面向低空无人机测控的GMSK-MIMO分集方法

A GMSK-MIMO diversity method for Low-Altitude UAV TT&C

摘要:

针对低空无人机测控链路复杂多径环境下高斯最小频移键控(Gaussian Minimum Shift Keying, GMSK)跳频体制信号结构与Alamouti(空时分组码提出者)空时分组码不兼容而难以引入多输入多输出(Multiple-Input Multiple-Output, MIMO)分集增益的问题,提出一种基于劳伦特(Laurent)分解与符号交替差分预编码联合映射的GMSK-MIMO分集方法。通过Laurent分解将GMSK信号映射至脉冲幅度调制(Pulse Amplitude Modulation, PAM)表示域,利用伪符号序列的相位周期性与符号交替差分预编码的耦合效应,在PAM域构造满足Alamouti正交条件的等效符号矩阵,从而在保持GMSK连续相位和恒包络的前提下实现发射天线分集。理论分析与仿真验证表明,该联合预编码方法在2×2准静态瑞利衰落信道下可实现满四阶分集增益。与标准正交相移键控(Quadrature Phase Shift Keying, QPSK)调制下Alamouti空时编码方案相比,编码增益损失约0.4 dB,且具有良好的频谱保持性和较低的接收机复杂度。

Abstract:

To address the incompatibility between Gaussian Minimum Shift Keying (GMSK) frequency-hopping signals and Alamouti space-time block codes, which prevents the introduction of Multiple-Input Multiple-Output (MIMO) transmit diversity gain in low-altitude UAV telemetry, tracking, and command (TT&C) links under complex multipath environments, a GMSK-MIMO diversity method based on Laurent decomposition and symbol-alternating differential precoding joint mapping is proposed. The GMSK signal is first transformed into the Pulse Amplitude Modulation (PAM) representation domain via Laurent decomposition. By exploiting the coupling effect between the phase periodicity of pseudo-symbol sequences and the symbol-alternating differential precoding, an equivalent symbol matrix satisfying the Alamouti orthogonality condition is constructed in the PAM domain, thereby achieving transmit antenna diversity while preserving the continuous phase and constant envelope properties of GMSK. Theoretical analysis and simulation results demonstrate that the proposed joint precoding scheme achieves full fourth-order diversity gain over a 2×2 quasi-static Rayleigh fading channel. Compared with the standard Alamouti space-time coded scheme employing Quadrature Phase Shift Keying (QPSK) modulation, the coding gain loss is approximately 0.4 dB, while maintaining favorable spectral containment and low receiver complexity.