Different from the existing isotropic path loss model, in this paper, we develop an anisotropic path loss model for the fifth generation (5G) multi-input multi-output (MIMO) fractal cellular networks, in which the coverage boundary has the fractal characteristics including the self-similarity and the detailed structure at arbitrarily small scales of the angle domain. Based on the real-world measurement data collected from the Zhangjiang Road in Shanghai, China, we analytically derive the coverage probability, the area spectral efficiency (ASE), and the sum rate for the fractal small cell networks, with the assumption that the path loss exponent follows the Gamma distribution. Simulation results indicate that compared with the conventional isotropic path loss model, the coverage probability under the anisotropic path loss model has been overestimated in small cell networks. With the anisotropic path loss model, the ASE with MIMO technologies is higher than that with single-input single-output (SISO) technologies in the low signal to interference ratio (SIR) regions and is lower than that with SISO technologies in the high SIR regions. Therefore, the coverage model of small cell network needs to be rethought by taking into account the fractal characteristic in wireless channels.

To meet massive wireless traffic demand in the future fifth generation (5G) cellular networks, small-cell networks are emerging as an attractive solution for 5G network deployments. The cellular coverage characteristic is a key issue for the deployment of the small-cell networks. Considering the anisotropic path loss in wireless channels of real cellular scenarios, in this paper the fractal coverage characteristic is first used to evaluate the performance of the small-cell networks. Moreover, the coverage probability, average achievable rate, and the area spectral efficiency are derived for fractal small-cell networks. Compared with the average achievable rate and area spectral efficiency with isotropic path loss models, the average achievable rate and area spectral efficiency with anisotropic path loss models have been underestimated in the fractal small-cell networks. Considering the impact of the anisotropic path loss on wireless channels, most of performances of wireless cellular networks need to be re-evaluated. This paper provides a tractable method to investigate the performance of the small-cell networks with fractal coverage characteristics.