# Ionospheric Polarization Techniques for Robust NVIS Remote Sensing Platforms
## 3 System Description  
The main challenge of the performed tests is to analyze the influence of the ordinary and extraordinary ionospheric waves at the same time through an NVIS channel. Furthermore, the system requires us to analyze at the same time both ionospheric waves and apply polarization techniques to reach the best results on the demodulation of the frame. To achieve the goal, it is necessary to have a reconfigurable software defined radio platform. One of the main disadvantages in the design of the transmission platform for an ionospheric link is having the ability to mitigate the effects of the channel. Due to new digital technologies, these processes are becoming more efficient and less costly. In addition, one of the main challenges is to develop a flexible platform that allows for the development of all the tests to be validated and for the implementation of all the tests and study hypotheses. All these requirements are solved with field-programmable gate array (FPGA) platforms. To do this and to develop a low-cost platform, we have chosen the STEMlab 125-14 board hardware from the manufacturer Red Pitaya [20]. The main core of this hardware is a Zynq-7010-SOC [21]. These products contain a one-core (or two cores depending on which product is chosen) ARM(r) Cortex(tm)-A9 [22] based processing system (PS), plus 28 nm of Xilinx programmable logic (PL). In addition, this board has different peripherals (see in Figure 1) that allow us to make a robust platform for remote sensing:  
* Zynq (FPGA/Cortex): The FPGA is essentially dedicated to data processing and the Cortex to time and data management. In the reception, this module allows us to receive and process the received signals in the analog digital converters (ADCs). This data is sent in real time to the Cortex via direct memory access (DMA). The Cortex is dedicated to saving the information on an external hard disk. In transmission, the Cortex contains the information that will be sent to the FPGA to be transmitted. * Converters: The board has two channels, with two analog digital converters (ADCs), with a resolution of 14 bits and a sampling frequency of 125 MSPS. These, along with the two SMA connectors, will allow us to receive the signal by the antenna, send it to the FPGA, and treat the signal. The two ADCs allow us to receive both ordinary and extraordinary waves at the same time. This synchronization allows us to make a fair comparison with all the different polarization techniques. As opposed to ADC converters, digital analog converters (DACs) have a single converter to manage the two converter channels from the digital to the analog world; this fact should be considered when implementing the transmitter of the system. These will allow us to take out the signal already treated by the antenna. Therefore, this peripheral is only used by the FPGA [23]. * RaspBerry PI-3 [24]: This second post-processor is added to help the main core to control peripherals and save the received frames. The connection between both boards is realized via Ethernet. * Amplifier: To carry out the various soundings with the FSK, QAM, and PSK modulations and with the transmissions of non-constant envelope modulations (QAM), using a class A amplifier is necessary. To carry out the study, an amplifier with a maximum power of 250 W was installed [25], although the maximum testbench power transmission is about 100 W and the minimum 3 W.
* Filters: The platform contains two band-pass filters (BPF). The first transmitter is a band HF band-pass filter from 3 to 10 MHz. This frequency range is defined for the NVIS channel. The second one, is defined from 4 to 6 MHz to filter possible interferences. * GPS: The synchronization in time between the transmitter and the receiver is accomplished via GPS. * Post-processing data: Finally, the post-processing of the data is performed by MATLAB software [26]. This is used to demodulate all the tests frames received and to obtain the results' graphics. The three antennas of the experiment are completely identical.