# Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links
###### Abstract  
In the current context of global communications, HF (High Frequency) NVIS (Near Vertical Incidence Skywave) data networks can be of strategic importance, providing short- and medium-range communication capabilities independent of terrestrial configuration and existing conventional communications infrastructure. They are essential in critical conditions, such as natural disasters or conflicts, when terrestrial networks are unavailable. This paper investigates the development of such systems for HF NVIS data communications by introducing a customized Orthogonal Frequency Division Multiplexing (OFDM) protocol with parameters adapted to HF ionospheric propagation, implemented on Software-Defined Radio (SDR) systems, which provide extensive configurability and high adaptability to varying HF channel conditions. This work presents an innovative approach to the application of OFDM narrow-channel aggregation in the HF spectrum, a technique that significantly enhances system performance. The aggregation enables a more efficient utilization of the available spectrum and an increase in the data transmission rate, which represents a substantial advancement in NVIS communications. The implementation was realized using an SDR system, which allows flexible integration of the new OFDM protocol and dynamic adaptation of resources. The work also includes the development of a messaging application capable of using this enhanced HF communication system, taking advantage of the new features of channel aggregation and SDR flexibility. This application demonstrates the applicability of the protocol in real-world scenarios and provides a robust platform for data transmission under conditions of limited access to other means of communication. Thus, this study contributes to the technological advancement of NVIS communications and opens new research and deployment directions in HF communications. OFDM; HF data communications; spectral resource allocation; SDR; NVIS; GNU radio; channel aggregation +
Footnote †: journal: applied sciences  
Article Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links  
[PERSON], et al. 1 Department of Electrotechnics and Measurements, Technical University of Cluj-Napoca, 400114 Cluj-Napoca, Romania; sorceau. to interference and multipath fading [2], thereby enhancing its robustness and reliability across a range of communication environments. A notable advancement in OFDM technology is the implementation of channel aggregation, which is also referred to as channel combining or carrier aggregation. The objective of this technology is to enhance overall data throughput by combining multiple narrowband channels to form a singular wideband channel [3; 4]. Channel aggregation is the process of combining several contiguous or non-contiguous frequency bands (channels) to form a larger channel bandwidth [5; 6; 7]. The aggregation process results in an improvement in the overall efficiency of data transmission [8], as the combined bandwidth of the aggregated channels is utilized more effectively. Two principal categories of aggregation technology exist. The first is continuous aggregation, in which adjacent channels are combined to form a wider channel. The second method, non-contiguous aggregation, involves the combination of non-contiguous channels [9]. This approach provides greater flexibility with regard to spectrum usage. OFDM channel aggregation offers a number of advantages. The technique increases data throughput by enhancing the total bandwidth, improving frequency efficiency through frequency slicing, and augmenting the system capacity to accommodate a greater number of users and more demanding data rates, particularly in high-density environments. In today's modern communications systems, such as those employing Wi-Fi and LTE-Advanced, channel aggregation is a widely utilized technique.