Optimized Synchronization Design for UAV Swarm Network Based on Sidelink

With the deployment and application of the fifth-generation communication technology as well as the research on the sixth-generation communication technology, the space-air-ground-sea integrated network has emerged as a key vision for future communications. Unmanned aerial vehicles (UAVs), serving as aerial nodes, can be utilized in emergency rescue and disaster relief, mapping, environmental monitoring, and enhancement of communication coverage, among other areas. In terms of enhancing communication coverage, the integrated space-ground network, with UAVs as an important component, can provide seamless communication coverage to remote areas, deserts, oceans, and other all-domain three-dimensional spaces. UAVs have become important research objects due to their low cost and high flexibility, and the enhancement of communication coverage in the form of base station-relay UAV-slave UAV based on one-hop relaying has become a significant direction. However, the high mobility and extensive coverage of UAVs also give rise to synchronization challenges. In this work, to tackle the challenges of round-trip delay (RTD) from long-distance transmission and Doppler frequency offset in uplink synchronization between ground base stations and relay UAVs, a long-range random access preamble design is proposed. An enhanced two-step detection framework is introduced, where two distinct root sequence preambles are utilized for RTD estimation and random access respectively, and Doppler frequency offset is mitigated via pre-compensation. For the uplink synchronization in the sidelink between slave UAVs and relay UAVs, to address Doppler frequency offset, improve access efficiency, reduce resource consumption, and simultaneously account for the asynchrony among different users, an asynchronous non-orthogonal multiple access (A-NOMA)-based two-step random access scheme is developed. The scheme leverages existing physical random access channel (PRACH) preamble sequences with paired indexing for Doppler frequency offset estimation; on this basis, a successive interference cancellation algorithm based on Doppler frequency offset and phase compensation is designed to demodulate user data. For downlink synchronization between slave UAVs and relay UAVs, improvements to frequency offset estimation are achieved through redesigned sidelink synchronization signal block (S-SSB) resource allocation. Alongside this, a down-sampling-based detection scheme is designed to reduce UAV power consumption given energy constraints, with a comprehensive link algorithm developed to support implementation.