On The Design And Implementation Of The LTE Downlink Synchronization Technique

3GPP Long Term Evolution(LTE) project is the evolution of 3G standards, which improves and enhances the 3G air-interface technologies. Orthognal frequency-divison multiplexing(OFDM) is one of the key technologies in the LTE downlink transmission system, and important features of OFDM systems include high spectrum efficiency, high data transmission rate and immunity to multipath fading and impulsive noise. However, OFDM systems are far more sensitive to time synchronization error and freqeucy offset than single-carrier systems. The performances are degraded dramatically by inter-symbol interference(IS I) because of time sychronization error or inter-carrier interference (ICI) for frequency offset. Therefore, synchronization is a key technical problem in OFDM systems, and synchronization techniques for OFDM systems are essential. The work of this thesis is to determine the sychronization scheme for LTE downlink OFDM systems on the basis of the analysis of the classical synchronization algorithms for OFDM systems, and design the sychronizer on the DSP platform.Firstly, the basic principles of OFDM technology and the influence of the synchronization errors on OFDM systems are addressed in this thesis. Then several classical OFDM synchronization schemes are introduced, including synchronization algorithms based on training sequences and blind synchronization algorithms based on structural characteristics of OFDM symbols like cyclic prefix(CP), and different synchronization algorithms are analyzed and compared from the simulation performance and computational complexity.Secondly, the synchronization scheme for LTE downlink OFDM system is determined according to the generating operations and time-frequency mapping characteristics of physical signals. The PSS is generated by Zadoff-chu(ZC) sequence, which has good correlation properties and sent twice in a radio frame. In this thesis, the algorithm based on PSS cross correlation is employed to get the coarse timing synchronization point; Meanwhile, the coarse fractional frequency offset estimation is also based on the PSS and the algorithm has a good performance and a relatively large estimated range. Fine timing synchronization and residual frequency offset estimation algorithm are performed by reference symbols (RS), but the length of CP has a influence on the estimated range of the residual frequency offset.Finally, the synchronization scheme for LTE downlink OFDM system is designed and implemented on the DSP platform based on the verification of LTE synchronization schemes, and the synchronization modules include the coarse timing module, coarse fractional frequency offset estimation module, fine fractional frequency offset estimation module, fine timing module, and frequency correction module. Test modules on the DSP platform include modulation module,. RS generation module, resource element mapping module, OFDM symbol generation module, OFDM demodulation module, and synchronization module. The analysis validates that, the reference symbols are generated in parallel to raise the operating speed and the coordinate rotation digital computer (CORDIC) algorithm is employed to design the frequency offset estimation and frequency offset correction modules in order to reduce the hardware consumption and computational complexity.