MILLIMETER-wave bands are being eyed for their potential in carrying broadband data transmissions. With the severe attenuation of millimeter-wave signals along even low-loss transmission lines, signal losses are excessive for long-haul transmissions. In addition, millimeter-wave wireless channels suffer from multipath fading and can be degraded by intersymbol interference (ISI). A solution may be to transfer millimeter-wave signals by means of optical-fiber transmission, which has low attenuation and no interference.
Recently, an indoor, wireless millimeter-wave system with an optical orthogonal-frequency-division-multiplexing (OFDM) signal link was developed by Ajung Kim and S. Nahm from the Sejong University School of Engineering and Applied Sciences, Harvard University. The optical link contains a distributed-feedback laser diode and a photodetector. To convert the S-band signals to OFDM signals with a center frequency of 59.7 GHz, the millimeter- wave transmitter mixes 57.2-GHz signals from the local oscillator (LO). The reverse-order action is performed in the millimeter-wave receiver.
Channel modeling was based on measurements of 60-GHz channels. To validate the model, the designers compared measured values with the simulated ones. Link performance was measured in an office environment under line-of-sight (LOS) conditions and with a 30-km optical OFDM link. The results are promising for linking communications infrastructure with optical backbone systems. See "Evaluations and Measurements of Indoor Wireless Systems with Optical Orthogonal Frequency Division Multiplexing Lines," Microwave And Optical Technology Letters, March 2011, p. 491.