Abstract-In this work, we study how to optimally manage the freshness of information updates sent from a source node to a destination via a channel. A proper metric for data freshness at the destination is the age-of-information, or simply age, which is defined as how old the freshest received update is since the moment that this update was generated at the source node (e.g., a sensor). A reasonable update policy is the zero-wait policy, i.e., the source node submits a fresh update once the previous update is delivered and the channel becomes free, which achieves the maximum throughput and the minimum delay. Surprisingly, this zero-wait policy does not always minimize the age. This counter-intuitive phenomenon motivates us to study how to optimally control information updates to keep the data fresh and to understand when the zero-wait policy is optimal. We introduce a general age penalty function to characterize the level of dissatisfaction on data staleness and formulate the average age penalty minimization problem as a constrained semiMarkov decision problem (SMDP) with an uncountable state space. We develop efficient algorithms to find the optimal update policy among all causal policies, and establish sufficient and necessary conditions for the optimality of the zero-wait policy. Our investigation shows that the zero-wait policy is far from the optimum if (i) the age penalty function grows quickly with respect to the age, (ii) the packet transmission times over the channel are positively correlated over time, or (iii) the packet transmission times are highly random (e.g., following a heavy-tail distribution).

No abstract

Theory of characteristic modes (TCM) can provide physical insight into the radiation mechanism of arbitrarily-shaped electromagnetic objects. However, how to compute the characteristic modes (CMs) of different structures is still an open problem. Even for the calculation of CMs of an isolated dielectric body, there are eleven integral equation (IE)-based formulations which result in different modal solutions. Such kind of non-uniqueness of solutions makes CMs community confused. One of the objectives of this paper is to outline the differences among all existing IE-based formulations for the CMs of dielectric bodies. The existing formulations are briefly reviewed and carefully compared. We present a procedure to implement the different formulations in a unified manner. Then, we make a complete comparison of the numerical results of existing methods for a dielectric cylinder, which also serves as cross-validation for these approaches. We hope that this paper will help researchers understand the calculation of CMs of dielectric bodies and explore the computation methods of CMs for more complex objects.

No abstract