Hybrid Precoding Design For Beam Squint In Terahertz UM-MIMO Systems

Terahertz communication technology is expected to become one of the key technologies of future 6G by virtue of its ultra-large bandwidth.Ultra-large-scale antenna technology can provide huge spatial diversity and improve spectral efficiency,which is also significant in 6G wireless communication systems.In the phase-shifter based hybrid precoding of UM-MIMO systems,due to the large bandwidth of the terahertz frequency band,the subcarrier channels of different frequencies have different equivalent spatial directions.When the beam is formed at the transmitter,a serious beam squint problem will occur because the phase generated by the phase shifter is independent of frequency.The beam squint effect results in a limited array gain of the antenna,which in turn affects the overall data transmission rate of the system.Therefore,the design of hybrid precoding for beam squint effect has become a research hotspot in terahertz communication.Based on improving the hybrid precoding structure,this thesis proposes two hybrid precoding schemes in terahertz UM-MIMO systems to eliminate the performance loss caused by the beam squint effect,namely,the hybrid precoding scheme based on two-layer phase shifter and the hybrid precoding scheme based on fixed delay true delayer.The main contributions of this thesis are as follows:(1)This thesis first introduces the causes and influencing factors of the beam squint phenomenon.In order to deal with this phenomenon,this thesis proposes a design on hybrid precoding based on two-layer phase shifters.Using 3D MIMO planar antenna array,the higher-dimension uniform linear antenna array steering vector with is converted into the form of the kronecker product of two lower-dimensional antenna steering vectors,reducing beam squint caused by array gain loss.In view of the amplification of the beam squint caused by the ultra-large bandwidth of the terahertz band,based on the 3D massive MIMO system,this thesis proposes a hybrid precoding scheme based on a two-layer phase shifter structure.The first layer phase shifter is used for frequency independent beamforming,and the second layer phase shifter is used for beam phase compensation of subcarriers with different frequencies.The simulation results show that the proposed scheme can achieve near-optimal array gain and obtain higher system performance.(2)In view of the beam squint effect,this thesis uses a new fixed delay true delayer to replace the phase shifter as the hardware device for analog precoding,and designs a hybrid precoding scheme with a fully connected structure.In the fixed delay true delay device,since the delay it can provide is fixed,it can effectively reduce hardware cost and energy consumption while providing frequency-dependent phase shift,and eliminate the influence of beam squint at a lower cost.In this thesis,the delay selection network is used to select the fixed delay true delayer with the closest phase for the transmitting antenna,so as to improve the array gain of the antenna and achieve high system performance.The simulation results show that the proposed scheme has a near-optimal achievable rate performance.