n; where to the place base stations or access points; which network topology to use; how to con- nect the base stations or access points to a core network; how to facilitate further evolution of the network's design and SO on. It must also decide whether or not to partner with other operators to share network resources such as base station sites and antennas. Frequency Reuse Another factor impacting cell design is spectrum resource allocation and frequency reuse factors as discussed in Chapter 10. The first generation (1G) and 2G standards reused the allo- cated frequency spectrum in every 7th or 3rd/4th cell site, respectively. While this process is easy to design and implement on paper as shown in Figure 13.13, designing for a real city with buildings, hills and valleys, and different population density requires different size and shape of cells. Frequency allocation was a main task of cell design companies. The Code Division Multiple Access (CDMA)-based 3G standards simplified frequency allocation as the same frequency could be used in all cells. Cell design for 3G systems how- ever required a good management inter-cell multi-user interference. The signal from any base station represented itself as interference to all neighbouring cells and therefore antennas had to be tuned to ensure signal leakage outside a cell was minimised. Figure 13.14 shows how two neighbouring cell antennas are tuned. As the signal from a base station appears as inter- ference to the neighbouring cell's users, the antennas of base stations are tilted in order to minimise the mutual interference. Adjusting the antenna tilts is one factor in controlling and tuning the network. Another challenge arises when a network is expanded and new base stations need to be introduced into the network. Such introduction is not trivial as there is no suitable place to install a new base station and create a cell around it, as can be seen from Figure 13.15. Antennas and networks need a retune to ensure optimised performance. Network Topologies, Design a