Investigations On Optical Millimeter-wave Generation In Radio Over Fiber Wireless Access Systems

Radio-Over-Fiber (ROF) technique, the integrated optical and wireless access system, carries broadband radio signal through fiber instead of cable. RoF not only has the merits of wide bandwidth and high immunity to electro-magnetic interferences,but also allows the sharing equipment at the CO and therefore enables simple radio distribution architecture to be implemented, and has become one of the most promising technologies in the next generation wireless access networks. While the generation of millimeter-wave signal with high quality is crucial in mm-wave-band communication systems, to generate optical mm-wave signal in the optical domain has the characters of high frequency, low phase noise, simple structure and considered to be cost-effective, thus becomes one of the key points in radio over fiber systems。In this paper, the backgrounds and practical significance of the RoF system are presented, which is the key to realize wireless high speed data transmission and wireless coverage. And the principles of the main modules and components that are vital in RoF systems are introduced. Then, a conclusion of various methods to generate millimeter-wave is made, including both advantages and disadvantages.In addition, the generation of mm-wave signal based on MZ modulator is specially analyzed theoretically and verified by simulation, which contains SSB modulation, DSB modulation and OCS modulation. Moreover, millimeter-wave generation methods of frequency quadrupling and octupling are theoretically analyzed.The main performances in the RoF system like link-gain, harmonic-distortion and inter-modulation distortion are studied. The periodical power fading effect of the generated mm-wave caused by chromatic dispersion is analyzed and verified by VPI simulation, which can be overcome by SSB modulation.Finally, a full duplex RoF system is proposed. For the downlink, a novel method using cascaded MZ modulators to realize frequency octupling is presented. For the uplink, no optical source is needed because the light from the center office plays the role of a DFB laser. In the simulation, 5Gbps transmission in the downlink and 2Gbps transmission in the uplink are realized. After 20km-fiber transmission, the suppression ratio of the generated millimeter wave is higher than that of MZ modulators connected in parallel. The desired BER, eye diagram were achieved without serious chromatic dispersion. And the variation of the suppression ratio with direct-current adrift is studied to satisfy actual applications.