Research On Synchronization Technology In OFDM Telemetry

Because of high bandwidth efficiency and good anti-multipath interference performance, OFDM will be a key technology in the next wireless communication systems, and has the potential advantages in unmanned aerial vehicles (UAV) telemetry. This paper studies the technology of OFDM synchronization for the UAV telemetry, and the particular contents are as follows:Firstly, the problems of previous algorithm based on cyclic prefix are analyzed in detail. The analysis results show that not only multipath interference affects the system performance, but also symbol timing error affects carrier frequency offset (CFO) estimation in this algorithm, and then the approach is presented for improving the performance. The symbol timing fine synchronization based on pilot tones are studied, and then a novel symbol timing fine synchronization algorithm based on constellation characteristics is proposed. First, a new timing metric function is defined via energy criterion in the constellation area, and then the symbol timing offset estimation is obtained by maximizing the timing metric function. The proposed algorithm has an impulse timing metric and not high sidelobe, so rigorous timing synchronization can be achieved.Secondly, the previous synchronization algorithms based on training symbol are analyzed. The analysis results show that they suffer timing ambiguity and have not good performance in multipath channels, and then according to the characteristics of UAV telemetry, two novel synchronization algorithms based on training symbols are proposed. The first one is based on repeat conjugate symbol. The second one is based on conjugate symmetry symbol. In these algorithms, the new training symbols are designed, and the new timing metric functions are defined, and the formulae of symbol timing offset estimation and CFO estimation are presented. They not only solve the problem of timing ambiguity, but also present good performance in multipath channels. Both of the algorithms have the ability of CFO estimation, and the maximum CFO estimation range are+2subcarrier spacing and±4subcarrier spacing respectively.Thirdly, the CFO tracking algorithms based on the characteristics of data subcarriers are studied. Two blind CFO tracking algorithms are proposed. The first one is suit for OFDM systems with BPSK modulation. In the first method, the CFO estimation is obtained by utilizing phase difference between two OFDM symbols through cross-correlation between them either in time domain or in frequency domain. In the second method, according to the conjugate symmetry properties of OFDM-BPSK symbols, the CFO estimation can be obtained. The second one is suit for OFDM systems with QPSK or QAM modulation. The residual CFO compensation in the time domain is achieved by utilizing characteristics of constellations and using a second order phase locked loop. Both of the new algorithms are low computation complexity and high bandwidth efficiency, because they don’t need pilot tones or training symbols.Finally, the UAV telemetry experiment system is built. Some of proposed algorithms are validated. The FPGA program for baseband transceiver is designed, and the performances are tested. The system experiments are developed, and the experiment results show that OFDM is suit for UAV telemetry.