In this letter, we demonstrate a 48-Gb/s polarization-division-multipexing (PDM) QPSK radio-over-fiber (RoF) system over 2$,times,$2 multiple-input–multiple-output (MIMO) wireless transmission. The PDM-QPSK signal is up-converted to a 40-GHz mm-wave and broadcasted through the 2$,times,$2 MIMO wireless system. The principle of the PDM-QPSK RoF system over 2$,times,$2 MIMO wireless transmission is presented. The conventional digital coherent demodulation algorithms for the compensation of chromatic dispersion, polarization demultiplexing and frequency offset, and phase recovery have been proved valid even if the 2$,times,$2 MIMO wireless transmission system is utilized. The BER performance after over 400-km fiber link and 100-cm wireless link clearly demonstrates the feasibility of this PDM-QPSK RoF system over 2$,times,$2 MIMO wireless link.