# Study of NVIS Channel for USN Protocol Definition in Antarctica
In our work, we compare a multicarrier modulation with narrowband modulations based on a study done in this work of the NVIS channel effects. Also in this article, we propose a new frame protocol to improve the communication. At last, [PERSON] [23] studies the improvement of the data rate transmission on a 3 kHz NVIS channel by the use of differential modulations. In contrast, our work will be focused on the improvement of the robustness of the transmission instead of the data rate. The HF communications frame protocols are defined at STANAG [24] and MIL-STD 188 110C Appendix D [25]. These HF communication frame protocols are designed for both oblique and NVIS communications which have different channel parameters. The NVIS communications can communicate from 0 km to 250 km and oblique communications more than 250 km. For short distances, depending on the terrain, depending on the terrain, the groundwave can be received. In contrast to the skywave, the groundwave is a wave which is propagated by the surface of the terrain. For distances shorter than 10 km you may receive both the groundwave and the skywave, which can be studied separately and will cause multipath interference. For vertical incidence, the sporadic-E layer often appears together with the F2 layer and also causes multipath interference [26]. In oblique communications the interferences produced by multiple paths are lower than NVIS communications. In this article, we study the main characteristics of the NVIS channel for high latitudes, particularly in Antarctica. The new frame protocol definition will be focused on the requirements of USN in remote areas based on NVIS communications with an electromagnetic coverage area from 0 km to 250 km. Although NVIS communications at distances between 0 km and 10 km are not normally used, there may be cases where several nodes can be distributed around the principal node from distances between 0 km and 250 km. This scenario case is the one we tested to define a new frame protocol. The groundwave may be received for short distances without a line of sight, making the correct -reception much harder. The current frame protocols do not consider this distance making very complex the reception of a message. For this reason, the purpose of this study is to define a frame protocol based on an orthogonal frequency division multiplexing (OFDM) modulation instead of the actual modulations used at the HF standards. The OFDM modulation will assure at all the communication of the USN. The definition of this frame protocol is set depending on the characteristics of different NVIS channel situations. We measure the Doppler shift and the delay spread of the channel [27] for both the groundwave, sporadic-E layer and F2 layer as a function of time in an NVIS area around the Spanish Antarctic Base Juan Carlos I in Livingston Island. This article is organized as follows. In Section 2, we detail the test location, the system used to perform the channel tests and the frame structure used. In Section 3, we describe the analysis signal processing to obtain the results and we show the NVIS channel results obtained. In Section 4, based on the results obtained, we explain the proposed frame protocol and the differences with the actual HF standards. Finally, Section 5 contains the conclusions achieved in this article.