Rapid Coherent Acquisition of DS/FH Measurement and Control Signals for Low-Altitude High-Mobility Communications

Intelligent low-altitude communication systems can support advanced economic development and the operation of integrated transportation networks. Direct-sequence/frequency hopping （DS/FH） hybrid spread-spectrum modulation techniques combine the advantages of direct-sequence spread-spectrum modulation and frequency hopping. This approach has shown strong anti-jamming and concealment capabilities and has thus been recognized as a key technology to build safe， reliable， and efficient networks for intelligent low-altitude communications. However， the strong Doppler effect enhances the complexity of coherent synchronization techniques and considerably reduces the capture accuracy in scenarios that require high dynamic range. In this study， a fast coherent synchronization technique is proposed to address this issue by balancing performance improvements with the system complexity. First， a variable-rate frame structure is designed using M-sequence and Barker codes to optimize the synchronization speed while retaining rate flexibility by adaptively adjusting the symbol period of the data segment to match the sensitivity required for the demodulation. Second， a two-stage phase-compensation mechanism is designed to eliminate phase residuals introduced by delays at the half-code slice and sampling-rate levels. Finally， a low-complexity cross-hop frequency bias estimation method is proposed to estimate the Doppler frequency bias with high precision. The results of a simulation show that the proposed scheme can improve gain by 8~9 dB with only a 20% increase in complexity compared to a conventional non-coherent algorithm. The root mean square of the time estimation was 0.38 ns with a large frequency bias， while that of the frequency estimation was 29.85 Hz. Thus， the proposed method realizes sub-nanosecond accuracy in estimating time and an accuracy of tens of hertz in estimating frequency. These findings imply the applicability of this approach in millimeter-wave anti-jamming measurement and control for fast capture in scenarios with high dynamic range.