 using macrocells expected subscriber density. More traffic capacity is required as the number of subscribers increases, and as each base station can service a limited number of users at any one time, the coverage area of a cell in a densely populated urban area is usually small. In suburban and rural areas, population density is relatively smaller and therefore a base station can cover a larger area. The cell radius therefore ranges from several hundred metres in dense urban areas, to several kilometres in suburban areas and to tens of kilometres in rural areas. The limit of coverage in rural areas is determined by the technology and standard constraints. Usually sector antennas are used to divide a cell into several (often three) parts, and thereby increase the capacity of a base station site by a corresponding factor of several (three) as illustrated in Figure 13.7. The height of base station antennas are a function of cell topography and building charac- teristics. A summary of macrocell designs is as follows: Dense urban Downtown areas in major cities, characterised by high-rise buildings and high population density. The cell radius is in the order of 300-500 m. Typical antenna heights are 35 m or higher. Cell sites and antennas are usually placed on rooftops. Network Topologies, Design and Convergence Figure 13.7 Sector antennas Urban Town centres, characterised by medium height buildings. Cell radius is in the order of 1 km and base station antenna heights are 25-40 m. Again cell sites and antennas are usu- ally placed on rooftops. Suburban Urbanised areas, with single to multi-storey buildings. Cell radius is in the order of a few kilometres and antenna height is 10-25 m. Cell sites are usually rooftops or in specially built towers. Rural Sparsely populated areas with little traffic. Here the cell radius is determined by a system's link budget (i.e. how far the signal can travel before received SNR falls below a required threshold). The cell radius also depends on the technology's coverage limitations, wh