Design And Performance Analysis Of Multi-antenna System For High Mobility Wireless Communications Systems With Imperfect Channel Estimation

In recent years,there has been an escalating demand for robust and highly reliable broadband wireless communications on high speed railway(HSR)systems.The predominant HSR wireless communication system is still the Global System for Mobile Communications for Railway(GSM-R).The GSM-R is based on the second generation(2G)wireless communications systems.GSM-R systems can support basic real-time train operations such as train dispatch and train control which do not require massive bandwidth(or high data rates).However,it lacks the capacity for supporting broadband multimedia applications,such as on-board video surveillance,which are increasingly becoming bandwidth intensive and broadband applications such as modern Internet services.The designs of robust and highly reliable broadband high mobility wireless communication systems for HSR systems remain a great challenge,due to Doppler effect caused by the high-speed relative movement between the transmitter and the receiver.The wireless channel suffers from fast fading due to the Doppler effect,which reduces the accuracy of channel estimation and data detection,thereby severely deteriorating system performance.Furthermore,high mobility has been made an integral part of the upcoming fifth generation(5G)network,which is expected to provide satisfactory services for users traveling at 500km/h.This further poses a great challenge for the design of robust and highly reliable broadband high mobility wireless systems for HSR systems.In this thesis,we will focus on the utmost challenging task of providing robust and highly reliable broadband wireless communications for high mobility scenarios such as HSR systems.In this thesis,Doppler diversity as well as space(antenna)diversity is applied to mitigate the negative effect of channel estimation errors due to the Doppler effect which is present in high mobility scenarios.Our effort is primarily focused on optimum Doppler diversity designs of practical single-input multiple-output(SIMO)high mobility systems in the presence of channel estimation errors.This effort which aims at providing guidelines for the designs of practical high mobility wireless communication systems is carried out in studies in this thesis as follows:Firstly,we establish the tradeoff relationship between Doppler diversity and channel estimation errors by studying the maximum Doppler diversity order of SIMO high mobility systems with imperfect channel state information(CSI).With the aid of a simple repetition code,pilotassisted transmission scheme and a pilot-assisted minimum mean squared error(MMSE)channel estimation algorithm,the fast time-varying channel is estimated.Based on the statistical analysis of the estimated channel,a new optimum SIMO diversity receiver,which incorporates the statistics of the channel estimation errors that can effectively harvest both space(antenna)diversity and Doppler diversity inherent in the system,is proposed.The symbol error rate(SER)of the new optimum receiver is analyzed,which quantifies the impact of channel estimation errors on system performance.The achievable maximum Doppler diversity order by a SIMO high mobility system with imperfect CSI is identified by performing asymptotic analysis of the analytical SER.Analytical results show that,if the energy of pilot symbols scales linearly with that of data symbols,then a SIMO system with imperfect CSI can achieve the same Doppler diversity order as those with perfect CSI in a high mobility environment.However,a non-linear scaling between the energies of pilot and data symbols always results in loss in Doppler diversity.It is also shown that,even though a SIMO system with imperfect CSI can achieve the same Doppler diversity order as a SIMO system with perfect CSI,there is always a nonnegligible loss in energy in the form of coding gain which can be minimized but not totally eliminated.Secondly,based on the above implementation,the fundamental performance limits of a SIMO high mobility wireless communication systems with imperfect CSI is studied.Based on the SER of the new proposed SIMO Doppler diversity receiver,the fundamental tradeoff between Doppler diversity and channel estimation errors is quantified through two performance metrics namely,the maximum achievable diversity(including both Doppler diversity and antenna diversity)in the presence of imperfect CSI and the loss in coding gain due to channel estimation errors when the signal-to-noise(SNR)and the coding block length are large.The analytical results are then used to optimally allocate transmission energy to pilot symbols and data symbols to simultaneously maximize the Doppler diversity order achievable by SIMO high mobility systems and minimize the loss in coding gain due to channel estimation errors.The results reveal the fundamental performance limits of SIMO Doppler diversity systems in the presence of imperfect CSI.These results can be used as a guide in the designs of optimum SIMO high mobility systems with performance approaching the fundamental performance limits.Finally,the study on optimum designs of SIMO high mobility systems with imperfect CSI in this thesis is done using a simple repetition code at transmitter side as a form of precoding to ensure maximum Doppler diversity order in the system.This precoding scheme can achieve maximum Doppler diversity order at the expense of low spectral efficiency in the system.To improve on the spectral efficiency of the system,a set of linear modulation schemes namely,MPSK,MASK and MQAM respectively are employed and the most spectral efficient modulation scheme among these linear modulation schemes is adopted to improve on the spectral efficiency of the system.Simulation results show that MQAM modulation scheme can achieve higher spectral efficiency than MPSK and MASK schemes.