Dynamic Access and Time–Frequency–Phase Synchronization for Multisatellite Carrier Coherent Superposition in LEO Satellite Systems

ABSTRACT Reliable low‐Earth‐orbit (LEO) direct‐to‐handheld transmission is constrained by the limited single‐satellite link budget, while rapid orbital motion continuously changes link signal‐to‐noise ratios (SNRs) and cooperative satellite availability, making it difficult for static multisatellite carrier coherent superposition schemes to sustain stable coherent gain. To address this problem, a dynamic access–oriented LEO multisatellite carrier coherent superposition framework is proposed. The key innovation lies in introducing an SNR threshold–driven satellite admission and replacement mechanism, which dynamically updates the cooperative satellite set and maintains coherent combining during satellite overflight and cooperative‐set switching. To support the seamless access of a new satellite, sparse demodulation reference signal (DMRS) reuse, joint time–frequency–phase synchronization tracking, and Kalman filter–based channel state information (CSI) feedback‐loss compensation are integrated to reduce pilot overhead, synchronization reconstruction errors, and the impact of CSI feedback packet loss. Simulation results show that stable link establishment is achieved within 100 ms under initial timing, frequency, and carrier‐phase offsets of 100 ns, 1 Hz, and 100°, respectively. In a representative four‐satellite dynamic cooperation scenario, an average SNR gain of approximately 7.43 dB over the strongest single‐satellite link is achieved. Under an acceptable SNR‐loss constraint of 0.5 dB, the Kalman filter–based compensation increases the tolerable CSI feedback packet‐loss rate from 5.6% to 29.7%, while the minimum sparse DMRS overheads required for the AWGN and NTN‐TDL‐D channels are approximately 3.12% and 4.76%, respectively.