The Research Of LTE Downlink Precoding Under The Environment Of High-speed Railway

China has built the world’s biggest and highest-speed railway network in record time. LTE (Long Term Evolution) has become the ideal choice for high-speed railway broadband wireless communications, in order to meeting the demand for high-rate transmission and multimedia communication. At the same time, it tallies with gradual tactics and evolution strategy from GSM-R to LTE-R defined by the International Union of Railways. As part of the standard for3GPP Long-Term Evolution (LTE) cellular communications systems, which relied on the accurate channel state information.Precoding at the transmitter can provide diversity gain and significantly increase the channel capacity, as well as simplify the signal processing of the receiver and reduce the implementation complexity.Most of researches focus on precoding for low mobility scenarios at present, high mobility causes the most serious effect-doppler.The current channel state information by feedback or reciprocity are no longer suitable for conducting transmitter to send downlink data in the future moments. Therefore, it is necessary to overcome the demand for channel state information. That is the question which the paper has researched on.First, precoding theory is introduced, we discuss and compare precoding strategies specified in LTE, the last choice is open loop based on the large delay CDD(Cyclic delay diversity), then explain codebook design method. Emphasis is placed on the detail of Codebook and effect of the delay parameter for precoding CDD based. Simulation is carried out under high-speed railway environment. On the other hand, unitary space-time is studied, including differential unitary space-time and double differential space-time coding. On this basis, a double differential space-time frequency combining OFDM precoding scheme is put forward, differential coding is introduced into both time domain and frequency domain in an OFDM system, which effectively resist frequency-selective and time-selective fading. What is more important, it shows to be robust to dopper shift. The resulting transceiver has very low complexity and is applicable to an arbitrary number of transmit and receive antennas. Simulation result and analysis prove that the DDSTF-OFDM can improve the performance. In the last part, it introduces the process of designing DDSTF-OFDM transceivers and realizing the OFDM system with performance analysis and test based on LabVIEW+USRP platform.