF range [8]. The approach to the characterization of the ionospheric performance as a communication channel is mainly conducted by means of a ionosonde, which is a transmitter that covers the entire HF frequency range with an antennapresenting an homogeneous efficiency in all the frequency range, and it is working with a vertical setup. The echoes of the transmitted signals are received, analyzed and displayed in a representation called _ionogram_, which shows the reflection altitude as a function of the emitted frequency, as observed in the example of ionogram in Figure 1. The black line corresponds to the computed plasma frequency in relation to the true altitude, and the ordinary frequencies following the right hand circular polarization are represented in red, while the extraordinary frequencies that follow the left hand circular polarization are shown in green (assuming upward wave propagation and a northern-hemisphere location [8]). Nevertheless, the communication characteristics nowadays require further methods to evaluate the quality of the communications, including not only narrowband tests but also wideband and other channel parameters required for the design of the HF system. In order to predict the channel performance, parameters as the best usable frequency, the expected signal-to-noise ratio (SNR) and the elevation angle, and of course, in some situations, the multipath spread estimation are crucial values to take into account when designing an HF ionospheric communication system. In this sense, it is worth mentioning IONCAP [9] and its variations VOACAP and ICEPAC (\"Voacap quick guide\", www.voacap.com, last access 30/10/2018. ), and also ASAPS [10]. All these simulators use virtual geometry (flat Earth models, laminar ionosphere, etc.) to minimize the computation time, at the cost of certain lack of accuracy in the outcome of the simulation depending on the latitude. These techniques are frequently used when the communication link under test is not yet available, or in the first stages of the design of the communication link. The most realistic approach to a channel performance prediction is to sound the link widely, both narrowband and wideband, at all available frequencies, hours of the day and time of the year throughput a whole solar cycle. This article pretends to review a set of relevant projects focused on designing and testing narrowband and wideband, both vertical and oblique, sounding techniques.