The growing demand for high-speed and reliable inflight broadband connectivity (IFC) presents unique challenges for direct air-to-ground communication (DA2GC) systems, especially when they operate in the millimeter-wave (mmWave) band. While mmWave frequencies offer high capacity and spectral efficiency, they are highly susceptible to atmospheric impairments such as rain attenuation, gaseous absorption, and Doppler effects (caused by aircraft mobility).
This study proposes a signal-to-noise ratio (SNR)-driven adaptive coding and modulation (ACM) framework specifically designed for DA2GC links. The goal is to enhance spectral efficiency and link reliability under time-varying channel conditions. The proposed ACM system dynamically selects the modulation and coding scheme (MCS) based on real-time channel feedback, ensuring robust performance in both favorable and adverse weather conditions.
A comprehensive DA2GC channel model is developed based on ITU-R recommendations. The model accounts for rain attenuation, polarization effects, antenna tilt, and altitude-dependent gas absorption. To capture the dynamics of high-mobility aeronautical environments, simulations employ a Rician fading model, with the Doppler coherence time used to define ACM block duration. The results indicate that rainfall can cause variations of up to 100 dB in the link budget, highlighting the importance of adaptive transmission strategies. Lower-order quadrature amplitude modulation (QAM) schemes with stronger coding ensure link availability during rain fades, while higher-order QAM schemes maximize throughput under clear-sky conditions.
The proposed ACM scheme demonstrates promising real-time adaptability, indicating its potential to support high-speed and reliable inflight connectivity. Future work will focus on uplink adaptation, energy-efficiency trade-offs, and the practical (particularly hardware) feasibility of onboard ACM implementation.
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