# SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications
## 4 System Design
### Platform Overview  
The current platform is based on the concept of Software-Defined Sadio (SDR). SDR allows the independence of certain radio hardware components to operate independently as they are implemented by software. This fact offers many possibilities, especially when the transmitted protocols are not the same and some values (the carrier frequency, for example) need to be set several times for different scenarios.  
The boards used for the signal processing were two Red Pitaya STEMLab 125-14 [45]. They present a ZYNQ SoC, including a field-programmable gate array (FPGA) and a central processing unit (CPU), an analog-digital converter (ADC) and a digital-analog converter (DAC). These converters present a resolution of 14 bits and are managed by a 125 MSPS clock.  
The operating schemes of the Red Pitaya were based on two blocks: the processing system (PS) and the programmable logic (PL). On one side, the PS refers to the CPU, in which, in our case, an operating system (OS) is installed over this PS. This is where the configuration files are located and the peripherals, such as the external memory, are managed. On the other hand, the PL manages all the upsampling and downsampling, making use of the DUC/DDC converters that have been programmed using mainly FIR and CIC filters. The convergence of the two worlds is done through the RAM which, thanks to the direct shared memory (DMA), allows the transmission of data from the PS to the PL. In this case, the PS shares the data to be processed with the DMA, and the DMA passes the data to the PL through a FIFO memory where the DUC/DDC performs all the processing.  
A microprocessor is connected to the Red Pitaya, which is in charge of time synchronization and the management of different peripherals. In addition, it is responsible for saving the raw files for transmission and also saving the received files for further post-processing. The microprocessor model chosen was the Raspberry Pi 3 [46].  
A class-A amplifier was chosen to transmit signals with amplitude or phase changes that require linearity. The chosen model was a Bonn BLWA 0103-250 [47], which works at frequencies within the NVIS band and performs well in terms of efficiency. It was  
Figure 3: System model deployment.  
connected to the output of the Red Pitaya and enabled output powers above 54 dBm, although we should mention here that we did not use more than 25 W in our tests.  
On the receiver side, an amplifier was used only to preamplify the signal by 30 dB without degrading its SNR. In addition, a Low Noise Amplifier (LNA) was used to perform proper demodulation. A Band Pass Filter (BPF) on both sides prevented external interference at frequencies around our carrier frequency. The carrier frequency was set to 5.4 MHz due to previous ionosphere behavior analysis. At the transmitter side, a horizontal dipole was used, and two inverted Vees were used at the receiver due to the straightforwardness of their installation, together with a respectable gain of 6.8 dB [48]. Having two antennas on the receiver increases the availability of communications by taking advantage of both the Ordinary and Extraordinary skywaves and using them to apply the Single Input Multiple Output (SIMO) technique, but in this case, we just used the Ordinary skywave due to its higher gain [49].  
Figure 4 exhibits an overview of the whole system in a schematic way.