Joint Mode Selection and Power Control in Hybrid Bit-Semantic Communications

In this letter, we propose a novel hybrid bit-semantic communication (BitCom/SemCom) system that dynamically switches between modes in each fading epoch based on the instantaneous BitCom and SemCom rates. Leveraging channel state information at both transmitter and receiver, we enhance the rates of both communication chains through: (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$i$ </tex-math></inline-formula>) channel inversion power control in SemCom mode, which mitigates the degradation of the semantic similarity index under fading, and (ii) water-filling power control in BitCom mode, which maximizes channel capacity under fading. We further maximize the equivalent semantic throughput by jointly optimizing the SemCom received and the BitCom transmit power using an efficient subgradient-based algorithm. The hybrid system successfully combines the strengths of its mono-modal counterparts, achieving SemCom-level performance at low and moderate SNRs and BitCom-level performance at high SNRs.