electronics Article SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications Tomas Gonzalez , Joaquim Porte , Jordi Male , Joan Navarro , Josep M. Maso , Agustín Zaballos , Joan L. Pijoan * and David Badia   Citation: Gonzalez, T.; Porte, J.; Male, J.; Navarro, J.; Maso, J.M.; Zaballos, A.; Pijoan, J.L.; Badia, D. SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications. Electronics 2021, 10, 1636. https:// doi.org/10.3390/electronics10141636 Academic Editor: Davide Brunelli Received: 12 June 2021 Accepted: 6 July 2021 Published: 9 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). La Salle Campus, Ramon Llull University, 08022 Barcelona, Spain; tomas.gonzalez@salle.url.edu (T.G.); joaquim.porte@salle.url.edu (J.P.); jordi.male@salle.url.edu (J.M.); jnavarro@salleurl.edu (J.N.); josep.maso@salle.url.edu (J.M.M.); agustin.zaballos@salle.url.edu (A.Z.); david.badia@salle.url.edu (D.B.) * Correspondence: joanlluis.pijoan@salle.url.edu Abstract: 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). Keywords: remote sensing; HF; NVIS; IoT; OFDM; SC-FDE; SDR 1. Introduction The use of sensor networks in remote areas plays a fundamental role in the develop- ment of applications such as fire detection and human rescue, among others. Monitoring these difficult-to-access areas and collecting in-field data have emerged as a popular re- search topic in the last 20 years [1,2]. This has motivated the conception of new distributed- computing paradigms such as the Internet of Things (IoT) or Ubiquitous Sensor Networks (USN) [1], in which several devices are deployed in a certain scenario and link together by means of a wireless (or not) network. There are many wireless technologies currently in use for IoT sensors [3]. Most of them (such as LoRa or Sigfox) rely entirely on already deployed infrastructure, which complicates their deployment in under-resourced or difficult-to- access areas. The most common way to install these networks for these kind of use-cases is using satellite communications. The main drawbacks of using satellites are their high installation cost [4,5] and their dependence on the satellite’s orbit, which sometimes makes coverage difficult due to loss of line of sight (LOS) or low signal-to-noise ratio (SNR). A cheaper and faster-to-deploy alternative without the need for LOS is Near Vertical Incidence Skywave (NVIS) that can be used to link nodes from a distributed sensor network spanning large-scale areas [6]. The NVIS technique is a good solution for deploying networks in infrastructureless (or remote) areas or in places where natural catastrophes have occurred. NVIS benefits from ionospheric reflection by transmitting high frequency (HF) signals with an angle of incidence between 70 ◦ and 90 ◦ . The ionosphere allows HF signals to bounce back due to ionization on some of its own layers, creating a coverage zone Electronics 2021, 10, 1636. https://doi.org/10.3390/electronics10141636 https://www.mdpi.com/journal/electronics