Research On Symbol Timing Synchronization And Peak-to-Average Power Ratio In Wavelet Packet Transform Based Multicarrier Modulation Communication System

While the need of real time multimedia and Internet access services increases rapidly, the design of the 4th generation wireless communication system (4G) is on the schedule. Multi-carrier modulation (MCM) has become a promising technique in 4G due to the extremely high information transmission speed. The prospect of MCM is broad because of its higher frequency efficiency. Wavelet packet transmission based multi-carrier modulation (WPT-MCM) is a new technique which combined the highlight of wavelet packet function and multi-carrier modulation; it provides high frequency efficiency, better immunity against interferences, and easily to be realized by the multi-speed transmission. In this dissertation, the symbol timing synchronization and peak-to-average power ratio (PAPR) of WPT-MCM were analyzed and researched, several improved methods and ideas have been demonstrated.At first, the basic theories of wavelet/wavelet packet were introduced and some WPT-MCM schemes were analyzed. Wavelet packet division multiplexing (WPDM) modulation scheme was chosen to be the module in the dissertation. Then the symbol timing synchronization of WPDM was studied, and a most effective method which is based maximum likelihood decision-directed (MLDD) algorithm was described and analyzed. Through the analysis, the shortage of MLDD algorithm was obtained: the synchronizer needs a very long time to return normal status when there is an outburst symbol time delay. In order to improve this, a new algorithm based fuzzy control was proposed. A fuzzy control module was introduced into MLDD algorithm to compensate the estimated value to accelerate the constringency, with the help of the experience data which was generated by the pre-added training symbol sequences.The simulation result indicated that the new algorithm improved the system capacity of synchronization recovery a lot.The issue of PAPR in WPDM was studied in the next parts. The influence of different wavelet packet functions and the number of sub-carriers to the PAPR performance was analyzed first. Subsequently, two important techniques in reducing PAPR in OFDM system were introduced detailedly; they are selected mapping (SLM) and partial transmit sequence (PTS). Aimed at the shortage of rotation vector generation method in the two techniques, the fractional factorial designs (FFD) which has been broadly used in engineering test was imported into the consideration, and an improved SLM scheme based on uniform design (UD) and an improved PTS scheme based on orthogonal design (OD) were proposed. In the improved SLM scheme, the rotation vector was generated by an existed uniform table which overcomes the drawback in conventional SLM. The simulation result proved that the new SLM scheme could obtain a better PAPR performance than conventional SLM without any system complexity increasing. The rotation generation algorithm in improved PTS scheme is the same as it in the improved SLM scheme, the only different is that the orthogonal table took place of uniform table. Because of the character of OD, there were some disciplinarian in the result of proposed PTS scheme, the optimal rotation vector can be concluded according to the parameter estimation. The simulation result proved that the improved PTS can reduce the computation notably and obtain an extremely good PAPR performance approaching the optimal PTS.Then some attempt to migrate the clipping technology from OFDM to WPDM to reduce PAPR was carried out in the dissertation. An interleaver and clipping technique was proposed. The interleaver belongs to the probability type technique, and the simulation proved that the PAPR performance of interleaver technique is nearly the same as SLM scheme, but the probability type method can't guarantee to reduce the PAPR below the threshold value. The simulation also proved that clipping method could be directly used in WPDM because the influence of non-linearity in power amplifier to system is bigger than clipping. So a combination of interleaver and clipping can improve the PAPR in WPDM better than one technique only. At the end, a new companding method based on a new transfer function was proposed; the new method extends the power of small signal, compresses the big signal and keeps the middle signal unchanged, in order to maintain the performance of system. The simulation result proved that the new method is effective.