Analysis of downlink power control in a GSM system

The control of the power transmitted by the radio base stations has been proposed to improve the quality or to increase the capacity of a GSM cellular system. The performances of an downlink power control algorithm proposed by one GSM manufacturer are investigated. This algorithm is based on a linear control technique taking into account both the received signal strength and quality of the radio link. The algorithm characteristics are first analysed from a theoretical point of view. Furthermore, a stability analysis of the proposed algorithm is provided. It is pointed out that unstable states exist also in the domain of validity of the algorithm parameters. The second step of the analysis consists in simulating the nonlinear nature of the GSM radio link parameters. Measurements have been undertaken in two cities in Switzerland to assess the performance of various parameter settings on the downlink power control. Measurement results are analysed and compared to the theoretical investigation. Possible performance degradation due to the simultaneous use of downlink power control and frequency hopping over all transmitted carriers are also shown