The recent tremendous growth of mobile broadband traffic demand forces mobile operators to find ways for offloading their macro-infrastructure. This can be achieved by a combination of two concepts, i.e., introducing/deploying a finer grid of base-stations (femto-cells) together with increasing the share of traffic volume that is handled by communication-links operating in unlicensed bands (e.g., IEEE802.11-based wireless-LAN). Both concepts are supported by the recent proliferation of wireline broadband (DSL/cable) which enables the cost effective backhauling of small sites using standard internet access. However, new challenges in terms of network-planning arise as these small cells are deployed randomly, their resulting spatial density in urban environments may get very high, and thus the aspect of interference-mitigation is of vital importance. The contribution of this paper is twofold: first we provide a comparison of licensed-band femto-cell versus unlicensed WiFi technologies. We try to outline characteristic differences and respective limitations in terms of efficiency of radio-resource sharing mechanisms, with a specific focus on macro-cell offloading scenarios. Second, we present the results of a measurement campaign that allows to quantify the current density of WiFi access points in the 2.4GHz WiFi band in urban environments. We subsequently use these data sets to extrapolate the coverage ratios that are achievable by aggregating low-power WiFi/femto cells. This can be used to provide novel insights into specific radio-resource management problems which a mobile operator needs to solve when moving towards smaller cells and less-controllable deployment procedures.
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