Modern cellular networks support a concept known as the heterogeneous network – the term ‘heterogeneous’ is generically defined as meaning ‘diverse in character or content’ and that is the sense in which it is employed here.
A HetNet is one in which many base stations, of varying types and sizes, share the same radio resource environment to provide a multi-layered or hierarchical service consisting of network of macro cells and small cells which can offer ‘multi-mode’ (a mixture of 2G, 3G and/or 4G services) and ‘multi-technology’ (a mix of cellular and Wi-Fi) services. Multi-technology deployments allow Wi-Fi access points to be integrated into a HetNet to provide seamless cell coverage with the capability to hand calls over between technology layers. In contrast, a traditional network that employed only macro cells would be classed as a ‘homogeneous’ network.
A HetNet can consist of large macro cells, providing umbrella coverage over wide areas and smaller mini and micro cells, providing fill-in coverage in hard-to-reach or high-demand areas. In essence this is the same coverage concept employed in legacy networks’ hierarchical designs but modern HetNets are also able to deploy additional layers of elements. Relay Nodes, repeaters and ‘home base stations’ are able to further focus coverage and/or capacity into areas that macro cells might struggle to serve, such as areas with challenging terrain, the interiors of large buildings and underground areas.
All cells in a HetNet will share the same set of radio resources (e.g. the licensed bandwidth assigned to the operator), which means that any coverage and capacity gains made as a result of deploying a HetNet must be weighed against the increases in interference that may be experienced in areas served by a variety of different types of site. Frequency planning techniques may go some way to mitigate these effects, as will the employment of techniques such as ICIC (InterCell Interference Coordination), which allows neighbouring base stations to coordinate their usage of cell edge resources to minimize inter-cell interference issues.
Further improvements in interference mitigation in a HetNet can be made by employing dynamic frequency planning techniques at the femtocell layer. Based on measurements and feedback taken by the femto base stations and mobile phones, these techniques allow a network to dynamically reconfigure femtocell resources over time to use channels that cause and experience the least amount of co- and adjacent channel interference.