Digital Predistortion For 5G Broadband Radio Frequency Power Amplifiers

This thesis focuses on the digital predistortion(DPD)techniques in 5G mobile communications,which provides introduction and detailed analysis in different aspects,such as the center frequencies,features of bands,and architectures of power amplifiers(PAs)and transmitters.More concretely,they can be divided into sub-6 GHz and millimeter-wave(mmWave)transmission according to center frequencies;broadband and concurrent multiband transmission based on features of bands;Doherty,envelope tracking(ET)and phased array transmitters on account of architectures,respectively.The nonlinearity problems in the following four scenarios are investigated in depth: concurrent dual-band Doherty PAs,concurrent dual-band ET PAs,mmWave ultra-broadband PAs,and concurrent dual-broadband phased array transmitters.In this thesis,novel DPD techniques have been proposed and experimentally verified with excellent results,which are briefly introduced as follows:1)Envelope preformulation(EPF)DPD has been put forward for concurrent dual-band Doherty PAs.The piecewise method is combined with preformulated envelopes to improve the stability of models based on memory polynomials like 2-D,which also leads to lower computational costs.In experimental verification,EPF DPD has successfully improved the inband linearity and suppressed the out-of-band spectrum regrowth in the meantime.The research has resulted in a publication in IEEE Microwave and Wireless Components Letters.2)Configurable multimode(CMM)DPD has been developed for concurrent dual-band Doherty PAs.A new structure of the DPD system is raised by revising formulas of the dualband model based on decomposed vector rotation(DVR).The interdependence between two bands is cancelled,e.g.,the linearization of band #1 no longer needs the PA output signal in #2.Based on CMM DPD,the distortion in five different modes is effectively removed,as evidenced by excellent experimental results.Compared to dual-band DVR model,switching among multiple modes brings better flexibility as well as reduced computational costs on hardware.This part of work has been published in IEEE International Conference on Microwave and Millimeter Wave Technology(ICMMT).3)Band-limited(B-L)DPD for concurrent dual-band ET PAs has been presented.By introducing band-pass filters into the feedback loop,the nonlinearity within a specified range can be effectively compensated by B-L DPD,which reduces the bandwidth requirement in both RF and envelope paths.This part of work has been published in IEEE Asia-Pacific Conference on Antennas and Propagation(APCAP).4)Uniform(UNF)DPD for concurrent dual-band ET PAs has been developed.Based on mathematical deduction,a uniform behavioral model is proposed,which can accurately describe characteristics of PAs supplied by any one of average envelope,general mean envelope,peak envelope,and weighting factor peak envelope.Compared to distributed memory polynomials,UNF DPD has a concise form and achieves better experimental results.Also,UNF technique consumes less computational resources because of improved generality and flexibility.This work has been published in IEEE Transactions on Microwave Theory and Techniques.5)Limited processing bandwidth(LBW)DPD for ultra-broadband mmWave PAs has been introduced.The proposed method adopts parallel processing and linear decomposition of signals to remove distortion within a specified range using very limited processing bandwidth.In experimental verification,LBW DPD has successfully linearized different carrier-aggregated signals of broad modulated bandwidth,which can achieve comparable performance with conventional techniques but requires much lower sampling rates.This research has been published in Wireless Communications and Mobile Computing.6)In-band(I-B)DPD for concurrent dual-broadband phased array transmitters has been proposed.In order to use limited processing bandwidth,I-B technique borrows the concept of LBW DPD: parallel signal processing and linear decomposition.In concurrent dual-broadband transmission at Ka-band,distorted beams radiated by phased array antennas are effectively linearized.Distortion within the modulated bandwidth,also known as in-band nonlinearity,is largely suppressed,especially.Additionally,a pruning method to reduce coefficients is also presented,and accordingly the complexity is significantly decreased.I-B DPD has been published in IEEE Microwave and Wireless Components Letters.