## 1 Introduction  
The ionosphere has an essential function for our planet, which is the protection against external radiations. It has been studied for a long time both in a physical way and also as a communication channel [1, 2], as the ionosphere behaves like a mirror for high frequency (3-30 MHz) signals. Using the ionosphere as a channel, and taking advantage of the benefits of signal reflection for HF, has wide use for emergency services and is also suitable for ubiquitous sensors networks (USN). In addition, it avoids the use of satellites and high infrastructure and operational costs.  
The behavior of the ionosphere is under continuous research due to its difficult prediction. Observatories, using an ionosonde, examine the quantity of ions and electrons produced in the atmosphere to get information of the radio wave refraction, and the generation of different waves due to these reflections (the ordinary and the extraordinary waves [1, 2]). There are studies such as those by the respective authors of [3, 4, 5], where a deterministic model of a narrowband and wideband HF channel was studied giving a prediction of quality parameters of ionospheric communications, but there was no distinction between ordinary and extraordinary waves. Doppler and multipath measurements on oblique and vertical ionospheric paths were performed in studies by the respective authors of [6; 7] and also in the Doppler And Multipath Sounding Network (DAMSON) project [8; 9]. Moreover, another sounding of narrowband and wideband ionospheric communications, was done with oblique transmissions [10]. It is known that the availability of the ionosphere as a communication channel is not remarkable, since it depends on its ionization. Its behavior varies throughout the day, which implies that different transmission frequencies must be used due to the change of the critical frequency. Due to its low bandwidth and coherence time, the ionosphere is not a solution for high-speed data transmissions but, on the other hand, its channel characteristics are suitable for sensor network deployment.  
In this article, we analyze the behavior of the ionosphere as a communication channel, using near vertical incidence skywave (NVIS) propagation and the transmission of different data frames. The transmit frequency used in our experiment was 5.4 MHz. NVIS propagation is based on the transmission of a signal with an incidence angle between 70\({}^{\circ}\) and 90\({}^{\circ}\) to the ionosphere. The properties of the ionosphere cause that signal to be reflected, obtaining a coverage of approximately a 250 km radius [11], which is a relevant fact for remote sensors or emergency communications for places without infrastructure [12]. The ionospheric reflection is frequency dependent, with typical frequencies ranging from 3 to 10 MHz. The coverage radius of 250 km corresponds with F\({}_{2}\)-layer propagation.  
In what follows, characteristics of the ionosphere and NVIS communications and polarization diversity are introduced in Section 2. The sounding system implemented is described in Section 3, where, in Section 3.1, the overall infrastructure is explained, the data frames designed and used are detailed in Section 3.2, and the test scenario is described in Section 3.3. Results of this study are presented in Section 4, and finally, the conclusions of this work are in Section 5.