# Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links
## 4 Conclusions  
The aim of this research was to develop an HF data communication system based on OFDM. This system introduces an important innovation for HF communications, the use of narrowband channel aggregation, which allows a significant increase in transmission rate and spectral efficiency. HF channel characteristics are highly variable due to ionospheric and noise variations, which makes the use of wideband channels (>12 KHz) for OFDM transmissions difficult and impossible due to peak-to-average variations, desynchronization, subcarrier frequency variations, etc. However, by using the higher bandwidths resulting from channel aggregation, the system has higher data transmission capabilities, but is less affected by the channel, making it ideal for applications requiring high throughput, such as advanced digital communication systems.  
Figure 25: Real field HF OFDM receiver tests—modified for NVIS propagation.  
The flexibility of deployment on Software Defined Radio (SDR) platforms facilitates rapid configuration and efficient deployment. Both laboratory and real-world tests have shown that the application operates without packet loss under favorable SNR conditions, confirming the robustness and performance of the system.  
An HF CHAT communication application has also been developed to integrate these innovative capabilities. The application provides an intuitive user interface that enables live chat and attachment transmission, enhancing the usability and efficiency of modern RF communications. A number of studies have been conducted on OFDM for HF links and channel aggregation into VHF/UHF broadband channels. However, many of these studies are still in the early stages of simulation and have produced no practical, usable systems. In contrast, our research has made significant progress, resulting in the prototyping of fully operational and efficient systems.  
Future developments will be oriented towards conducting further field trials of the OFDM aggregation system in order to validate its performance in dynamic and challenging radio frequency environments. These tests will assess the system's resilience to ionospheric variations, interference, and SNR, and will include long-distance communication trials. By knowing the lost packets at the receiver, the system will be set to resend only that packet, helping when SNR has low values. Moreover, in order to optimize the system's performance under varying channel conditions, further improvements in adaptive modulation and encryption techniques, such as the Advanced Encryption Standard (AES), will be explored. Continued advances in SDR technology will support these innovations, paving the way for more efficient and reliable HF communications systems. Finally, the system will be modified to operate in full duplex, enabling it to function as a transceiver. This will be followed by a transition from the development stage to real-field testing. This marks a critical milestone in the research, where the system will be deployed in an actual field environment to be used under real applications.  
Conceptualization, P.B.; Methodology, E.S. and M.S.; Software, E.S.; Validation, M.S.; Writing--review & editing, M.S.; Supervision, P.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data are available upon reasonable request.  
The authors declare no conflicts of interest.