This paper describes a four-element multiple-input multiple-output (MIMO) transmitter (TX) system that features an analog spatial de-interleaver to simplify the baseband-input complexity and increase the spatial matching of the sub-TXs. The MIMO diversity gain and power-combining gain are jointly exploited to relax the output power of the four power amplifiers and eliminate their output matching networks, leading to a compact implementation of the entire TX system. The MIMO effectiveness is improved by introducing an radio-frequency to baseband dc feedback technique that enhances the matching among the sub-TXs against process variation. In the verification, the TX system is co-designed with a compact antenna array-on-PCB that generates a null zone in the propagation pattern, and the electric-field polarization angle, to achieve diversity propagation. All techniques together improve the signal-to-noise ratio or data rate by generating multiple data streams according to the signal power arriving at the receiver over different fading channels. The four-element TX chip fabricated in 65-nm CMOS occupies a die area of 1.44 mm(2). It covers an RF range of 0.7-2.5 GHz, and shows an equivalent isotropically radiated power of 23.9 dBm. When transmitting a 20-MHz 64-QAM orthogonal frequency division multiplexing signal at 2.3 GHz, the average system efficiency is 61% and error-vector magnitude is -26 dB.