Doppler Diversity Transmissions In High Mobility Wireless Communication Systems With Imperfect Channel Estimation

As the crucial support for the national economic growth, transportation and wireless communications develop dramatically in China in recent years. By the end of 2015, Chi-na's operational high-speed railway (HSR) mileage reached 19,000 kilometers, occupying 60%of the whole world's HSR operational mileage. On the other hand, China has also made significant breakthrough in the field of wireless communications. Time duplex-long term evolution (TD-LTE), the fourth generation of mobile communication (4G) proposed by China has entered into the large-scale commercial stage. Meanwhile, the connection and integration of transportation and wireless communications are highly demanded. To ensure a safe, steady and efficient operation of the train, advanced information and com-munication technology (ICT) should be provided to meet the demand of information transmission for the train dispatching and controlling system, especially for the HSR. In addition, explosive growth of data traffic, massive connections and continuous emergence of new services are beyond and exhausting the capabilities of existing wireless networks. The required reliable, steady, fast and efficient wide-band wireless communication services for high-mobility scenarios are thus the important challenges of the fifth-generation (5G) mobile communications systems.Although the requirements of train dispatch and control are basically satisfied by the mobile communication network based on Global System for Mobile communications-Railway (GSM-R), there are bottlenecks for the existing wireless communication tech-nologies to meet the demand, such as the broadband wireless access for passengers and the massive data transmission for wireless sensor networks under the high mobility sce-narios. In this thesis, we will focus on one of the most challenging problem of broadband wireless communications in high-mobility scenario, i.e. Doppler effect induced by the relative movement between transmitter and receiver. The wireless channel is fast fading due to Doppler effect, which reduces the accuracy of channel estimation and data detec-tion, thus seriously deteriorating system performance. In this thesis, Doppler diversity transmission is applied to combat Doppler effect, we put our effort mainly in the research on Doppler diversity transmissions with imperfect channel estimation. The contributions include analysis and improvement of the classical precoder to achieve Doppler diversity and, simple implementation, fundamental limits and practical design of Doppler diversity transmission in the presence of imperfect channel state information (CSI).Firstly, many existing Doppler diversity transmission schemes require joint transmit-receive design, i.e. precoding at the transmitter and abstract the useful signal by ap-propriate combining technique at the receiver, in order to achieve maximum Doppler diversity. The well-known precoder would introduce different amount of redundancy ac-cording to the movement velocity, which sacrifices the spectral efficiency, increases the implementation complexity, and is difficult to perform velocity adaptation. Based on the design criterion for achieving maximum Doppler diversity, we extract the idea of maxi-mum Doppler diversity transmission and propose a strategy to improve the precoder. In addition, it is proved that the system with Doppler domain multipexing (DDM) or other spectral efficient precoders can achieve maximum Doppler diversity. The proposed pre-coder, such as inverse discrete Fourier transform (IDFT) matrix, spread the data symbol and then transmit to the time-varying channel. While at the receiver, the useful signal are separated from the interference and combined to achieve maximum Doppler diversi-ty. Compared to the redundancy-based precoder, the proposed precoding scheme can be used under arbitrary constellation and arbitrary movement velocity without adaptation, because it does not require the knowledge of the Doppler spread. Further, the proposed precoder can achieve a much higher spectral efficiency since no redundancy is inserted.Secondly, most existing Doppler diversity transmission schemes assume perfect chan-nel state information. However it is a non-trivial task to get an accurate and timely estimate of the fast time-varying fading. Thus, it is much more challenging while re-warding to study Doppler diversity with imperfect CSI. To figure out the relationship between Doppler diversity and channel estimation error, and to analyze and design prac-tical Doppler diversity systems in the presence of channel estimation error, a simple implementation is firstly presented based on the pilot-assisted techniques for fast time-varying channel estimation. With the statistical properties of the estimated channel and the corresponding channel estimation error, the design of the optimum diversity receiver and its analytical symbol error rate (SER) can be facilitated. The SER expression can quantitatively capture both the impacts of imperfect CSI on the performance and the tradeoff between Doppler diversity and channel estimation error. It is shown that the SER is quasi-convex in Doppler spread and decreasing monotonically in the pilot per-centage, and the performance of systems with high enough pilot percentage can approach that of a system with perfect CSI. On the other hand, when the pilot percentage is too low, the benefits of Doppler diversity are offset by channel estimation error.Next, the fundamental performance limits of the Doppler diversity transmission are investigated based on the above basic implementation. The tradeoff between Doppler diversity and channel estimation error determines the performance, in terms of pilot per-centage and the energy allocation between the data symbols and pilot symbols. Based on the analytical SER expression for the optimum Doppler diversity receiver, two perfor-mance metrics can be identified with the help of asymptotic analysis, i.e. the maximum Doppler diversity order achievable with both perfect and imperfect CSI, and the loss in signal-to-noise ratio (SNR) caused by channel estimation errors. It is shown that, when the pilot percentage is sufficiently high meanwhile the energy allocated to the pilot and data symbols scale linearly with each other, the systems with or without perfect CSI can achieve the same Doppler diversity order (over a unit time) which is twice of the Doppler spread. However, there is always a gap between their error performance, and it can not be eliminated but can be minimized through optimum energy allocation.Finally, stimulated by the fundamental performance limits of Doppler diversity trans-mission with imperfect channel estimation, three criteria are proposed to guide the de-sign of practical Doppler diversity systems, respectively related to channel estimation error, Doppler diversity order and SNR loss. Thus, this thesis aims at designing practical Doppler diversity transmission schemes for high-mobility systems with imperfect CSI, with performance approaching the fundamental limits while keeping high energy and spectral efficiency. At the transmitter, a spectral efficient precoder, Doppler domain multiplexing (DDM) is employed to achieve high spectral efficiency and, advanced algorithm is used to optimize the energy allocation between data and pilot symbols. While at the receiv-er, a suboptimum receiver with low complexity is developed by studying the statistical properties of the estimated channel coefficients, and the corresponding analytical error performance is derived. With such a joint transmit-receive design, the proposed Doppler diversity system can achieve high energy and spectral efficiency. It has been shown, under high mobility scenarios energy and spectral efficient Doppler diversity transmissions can work well if properly designed. Although there is an additional loss compared to the theoretical lower bound, the practical systems can achieve the same diversity order with satisfactory performance.