Voice quality and capacity enhancement in GSM system using the features of adaptive multirate and M-ary PSK modulation

Abstract

GSM (Global System for Mobile communication) is a popular mobile communication system especially for its roaming facility and handset changing flexibility. Due to increasing demand it is now essential to increase the cell capacity. In GSM release 99, AMR (adaptive multi rate) speech coding rates are specified which can ensure toll grade voice quality even for HR (Half Rate) traffic channel. This makes the cell capacity doubled. In this thesis a new technique has been presented which can extend the cell capacity further by introducing a QR (Quarter Rate) traffic channel in combination with the QPSK and 8PSK modulation scheme. Using the proposed technique it is also possible to provide high quality voice using high AMR speech coding rates. A link adaptation algorithm has been developed for selecting the link condition such as speech coding rate, traffic channel rate and modulation scheme for the call set up by a mobile terminal. If the carrier to noise ratio (CIN) experienced by the mobile terminal is more than the minimum allowable CIN ratio of a certain link condition, the link condition is assumed to be feasible for the mobile terminal. This minimum allowable CIN has been determined by running a MATLAB simulation for the proposed coding scheme. The CIN ratio for an acceptable voice quality (i.e., 10.5 BER) is considered as the minimum allowable CIN ratio. The formation of voice blocks from the raw voice packet 'and mapping of this voice block to TDMA frames has also been presented. The framework of signalling for the proposed QR traffic has been presented with the analysis of required CIN for acceptable bit error rate ..The proposed technique is devised in such a way that it can be applied in the GSM system without the change of installed hardware. The performance of the proposed scheme has been compared with that using Grameen Phone's coverage prediction model, by running the developed algorithm for link adaptation. It is observed that the cell capacity is increased by 63.5% on the average. It has also been found that for FR and QR, QPSK modulation shows better performance than GMSK and 8PSK. Whereas GMSK performs better in HR. 8PSK can only be used for few higher speech coding rates which are not feasible for GMSK and QPSK.