trength and the interference level for all locations in a cell. Generally this is done using a computer based tool which can There are a number of elements which contribute to the received draw upon a database of cell site information and terrain data. Some signal strength at a mobile. Firstly, for a line of sight path, there is a advanced tools can also take account of diffraction losses. For free space path loss which is related to the radial distance between practical purposes a planner will aim to achieve the required signal base station and mobile. strength and C/I ratio over 90% of the cell coverage area, by varying In addition to this loss, where there is no direct line of sight path, antenna heights, transmitter powers, frequency allocations and there will be a diffraction loss resulting from obstructions in the other factors as appropriate. path. In general there will also be an effect due to multiple signals To simplify calculations, an allowance for Rayleigh fading and arriving at the mobile due to reflections from buildings and other shadow fading is usually made within the system power budget. A terrain features. This multi-path effect will result in signals either typical power budget is shown in Table 47.1. adding constructively or destructively. As a mobile moves around within a cell it will experience varying 47.3.5 Adding capacity signal, as shown in Figure 47.6, due to these factors. Fast fading is caused by the multipath effect, and occurs with only a small move- Once a cellular network has been planned to provide overall cover- ment of the mobile. This is also known as Rayleigh fading. Slow age, there are a number of ways of adding additional capacity. A Overview of systems 47/7 set. One way of cutting significantly the level of interference is to Table 47.1 Typical power budget (TACS) use several directional antennas at the base stations, with each (1 = Key planning parameters) antenna illuminating a sector of the cell, and with a separate channel Signal strength budget Downlink Uplink set a