Study On Timing Synchronization In OFDM Communication Link Of UAV

UAV has the advantage of low cost, flexible deployment and no casualties etc. It is widely used in military and civil applications. OFDM(Orthogonal Frequency Division Multiplexing) modulation technology with high transmission rate, spectral efficiency and excellent anti-multipath ability has been a research hotspot in the UAV communication downlink. As a critical technology of the OFDM system, timing errors may destroy orthogonality between subcarriers and decrease demodulation performance of the system. The reliable transmission of information can not be achieved.The thesis mainly focuses on the timing synchronization algorithm of OFDM date link in the UAV ground-air communication environment. The main contents are displayed as follows:Firstly, the principle of UAV OFDM communication link is introduced. Then, study on the propagation characteristics of wireless channel and several typical channel model. Lastly, UAV ground-air communication system is analyzed. Combine with application scenarios to determine the channel simulation parameters, which are the foundation of the following research.In order to solve the problem that the existing synchronization algorithm has a high probability of miss detection in UAV fast time-varying channel, a new algorithm based on channel estimation is proposed, which contains coarse synchronization and fine synchronization. Then, channel impulse response is obtained using the least square(LS) algorithm, based on which the location of each path was determined according to the difference in a sliding window. The first path was finally detected via the minimum value or adaptive threshold approach. Theoretical analysis and simulation results show that, the proposed algorithm can decrease the probability of miss detection, improve the system timing performance and meet the requirement of UAV system.In order to deal with the problem that the above proposed algorithm need obtain the channel impulse response of cycle-right-shift property by the transformation between time and frequency domain, which has complex calculation and is not flexible, a timing synchronization algorithm based on CAZAC sequence is proposed. The training symbol designed in this paper is composed of four repetition structure. Coarse synchronization locates the strongest path by using the method of correlation peak accumulation. On the basis of that, fine synchronization multiplies multiple short-time correlation peaks to enhance the correlation peak, thereby improving the signal timing performance. Theoretical analysis and simulation results show that, as compared with Yang algorithm, the proposed algorithm has higher detection probability, while reducing the amount of calculation, is more suitable for fast time-varying multipath channel of UAV ground to air communications environment.