  It is anticipated that much higher network capacity will be achieved by the
fifth generation (5G) small cell networks incorporated with the millimeter wave
(mmWave) technology. However, mmWave signals are more sensitive to blockages
than signals in lower frequency bands, which highlights the effect of
anisotropic path loss in network coverage. According to the fractal
characteristics of cellular coverage, a multi-directional path loss model is
proposed for 5G small cell networks, where different directions are subject to
different path loss exponents. Furthermore, the coverage probability,
association probability, and the handoff probability are derived for 5G fractal
small cell networks based on the proposed multi-directional path loss model.
Numerical results indicate that the coverage probability with the
multi-directional path loss model is less than that with the isotropic path
loss model, and the association probability with long link distance, e.g.,
150m, increases obviously with the increase of the effect of anisotropic path
loss in 5G fractal small cell networks. Moreover, it is observed that the
anisotropic propagation environment is having a profound impact on the handoff
performance. Meanwhile, we could conclude that the resulting heavy handoff
overhead is emerging as a new challenge for 5G fractal small cell networks.
