Synchronization Techniques For Low Snr Tdma Systems

With the development of communication technology, the received signal is appeared to be poorer and poorer, even lower than threshold for direct demodulation, so the demand for dealing with signals under low signal-noise-ratio (SNR) is increasing. Focusing on the object of the study, synchronization of TDMA signals under low SNR is researched in this paper. The main research working can be summarized as follows:Firstly, the feed-forward symbol timing synchronization which fits for TDMA signals under low SNR is researched, and the interpolation filter with flexible parameter is introduced; using the estimation of timing delay, the estimation value of signals on the optimal sampling time can be got by the interpolation filter.Secondly, the feed-forward carrier synchronization of QPSK modulation signals based on maximum likelihood (ML) criterion is lucubrated, combining with the ML algorithms and the character of TDMA signals frame, a carrier synchronization satisfied with TDMA signals under low SNR is proposed. The COSTAS loop is a common and efficient structure for carrier synchronization of OQPSK modulation signals, and in order to avoid the shortcoming of the COSTAS loop, an improved structure is given.Finally, turbo synchronization based on soft symbol information is introduced, and after the analysis of usually used soft synchronization algorithms, a soft synchronization and soft decoding combining structure based on ML criterion and exception-maximum (EM) algorithm is described in this paper.The contributions of this paper can be summarized as follows:1. According to the frame structure information of TDMA signals, an improved algorithm satisfied for QPSK signal with large frequency offset is proposed. In the improved algorithm, data-aided(DA) estimation with the pilot data is used to get the coarse frequency offset and the accurate frequency offset is got by the non-data-aided(NDA) processing based on the whole frame data;2. Associating with cycle-spectrum and COSTAS loop, a fast and efficient carrier synchronization structure of OQPSK signals is designed, which can be used to deal with signals which have large frequency offset;3. After researching on the performance of the timing synchronization decisided by soft decoding and the CA synchronization based on EM algorithm, utilizing the advantages of these two algorithms, a searching algorithm based on coding information is got;4. The performance of CA carrier synchronization is poor, if there is a large frequency offset in the signal. In order to reduce the affection of the frequency offset, a carrier pre-processing module is operated before CA synchronization;5. Jointing with the CA timing delay and CA frequency offset estimation, a soft synchronization and soft decoding combining structure, which fit for TDMA signals under low SNR, is given in this paper.