A photonics-assisted D-band wireless transmission system employing an in-phase/quadrature (I/Q) mixer and maximum-ratio combining (MRC) is experimentally demonstrated. Two intermediate-frequency (IF) outputs from a single radio-frequency input are digitized and processed separately. After channel estimation and equalization, the recovered complex baseband signals are coherently combined. A 32-Gbaud quadrature phase-shift keying orthogonal frequency-division multiplexing (QPSK-OFDM) signal with an effective payload rate of 38.65 Gb/s is transmitted over a 20 m indoor wireless link with a calculated optical-beat carrier frequency of 138.5 GHz. Across seven input optical power settings from −8 to −2 dBm, MRC provides an average signal-to-noise ratio (SNR) improvement of 1.78 dB and a maximum improvement of 2.32 dB relative to the higher-SNR single branch at each power. Combining also improves constellation convergence and reduces the bit error rate (BER), with the MRC output falling below the 7% hard-decision forward-error-correction threshold of 3.8 × 10⁻³ at the sampled power settings of −5 dBm and above. These results demonstrate improved signal quality through I/Q-branch combining in the investigated D-band link.