Recently, semantic communications have been considered as a promising beyond-Shannon paradigm to reduce network traffic and increase reliability, thus making wireless networks more energy efficient, robust, and sustainable. However, the performance is limited by the efficiency of the semantic transceivers, i.e., the achievable “similarity” between the transmitted and the received signal. If a strict similarity requirement holds, semantic transmission may not be applicable, thus Shannon communication is mandatory. In this paper, for the first time in the literature, we propose a multi-carrier Hybrid Semantic-Shannon communication system, where without loss of generality, the case of text transmission is investigated. To this end, a joint semantic-Shannon transmission selection and power allocation optimization problem is formulated, aiming to minimize the transmission delay, while it guarantees a strict similarity threshold. Despite its non-convexity, by utilizing alternating optimization the problem is separated into a convex and a mixed linear integer programming one, both of which can be optimally solved. Moreover, to improve the performance of the proposed hybrid scheme, a novel association of text sentences to subcarriers is proposed based on the sentence’s data size and subcarriers’ channel gains. We show that the proposed association is optimal in terms of transmission time delay. Numerical results and simulations verify the effectiveness of the proposed hybrid semantic-Shannon communication method and the derived sentence-to-subcarrier association, and provide useful insights on the designing parameters of such systems.
