ll case. Sectorisation means using three sector cells instead of omnidirectional antennas. This method helps to re-use the frequency pattern more efficiently by setting a smaller frequency re-use factor. Cell splitting, on the other hand, means adding a new site between two existing sites to share the traffic load with them. This solution is limited because at a certain site-to-site distance, it is not possible to add any more macro sites. Then, the other option is to add micros and in-building cells, especially at hotspot area levels. Multilayering the network means structuring overlapping cells into upper and lower layer cells in order to allocate users to these layers according to their speed. Slow moving users camp on small cells and fast moving mobiles (i.e. vehicles) use the macro layer. This separation optimises the number of handovers and distributes traffic to reduce overload. AMR for Both Coverage and Capacity Enhancements The adaptive multirate (AMR) is a kind of tradeoff between source coding and channel coding in a TDMA frame. The AMR provides a set of codecs fitting in full rate (FR) and half rate (HR) physical channels. These codecs provide a variable tradeoff between error protection and speech quality (see Figure 2.65). The aim is to improve speech quality by adapting the most appropriate channel mode and codec based on radio conditions. It consists of two algorithms: link or codec mode adaptation (choice of codecs); channel mode adaptation (half rate or full rate). The AMR takes into consideration the radio conditions (C/I) and the load conditions (EFL %). It uses 14 codecs, 8 in FR ranging from 4.75 to 12.2 kbps and 6 in HR ranging from 4.75 to 7.95 kbps. In GSM FR/EFR, the channel bit rate is 22.8 kbps = 13 kbps (speech coding) + 9.8 kbps (channel coding) In the AMR case, the codec mode can be changed and more error correction bits can be used whenever the channel requires. The best codec choice is done through the link adaptation algorithm where, to each C/I value, the codec providing the bes