Causal Reasoning Foundations of Semantic Communication Systems
Christo Kurisummoottil Thomas, Christina Chaccour, Walid Saad, Merouane Abdelkader Debbah · 2025
Semantic communication (SC) is an effective approach to communicate reliably with minimal data transfer while simultaneously providing seamless connectivity. In this chapter, the foundations of a novel emergent SC (ESC) framework is proposed. This ESC system is composed of two key components: a signaling game for emergent language design and causal reasoning for extracting the structure present in the data (defined as semantics). First, the fundamental principles of causal reasoning such as causal discovery, representation learning, and causal inference are outlined. Additionally, novel semantic capacity definitions are introduced, incorporating the causal reasoning capabilities at the receiver's end. In order to design the language, the signaling game is solved using an alternating maximization between the transmit and receive nodes utilities. The generalized Nash equilibrium is characterized, and it is shown that the resulting transmit and receive signaling strategies lead to a local equilibrium solution. As such, the emergent language not only creates an efficient (in physical bits transmitted) transmit vocabulary dependent on communication contexts, but it also aids the reasoning process (and enables generalization to unseen scenarios) by splitting complex received messages into simpler reasoning tasks for the receiver. The ESC system is also designed to enhance the novel semantic metrics of information, reliability, distortion, and similarity that are designed using rigorous algebraic properties from category theory, thereby generalizing the metrics beyond Shannon's notion of uncertainty. Simulation results confirm that the ESC system effectively communicates with reduced bits and achieves superior semantic reliability compared to conventional wireless systems and state-of-the-art SC systems lacking causal reasoning capabilities. Additionally, the overhead involved in language creation diminishes over time, validating the system's ability to generalize across multiple tasks.