Research On Random Access Sequence Design And Detection Algorithm In LTE-based Satellite Communication

With the popularity of satellite mobile communication system,the combination of highspeed LTE technology and satellite mobile communication system has become an inevitable trend.The LTE system establishes the uplink timing synchronization through the uplink random access procedure,so it is important for the random access to connect the user to the communication network.Due to the unique channel characteristics of the satellite communication system,the terrestrial LTE random access technology can not be directly applied to the satellite communication system.Firstly,this paper describes the satellite communication system architecture and channel characteristics and analyzes the research situation of random access technology in satellite communication system at home and abroad.In addition,the role of random access in LTE systems and random access processes are amply discussed,the generation and detection process of terrestrial LTE system random access signal are analyzed detailly.The paper focuses on the random access technology in satellite mobile communications with the background of the terrestrial LTE random access technology.Secondly,considering the large transmission delay of the satellite mobile communication system,the random access preamble sequence must be redesigned.Based on the design principle of random access signal and the characteristics of satellite mobile communication system,this paper presents two design Solutions of universal sequence and cascade sequence.The duration of the two sequences is consistent,but the universal sequence is a complete ZC sequence with small subcarrier spacing,long sequence length and many DFT points.And the cascade sequence is cascades with a multi-stage short ZC sequence,short ZC sequence can still follow the terrestrial LTE system format.The subcarrier space of the cascade sequence remains unchanged at 1.25 k Hz,it has less DFT points and less computational complexity than universal sequence.The simulation results show that the cascade sequence has better anti-frequency deviation performance and is more suitable for satellite mobile communication system with larger Doppler expansion.Finally,in order to eliminate the influence of the larger Doppler spread on the random access in the satellite communication system,this paper proposes an improved timing estimationalgorithm based on the TAE-MCD algorithm.The TAE-MCD algorithm directly combines the conjugate of the ZC sequence and the ZC sequence in time domains to form a new preamble sequence.When calculating the DFT spectrum of one of the sequences,the result of the other sequence is added as the additional noise of the DFT spectrum.The algorithm proposed in this paper is to reduce the additional noise to improve the timing performance.The algorithm requires the terminals to transmit ZC sequence and the conjugate of the ZC sequence as a preamble in the time domain continuously.the DFT operation is performed after multiplication of the local ZC sequence and the received signal,The DFT spectrum are independent of each other.The peak positions in both DFT spectrum are related to the frequency offset,by subtracting the two peak positions,we can eliminate the effects of Doppler shift and obtain the correct timing.Although the algorithm increases the time domain resource,the simulation results show that the performance of the algorithm is about 2d B higher than that of the TAE-MCD algorithm when the probability of false detection is 1%.At low signal-to-noise ratio,this algorithm is superior to TAE-MCD algorithm and the performance this algorithm is very close to that of the existing LTE system timing estimation algorithm.