  To meet the surging demand for throughput, 5G cellular networks need to be
more heterogeneous and much denser, by deploying more and more small cells. In
particular, the number of users in each small cell can change dramatically due
to users' mobility, resulting in random and time varying uplink interference.
This paper considers the uplink interference in a 5G heterogeneous network
which is jointly covered by one macro cell and several small cells. Based on
the L\'{e}vy flight moving model, a mobility-aware interference model is
proposed to characterize the uplink interference from macro cell users to small
cell users. In this model, the total uplink interference is characterized by
its moment generating function, for both closed subscriber group (CSG) and open
subscriber group (CSG) femto cells. In addition, the proposed interference
model is a function of basic step length, which is a key velocity parameter of
L\'{e}vy flights. It is shown by both theoretical analysis and simulation
results that the proposed interference model provides a flexible way of
evaluating the system performance in terms of success probability and average
rate.
