A radio-over-fiber(RoF) system with 56 GHz optical millimeter-wave(mm-wave) generation using an optical phase modulator(PM) based on frequency-doubling technology is proposed and demonstrated experimentally.Firstly,at the central station(CS),a 56 GHz optical mm-wave is generated by a 28 GHz microwave-signal source,which is applied to the phase modulator;secondly,2.8 Gbit/s OOK downstream data is modulated onto 56 GHz optical carrier and then transmitted over 20 km of standard single-mode fiber(SSMF) to the ...