# Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links
## 3 Results and Discussion
### Real Field Conditions
#### Mobile Receiver System (Figure 19b)  
Similarly, the USRP N210 was used as the receiving hardware, in a manner similar to that of the transmitter. This approach ensured consistency in the hardware used, allowing any variations in performance to be attributed to environmental factors rather than differences in equipment. Laptop: The laptop, which was running the standalone HF OFDM receiver application, was connected to the USRP N210. The application in question was responsible for the demodulation, decoding, and display of received messages and files. A whip, HF antenna was also used. This antenna was selected on the grounds of its portability and ease of installation on the mobile platform. Despite its compact nature, the whip antenna proved to be an adequate means of capturing HF signals over moderate distances. The transmission and reception of messages:  
The testing started with the transmission of basic text messages from the fixed transmitter (Figure 20). The mobile receiver, situated at different distances, successfully received the transmitted messages (Figures 21-24). The process was repeated with different message lengths, including longer text messages and small file transfers, in order to assess the system's capability to handle varying data sizes. The quality of the received signal was monitored continuously using the receiver application's real-time spectrum and waterfall displays. The analysis of the quality of the received signal was conducted in terms of SNR (the results were analyzed only for the test that met minimum 20 dB), PER (Figure 21), and the ability to decode the message correctly (Figures 22 and 23). To assess the system's performance in the context of typical HF communication propagation challenges, a series of scenarios were tested. These included line-of-sight conditions, obstructed paths (e.g., behind hills or buildings), and varying distances. Figure 19: Real-world system tests: (**a**) transmitter fixed system and (**b**) mobile receiver system. Figure 21: Real field HF OFDM receiver tests (receiving mayday messages)—investigating PER. Figure 22: Real field HF OFDM receiver tests—receiving mayday messages. Figure 20: Real field HF OFDM transmitter tests—sending mayday messages. The subsequent tests were conducted with the objective of evaluating the transmission of attached files. The system continues to demonstrate the same optimal performance as previously observed, in accordance with the specified SNR requirements. To expand the scope of our field testing, additional experiments were conducted which focused on NVIS propagation, a technique commonly used for short to medium-range communication over distances typically ranging from 50 to 650 km. The utility of NVIS propagation lies in its capacity to facilitate communication in situations where direct, unobstructed line-of-sight is impeded by terrain, such as in mountainous or forested areas. The modification of the whip antenna on the mobile receiver enabled the experimental setup to be adapted for the assessment of NVIS propagation. The assessment was conducted with the objective of evaluating system performance in conditions that emulate real-world scenarios. The propagation of NVIS is dependent upon the transmission of signals at an angle approaching vertical into the ionosphere. Subsequently, the signal is reflected at an acute angle back to Earth. This propagation mode results in a substantial increase in the coverage area surrounding the transmitter. Consequently, this approach represents an effective method for regional communication, eliminating the necessity for relay stations or repeaters. In order to facilitate typical NVIS communication, the antennas in question must radiate the signal in a near-vertical direction. The transmitter is equipped with an inverted V antenna, which enables the achievement of NVIS propagation. To achieve this with the  
Figure 23: Real field HF OFDM receiver tests—receiving mayday attach file. Figure 24: Real field HF OFDM receiver testing—PER analysis.