Joint Source-Channel Coding for Channel-Adaptive Digital Semantic Communications

In this paper, we propose a novel joint source-channel coding (JSCC) approach for channel-adaptive digital semantic communications. In semantic communication systems with digital modulation and demodulation, end-to-end training and robust design of JSCC encoder and decoder becomes challenging due to the nonlinearity of modulation and demodulation processes, as well as diverse channel conditions and modulation orders. To address this challenge, we first develop a new demodulation method which assesses the uncertainty of the demodulation output to improve the robustness of the digital semantic communication system. We then devise a robust training strategy that facilitates end-to-end training of the JSCC encoder and decoder, while enhancing their robustness and flexibility. To this end, we model the relationship between the encoder's output and decoder's input using binary symmetric erasure channels and then sample the parameters of these channels from diverse distributions. We also develop a channel-adaptive modulation technique for an inference phase, in order to reduce the communication latency while maintaining task performance. In this technique, we adaptively determine modulation orders for the latent variables based on channel conditions. Using simulations, we demonstrate the superior performance of the proposed JSCC approach for both image classification and reconstruction tasks compared to existing JSCC approaches.