A cost-efficient radio over fiber system based on a 21-tuple frequency binary phase-shift keying (BPSK) millimeter wave using a phase modulator (PM) and semiconductor optical amplifier (SOA) is theoretically proposed and demonstrated. In this scheme, the PM is driven by a radio frequency BPSK downlink signal at an appropriate modulation index to generate the +3rd-order sideband bearing BPSK signal and −4th-order sideband unmodulated generated signal, which is similar to the single sideband signal. New frequencies of sidebands are obtained after four-wave mixing in SOA. At the base station, after filtering, the −11th-order sideband bearing BPSK signal and +10th-order sideband unmodulated signal are abstracted, and then a photodiode is input to generate a 21-tuple frequency BPSK mm-wave signal, whereas the −4th-order sideband is used as the carrier for uplink. Simulation results show that, without precoding, a wideband 21-tuple frequency BPSK mm-wave signal with the frequency varying from 21 to 210 GHz can be obtained. Furthermore, the impact of SOA’s current and the laser linewidth on the system performance is also studied. The power penalty for the downlink signal is 2 dB and for the uplink signal is 1.7 dB after 50 km single-mode fiber transmission at a BER of 10 ^{− 9}, which shows good system performance.