The Researches On The Peak To Average Power Ratio In CO-OFDM System Based On FRFT Algorithm

Optical orthogonal frequency division multiplexing (OOFDM) is a novel modulationtechnique using orthogonal frequency division multiplexing (OFDM) in opticalcommunication system. Coherent optical orthogonal frequency division multiplexing(CO-OFDM) combined coherent optical detection and OFDM. Coherent system has highcomputation efficiency and is easily estimated in channel and phase for the combination withOFDM. Likewise, OFDM can maintain the linearity for the introduction of coherent detectiontechnology in the process of RF-to-optical and optical-to-RF. CO-OFDM technique hasbecome a popular research in optical communication field in recent years.For one of the major drawbacks of CO-OFDM systems is high peak to average powerratio (PAPR), a novel CO-OFDM system based on fractional Fourier transform (FRFT) isproposed in this paper. The principle of PAPR reduction with FRFT is obtained by theoreticalanalysis and the derivation of equation. The CO-OFDM system based on FRFT is built by useof Matlab and Optisystem software. Then, the validity of the results is verified by simulation.Aiming at the shortage of the conventional PAPR reduction techniques, the combinedalgorithms based on FRFT with two different kinds of schemes is put forward. The simulationanalyses have three aspects including compatibility of the combination, inhibition effect ofpeak to average power ratio and improvement of the negative effects in the reductionalgorithm. The probability of the selective mapping algorithm combined with fractionalFourier transform is104less than that of the single selective mapping algorithm when therotation factor is4and the peak to average power ratio is higher than6.6dB. When the samedegree of inhibition of PAPR is chosen, the time complexity of the combined algorithmdecrease by8times than single algorithm. The simulation results using clipping algorithmcombined with fractional Fourier transform have great improvement than single algorithm,which shows combined algorithm has good performance of compatibility.The research emphasis in this paper is CO-OFDM systems and the paper is organizedas follows:1. The development situation, research significance, and the basic principle of relevanttechnology in CO-OFDM system are expounded in detail. The reasons why the PAPR ofCO-OFDM system is high and the negative influences of high PAPR on system performanceare discussed primarily.2. A novel FRFT algorithm scheme is proposed to reduce PAPR. The CO-OFDM systembased on FRFT is set up by using Matlab and Optisystem. The principle of this algorithm isanalyzed and its validity is demonstrated by simulation which is consistent with theoreticalderivation. Furthermore, the algorithm will not bring in other effect to the system. A methodto analyze PAPR in optical field combining with the novel algorithm is proposed, which can be used to observe the peak value of the superposed signals in optical time domain. Finally,the validity of PAPR reduction with different order is compared under the premise that FRFTdoesn’t have any effect on the average power of signal.3. For the problem that some PAPR reduction techniques will introduce adverse impactto system, the availability and limitation of a number of normal joint algorithms are analyzed,including their combination ways. Then, the compatibility, PAPR reduction validity, andnegative influences on systems of two different types of PAPR reduction techniques combinedwith FRFT are analyzed. The numerical results indicate that the novel schemes proposed inthis paper can decrease the degradation of system caused by these algorithms themselves.Additionally, the PAPR reduction effectiveness of such techniques with limited reductionfunction can be enhanced distinctly by combining with FRFT. The ways of combinedalgorithms mentioned in this paper have no difficulty in the compatibility in theory, and theycan reduce the PAPR of system effectively.