| Recent years,the rapid development of the mobile Internet has upended the traditional mobile communication business model and provided users with unprecedented user experience,which profoundly affected all aspects of people's life.The further development of the mobile Internet will bring thousands-fold increase in mobile data traffic in the future,arising people's requirements for data transmission speeds of mobile terminals rapidly increase.With the rise of the Internet of Things,mobile communication technologies penetrate into a wider range of industries and fields.The explosive growth of future applications in the development of the Internet of Things,such as future mobile medical,Internet of vehicles,home automation makes thousands of equipment network access to mobile communication,which brings infinite vitality and new challenges to mobile communication.With the increasing demand for higher signal transmission rates and the gradual crowding of low-band spectrum resources,wireless communication systems require more abundant spectrum resources.Millimeter-wave frequency,ranging from 30 GHz to 300 GHz,has a rich spectrum of resources,can meet the current traffic demand,which is considered to be a promising choice for next-generation wireless communication technology.However,it is difficult to directly use the millimeter-wave as the carrier of wireless systems due to the large transmission loss and the difficult signal processing in the electric field.Radio over fiber(RoF),by which the millimeter-wave signal is modulated on the lightwave and then is recovered to electrical domain after transmitted over fiber,is becoming an attractive transmission technology for the future wireless access network due to its large bandwidth,high mobility and low propagation loss,and can effectively overcome the large transmission loss of the wireless millimeter-wave signals transmitting directly in the air.Optical millimeter-wave generation technology is one of the key technologies of the RoF system,and so the generation technology of high-frequency and high-quality digital modulation millimeter-wave signal is a hot research direction in this field.In this paper,we firstly review the principle of RoF system and then deeply analyze the generation technology of optical millimeter wave in RoF system.Then the external modulation technology and precoding technology,which are used in the generation of vector millimeter wave signals,are described in detail.After that,we theoretically analyze and derive the effects of fiber dispersion on millimeter-wave transmission.Based on the research of the frequency multiplication technology of vector millimeter-wave signals,an 8PSK signal generation scheme based on phase modulation with frequency-quintupling is proposed and demonstrated,then an adaptive 8QAM/16-star QAM millimeter-wave signal generation scheme with frequency-nonupling is given.(1)According to the research of the nonlinear transformation of vector signal during the process of generating millimeter-wave signal via phase modulation(PM)with frequency multiplication in RoF system,we find that the 1st and 4th order sidebands of the output spectrum from the optical phase modulator get the maximum amplitudes at the same time and the others are relatively small.Considering the phase characteristics of the 8PSK signal,an 8PSK signal generation scheme is proposed based on phase modulation with frequency-quintupling with simple precoding.Because the Euclidean distance of constellation of signal keeps constant after precoding,the 8PSK signal generated by the scheme has better noise resistibility than that generated by the conventional generation scheme of millimeter-wave signal with pre-coding and frequency multiplication technology.After a detail theoretical analysis of the proposed scheme,the simulation link is built.The constellation diagram and Bit Error Ratio(BER)curves based on the simulation link show that the generated 8PSK millimeter-wave signals have good performance,which verified the feasibility of the proposed scheme.(2)According to the study on the nonlinear generation of vector millimeter-wave signal via phase modulation,we present an adaptive 8QAM or 16-star QAM millimeter-wave signal generation scheme with frequency-nonupling.Considering the phase characteristics of the 8QAM and 16-star QAM signal,it is found that the phase information can be recovered directly at the receiving end without phase precoding to the driving RF signal.The theoretical derivation shows that the scheme has a simple structure and the generated 16-star QAM millimeter-wave signal has better noise resistibility than the rectangular 16QAM signal in the wireless transmission scheme.The RoF link is built on the simulation platform and the simulation constellation diagram and BER curves show that the generated QAM millimeter-wave signals have good performance. |
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