Insufficient link budget has become a bottleneck problem for direct access in current satellite communications. In this paper, we develop a semantic transmission framework for direct satellite communications as an effective and viable solution to tackle this problem. To measure the tradeoffs between communication, computation, and generation quality, we introduce a semantic efficiency metric with optimized weights. The optimization aims to maximize the average semantic efficiency metric by jointly optimizing transmission mode selection, satellite-user association, ISL task migration, denoising steps, and adaptive weights, which is a complex nonlinear integer programming problem. To maximize the average semantic efficiency metric, we propose a decision-assisted REINFORCE++ algorithm that utilizes feasibility-aware action space and a critic-free stabilized policy update. Numerical results show that the proposed algorithm achieves higher semantic efficiency than baselines.

翻译：链路预算不足已成为当前卫星通信中直连接入的瓶颈问题。本文提出了一种面向卫星直连通信的语义传输框架，作为解决该问题的有效可行方案。为了衡量通信、计算与生成质量之间的权衡，我们引入了一种带优化权重的语义效率度量指标。优化目标是通过联合优化传输模式选择、星-用户关联、星间链路任务迁移、去噪步数与自适应权重，最大化平均语义效率度量，这是一个复杂的非线性整数规划问题。为最大化平均语义效率度量，我们提出了一种决策辅助的REINFORCE++算法，该算法利用了可行性感知的动作空间和无评论家的稳定策略更新。数值结果表明，所提算法相较于基线方法实现了更高的语义效率。