# SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications
## 1 Introduction  
The use of sensor networks in remote areas plays a fundamental role in the development 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 research 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\({}^{\circ}\) and 90\({}^{\circ}\). The ionosphere allows HF signals to bounce back due to ionization on some of its own layers, creating a coverage zoneof approximately 250 km (depending on many factors, such as the frequency or the time of day). Despite that, not all of the HF band can be used in a NVIS link [7] and the frequencies at which radio signals refract are between 3 MHz and 10 MHz [7]. Currently, NVIS nodes can be implemented on SDR platforms [8; 9] and can achieve bit rates of up to thousands of bps (if higher data rates were needed, multiple antennas could be used to take advantage of the multipath effect or with multiple antennas using different channels [10; 11]). NVIS have to follow HF standards, which usually work with single carriers such as Quadrature Amplitude Modulation (QAM) or Phase-shift keying (PSK). These modulations require time-domain equalization to avoid multipath effects of the channel. Time-domain equalization is a computationally intensive implementation [12], which makes it unsuitable for low-cost platforms. Most current communications standards use the multicarrier scheme because of its adaptability and simple equalization [13]. These modulations have become widespread thanks to the exploitation of spectral bandwidth and the usage of mathematical expressions such as Fast Fourier Transform and the Inverse Fast Fourier Transform (FFT and IFFT) that optimize performance. For instance, Orthogonal Frequency-Domain Multiplexing (OFDM) was used in older versions of Wi-Fi [14]. Additionally, Orthogonal Frequency-Division Multiple Access (OFDMA) and Single-Carrier Frequency Division Multiple Access (SC-FDMA) are currently used for LTE and 5G in downlink (DL) and up-link (UL), respectively [15]. Indeed, recent studies confirm that such modulations could be used for resource-limited IoT devices [16; 17; 18]. Considering these modulations in the context of ionospheric communications, the study performed in [19] compared SC-FDE with OFDMA and SC-FDMA with ionospheric communications in an oblique way. In [20], an alternative solution with an OFDM transmission was proposed to avoid the complex equalization (OFDM requires frequency-domain equalization) for NVIS communications using the Zero Forcing (ZF) equalization method.