AI 中文总结
针对低轨卫星到地面相干自由空间光通信中多 GHz 多普勒频移带来的频偏问题，提出 HTF-FOC 接收机架构，通过混合捕获和跟踪过程，包括基于 4 次方 FFT 的粗 CFO 捕获等，能有效跟踪频移并降低残留 CFO，成功率达 100%。
AI 中文摘要
相干自由空间光（FSO）通信是低地球轨道（LEO）卫星下行链路传输的一个有前景的解决方案。然而，高轨道速度引入多 GHz 多普勒频移，表现为快速时变载波频率偏移（CFO），这对传统相干光接收机是重大挑战。窄范围数字环路无法获取初始偏移，宽范围前馈或光域解决方案要么残留大误差要么成本高。本文提出一种多普勒感知混合时频频偏补偿（HTF-FOC）接收机架构，采用累积时变随机过程模型处理动态多普勒引起的相移。该接收机实现混合捕获和跟踪过程来获取和补偿多 GHz 多普勒变化，包括基于 4 次方 FFT 的粗 CFO 捕获、残留 CFO 切换验证和低复杂度判决导向（DD）锁频环（FLL）跟踪。推导了相位平均成对错误概率（PEP）和联合界符号错误率（SER）表达式并通过蒙特卡罗模拟验证。结果表明，对于 400 - 800 km 的典型 LEO 高度和 7.3 - 7.9 km/s 的轨道速度，所提出的 HTF-FOC 方法能跟踪超过±5 GHz 的多普勒频移，同时将残留 CFO 保持在 80 MHz 以下，成功率达 100%。
英文摘要
Coherent free-space optical (FSO) communication is a promising solution for low Earth orbit (LEO) satellite downlink transmission. However, high orbital velocity introduces multi-GHz Doppler shifts that appear as a rapidly time-varying carrier frequency offset (CFO), which is a major challenge for conventional coherent optical receivers. Narrow-range digital loops cannot acquire the initial offset, while wide-range feedforward or optical domain solutions either leave large residual errors or impose substantial implementation cost. In this paper, a Doppler-aware hybrid time-frequency frequency offset compensation (HTF-FOC) receiver architecture is proposed for coherent LEO satellite-to-ground FSO links using a cumulative time-varying random process model for the dynamic Doppler-induced phase shift. The proposed receiver implements a hybrid acquisition and tracking procedure to acquire and compensate for multi-GHz Doppler variations, including 4th-power FFT-based coarse CFO acquisition, residual CFO handover verification, and low-complexity decision-directed (DD) frequency-locked loop (FLL) tracking. The phase-averaged pairwise error probability (PEP) and union-bound symbol error rate (SER) expressions are derived and verified using Monte Carlo simulations. The results demonstrate that the proposed HTF-FOC method tracks Doppler shifts beyond $\pm5$ GHz while keeping the residual CFO below $80$ MHz with a success rate of $100\%$ for typical LEO altitudes of $400{\!-\!}800$ km and orbital speeds of $7.3{\!-\!}7.9$ km/s.
Comments14 pages, 9 figures, 3 tables
