This work is a study about the performance of a typical UMTS/WCDMA link for third generation mobile communications systems which uses the adaptive modulation technique. This technique consists of choosing, for each transmitted frame, the most efficient modulation according to the channel conditions. The basic aspects of the problem were analytically examined considering transmissions with PSK and QAM coherent techniques in a flat fading channel with Doppler effect. A simulator was developed in order to consider the application in a more realistic model. This simulator was capable of modeling this problem considering the main aspects of the HSDPA (High Speed Data Packet Access) system, which is a subsystem of UMTS/WCDMA that is standardized for high-speed data transmission in the downlink. The link level performance was evaluated for many different situations through average transmission bit error rate and throughput, investigating the influence of the system parameters in this performance.