Research On Peak-to-average-power Ratio Reduction For Hybrid Carrier System

In recent years, Hybrid Carrier (HC) systems based on4-weighted fractionalFourier transform (4-WFRFT) have been proposed, and the system can transmitsimultaneously single-carrier (SC) and multi-carrier (MC) at the time-frequencydomain. It is a real HC system in blend of SC and MC characteristics. HC basedon4-WFRFT contains MC modulation signals, and when a plurality of signalswith the same phase are superimposed, it may produce relatively large transientspikes. So there is peak to average power ratio (PAPR) problem in HC. HighPAPR causes nonlinear distortion of amplified signals from Power Amplifier(PA), and also reduce PA efficiency and signals coverage, etc. High PAPR willalso require the median increase of digital to analog converter (DAC), whichwould increase the cost of the DAC device. Therefore, high PAPR is always amajor problem which limits the development and the application of HC systems.In this article, We research on reducing the PAPR and related issues for HCsystem.In order to make the HC discrete-time signals approximate thecontinuous-time signals in PAPR, we propose an oversampling method for HCdiscrete-time signal, and the average power and the power spectral density ofoversampled signals are analyzed. Through the simulation and analysis of theoversampled signals in PAPR performance, we conclude that when theoversampling rate L=8, the baseband discrete-time signals can accurately reflectthe PAPR performance of the baseband continuous-time signals.We use Clipping technique to suppress HC system PAPR. By HC systemssimulation, we can get that Clipping processing introduces in-band noise andout-band radiation, and by the cost of the reducing system BER performance,Clipping technique can effectively inhibit HC PAPR in different orders. Highpeak HC signals through PA causes nonlinear distortion in the nonlinear regions,this process can be seen as the Clipping process. Therefore, we discuss theimpact of Clipping process on the system performance, which can serve as areference on the nonlinear distortion caused by high peak value of the HC signalsafter PA.As doing the clipping process to the HC signals brings in-band and out-bandnoise radiation, so we will use the Clipping and filtering technology to filterout-band radiation. By analyzing Clipping signals and Clipping and filteringsignals in BER performance, we obtain that when Eb/N0is larger, clipping and filtering BER performance can be better than the clipped signals. And at the sameclipping rate, when the MC has greater proportion of the HC, the Clipping BERperformance can be better than filtered signals at smaller Eb/N0. By thesimulation and the analysis of the clipped and filtered signals PAPR, we drawthat Clipping and filtering technique can effectively suppresses HC system PAPRand the out-band interference, but the filtering process produces peakregeneration phenomenon. In order to suppress the peak regenerationphenomenon caused by the filter, we use the first and second repeated clippingand filtering techniques to suppress HC system PAPR. By simulation, we obtainthat the more times of the repeated clipping and filtering, the system PAPR issmaller, but the system BER is greater. Compared to the first repeated clippingand filtering technique, the second technique can suppress PAPR moreeffectively, but the BER increases more. Finally, we compare the advantages anddisadvantages of the clipping-type techniques used in the HC systems. Theresults of this article have certain reference in the actual HC systems for usingclipping-type techniques in combination with other techniques to suppress theHC systems PAPR, and the future studies will focus on the cancellation of theHC clipping-noise at the receiving terminal to improve the system BERperformance.