In this paper, we propose and experimentally demonstrate the first single-?? 40 Gb/s and 108 Gb/s multiple-input multiple-output orthogonal frequency-division multiple access (OFDMA) passive optical networks (PON) architecture for next-generation PON systems based on OFDM, polarization multiplexing (POLMUX), and direct detection. Superior performance was exhibited after 20 km SSMF transmission and a 1:32 optical split. The novel POLMUX approach greatly simplified receiver-end hardware compared to coherent detectors, while increasing spectral efficiency to enable 40+ Gb/s data rates. Moreover, the proposed solution achieved the highest single-wavelength downstream transmission reported to date in any PON system. As such, the introduced architecture may be viewed as a highly attractive candidate for next-generation optical access.