Rearch On Reducing Peak-to-Average Power Ratio Of OFDM Signals In Optical Transmission System

With the development of communication technology, the demand for the largecapacity, high-speed and multiple services communication is significantly increased,and the development of the fourth generation (4G) mobile communication willbecome an irresistible trend. The orthogonal frequency-division multiplexing(OFDM) technology has been recognized as the core technology of4G due to theseadvantages, such as elimination of multi-path fading and inter-symbol interference(ISI), high spectral efficiency, and large capacity. The radio over fiber (ROF)technology, which is a combination of optical communication and wirelesscommunication, has the advantages of high-capacity, flexibility, low cost, simplestructure of the base station and so on. So it is considered as one of the4G wirelessaccess technologies. Therefore, the research on the OFDM and ROF technology is ofgreat importance.High peak-to-average power ration (PAPR) of OFDM signals not only result inthe amplifiers, Mach-Zehnder modulator (MZM) and other optoelectronic devicesbeyond their linear ranges, but also bring fiber nonlinearity. This paper deals withthe methods on how to reduce the PAPR of OFDM signals. We introducecompanding transform into the high-speed optical baseband transmission system andthe ROF system to reduce the PAPR. And we get some positive outcomes:Firstly, the direct-detection optical orthogonal frequency-division multiplexing(DD-OOFDM) system with Companding Transform (CT) technique is investigatedby theoretical analysis, simulation and experiment. The results show that the CTtechnique in optical transmission system can both reduce the PAPR of OFDM signalsand improve the receiver sensitivity. The simulation results show that the PAPR ofthe OFDM signal decreases and the BER of the system firstly decreases and thenincreases when the increase of the companding coefficient μ. We can choose a properμ compared with the PAPR and BER of the system. Our experiment results confirmthat the PAPR of the OFDM signal decreases by3dB, and the receiver sensitivity isimproved by2.5dB at BER of10-4after transmission over100-km single-mode fiberwhen μ is2.Secondly, a60-GHz OFDM-ROF system based on CT technique was proposedand experimentally demonstrated. It can reduce the PAPR of OFDM signals and improve the sensitivity of the receiver. In the central station (CS),58GHz opticalmillimeter-wave based on optical carrier suppression (OCS) modulation wasgenerated by using one dual-arm Mach-Zehnder intensity modulator (MZM-IM) anda Fiber Bragg Grating (FBG).2.5Gbit/s QPSK-OFDM signals with CT weremodulated to the optical carrier by another MZM-IM, and then the O-OFDM signalswere transmitted over50-km standard single-mode fiber (SSMF) to the base station(BS). In the BS, we got the BER and constellation of the OFDM signals by changingthe received power of the optical milli-meter wave. The experimental results showthat the PAPR of the OFDM signal is decreased by3dB, and the receiver sensitivityis improved by2.3dB for launch power of12dBm at the BER of10-3and withoutdispersion compensation.