Age of Information (AoI) is an end-to-end metric that can capture one semantic property known as the timeliness of information, which is essential in status-updating systems. Semantics of Information (SoI) captures properties of information that relate to the “goal” and “purpose” of information, which is to improve the performance of systems using the information. As a new paradigm shift toward 6G communications systems, SoI will create systems that generate, process, and transmit only a small fraction of information. Thus, they consume less energy and channel resources without sacrificing their effectiveness.

This special issue honors Prof. Anthony Ephremides' contribution to the area and celebrates his 80th birthday. Prof. Ephremides is among the very first who foresaw the implications of the Age of Information as a paradigm shift in communication networks and advocated for a more general theory, the semantics of information based on rigorous foundations.

The editors would like to thank the authors of all papers for their submissions. In addition, special thanks go to the reviewers for their help in allowing us to complete the reviews and decisions on time. The papers in this special issue will report research advances in the semantics of information. More specifically, seven accepted papers deal with the age of information, and three investigate semantics beyond the age of information.

Age of Information: In this part we present the papers that considered AoI. The paper “Fresh-CSMA: A Distributed Protocol for Minimizing Age of Information” by Tripathi, Jones, and Modiano considers the design of distributed scheduling algorithms that minimize age of information (AoI) in single-hop wireless networks. The paper “Age-Optimal Multi-Flow Status Updating with Errors: A Sample-Path Approach” by Sun and Kompella studies an age of information minimization problem in continuous-time and discrete-time status updating systems that involve multiple packet flows, multiple servers, and transmission errors; and four scheduling policies are proposed. The paper “AoI Analysis and Optimization in Systems with Computations-Intensive Updates” by Vilni, Moltafet, Leinonen, and Codreanu considers a computation-intensive status update system with a data pre- processing server and a transmit server. The paper “Fresh Multiple Access: A Unified Framework Based on Large Models and Mean-Field Approximations” by Hui, Wei, and Chen builds a unified framework based on large models and mean-field approximations. Freshness-oriented multiple access is studied, focusing on Age of Incorrect Informatioin (AoII) and peak AoII scenarios. The paper “Timely and Covert Communications under Deep Learning-Based Eavesdropping and Jamming Effects” by Costa and Sagduyu explores the concept of timeliness in covert communications when faced with eavesdropping and jamming. Time-sensitive information is transmitted through a wireless channel between a transmitter and a receiver, while an adversary seeks to detect the communication attempts with a deep learning-based classifier. The paper “Age of Information Games Between Power Constrained Schedulers and Adversaries” by Banerjee, Ulukus, and Ephremides considers AoI games in a cellular network with multiple users, one base station (BS), multiple channels, and one adversary. The paper “On the Age of Information of Processor Sharing Systems” by Gandarias, Doncel, and Assaad examines the AoI of a source sending status updates to a monitor through a queue operating under the processor sharing discipline.

Semantics beyond Age of Information: Here we present the three papers that consider SoI beyond AoI. The paper “State-aware Real-time Tracking and Remote Reconstruction of a Markov Source” by Salimnejad, Kountouris, and Pappas considers the real-time remote tracking and reconstruction of a two-state Markov process. A state-aware randomized stationary sampling and transmission policy is proposed which accounts for the importance of different states of the information source, and their impact on the goal of the communication process. The paper “Sampling for Remote Estimation of an Ornstein-Uhlenbeck Process through Channel with Unknown Delay Statistics” by Chen, Tang, Wang, Yang, and Tassiulas considers the sampling of an Ornstein-Uhlenbeck process through a channel for remote estimation to minimize the mean square error at the estimator under a sampling frequency constraint. The paper “Control-Aware Scheduling over Multi-hop Networks” by Kutsevol, Ayan, and Kellerer demonstrates the benefits of the application-oriented design of networking algorithms for control. The transmission scheduling over a multi-hop network that connects the WNCS components is considered.