This paper analyses the outdoor small-cell millimeter-wave (mmW) propagation using ray-based simulation tools. The main characteristics of the wireless channel, i.e. pathloss, delay and angular spreads, are characterized through various small-cell scenarios, and by comparison between several frequency bands. The 2.4 GHz and 5 GHz bands represent the traditional sub-6GHz frequencies in public radio-communication networks, while 28 GHz and 60 GHz stand for the mmW 5G candidate frequencies. A first analysis compares the downlink Signal-to-Noise Ratio (SNR) in an urban small-cell network, showing how severely the coverage beyond building or vegetation blockage is degraded at mmW. Then, the shadowing and channel scattering created by a small house is studied; the delay and angle spreads behind the house are divided by respectively 5.8 and 2.8 when going from 5 GHz to 60 GHz. Finally, the paper makes recommendations for representation of small objects, trees in particular, to achieve accurate channel or coverage simulations.Index Terms -millimeter-wave propagation, ray-based modeling, shadowing, scattering, 3D geographical map data, 5G, outdoor small-cell.I.

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Femtocell technology recently gained attention due to its potential benefits for mobile operators (significant capacity offload and extension of the coverage at low cost) but there are still hard technical challenges to be addressed (e.g., the optimization of the interference management). Furthermore, the deployment strategies are still in question, such as the femtocell access-mode (open or closed) and the spectrum usage to adopt. Consequently, reliable simulations are necessary in the perspective of massive femtocell deployments, in particular for characterization of the impact on the coverage quality. An original solution is introduced and exploited in this article. It offers two complementary approaches for two different applications: a synthetic model for realization of small-scale or illustrative case studies and a real model for realistic and large-scale heterogeneous network performance evaluation. It relies on a suite of simulation tools including the generation of random 3D femtocell deployments in synthetic or real environments; realistic pathloss predictions; and a 3D downlink coverage performance analysis (i.e., considering all floors) of long-term evolution heterogeneous networks. A first study shows not only a large improvement of coverage quality for femtocell users, but also a very significant degradation for non-subscribers in the vicinity of closed-access femtocells. The femtocells have a strong impact locally (gain or degradation depending on the access-mode and user type) and not only at their own floor. Therefore, a 3D evaluation is relevant. Then, a second study offers realistic and large-scale analyses of the coverage evolution after corporate femtocells have massively been deployed in urban macrocells. The results show a moderate impact on the average spectral efficiencies but a strong impact locally. In this study, closed-access femtocells cause dead zones for non-subscribers in 15% of indoor areas leading to non-uniform service coverage, whereas they increase the spectral efficiency of femtocell subscribers (by 1.5 bps/Hz in 20% of indoor areas). These are critical information for a mobile operator since the experience of its customers is much affected by the femtocell deployment and by the selected access mode.

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