Introduction

The emerging applications for mobile internet in different areas such as education, health care, smart grids, and security are growing very fast. Due to the integration of these applications with people's lives it is inevitable that over then the next ten years there will be huge increases in the required user data rate per area and the spectral efficiency. On the other hand, delivering a higher data rate per area requires more transmission power, which is constrained not only by safety limits but also by the importance of the global warming issues and need for green communications. Therefore, high-speed transmission would mean a diminishing coverage range, as otherwise, an enormous increase of transmission power is required by both mobile terminals and base stations to maintain the current cell size and achieve the ambitious targets of the beyond-IMT advanced technologies.

Cell splitting, i.e. dividing large cells into a number of smaller cells, is a promising method that can significantly increase both the capacity and the coverage in future cellular networks. Since the divided cells have to operate with full spectrum reuse across all base stations, i.e. due to the scarcity of bandwidth resources, co-channel interference becomes the major issue in cell splitting. On the other hand, the spatial dimension can be more effectively exploited as a complementary communication resource to the traditional ones. In a cellular network, spatial dimensions are available either locally, i.e. through multiple antennas at BSs, terminals, relays, vehicles, or in a distributed configuration such as an array of multiple antenna BSs within a cluster of cells.