  Each generation of cellular networks is characterized by its distinct
capabilities and innovations, which reflect the significant milestones reached
with each new release. 5G has made substantial progress through the deployment
of advanced encoding and modulation techniques, nearly reaching the Shannon
physical capacity limit. In light of the requirements of Beyond-5G
technologies, there is the need of a paradigm shift in the development of
communication systems.
  Recent developments in the realm of Artificial Intelligence (AI) have enabled
the deployment of tools with high abstraction capabilities, relevant for
feature extraction processes and End-to-End system optimization tasks. In this
context, Semantic Communications has emerged as a novel information
transmission system, with AI as one of the core components in its
implementation. This communication paradigm relies on the extraction and
transmission of the "semantic meaning" of the source information using AI
techniques, diverging from the conventional systems that primarily focus on
ensuring the successful reception of the transmitted bits.
  The purpose of this survey is to provide a comprehensive overview of the
fundamental concepts underlying Semantic Communications, including Shannon's
Information Theory, classical and modern theories of semantic information, and
an examination of the framework and system design of Semantic Communications.
Additionally, recent implementations are reviwed, including the analysis of
Semantic Communications systems according to the information object transmitted
and the objective of the information transmission. Moreover, an in-depth study
of prototypes and demonstrations are presented, supporting the viability of the
Semantic Communications systems. Finally, some of the most relevant open
challenges are detailed, highlighting open research questions to be pursued in
Semantic Communications.
