Research And Implementation On Uplink And Downlink Synchronization For LTE System

Mobile communication has developed rapidly in the last thirty years, particularly the "quasi-4G" standard LTE proposed by 3GPP, which is a research hotspot in recent years. It utilizes OFDM technology so as to improve the spectrum efficiency. However, OFDM is so sensitive to the time and frequency deviation that the synchronization procedure becomes a great challenge of the LTE system. This paper studies the synchronization process for LTE uplink and downlink based on the open source software-defined radio platform, namely GNU Radio and USRP in Linux.First of all, this thesis analyzes the technical foundation of LTE physical layer, including the transmission structure and the process flow of uplink and downlink in LTE system. Furthermore, the synchronization signals(PSS, SSS) as well as the reference signals are introduced as pilot signals used in the correction for the deviation of time and frequency.Then, synchronization scheme is studied according to the impact analysis on the deviation of time and frequency. Firstly, an overall downlink synchronization scheme is proposed in this paper, which is composed of two parts:the cell search procedure, during which the coarse timing and frequency synchronization is achieved based on PSS and SSS sequence, and the estimation and compensation based on RS sequence for channel response and the residual errors of timing and frequency. Secondly, this thesis designs an uplink synchronization scheme, wherein the timing synchronization is achieved by timing adjustment in UE, in which the adjustment amounts is obtained by SRS detection in eNB, and the frequency synchronization is accomplished in UE rely on the pre-compensation before the transmission. Thirdly, we provide a scheme for the synchronization between uplink and downlink by calculating the time interval for uplink transmission on the basis of downlink timing synchronization and TA procedure. Fourthly, the overall synchronization process for LTE system, as well as the optimization program, is proposed taking the features of GNU Radio platform into account.Finally, the scheme above is implemented using C++based on GNU Radio, as well as the test in interior wireless environment and Rice channel respectively. The results show that:1) the acquire time ranges among 0.35s-0.45s under different traffics in interior wireless environment, and the process time of each subframe is in 0.25ms-0.31ms after the system is stable, which lead to the conclusion that the synchronization scheme proposed in this paper ensures the real time processing of received data.2) The residual deviation of timing and frequency can converge within 5ms in interior wireless environment.3) In Rice channel, the timing detection error rate reaches 10%, the MSE of the residual timing and frequency error reach 10-2 and 10-3 respectively under the condition of OdB SNR. Meanwhile the timing detection error rate reaches 0 under 6dB SNR.