  Digital and analog semantic communications (SemCom) face inherent limitations
such as data security concerns in analog SemCom, as well as leveling-off and
cliff-edge effects in digital SemCom. In order to overcome these challenges, we
propose a novel SemCom framework and a corresponding system called HDA-DeepSC,
which leverages a hybrid digital-analog approach for multimedia transmission.
This is achieved through the introduction of digital-analog allocation and
fusion modules. To strike a balance between data rate and distortion, we design
new loss functions that take into account long-distance dependencies in the
semantic distortion constraint, essential information recovery in the channel
distortion constraint, and optimal bit stream generation in the rate
constraint. Additionally, we propose denoising diffusion-based signal detection
techniques, which involve carefully designed variance schedules and sampling
algorithms to refine transmitted signals. Through extensive numerical
experiments, we will demonstrate that HDA-DeepSC exhibits robustness to channel
variations and is capable of supporting various communication scenarios. Our
proposed framework outperforms existing benchmarks in terms of peak
signal-to-noise ratio and multi-scale structural similarity, showcasing its
superiority in semantic communication quality.
