Adaptive Payload-Aided Near-Field Beam Tracking via Thompson Sampling

Abstract

Extremely large antenna arrays at high frequencies substantially extend the radiative near-field region in 6G networks, making mobile beam alignment depend jointly on user angle and range. The added range dimension enlarges the beam-search space, making repeated pilot-based sweeping costly under mobility, while sensing-assisted tracking depends on propagation conditions, echo quality, and target reflectivity. We propose an adaptive payload-aided near-field beam-tracking framework based on maximum likelihood estimation (MLE) and Thompson sampling (TS). Selected received payload samples are fed back and reused as tracking observations, avoiding dedicated beam-sweeping symbols during tracking. Within each sliding window, local angle and range trajectories are modeled by low-order polynomials to capture velocity, acceleration, and higher-order motion variations, and are estimated by MLE using the spherical-wave channel model. A local Gaussian approximation centered at the MLE, with covariance from the inverse observed Fisher information, represents trajectory uncertainty. For payload transmissions selected for feedback, TS samples a trajectory hypothesis and maps it to a payload beam toward the sampled state, while the remaining transmissions use the MLE-predicted beam. To handle nonstationary mobility, an asymptotic chi-square characterization of in-window estimation risk motivates joint adaptation of observation-window length and polynomial degree, while online residual statistics adjust the update interval and feedback ratio. The framework is also extended to uniform planar arrays with elevation tracking. Simulations under smooth and sharp-turn trajectories show high payload-accounted mean normalized beamforming gain, low normalized-gain variance, and high effective-symbol reliability, while the adaptive mechanism provides substantial robustness under nonstationary sharp-turn mobility.

Abstract shortened in the arXiv metadata due to the 1,920-character limit; the full abstract appears in the manuscript