Research On Random Access Algorithm In LTE System Under High-speed Railway Environment

Nowadays,LTE technology has been used commercially on a significant scale owing to several competitive edges,such as high-frequency spectrum efficiency,high rate,low delay and high mobility.With the development of LTE technology,the technique of high-speed railway is increasingly developing at the same time in China.LTE technology is applied to LTE-R for its incomparable advantages to solving plenty of problems under the high-speed railway environment,which are the high moving speed of UEs and the service diversification.Random access process is a significant step before users access network,and its performance directly affects the overall performance of the system.Therefore,it is necessary to guarantee the good performance of the random access process to ensure reliable communication in the complex high-speed railway environment.This paper could be employed in the industrial background and severed as reference for other researchers.The main works of this paper are as follows:(1)The LTE technology,the communication characteristics of high-speed railway environment and the advantages of LTE technology applied to high-speed railway communication is introduced in this paper.After elaborating the purpose and process of random access this paper focuses on the generation of random access preamble and backoff process of random access process.Discussing and simulating the characteristics of the ZC sequence which is the root sequence of the LTE random access preamble.The improved direction of LTE random access is analyzed based on the communication characteristics of high-speed railway.(2)The random access preamble detection algorithm under the high-speed railway environment is studied in this paper.As we all know,high-speed railway environment will bring large Doppler frequency offset.The LTE system random access detection technology relies on the good correlation of its preamble and frequency offset has an obvious influence on it.On the basis of theoretical deduction and simulation analysis to the correlation characteristic of the random access preamble,which is influenced by the frequency offset.The random access detection algorithm in the high-speed mode of LTE standard is studied,also we analyzes the shortcomings of the algorithm in the ultra-highspeed environment are analyzed.In order to make up the limitation of this algorithm,an improved single window detection algorithm is proposed in this paper.As a result,via taking into account the improved algorithm takes into account the frequency offset of more than one PRACH subcarrier interval.The complexity of the algorithm is reduced by using a large enough single window.The pseudo peak offset is limited to a smaller range by a certain limit.Additionally,the performance of the improved algorithm and the LTE standard algorithm under different channels and speed conditions is compared in this paper.The results show that the improved algorithm can meet the need of detection at higher speed,meanwhile,the miss detection probability declines 0.5-2dB under different conditions.(3)The random access algorithm based on the division of prioritization is studied in this paper.The LTE-R system carries many subsystems of the railway safety and non-security services.With the analysis of the LTE-R system accessing requirements for random access differentiation,two implementations to divide prioritization and a random access algorithm based on division of prioritization is proposed.This algorithm is based on the allocation of random access preamble and the backoff algorithm.Via providing more random access code resources and shorter backoff time for high priority users,the access opportunities of high priority users are increased and the access delay is reduced.The simulation compares the performances of the proposed algorithm and the LTE public network random access algorithm.The results show that the proposed algorithm improves the random access success rate and reduces access delay of higher priority users.