## 5 IoRT Systems Based on Space-Borne NTN Networks
### Enhanced Data Transmission Reliability
As a result, transmission times are reduced, and the compression rate is enhanced. The simulation results indicate that the HARQ-GD protocol reduces the Age of Information (AoI), a metric that quantifies the freshness of data, more effectively than conventional Generalized Deduplication (GD) and Hybrid Automatic Repeat Request With Chase Combining (HARQ-CC) schemes. LEO satellite networks have emerged as a promising solution for ubiquitous IoT connectivity. A Grant-Free Random Access (GF-RA) mechanism is proposed in [111] to address the dynamic nature of IoT traffic and intermittent transmissions from randomly activated devices. This GF-RA system employs a modified Bernoulli-Rician Message Passing (BR-MP) technique with Expectation-Maximization (EM) for User Activity Detection (UAD) and Channel Estimation (CE). Extensive simulation results of the proposed BR-MP-EM algorithm validate its accuracy and show its resilience to channel impairments. In [112], researchers propose an integrated satellite-terrestrial IoT network to extend coverage to areas not serviced by terrestrial cellular networks. The system employs Power Domain Non-Orthogonal Multiple Access for both LEO satellites and terrestrial base stations. However, interference arises between LEOs and base stations due to limited feedback line capacity. To mitigate this issue, researchers aim to minimize the overall network's transmission power while ensuring acceptable IoT device communication rates. To tackle this challenge, an Iterative Penalty Function (IPF)-based scheme is proposed for designing robust beamforming for satellites and base stations. Simulation results demonstrate the effectiveness of the proposed scheme in satellite-terrestrial IoT scenarios. Table 7 summarizes the proposed schemes to enhance the reliability of IoT data transmission. The mitigation of interference, the development of novel modulation and signaling schemes, the use of RL-based resource allocation methods, HARQ feedback enhancement, the design of novel data encoding schemes and collision detection techniques, and robust beamforming schemes for NOMA-based networks are among the highlighted approaches. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Refs.** & **NTN Architecture** & **Description** \\ \hline
[103] & LEO & Mathematical derivation of an electromagnetic Power Flux Density (PFD) mask for mitigating satellite interference on terrestrial communication systems in the same frequency band. \\ \hline
[104; 105] & GEO & Development of a novel modulation and signaling scheme based on Chirp Spread Spectrum (CSS). \\ \hline
[106] & LEO-GEO & Development of a reinforcement learning framework to optimize resource allocation and enable robust data transmissions. \\ \hline
[107] & Generic & Use of Network Coding to enhance the feedback mechanism of the HARQ process. \\ \hline
[108] & LEO-LoRA & A system architecture is proposed, along with the development of a data encoding and packaging scheme for the reliable transmission of IoT data over low-speed satellite links. \\ \hline
[109] & Generic & Development of a novel collision detection scheme for satellite-based IoT that enables rapid collision detection and load estimation while being robust to non-orthogonal interference. \\ \hline
[110] & Generic & Use of data compression and transmission optimization to improve the HARQ process as well as the timeliness of data. \\ \hline \hline \end{tabular}
\end{table} 
Table 7: IoRT systems based on space-borne NTN networks: **enhanced data transmission reliability**.