Lossless pixel-level image transmission is a fundamental regime beyond semantic communications, because exact recovery requires both accurate symbol probability modeling and reliable delivery over noisy channels. This paper proposes DDM-SSCC, a discrete-diffusion-model-based separate source-channel coding framework for lossless image transmission. Different from raster-order autoregressive coding, the proposed source codec adapts a diffusion language model to pixel-token restoration and performs synchronized reverse arithmetic coding under bidirectional attention, allowing multiple masked tokens to be coded within one reverse denoising step. This progressive restoration process also yields a more favorable source representation for noisy transmission, since newly restored tokens can serve as bidirectional context in subsequent denoising steps. To bridge the gap between generation-oriented masked denoising and lossless arithmetic coding, we further introduce a Halton-guided denoising order, a mask-ratio-aware cosine schedule, and a lightweight temperature calibration module. These designs respectively improve spatial coverage, adapt the denoising pace to context reliability, and calibrate the probability tables used by arithmetic coding. Experiments on CIFAR10, DIV2K-LR-X4, and Kodak over additive white Gaussian noise and Rayleigh fading channels show that DDM-SSCC achieves better exact-recovery performance than representative lossless and semantic communication baselines, while ablation studies verify the effectiveness of the proposed denoising order, schedule, and calibration modules.

翻译：无损像素级图像传输是超越语义通信的基础范式，因为精确恢复需要同时实现准确的符号概率建模和在噪声信道上的可靠传输。本文提出DDM-SSCC，一种基于离散扩散模型的分离式源信道编码框架，用于无损图像传输。与光栅顺序自回归编码不同，所提出的源编码器将扩散语言模型适配于像素令牌恢复，并在双向注意力下执行同步反向算术编码，允许在一个反向去噪步骤中对多个掩码令牌进行编码。这种渐进式恢复过程还为噪声传输产生了更有利的源表示，因为新恢复的令牌可以在后续去噪步骤中作为双向上下文。为弥合面向生成的掩码去噪与无损算术编码之间的差距，我们进一步引入哈尔顿引导的去噪顺序、掩码比率感知的余弦调度以及轻量级温度校准模块。这些设计分别改善了空间覆盖、使去噪速度适应上下文可靠性，并校准算术编码使用的概率表。在加性高斯白噪声和瑞利衰落信道下的CIFAR10、DIV2K-LR-X4和Kodak数据集上的实验表明，DDM-SSCC在精确恢复性能上优于代表性的无损和语义通信基线，而消融研究验证了所提出的去噪顺序、调度和校准模块的有效性。