t MOS is chosen. The AMR has several benefits: Speech quality (MOS). Due to the use of new codecs that are more robust to interference than the EFR codec, the AMR can maintain good speech quality in poor C/I conditions. Audio quality is proved Radio Network Optimisation Controlled output power Upper threshold Lower threshold Figure 2.66 Variation of controlled output power to be better than with the EFR in bad C/I conditions. In addition, AMR HR speech quality (MOS) is significantly better than GSM HR and is comparable with GSM FR in good link conditions. AMR coverage gain. Performance at the cell edge is improved thanks to the use of more robust codecs. The most robust codec (4.75kbps) seems to provide, according to laboratory tests, a 5 dB gain compared to the EFR performance. Capacity gain. AMR robustness allows tightening frequency re-use patterns, increasing spectral effi- ciency. The AMR-FR can provide around 140 % capacity gain versus EFR in the hopping layer. The AMR is at the end a speech codec, which adapts its operation optimally according to the prevailing channel conditions. The adaptation is based on the received channel quality estimation in both the mobile station and base transceiver station (BTS). Conclusion A multitude of methods exist to enhance capacity and coverage. They are therefore interdependent. The issue involves how to chose from several methods and when. Once implemented, these solutions increase the complexity of quality degradation analysis. Handover Performance The main idea of mobile cellular networks is enabling mobility. Mobility means allowing the subscriber to make or receive calls when moving within an own network or between different networks (e.g. different cities or countries, if a roaming agreement exists between the operators). To ensure this, two elements are important: firstly, cells constituting the network should overlap between them at an acceptable level and, secondly, there should be a software feature that handles mobility management. This section concentrates o