Channel Estimation under Large Doppler Shifts and Channel Aging in NOMA-Based Air-Ground Communications

Kurzfassung

The growing number of aircraft, combined with the limited available spectrum, poses a challenge for air traffic management communications. One promising solution to the problem of spectrum scarcity is the use of multiple antenna systems with non-orthogonal multiple access (NOMA). While NOMA techniques enhance spectral efficiency, their application to air traffic management communications is challenged by the high speed of the aircraft (up to 214 m/s) and the long communication ranges (up to 250 km), resulting in significant Doppler shifts and low signal-to-noise ratios, respectively. This study explores the feasibility of NOMA in air-ground communications by employing a realistic geometry-based stochastic air-ground channel model, derived from dedicated flight measurement campaigns. We assume multiple aircraft simultaneously transmit data to a ground station. Our investigation is twofold. First, we study channel estimation using orthogonal pilot sequences. Second, we characterize channel aging, which is defined as the time period after which the previously obtained channel estimation becomes outdated. The results demonstrate that a Doppler-robust channel estimation method is required to fully exploit the potential of NOMA-based air-ground communications. Our analysis further reveals that the channel estimate remains valid over extended periods during the climb & descent and enroute cruise scenarios.