Satellite-terrestrial communications are severely constrained by high path loss, limited spectrum resources, and time-varying channel conditions, rendering conventional bit-level transmission schemes inefficient and fragile, particularly in low signal-to-noise ratio (SNR) regimes. Semantic communication has emerged as a promising paradigm to address these challenges by prioritizing task-relevant information over exact bit recovery. In this paper, we propose a semantic forwarding-based semantic communication (SFSC) framework optimized for satellite-terrestrial networks. Specifically, we develop a vector-quantized joint semantic coding and modulation scheme, in which the semantic encoder and semantic codebook are jointly optimized to shape the constellation symbol distribution, improving channel adaptability and semantic compression efficiency. To mitigate noise accumulation and reduce on-board computational burden, we introduce a satellite semantic forwarding mechanism, enabling relay satellites to forward signals directly at the semantic level without full decoding and re-encoding. Furthermore, we design a channel-aware semantic reconstruction scheme based on feature-wise linear modulation (FiLM) to fuse the received SNR with semantic features, enhancing robustness under dynamic channel conditions. To support multi-user access, we further propose a codebook split-enhanced model division multiple access (CS-MDMA) method to improve spectral efficiency. Simulation results show that the proposed SFSC framework achieves a peak signal-to-noise ratio (PSNR) gain of approximately 7.9 dB over existing benchmarks in the low-SNR regime, demonstrating its effectiveness for robust and spectrum-efficient semantic transmission in satellite-terrestrial networks.

翻译：星地通信受限于高路径损耗、有限频谱资源和时变信道条件，导致传统比特级传输方案效率低下且脆弱，尤其在低信噪比（SNR）场景下。语义通信通过优先传输任务相关信息而非精确比特恢复，成为应对这些挑战的前沿范式。本文提出一种面向星地网络优化的语义转发式语义通信（SFSC）框架。具体而言，我们设计了一种矢量量化联合语义编码与调制方案，其中语义编码器与语义码本协同优化以重塑星座符号分布，从而提升信道适应性与语义压缩效率。为减轻噪声累积并降低星上计算负担，我们引入了卫星语义转发机制，使中继卫星能够在语义层面直接转发信号，无需完全解码与重新编码。此外，我们基于特征级线性调制（FiLM）设计了一种信道感知的语义重建方案，将接收信噪比与语义特征相融合，增强了动态信道条件下的鲁棒性。为支持多用户接入，我们进一步提出码本分割增强型模型分割多址接入（CS-MDMA）方法以提高频谱效率。仿真结果表明，所提出的SFSC框架在低信噪比区域相比现有基准方法实现了约7.9 dB的峰值信噪比（PSNR）增益，验证了其在星地网络中实现鲁棒且频谱高效语义传输的有效性。