Research On Uplink Synchronization Techniques Based On Joint Timing And Frequency Estimation

With the rapid development of mobile Internet and mobile communication technology, 3G mobile communication system has been unable to meet the needs of customers. LTE-Advanced(4G) adopted in OFDM(Orthogonal Frequency Division Multiplexing) which is based on multiple access multiplexing technology. The neutron spectrum of subcarriers in the system cover each other, with strict orthogonality. Frequency offset will destroy the orthogonality of subcarriers, causing inter-carrier interference(ICI); the time offset will introduce inter-symbol interference(ISI). In LTE-A uplink synchronous, using frequency and time synchronized manner to reduce the effect of ICI and ISI system performance. Frequency synchronization process is to get the offset casused by time-varying wireless channel and Doppler shift caused by the estimate, and then do the frequency compensation; time synchronization is required to estimate the time offset due to multipath and other causes, and then convert to timing advance(TA), to notify the user equipment(UE) to adjust the uplink transmission time. Offset and time offset estimation accuracy will directly affect the demodulation performance of the system, while a good TA management program can effectively reduce the number of TA transmission of the base station, reducing the load of the base station. The main work on this paper, as follows:(1) First study the principle of LTE-A system uplink synchronization, and learn the frequency and time offset estimation algorithm based DMRS, then made two improvements. First, the use of cross-correlation, get the time offset and frequency offset together to reduce computational complexity; second, do statistical error values associated with different sub-carrier spacing frequency offset calculation as well as the computational complexity, then adjustment the sub-carrier spacing. Simulation results show that the improved algorithm reduces the computational complexity by 25%, and the accuracy has improved.(2) Study the TA management scheme based on threshold. When the uplink channel quality is good may lead to the advance command is small then the threshold, and UE cannot receive TA for a long time, which will lead to a misjudgment. To avoid this situation by adding heartbeat module in the base station side, regularly send TA command to UE, to reduce the probability of misjudgment. MATLAB simulation results show that the improved scheme without affecting the BLER, can effectively reduce the probability of misjudgment.(3) In this paper, realize the estimation algorithm on FPGA. Use DSP Builder complete the pilot channel estimation and accomplish the estimation module design, the use of Signal Compile to generated Quartus II project, downloaded to the FPGA development board, and finally capture the result from FPGA through SignalTap II to complete the FPGA for time offset estimation algorithm verification.