Research And Implementation On Combined Technology Of PAPR Reduction And Pre-distortion In OFDM Systems

Time Domain Synchronous-Orthogonal Frequency Division Multiplexing(TDS-OFDM) modulation technology is employed in the multi-carrier mode ofChina’s Digital Television Terrestrial Broadcasting Standard DTMB. OFDM has beenused more and more widely in wireless communication field for its tremendousadvantages, such as strong anti multi-path fading capability, high spectrum efficiency,adoption of IFFT/FFT for modulation and demodulation, and so on. Besides DTMB,OFDM is also widely used in other systems, such as DAB, ADSL, WLAN, etc.However, one major drawback of OFDM technology is its high Peak to AveragePower Ratio, PAPR for short.High Power Amplifier (HPA) is an indispensable part in digital television signallaunch system. In DTMB system, the wide-band and high-PAPR signals areparticularly sensitive to the nonlinearity of HPA. Serious nonlinear distortion will beproduced when the high-PAPR signals go through HPA, which in turn causes in-bandand out-band distortion, reduces system’s anti error codes performance, causes powerspectrum leakage and interference to adjacent-channel signals, and so on.In order to reduce the nonlinear distortion of OFDM signals caused by the poweramplifier, and enhance the working efficiency of the power amplifier, this paper aimsat two aspects. On the one hand, directly from the perspective of PAPR reduction ofOFDM signals, iterative clipping and filtering algorithm is studied in this paper andsimulated through MATLAB. Compared with traditional clipping algorithm on theeffects of PAPR reduction, the problem of signal peak reincreasing will be overcome,and the system’s PAPR performance will be improved better, without obviouslyaffecting bit error rate in the system receiver. On the other hand, from the perspectiveof linearization of the power amplifier, LUT adaptive pre-distortion technology basedon polar coordinates method and feedback loop delay estimation method in thepre-distortion system are studied in this paper, and the effects of in-band and out-banddistortion reduction are analysed through MATLAB simulation. The logic design andverification of the pre-distortion system are also realized based on FPGA hardware platform.In the end, aiming at the performance limitation of pre-distortion technology, animproving method, combining the two technologies of iterative clipping and filteringalgorithm and LUT adaptive pre-distortion technology based on polar coordinatesmethod, is put forward in this paper. And through MATLAB simulation, theinfluences of different clipping thresholds over the effectiveness of compensating thenonlinear distortion of the power amplifier in the combining scheme, are investigatedfrom the perspectives of amplitude compensation, constellation demodulation, biterror rate, power spectral density, etc. Once the clipping threshold is reasonably set,by which the dynamic range of input signals are narrowed, the combining schemewill reduce the system’s in-band distortion, significantly improve the system’s powerspectrum leakage, reduce adjacent-channel interference, compensate the nonlineardistortion of the power amplifier, accordingly enhance the working efficiency of thepower amplifier, without obviously affecting bit error rate in the system receiver.