Analyzing D2D mobility: Framework for steady communications and outage periods prediction

The use of direct device-to-device (D2D) communications is expected to drastically increase the spatial frequency reuse significantly improving the capacity of forthcoming fifth generation (5G) cellular systems. The number of D2D pairs operating within the same channel that can be supported in a certain area of interest is limited by the interference imposed by them on each other. In an inherently mobile environment where the users are constantly mobile the decision on enabling a certain number of direct connections should not only be based on the static interference picture but shall consider the changes in the signal-to-interference ratio caused by the user mobility. These changes manifest themselves into steady communications and outages time periods interchanging each other. In this paper, the statistical characteristics of the user mobility periods as a function of the D2D communications range, speed of users, number of communicating pairs and the area of interest is characterized utilizing the generic approach based on the Fokker-Plank equation. We show that although the time-evolution equation for the cumulative distribution function (CDF) of SIR can be written in explicit form for non-stationary movements of nodes, the solution cannot be obtained in closed-form. Using the mixed simulation-analytic approach we propose a generic methodology for performance assessment of time-dependent characteristics such as steady communications and outage time periods.