  As two major players in terrestrial wireless communications, Wi-Fi systems
and cellular networks have different origins and have largely evolved
separately. Motivated by the exponentially increasing wireless data demand,
cellular networks are evolving towards a heterogeneous and small cell network
architecture, wherein small cells are expected to provide very high capacity.
However, due to the limited licensed spectrum for cellular networks, any effort
to achieve capacity growth through network densification will face the
challenge of severe inter-cell interference. In view of this, recent
standardization developments have started to consider the opportunities for
cellular networks to use the unlicensed spectrum bands, including the 2.4 GHz
and 5 GHz bands that are currently used by Wi-Fi, Zigbee and some other
communication systems. In this article, we look into the coexistence of Wi-Fi
and 4G cellular networks sharing the unlicensed spectrum. We introduce a
network architecture where small cells use the same unlicensed spectrum that
Wi-Fi systems operate in without affecting the performance of Wi-Fi systems. We
present an almost blank subframe (ABS) scheme without priority to mitigate the
co-channel interference from small cells to Wi-Fi systems, and propose an
interference avoidance scheme based on small cells estimating the density of
nearby Wi-Fi access points to facilitate their coexistence while sharing the
same unlicensed spectrum. Simulation results show that the proposed network
architecture and interference avoidance schemes can significantly increase the
capacity of 4G heterogeneous cellular networks while maintaining the service
quality of Wi-Fi systems.
