A full-duplex radio-over-fiber system using frequency-decupling optical millimeter-wave based on external modulation via a Mach–Zehnder modulator is proposed and analyzed theoretically. The simulation results show that the power penalties for both the downstream and upstream signals are less than 0.5dB. In this scheme, the configuration of a base station is simplified due to no laser at a base station, and the frequency of local oscillator signal is largely reduced due to the frequency-decupling millimeter-wave technique. The costs of central office and base station are largely reduced.