# Advanced SDR-Based Custom OFDM Protocol for Improved Data Rates in HF-NVIS Links
###### Abstract
Channel aggregation involves the combination of 20 MHz channels with higher bandwidths (40 MHz, 80 MHz, or 160 MHz) to achieve enhanced throughput in Wi-Fi standards, including Wi-Fi 4/5/6 [10]. Similarly, carrier aggregation permits the combination of up to five 20 MHz carriers, thereby achieving a bandwidth of up to 100 MHz in LTE-Advanced [11] and 5G [12; 13]. OFDM is also becoming a popular choice in high-frequency (HF) data communications [14; 15; 16], where it is known to be a robust and efficient technology. The 3 to 30 MHz range is employed for long-range transmissions due to its capacity to reflect off the ionosphere [17]. Nevertheless, the attainment of high data rates and reliable communications in this band is challenging due to its narrow bandwidth. OFDM addresses these challenges by dividing the available bandwidth into a number of orthogonal subcarriers, each of which is modulated with a low-speed data stream [18; 19]. The use of SDRs in the implementation of OFDM with channel aggregation in HF data communications offers significant advantages [20; 21; 22]. The flexibility and reconfigurability of SDR allows for dynamic software adjustments [23; 24; 25; 26], enabling rapid protocol and bandwidth adaptation essential for efficient OFDM and channel aggregation. A notable example is the study entitled \"Channel Acquisition for HF Skywave Massive MIMO-OFDM Communications\" [27], which investigates channel acquisition for HF skywave communications using Multiple-Input Multiple-Output (MIMO) and OFDM modulation techniques. This paper introduces the concept of \"triple beams\" in the space-frequency-time domain and proposes channel estimation and prediction methods, demonstrating superior channel acquisition performance. Another relevant study is \"OFDM over wideband ionospheric HF channel: Channel modeling & optimal system design\" [28], which discusses wideband HF channel modeling and optimal OFDM system design for such channels. The paper introduces a 24 kHz bandwidth HF channel model and compares the performance of OFDM and single-carrier systems under various channel conditions. Also, the paper \"On the performance of OFDM and single-carrier communication over wideband HF channels\" [28] provides a detailed comparison between OFDM and single-carrier communication systems over wideband HF channels. The study analyzes the performance of both modulation schemes under varying channel conditions and proposes the implementation of a wideband HF OFDM HF transceiver using SDR technology. Most of these studies, as presented, have focused predominantly on theoretical simulations, without practical implementations to validate the effectiveness of the proposed solutions in real scenarios. This limited approach emphasizes the need for practical experimentsto evaluate the performance and challenges associated with the actual implementation of these technologies in HF communications. This paper extends the authors' previous research on HF ionospheric propagation, SNR analysis, and OFDM implementation for HF data communications. It addresses the following objectives/contributions:  
1. HF propagation and SNR analysis: an extension of the HF NVIS ionospheric propagation studies and an analysis of the potential for implementing new protocols in the HF range to enhance data communications. 2. Implementation and evaluation of a novel concept for the HF range--a custom OFDM system with aggregated channels using SDR: practical implementation and evaluation of an OFDM system with aggregated channels for HF data communication under laboratory and real conditions. 3. Development of digital signal processing hardware and software: development of the hardware system and software applications required for the efficient transmission and reception of data in the RF spectrum. 4. User interface development: develop an intuitive interface for HF data communication. 5.