# Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links
## 3 Results and Discussion
### Laboratory Conditions
The system demonstrated a consistent packet reception with stable synchronization and no loss if the SNR met the minimum requirement level packet in both contiguous and non-contiguous aggregation configuration proximity (the transmitted message reaching 100% of the received packet, as shown in Figures 14 and 15). In the contiguous aggregation configuration, in which the subcarriers are located in close proximity, the system exhibited very good packet reception and synchronization, along with an absence of packet loss. In the non-contiguous aggregation scenario, in which distinct transmission frequencies were used, the system demonstrated its ability to reliably decode packets, despite the distance between them. This flexibility in channel arrangement allows the OFDM system to efficiently utilize the available spectral gaps, making it suitable for dynamic spectrum access. Figure 14: Analyzing tag debug file and calculate PER. Figure 13: Check tag debug data saved in *.txt file after each reception. The HF communications channel is highly variable and susceptible to a multitude of influences, including those emanating from the ionospheric environment, interference, and noise. The aforementioned factors result in the manifestation of high levels of signal degradation across the spectrum of channels. The use of different modulation schemes on each OFDM channel allows for the optimization of data rate and robustness in accordance with the specific conditions of each channel. The overall spectral efficiency of the system can be optimized by matching the modulation type to the specific signal-to-noise ratio (SNR) of each aggregated channel. The use of higher-order modulations is advantageous when the SNR is high, as this allows for more efficient utilization of the spectrum and an increase in the overall data throughput. The implementation of various modulation types on each OFDM channel allows for the development of adaptive communication strategies. Adaptive modulation and coding (AMC) enable the modulation scheme to be adjusted in real-time based on the evaluation of the channel conditions. This adaptability ensures that the system can respond to changing conditions and continue to operate at optimal efficiency. It also allows the better utilization of available bandwidth and power resources, which is essential for long-distance HF communications. The figure below (Figure 16) illustrates the outcomes of an OFDM aggregation test in an HF data link. One channel employs QPSK modulation, while another employs 8-PSK modulation. The objective of the test is to assess the functionality and performance of these two modulation schemes when combined in a non-contiguous aggregation configuration. Figure 16: Monitoring received packages on each different modulated channel. The utilization of Channel 1 with QPSK has been demonstrated to offer enhanced robustness and reliability, as evidenced by the uninterrupted reception of packets and the absence of gaps in the received data (packets 498-504) (Figures 16 and 17). The relatively simple modulation scheme of QPSK provides superior resistance to noise and interference, thereby ensuring stable communication at 20 dB SNR. In contrast, on channel 2, which employs 8-PSK modulation, there are numerous gaps in packet reception (packet numbers 576-597). Of the 21 packets analyzed, only 8 were correctly received at the receiving end (Figure 16). When the PER was analyzed, there was 38.72% of packets lost (Figure 18). Although 8-PSK modulation facilitates higher data transfer rates (approximately 327 packets more than QPSK over the analyzed time period T), its elevated susceptibility to signal degradation results in diminished packet reliability. In a challenging HF environment, the elevated complexity of 8-PSK increases its susceptibility to errors and need higher SNR to have low PER.