We quantitatively investigate an influence of residual downlink optical sideband on uplink transmission in full-duplex dense wavelength division multiplexed (DWDM) millimeter-wave-band (mm-wave-band) radio-over-fiber (RoF) system. From our previous experiment, it was obtained that the influence was negligible when the undesired downlink sideband was suppressed by more than 20 dB. In this paper, we show the relationship between the downlink sideband suppression and the uplink transmission performance by numerical simulation, which was difficult to be scrutinized in practical experiment.