Optical frequency-division multiplexing (FDM) techniques offer a means of circumventing the bandwidth limitation of semiconductor lasers and photodiodes, utilizing the large bandwidth potential of optical carriers. Although present intensity modulation fiber-optic communication has a larger transmission capacity, the present fiber-optic system frequency utilization efficiency is 10−6-10−7 b/s/Hz. As radio systems share the same propagation media, extremely high-frequency utilization efficiencies are required. For example, an NTT 256 QAM radio system achieves ~10-b/s/Hz frequency utilization efficiency, but this system sacrifices relay span distance.