| Millimeter wave frequency band technology has been applied to the commercial stage of 5G,which can meet the higher-speed data transmission requirements of 5G largescale communication systems,and can alleviate the increasingly serious problem of frequency band spectrum shortage.In the 5G stage,the millimeter wave frequency band is gradually included in the considered communication frequency band.In order to further improve the communication quality,beamforming technology is also widely used.By cooperatively transmitting signals from multiple antennas,the signal power can be controlled to concentrate in a certain direction.Concentrating the signal power in the direction of the receiver to counteract the pressure from multipath channels and the high fading of mm Wave in free space is also a prerequisite for data communication in the mm Wave frequency band.In order to make better use of beamforming technology,5G NR specifies a fixed-structure signal,synchronization signal and PBCH block(SSB for short),which is used to serve users for initial beam scanning and time-frequency synchronization during the initial user access phase and so on.This paper first introduces the background significance and research status of millimeter-wave MIMO communication system,reviews the application of MIMO technology in communication systems in the past LTE era,and analyzes the difference between millimeter-wave MIMO technology in the 5G era and the past combined with the 5G NR standard.On this basis,for the beam scanning technology widely used in 5G NR,an initial access scheme based on beam scanning in the 5G NR standard is given,and the synchronization signal structure and time domain transmission mode used in 5G NR are introduced in the scheme.And a more widely used millimeter-wave channel model is given.Next,starting from the problem of low utilization rate of synchronization signals that may be caused by beam scanning,the time-frequency precise synchronization algorithm based on the traditional phase-locked loop algorithm and the Kalman filter proposed in this paper is introduced.Through simulation,the time-frequency synchronization performance of the two algorithms is compared under the condition of low signal-to-noise ratio brought by the users not covered by the beam,and the advantages of the Kalman filter time-frequency precise synchronization algorithm proposed in this paper are verified.After that,the preliminary hardware implementation of the time-frequency synchronization system is carried out,and the time-frequency synchronization circuit diagram under the condition of 4 times oversampling is given.With the dynamic threshold module,the synchronization sequence is locked by continuously detecting the synchronization peak,and it has a certain frequency offset correction.ability.Finally,the follow-up improvement direction of hardware circuit to realize precise time-frequency synchronization is analyzed. |
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