We demonstrate a power-efficient coherent interleaved frequency domain multiple access passive optical network (IFDMA-PON) uplink system. The digital signal processing circuits for coherent IFDMA-based quadrature phase-shift keying (PSK) are implemented on 40-nm field programmable gate arrays. By employing polar-coordinate transformed mapping of the PSK signal, the power consumption of the optical network unit is reduced to one-tenth that of conventional orthogonal frequency-division multiple access-PON systems. The transmission of a 6 Gb/s real-time coherent IFDMA-PON uplink signal across 20 km of single mode fiber is demonstrated for the first time.