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

TL;DR

This work tackles channel estimation and outage analysis for NOMA-based air–ground communications with large Doppler shifts using a geometry-based stochastic channel model. It compares Time-Division and Zadoff-Chu pilots and evaluates Zero Forcing and V-BLAST-MMSE-SIC detectors under channel aging across TL, CD, and EC flight phases, highlighting phase-dependent optimal estimator–detector pairings. The results show TL favors TD+ZF due to high SNR, while CD and EC favor ZC+VMS when CFO pre-compensation accuracy is high, with CFO accuracy thresholds rising with the number of simultaneous aircraft. The findings inform phase-aware receiver design and CFO management to maximize spectral efficiency in safety-critical aeronautical networks.

Abstract

This paper investigates a multiple antenna system with non-orthogonal multiple access (NOMA) for the exchange of air traffic management data between commercial aircraft pilots and ground-based air traffic controllers. While NOMA techniques enhance spectral efficiency, their application to aircraft 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. To accurately assess these challenges, we employ a realistic geometry-based stochastic air-ground channel model, derived from dedicated flight measurement campaigns. In this paper, multiple aircraft simultaneously transmit data to the ground station. We focus on the channel estimation problem at the ground station under high carrier frequency offsets and the effects of channel aging due to channel's time-varying nature. For the channel estimation problem, we compare the Zadoff-Chu sequences with time-division approach under varying carrier frequency offset pre-compensation accuracies at the aircraft transmitter. For the channel aging problem and performance evaluation of channel estimators, we compute the outage probability for both the zero-forcing detector and the minimum mean squared error detector with successive interference cancellation. The results show that the favorable channel estimator-detector combinations differ between the takeoff & landing phase and the enroute cruise phase of the flight, due to the distinct channel propagation characteristics of each phase.

Figures (2)

• Figure 1: Outage probability, $P_{\text{out}}$, for different flight scenarios with a rec of $M=64$ and varying number of ac, $K$. The channel is estimated using zc sequences, assuming perfect cfo compensation at the ac tx. The dashed line represents the zf detector, and the solid line corresponds to the vms detector.
• Figure 2: Outage probability, $P_{\text{out}}$, after 240 elapsed since channel estimation for $K=8$ and a rec of $M=64$. The dashed line represents the zf detector, and the solid line corresponds to the vms detector.