| The millimeter wave band will be used in the 5G commercial frequency band,which will transmit high-speed data for the 5G cellular communication system and alleviate the spectrum shortage for the sub-6GHz band.The millimeter wave with high frequency and short wavelength makes the antenna and related hardware easy to integrate.The millimeter wave system combined with massive MIMO can effectively resist the serious fading of millimeter wave in free space,combined with hybrid precoding and OFDM that can effectively resist multipath fading,it will provide high beam gain and ultra-high transmission rate for massive MIMO multi-user systems to meet the transmission requirement of 5G.The RF chain in traditional full-digital MIMO system is connected with specific antenna,and the signal is preprocessed by the digital precoding only.Massive MIMO systems with full digital structure will have huge cost and ultra-high energy consumption.The hybrid analog/digital precoding scheme can reduce the processing dimension of signal with the analog precoding.The structure can effectively reduce the number of RF chains,the cost of hardware and the energy consumption of system.And the performance is close to the full-digital precoding.The research on narrow-band hybrid precoding has been mature.The multi-user hybrid precoding under the frequency-selective fading channel is the focus of this paper.This paper first reviews the full-digital precoding structure and the common processing method of traditional single-user,multi-user and OFDM systems.Then,we introduce analog precoders with different structures,the narrow-band and wide-band channel models of millimeter wave.The analog precoder in wide-band and multi-user hybrid precoding system is shared by all carriers and users.Using the characteristics of broadband millimeter wave channel,its hybrid precoding design can be transformed into hybrid precoding design of narrow-band and single-user system,and two-level digital precoding must be used,the first level is used to assist the analog precoder to align beam,the other stage is used to control the inter-user interference with the hybrid block diagonalization algorithm.For the analog precoder of traditional fully-connected structure,the wide-band and multi-user hybrid precoding algorithms based on orthogonal matching pursuit,alternating minimization and beamforming algorithm are studied,which approximate the optimal full-digital precoding matrix and with high complexity.In order to reduce the complexity,this paper proposes an improved wide-band and multi-user hybrid precoding algorithm based on the mean channel covariance matrix,which avoids the calculation of the optimal precoding matrix and has good performance.Then the algorithms of the fully connected structure are improved to be suitable for the traditional precoding system of the sub-connection structure with higher energy efficiency and easier implementation.The simulation results show that the performance of sub-connection structure is better when each user only transmits one stream.Finally,the wide-band and multi-user hybrid precoding schemes of three improved analog precoders are studied,which have higher spectral efficiency than the traditional structure,because the improved analog precoders have the modulus-limited constraint instead of the constant modulus constraint.Considering the feasibility of hardware implementation,a dynamic antenna selection structure based on phase shifter,adjustable power amplifier and single-pole multi-throw switch is proposed,which can realize dual control of signal phase and amplitude,and we propose a wide-band and multi-user hybrid algorithm for this structure.The single-pole multi-throw switch can bring dynamic gain,and the structure is an extremely energy-efficient sub-connection mapping structure.When considering the quantization error,the quantization error of the structure that is composed of phase shifters and adjustable power amplifiers is very small. |
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