| With the rapid development of mobile internet and the multi-media service, the need for the transmission rate of the wireless communication is becoming higher, so the high-speed wireless communication has already become an important part to be developed. But now, the low frequency microwave band becomes more and more congested and these band can't meet with the increasing need for the bandwidth. The inherent wide bandwidth of millimeter-wave makes it the promising option to support the high-speed wireless communication in the future. However,the coverage of the base station (BS) of the millimeter-wave communication system is extremely limited for its high transmission loss in the air. Recently, the RoF has already overcome this issue. RoF technology takes advantage of the low transmission loss of the optical fiber, modulates the millimeter-wave signal on the lightwave and make it transmits over the optical fiber to extend the transmission distance to a great extent. Now, in the optical fiber wireless communication system,the generation of high quality digital mm-wave signal is also a important part and is studied by many researchers.In this paper, we first introduce the key RoF technology in the optical fiber wireless communication system and depict its principle and characteristic. Then, the generation of optical mm-wave signal in optical fiber wireless communication system by RoF technology is introduced,and the generation of digital mm-wave signal are emphatically researched.Through analyzing the spectrum generated by digital signal modulating the lightwave, a generation scheme of high quality binary phase shift keying (BPSK) mm-wave signal by frequency multiplication and a generation scheme of multi-band BPSK mm-wave RoF access signal are proposed. Then, according to our research on the linearity of the Bessel function, a generation scheme of SSB-4ASK millimeter-wave by sextupling frequency with a single PM is proposed.(1) Based on the study of the generation of mm-wave signal by frequency multiplication via a PM and the analysis on the spectrum generated by digital signal modulating the lightwave, we propose a BPSK mm-wave generation scheme by frequency multiplication via a PM with the aid of spectrum generated by BPSK RF signal modulating the lightwave. In this special spectrum, only the odd-order sidebands bear the BPSK signal while the even-order sidebands are unmodulated, an interleave is ably used as a wavelength selective switch, and the+3rd-order sideband and the unmodulated -4th-order sideband are ed to form the optical mm-wave signal transmits over the optical fiber, after beating in the photodiode, a 70GHz 3Gbps BPSK mm-wave signal is generated. And on the basis of the above scheme, a multi-band BPSK millimeter-wave RoF access signal generation scheme is proposed.In this scheme, through changing the structure of the above scheme and adjusting the modulation index of the PM, a tone bearing the BPSK signal and other three unmodulated tones are abstracted to for the optical millimeter-wave signal, and then three BPSK millimeter-wave signal is generated after beating in the PD. Because only one tones bears the BPSK signal in the above scheme, the optical millimeter-wave have high tolerance to the fiber chromatic dispersion. And the cost and the structure of the system is greatly reduced for using this special spectrum(2) Through researching on the theory of the PM and the Bessel function of the first kind , we find that some Bessel curve can have relatively horizontal section while other Bessel curve have constant slope at a certain range of the modulation index. By using this linear characteristic, the +2nd-order sideband bears the signal while the-4th-order sideband is unmodulated after the proper 4ASK RF signal with a proper direct current bias modulating the lightwave. This two sideband can be abstracted to form the SSB-4ASK optical millimeter-wave signal,this signal also has the high tolerance to the fiber chromatic dispersion for the special spectrum. |
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