Design And Implementation Of LTE Physical Layer Downlink On Software-defined Radio Platform

Mobile communication has developed rapidly in recent years, and the “quasi-4G” standard LTE proposed by 3GPP is a research hotspot in recent years. Compared to 3G standards, LTE has many advantages such as wide r spectrum, higher rate and lower delay. Today, most of the study on LTE system rely on the software simulation platform, but it is difficult for software modeling to simulate the changing wireless transmission environment accurately, while hardware platforms have their shortcomings because of high-cost and complexity of exploitation. In order to improve the efficiency and accuracy in the research of LTE system, this paper designs a new prototype of LTE physical layer downlink based on software defined radio USRP2. The prototype is of higher efficiency, more flexible simulation methods and more accurate imitation of wirelessly time-varying scenario.This paper firstly introduces the framework and protocol stack of LTE network, then conducts a study on frame format, physical channels, modulation, key sequence and so forth. Afterwards, the paper designs the overall framework of baseband signa l processing and implements the key modules: Firstly, the design of channel processing module, the design and implementation of bit signal processing with reference to the standard. Secondly, encoding and decoding algorithms of different physical channels : control channel uses a tail biting convolutional code which is reliable and shared channel uses Turbo code which is decoded by MAX_LOG_MAP algorithm. Thirdly, the channel estimation and equalization is achieved by LS algorithm and two-dimensional interpo lation based on RS sequence with the consideration of real-time transmission and computing ability. Fourthly, the impact of time and frequency offset on the system is studied. A synchronization scheme based on training sequence is proposed in this paper. T he paper realizes coarse time and frequency synchronization, cell research based on PSS and SSS sequence, and then completes the residual errors of timing and frequency based on results of channel estimation.Finally this paper implements the physical link design based on SDR platform which consists of USRP2 and UHD. QPSK and 16 QAM modulation under the 1.4MHz and 5MHz bandwidth of wireless environment are tested. The results show that this platform can realize timing and frequency synchronization, and the reliable transmission of information. This prototype fulfills the design requirements and verifies the function of the LTE physical layer downlink.