In the UMTS cellular radio system the so-called Adaptive Multi-Rate codec (AMR) will be used for speech transmission. The bit rate of the AMR codec can be controlled by the Radio Resource Management (RRM). The highest speech quality can be achieved at a net bit rate of 12.2 kbit/s whereas the highest traffic capacity corresponds to the lowest bit rate of 4.75 kbit/s. Simulations of the uplink of the UMTS physical layer have shown that the number of simultaneous speech traffic channels is limited to 73...149 per cell depending on the speech codec mode. These capacity figures are valid under some idealizing assumptions such as perfect power control and multipath compensation. An algorithm for AMR mode switching is presented which which can significantly improve the speech quality in UMTS systems. The algorithm is controlled by evaluating the soft output likelihood values of the channel decoder. The speech quality depends on the allowed bit rate which itself is determined by the number of users in the UMTS system. Therefore a comparative analysis of the spectral efficiency and the system capacity of UMTS and GSM is given.
