| Low-earth-orbit(LEO)satellite mobile communication system has become an indispensable and important part of the future mobile communication network,because of its global seamless coverage and sufficient system capacity.In order to meet the increasing demand for broadband data transmission services of military and civilian users,and by considering the rapid development of terrestrial mobile communication technologies,it is imperative to build a new generation of long term evolution(LTE)-based LEO satellite mobile communication systems with multiple services,high transmission rate and spectrum efficiency.As one of the pivotal technologies for the LTE air interface,the uplink synchronization technology is a crucial prerequisite for effectively establishing the communication uplink and implementing reliable data transmission.However,unlike terrestrial mobile communication systems,LEO satellite mobile communication systems have the characteristics of long transmission distance,wide beam coverage,fast relative movement,and limited load.It is obvious that,the existing LTE uplink synchronization technology is incapable of coping with such a high-dynamic environment with large attenuation,and consequently cannot satisfy the design requirements for low-complexity satellite-borne receivers.Taking the LTE-based LEO satellite mobile communication system as the research background,this dissertation focuses on the fast and effectively achievement of uplink synchronization for the satellite-borne receiver in high-dynamic environment,and carries out the relative researches from the aspects of random access preamble design and timing detection algorithm,as well as the uplink timing advance(TA)renovation algorithm.The main research contents and contributions are listed as follows.1.In consideration of the weaknesses of the universal preamble design scheme for LTE-satellite,and by combining the characteristics of LEO satellite-earth links,a preamble,with the robustness to satellite environment,low-complexity detection and excellent multi-access performance,is designed.First,by cascading multiple short ZC sequences robust to the carrier frequency offset(CFO),a long preamble format is constructed.While satisfying the large inter-beam time uncertainty and coverage performance requirements,the long preamble can also mitigate the adverse effects of the CFO on detection process in advance.Then,by utilizing the cascaded preamble format,three preamble sequence design schemes are further proposed according to different combinations of root sequence and cyclic shift offset.From the perspective of multi-user detection complexity and performance,the superiority of single-root cascaded long sequence(SCLS)composed of different cyclic shift sequences of a single root ZC sequence is verified.Compared with other two schemes,this scheme uses a minimum number of root sequences for multi-user access,and its cyclic shift offset is independent of the beam radius.The results show that the presented SCLS not only satisfies the requirements of false alarm probability and missed detection probability for LTE random access,but also achieves relative high success rates of first access in the scenarios such as large CFO,low SNR,and multi-user access,respectively.2.Two timing detection algorithms are designed for different communication scenarios,respectively.For satellite line-of-sight(LOS)channel,in utilization of the SCLS sequence,a low-complexity timing detection algorithm based on piecewise accumulation detection and multi-peak combinated estimation(PAD&MCE)is proposed.First,according to the PAD method,all the correlation peaks related to the same root sequence can be obtained by only one power delay profile(PDP)calculation.Then,based on the false alarm probability requirement of the receiver,a multi-user dynamic detection threshold is deduced,and the PDP peaks of all users in the detection window are captured.Finally,by considering the features of the cyclic shift offset in SCLS,the peak position set can be separately extracted for each user,and so that both the fractional and integral values of TA(normalized to the sequence length)can be determined in one step,according to the proposed MCE method.The results demonstrate that,the PAD&MCE algorithm obtains a significantly improvement in terms of detection success rate,timing estimation accuracy,and computational complexity,compared to the existing algorithms.For satellite multi-path fading channel,another timing detection algorithm is presented.By using the enhanced SCLS(E-SCLS),a sequence-joint timing metric based on length-variable differential cross-correlation(LDCC)is constructed,which not only can obtain a unique correlation peak,but also eliminates the effect of larger CFOs on timing estimation.Further,the statistical distributions of the proposed timing metric are derived at the correct and wrong timing positions,respectively.It is shown that the LDCC method can flexibly adjust the correlation length to suppress noise.Also,a detection threshold,that is independent of the noise variance,is obtained to determine the first arrival path.Numerical results are consistent with theoretical analysis,and demonstrate that the proposed algorithm not only can be applied to the satellite LOS channel,but has a better timing estimation performance under satellite multipath fading channels.3.Two TA renovation algorithms are designed for different communication scenarios.For the satellite LOS channel,a partial symmetric correlation(PSC)-based TA renovation algorithm is first proposed,to track the real-time timing of each received sounding reference signal(SRS).Due to the flexible variable segment length,the PSC has an adjustable CFO-tolerance range,which can effectively mitigate the negative impact of large CFOs at the receiver.In order to suppress the unwanted mirror peak caused by the PSC operation,peak-joint detection(PJD)is further used to improve the reliability of the timing estimation.Moreover,a low-complexity estimator has also been proposed to improve the system efficiency.The results show that the proposed method outperforms the related methods in the typical satellite LOS channel.For satellite multipath fading channel,by using all autocorrelation products between two adjacent SRS signals,a TA renovation algorithm based on high-order statistics is also presented.Through the reasonable design of the forth-order autocorrelation and differential normalization function for the timing metric,the extended range of the correlation length and the sharpness of the timing peak are effectively improved.Taking class separability,the robustness to multipath effect and CFO,and computational complexity as the criteria,the superior performance of the proposed method is theoretically evaluated.Simulation results under the typical satellite multi-path channel model verify the correctness of the theoretical analysis,and show that the proposed method can achieve lower false alarm probability,missed detection probability and timing estimation error,and makes the range of appropriate detection threshold much wider,compared with the related works. |
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