nificantly degrade. In addition to the above remedies, the operators have integrated small cells into their macro-networks to effi- ciently distribute network loading, and to maintain performance and service quality while reusing spectrum more efficiently. One solution to expanding an existing macro-network, while maintaining it as homogeneous, is to add more sectors per base station or deploying more macro-base stations. However, reducing the site-to-site distance in the macro-cell layout can only be pursued to a certain extent because finding new macro-sites becomes increasingly difficult and can be expensive, especially in dense urban areas. An alternative is to introduce small cells through addition of low-power 5G Network Architecture 47 access nodes or RRH to the existing [overlaid] macro-cells due to their more economical site acquisition and equipment installation. Small cells are primarily added to increase capacity in hotspots with high user demand and to fill coverage holes in the macro-network in both outdoor and indoor environments. They also improve network performance and service quality by off- loading the overlaid macro-cells. The result is a heterogeneous network topology with large macro-cells in conjunction with small cells providing increased capacity per unit area. Heterogeneous network planning dates back to GSM era where cells were separated through the use of frequency reuse. While this approach could still be taken in LTE, LTE networks pri- marily use a frequency reuse of one to maximize utilization of the licensed bandwidth. In het- erogeneous networks, the cells of different sizes are referred to as macro-cells, micro-cells, pico-cells, and femto-cells in the order of decreasing transmit power. The actual cell size depends not only on the access node power but also on physical antenna positions, as well as the topology of the cells and propagation conditions. In general, the small cells in an ultra-dense network (UDN) are classified into full-functional base stations (pico-cells and fe