In this paper, a multiple-input multiple-output (MIMO) extension for a third generation (3G) wireless system is described. The integration of MIMO concepts within the existing UMTS standard and the associated space-time RAKE receiver are explained. An analysis is followed by a description of an actual experimental MIMO transmitter and receiver architecture, both realized on DSPs and FPGAs within a precommercial OneBTS base station. It uses four transmit and four receive antennas to achieve downlink data rates up to 1Mbps per user with a spreading factor of 32 and the UMTS chip rate of 3.84 MHz. Furthermore, different MIMO detectors are evaluated, comparing their performance and complexity. System performance is evaluated through simulations and indoor over-the-air measurements. Capacity and BER measurement results are presented.