Research On The Generation Of Optical MM-Wave Signal In ROF System

Increasing operation frequency to achieve higher bandwidth and lager capacity is necessary in future broadband wireless communications.However, loss of wireless channel becomes larger as frequency increases.ROF technology, combining the advantages of optical fiber communications and wireless communications, which can provide high bandwidth and large capacity (at Gbit/s level) as well as mobility for users, is proposed to extend the transmission distance of mm-wave effectively owing to low loss.Optical mm-wave signal needs to be generated in the central station of ROF system. Generation of optical mm-wave signals is investigated in this article, including two aspects as follows. On one hand, existent methods of generation of optical mm-wave signals are summarized and analyzed. On the other hand, several predominant and novel methods are proposed.The first aspect includes the following parts.The first, we introduce the direct modulation technology, and calculate the spectrum of optical mm-wave signal generated by this technology. The second, we summarize the optical heterodyning modulation technology, and calculate the spectrum of optical mm-wave signal generated by this modulation technology.What's more, we analyze the most disadvantage of this technology that it needs excellent phase relativity of two beating beams, with which we propose four methods to deal, including the method of selecting optical carrier from the wide spectrum optical source, the method of optical injection lock (OIL), optical phase-locked loop (OPLL),and optical injection phase-locked loop (OIPLL).The third, frequency up-conversion technology is introduced. We use FWM effect or phase modulation technology to realize frequency up-conversion. The fourth, we summarize the conventional external modulation technology, and illustrate that three different spectral structures can be generated by modulator with different configurations, including double side band (DSB) structure, single side band (SSB) structure and optical carrier suppress (OCS) structure.We analyze link performance of these three structures of optical MM-Wave signals through simulation.The second aspect is that we propose the optimum external modulation technology. In order to attain transmission with a better performance and a farther distance,it is investigated to generate a kind of optical mm-wave signal, comprising two frequency components that one carries baseband signal while the other is blank, to overcome power periodical fading and time shift of the sidebands which always exist in conventional generation schemes.Three optimum external modulation schemes, including SSB structure optical mm-wave with signal only on optical carrier, OCS structure optical mm-wave with signal only on one sideband, carrier and odd sidebands suppressed optical mm-wave with signal only on one sideband are demonstrated. Here, we emphasize the last scheme.It is a novel scheme to generate mm-wave where the quadrupling of local radio frequency is formed by using an external integrated Mach-Zehnder modulator through intensity modulation or phase modulation. Receiver sensitivity of 10 Gbit/s ROF system based on this scheme is-28.3 dBm under intensity modulation while-24 dBm under phase modulation after 65 km transmission,and Bit Error Rate(BER) is at 10-4 level after 100 km transmission. Optical carrier in uplink is provided by central station to simplify the base station, which also reduces the cost of base station.