A transport-theoretic freshness–reliability–delay framework for cooperative balloon-assisted UAV relay networks

The cooperative relay network utilizing the combination of balloons and UAVs can be seen as a promising strategy for increasing the coverage and improving information transmission in the large wireless network environment. Unfortunately, the existing communication protocols mainly focus on optimizing the individual measures of freshness, delay, or reliability, making it impossible to describe how those three metrics interact with each other. In this paper, we develop the FRD utility functional that provides a unified treatment of cooperative balloon-assisted UAV relay networks. To do so, we formulate the transport theoretic state of the system as the age density field, congestion field, and reliability field, which evolve in accordance with a coupled system of partial differential equations. The theoretical study confirms fundamental characteristics of the suggested framework such as the information mass conservation property, invariant property of admissible states, equilibrating property, and the transport efficiency-reliability coupling property. The numerical study is conducted via the Monte Carlo simulations based on heterogeneity of user demand and different balloon-assisted transport designs. The results confirm stable conservation property, limited dynamics of state evolution, convergence to theoretical equilibria, and the high accuracy of transport-coupling relationships. Comparing the proposed framework to benchmark models with the freshness objective, delay objective, and reliability objective, respectively, it is shown that the FRD approach yields the best overall utility of the system performance.