Research On Carrier And Symbol Synchronization Techniques In Wireless Transmission Systems

With the social continuous progress and the economic development,human requirements for high data rate,large capacity,high communication quality,diversified data services of wireless communication are increasing.To meet these new growing requirements,wireless transmission techniques have been in constant development and evolution.Currently,any wireless transmission systems have to be faced with the issue of system accurate synchronization.System synchronization decides the success or failure of the digital receiver designs,and that it has a great impact on communication quality.In wireless transmission systems,all existing synchronization methods have more or less deficiencies which cannot be ignored,including non-intuitiveness of the determination process of loop parameters,smaller parameter synchronization range,higher computational complexity,more difficulties for hardware implementation,poorer scalability,unsatisfactory synchronization performance in the case of lower signal-to-noise ratios(SNRs)and high dynamics and so on.Therefore,those algorithms cannot be adapted well to the further development of next generation wireless transmission systems.To solve these issues,the closed-loop synchronization technique of digital modulation satellite communication systems,carrier tracking technique of high-data-rate transmission systems under the condition of symbol rate sampling and low SNRs,frequency and symbol synchronization techniques for universal filtered multi-carrier(UFMC)based systems are primarily investigated in this dissertation.Moreover,the corresponding modified algorithms are also proposed to improve the parameter synchronization performance of wireless transmission systems,which promotes the rapid development of next generation wireless communication techniques.The main research contents and innovation achievements of this dissertation are summarized as follows:1.The closed-loop synchronization technique of digital modulation satellite communication systems is investigated.The commonly used parameter determination methods for digital synchronization loops are firstly analyzed.In these methods,digital loop parameters are always obtained through mapping transformations from s-domain to z-domain.However,these methods have the disadvantages of high complexity and non-intuitive parameter determination process.Moreover,when the sampling rate of received signals is close to the symbol transmission rate,there is a big difference between the practically obtained loop performance and the tracking performance required by the designers,which results in the fact that loop parameters have to be adjusted again.In order to solve this issue well,two effective z-domain parameter determination approaches are proposed.In the first approach,the positions of the phase-locked tracking system's poles are directly and flexibly ascertained in the z-domain by introducing the new adjustable variables.And then,unknown loop parameters are calculated according to the constraint relationship between loop parameters and introduced variables.For the second approach,by analyzing the stability and noise-suppressed characteristics of closed-loop synchronization systems,a smaller range of loop parameters is firstly attained.Afterwards,the unit-step response characteristic of these systems is adjusted and the parameter solving problem is translated into the conditional minimization problem of the phase error noise variance.Moreover,this minimization problem is further solved to acquire optimum loop parameters.Compared with traditional mapping transformation methods,two proposed approaches not only don't require the mapping transformation from the s-domain to z-domain,but also have the features of good intuitiveness and low complexity,and they can also ensure that the design of digital synchronization loops is in agreement with their practical realization.Furthermore,the second method is able to be presented in the form of a parameter solution software,which enables the parameter determinations of digital synchronization loops to become more flexible and more convenient.2.Carrier tracking technique of high-rate wireless transmission systems is investigated in the case of symbol rate sampling.A novel and robust non-data aided(NDA)carrier tracking algorithm is proposed for high-rate data transmission systems.This algorithm makes good use of a new and hybrid symbol decision-directed carrier synchronization architecture.In this algorithm,owing to the introduction of sampling rate conversion blocks,the digital frequency-locked loop(DFLL)operates under the condition of symbol rate sampling,whereas the working frequency of the digital phase-locked loop(DPLL)is larger than the symbol transmission rate,which enables the DPLL to estimate and compensate the phase offset of received signals for many times in the symbol period.Moreover,in order to lower the error probability of data symbol decision,the original hard decision strategy is improved.Compared to data-aided(DA)methods,the proposed method does not require to occupy the additional system spectrum resource and signal transmitting power.Additionally,the complexity of this novel algorithm is much lower than that of code-aided(CA)algorithms.Simulation results are provided to demonstrate that the proposed algorithm is able to achieve better synchronization performance than traditional carrier tracking loops under the condition of relatively low SNRs and symbol rate sampling.3.Carrier synchronization technique for UFMC-based systems is studied in the case of multipath Rayleigh fading channels.Different from orthogonal frequency division multiplexing(OFDM)technique,the inter-block interference(IBI)between adjacent UFMC modulated symbols cannot be removed in UFMC-based systems after passing through multipath fading channels.This is because the cyclic prefix(CP)or guard interval is not appended between adjacent UFMC modulated symbols.Accordingly,the corresponding carrier frequency offset(CFO)estimation approaches cannot be directly devised by only using a separate UFMC modulated symbol.To solve this problem well,a novel and hybrid carrier synchronization algorithm relying on two identical consecutive training symbol sequences is proposed for UFMC-based systems.Since both training sequences are adjacent and identical in this proposed approach,the least square(LS)method is directly applied to the second received training symbol firstly.Furthermore,the CFO estimation expression based on the LS criterion is derived in detail for achieving the coarse estimation of system CFOs.And then,the auto-correlation calculation operation is performed through utilizing the part of two received training symbols which are not affected by the previous UFMC modulated symbol,in order to estimate the residual CFOs precisely.Theoretical analyses and numerical simulations indicate that the proposed algorithm not only has the features of larger frequency offset estimation range,but also is able to achieve higher frequency offset estimation precision in comparison with existing CFO estimated algorithms.4.Symbol timing synchronization technique for UFMC-based systems is studied over multipath Rayleigh fading channels.Since UFMC is a modification of OFDM,some timing synchronization approaches for OFDM-based systems can be extended and applied to UFMC-based systems.However,these approaches have some disadvantages of insufficient estimation range,large estimation variance or high computational complexity.To overcome the deficiencies of available timing offset(TO)estimation approaches and further improve their synchronization precision,a new preamble synchronous signal architecture based on the pseudo-random noise(PN)sequences is devised and a robust TO estimation algorithm with low complexity is also proposed for UFMC-based systems according to this designed signal architecture.In this algorithm,a timing metric is firstly improved to enhance the rough TO estimation performance.Moreover,a smaller fine adjustable parameter range is also offered.And then,with combining the threshold criterion,the cross-correlation calculation between received signals and the local PN sequence is carried out in this fine adjustable range,in order to obtain the accurate estimation of residual TOs.Theoretical analyses and simulation results are presented to confirm that the proposed algorithm has lower computational complexity and that its synchronization performance is better than that of conventional TO estimation algorithms under the condition of multipath fading channels.