This article presents a novel photomixing heterodyne THz receiver based on waveguide-integrated (win) photoconductive antennas (PCAs). Conversion gain of -22 dB, a 25 dB increase in gain compared to previous work, was accomplished by an innovative optical waveguide coupling design. The conversion gain measured for this novel win-PAC receiver design compares impressively with -15 dB typical of Schottky-diode mixer approaches. Ultra-broadband performance from 100 to 600 GHz with 84 Gbit/s data rates was achieved with 4-QAM modulation over 1 meter with < 2.2^-2 BER and < 46% EVM performance, a record data rate for photonic wireless links with PCA receivers.