Design And Performance Analysis Of Extended Hybrid Carrier Signals Based On WFRFT

Throughout the evolution of mobile communication,modern mobile communication system undertakes a remarkable transform every ten years,mostly led by new technologies in physical layer.As an important part of the physical layer,the carrier scheme has also evolved with the development of communication technology,with the traditional single carrier(SC)scheme and multicarrier(MC)scheme gradually turning to unified and compatible architectures.Recently,researchers introduced Weighted Fractional Fourier Transform(WFRFT)into communication system,and proposed a novel hybrid carrier(HC)scheme.This scheme not only meets the evolution rule and realizes the compatibility between SC and MC,but obtains better BER performance than the other two resulting from a more uniform signal energy distribution in the time frequency plane.However,flaws of the existing classical HC system are gradually revealed with the development of research.First,though signal energy distributes more uniformly in timefrequency plane in HC scheme than that in SC or MC scheme,it must be admitted that the distribution tends relatively uniform instead of entirely uniform due to the mathematical limitation of WFRFT theory.Next,HC scheme only shows superiority over doubly selective fading channels but maintains an in-between performance under single dispersive channels compared with SC and MC scheme,which means the scheme needs back off to SC or MC to guarantee performance.Third,HC system can match signals with channels by choosing the transform order,whereas few researches that design matched signals according to channel characteristics exist because of the complex relationship between transform order and channel fading characteristics,which further limits the actual application of HC system.With a view to the flaws of current HC system,researches in this paper are shown as follows:Considering the limited flexibility of signal design in classic HC scheme,this paper proposes Extended Weighted Fractional Fourier Transform(EWFRFT)by eliminating the restriction of transform order.Besides,the basic properties of EWFRFT are investigated,and the corresponding extended forms of the main properties in classic WFRFT are present under the EWFRFT scheme.On this basis,the relationship between EWFRFT and classic WFRFT is analyzed in this paper.As illustrated in the results,EWFRFT possesses equivalent forms of single parameter 4-WFRFT,single parameter M-WFRFT andmulti-parameters 4-WFRFT if specific parameters are selected.When other parameters are selected,EWFRFT-based hybrid carrier system enables to generate some signal forms cannot achieved in classical HC system.In addition,waveform and spectral characteristics of EWFRFT signals are studied in this paper,proving that no additional time and frequency resources are occupied in EWFRFT system compared with classic HC system.In terms of the performance limitation of classic HC system under single dispersive channel,an EWFRFT based two component combination scheme is proposed in this paper specifically towards SC and MC system,taking advantage of the high flexibility of signal design in the extended HC system.The proposed scheme can provide additional gain without changing the traditional SC or MC scheme.There is no extra time,frequency and space resources being utilized in the proposed scheme,but BER performance is greatly improved at the expense of a small increase in implementation complexity.To further dig out the potential of double component signal,factors effecting on additional gain are investigated in this paper.From theoretical derivation,it is proved that the more equal power distributes in the two components,the more independent channel fading in the same symbol of the two components appears,and more obvious the additional gain can be obtained.Based on this conclusion,equal power allocation schemes for time domain double component EWFRFT signal and frequency domain double component EWFRFT signal are presented,in addition,an optimal design based on half-block inversion method to increase the fading independence in the middle part of the symbol is given.The proposed signal design scheme enables to improve the anti-fading performance of SC and MC system.The matching problem between signals and channels restricts the application of both classic HC system and extended hybrid carrier system.Aiming to design a proper signal form that matches with time-frequency double selective fading channel characteristics under extended hybrid carrier scheme,a matching criterion with minimized interference power variance as the objective function is proposed based on the relationship between interference power distribution and system BER.The energy ratio of the time/frequency domain components in the matched signal is studied according to channel fading characteristics,and specific method of calculating the energy ratio based on the channel state information is given.What's more,four-component signals with equal power allocation are designed by algebraic method.It solves that energy of the classical HC signal cannot be completely uniformly distributed in the time-frequency plane,thus further improvingthe performance of the HC system.Even so,it is still difficult to design HC signals with arbitrary time-frequency energy ratio in algebraic way.A signal design scheme based on geometric method is then present in this paper.Signals with various time-frequency energy ratios as required can be generated under the extended hybrid carrier scheme in that case,hence matching the signals and channel fading characteristics.Besides,BER performance of the hybrid carrier system is also improved under the time-frequency double selective channel.