Research On The Digital Predistrotion In OFDM-ROF System

With the rapid development of modern communication technology, wirelesscommunication technology makes the communication in anywhere and anytime whenpeople need, wideband communication technology can provide enough bandwidth forfaster transmission for voice, video and network data. The combination of wirelesscommunications and wideband communication has become a hot research in the fieldof communication. The Radio Over Fiber technology is the combination of wirelesscommunications technology and wideband communication technology with theadvantages of large capacity, large bandwidth, low attenuation loss, the immunity toradio frequency interference, flexible and easy installation, etc..Orthogonal Frequency Division Multiplexing technology is a multi-carriermodulation technology which has high robustness against multi-path fading andhigher spectrum efficiency. The combination of OFDM modulation technology andROF system takes the advantages of low complexity, high efficiency, low cost, it is ancompetitive and potential solution for wireless communication, mobile phones andwideband network field in the future.Subcarrier modulation (SCM) is utilized to modulate a RF signal on light, whichin turn will be transmitted by fiber. Unfortunately, the transmission in most cases maybecome corrupted by nonlinear distortion that is induced by the nonlinear response ofthe optical transmitter, optical receiver and chromatic dispersion of the single modefiber (SMF). The nonlinear distortion degrades the receiver sensitivity, which leads toa poor bit error rate (BER) and spurious free dynamic range (SFDR). Ultimately, thiswill increase RoF system costs and render it impractical. This article will use thedigital pre-distortion technology to improve the the ROF system in the nonlinearproblem.Firstly，the ROF technology and OFDM technology are introduced, and thetechnology characteristics, principle and implementation of both are analyzed. Thencommon linearization method especially digital predistortion technology is discussed.After that, the nonlinear model and predistortion structure are established thereby theanalysis of which causes the nonlinearity of the ROF system. A digital predistortionsimulation for wireless communications system of radio-over-fiber combined with power amplifier is designed, the preditortion system and the mode of ROF and PA isestablished by Matlab software. The different compensation architectures of jointcompensation and separated compensation are used, and analysis of the advantagesand disadvantages of the two structures, the indirect learning architecturepredistortion method is used with minimum mean square error (LMS) algorithm.Through the contrast of the power spectral density, the effect of the two compensationarchitectures is analyzed. Simulation results show that the nonlinearity of ROF systemis compensated effectively, about18dB ACPR improvement is obtained.