| Since 2020,The 5th Generation Mobile Communication System(5G)has gradually been put into commercial use.5G meets the requirements of ultra-low latency,low power consumption,high reliability and high transmission rate of future mobile communication systems,laying the foundation for emerging industries such as the Internet of Things and artificial intelligence.In the 5G system,only after the terminal device completes the cell search can it receive downlink data correctly and start the communication connection with the base station.Therefore,the cell search process is the first step for the terminal device to access the 5G cell.Cell search is an important part of wireless communication,and its performance will directly affect the subsequent communication capability of terminal devices.Regarding this performance requirement,this thesis studies the cell search process of the 5G communication system,which mainly includes two parts:5G downlink synchronization and Physical Broadcast Channel reception.Among them,in order to improve the decoding ability of Physical Broadcast Channel,this thesis studies the channel estimation algorithm and channel decoding algorithm of Physical Broadcast Channel.The work can be summarized as follows:Firstly,this thesis builds a simulation platform for 5G cell search,studies the cell search process in 5G scenarios,and designs a cell search scheme for 5G system.The method of 5G downlink synchronization is introduced,several common time synchronization algorithms and frequency offset estimation algorithms are described,and the algorithms are simulated and analyzed.Secondly,this thesis studies the channel estimation method of Physical Broadcast Channel in cell search.This thesis firstly introduces several traditional channel estimation algorithms in OFDM system,and then discusses the defects of the DFT-based channel estimation algorithms.Combined with the time-frequency characteristics of Physical Broadcast Channel,an improved DFT-based channel estimation algorithm is proposed.The simulation results show that the algorithm proposed in this thesis has a great improvement in performance compared with the original algorithm.Thirdly,this thesis studies the channel decoding process of Physical Broadcast Channel.Firstly,the existing common polar code decoding algorithms are compared and analyzed.Then,the implementation scheme of distributed cyclic redundancy check in Physical Broadcast Channel is introduced.On this basis,an improved early termination algorithm based on path tracking is proposed.The simulation results show that the early termination algorithm proposed in this thesis can further improve the early termination probability in the decoding process,thereby reducing the computational complexity of the system when the decoding fails.Fourthly,this thesis implements a 5G cell search system on an embedded board based on the improved channel estimation algorithm and polar code decoding algorithm.several functional modules such as downlink synchronization,reference signal processing and Physical Broadcast Channel reception are designed,and the system is optimized by methods such as look-up table and identifying priorities.The test results show that the implemented system can accurately search the 5G cell in the existing network environment and obtain the system information of the cell,so the research in this thesis has strong practicability. |
