Research On Doppler Diversity Over Time-varying Channel

In wireless communication system, a relative movement between transmitter andreceiver leads to Doppler spread, strengthens the channel’s time variance, especially inhigh-speed scenario, Doppler spread plays a significant factor in decreasing systemperformance. To hurdle or even make the most use of Doppler spread, this paperprovides an intensive research in Doppler diversity.Firstly, this paperconsidered the transmitter Doppler diversity.This thesis analyzedthe diversity effect of bit interleaved code under a time-varying-channel situation. Butit’s different in multi-carrier transmission system as the stronger the time-varyingchannel is, the quicker the inter-carrier interference (ICI), will increase, thus, turns outthat the system performance falls. So it is necessary to come up with an optimizedsystem design between diversity gain and ICI. The essence of transmitter Dopplerdiversity is to attach different phase shifts to single transmit signal and send themrespectively via multi-antenna so that the equivalent channel’s time variance will bestrengthened by magnifying Dopplerspreadof the equivalent channel. Both theoreticalanalysis and digital simulation prove that, like bit interleaved code’s conclusion, thesystem performance of transmitter Doppler diversity gets improved by increasingDoppler spread artificially while it grows impaired on the contrary due to Dopplerspread is overly strong. This paper analysed and simulatedan accurate figure of Dopplerspread which makes the system performance optimized. At the same time, this studycontrasted two typical ways of transmitter Doppler diversity, within a symbol,based onthe continuous phase shift diversity and fixed phase shift diversity. According totheoretical analysis and digital simulation, based on the continuous phase shift diversityis better than fixed phase shift diversity since there isn’t any extra influence of ICI whenchannel’s time variance is getting amplified.Secondly, this paper discussed receiver Doppler diversity which obtains multiplebranch signals via different frequency shift made by receiving time-domain signal andmaximal diversity gain via modified maximum ratio combining (MRC). Also, this paper inferred and validated theoretically the expression of each branch signals’ power andinterference power before combination, and got a theoretical expression of systemsignal to interference ratio after modified MRC. This ratio is limited to each branch’sfrequency shift and weighting coefficient.Firstly, assume that the weighting coefficientis confirmed, to maximize system signal to interference ratio, we’ll get an optimizedfigure of each branch’s frequency shift; and in the next place suppose that each branch’sfrequency shift is confirmed, to maximize that ratio after combination, we’ll get anoptimized weighting coefficient of each branch. Theoretical analysis and digitalsimulation works prove that the study is proper and this system design can make systemperformance optimized.Finally, Joint transmit-receive Dopplerdiversitywasalso been studied.Basis-expansion Model (BEM) is fitter to fast time-varying channel. First this paperdiscussed channel estimation techniques based on BEM. Theory attests that themaximum achievable Dopplerdiversity is determined by the rank of the correlationmatrix of the channel’s coefficients. When precoder’s equivalent channel matrix has fullrank, the system will achieve a maximum diversity order gain. For time-selectivechannel, Doppler spread determines maximumachieveableDoppler diversity order. Thisdesign validated that the VandermondePrecoder guarantees the system will obtain a fulldiversity when equivalent channel matrix has full rank. And for doubly-selectivechannel, we theoretically analysedthatDoppler spread and multipath numberjointlydetermine the maximum achieveablejoint multipath-Doppler diversity. Digitalsimulation indicates that the system can receive the diversity order effectively.This paper deeply researched the transmitter, receiver and joint transmit-receiveDoppler. Research proved that these three methods can effectively weaken the Dopplerspread’s adverse effects, and even make use of the Doppler spread for diversity.