# A Novel Multi-User Codebook Design for 5G in 3D-MIMO Heterogeneous Networks
## 2 Related Work  
Massive multiple-input multiple-output (MIMO) has become one of the most important technologies in wireless communication in the future [9]. Three-dimensional MIMO (3D-MIMO) is one of the main application scenarios of massive MIMO. Due to the limited space of these base station (BS), 3D-MIMO can use the Uniform Planar Arrays (UPA) to place hundreds of antennas in a relatively small space. In addition, 3D-MIMO can control the beam in the horizontal and vertical directions of the signal. Reasonable transmission and reception techniques can reduce multi-user interference and greatly improve system performance [10]. As the urban environment becomes more and more modern, the number of users in the community is increasing, and users are more and more dispersed in three-dimensional space. Since traditional base station transmitter beam can only be adjusted in the horizontal dimension, the vertical dimension is a fixed down dip for each user. However, the actual signal propagation channel has three-dimensional space characteristics, which can lead to serious inter-cell interference among the users of the cell, which greatly reduces the cell edge user throughput [11]. The 3D-MIMO precoding technique can divide each vertical antenna element into a plurality of elements without changing the size of the existing antenna so that the vertical dimension has a distinguishable pitch angle, which performed in the horizontal and vertical dimensions. Joint beamforming design can make full use of three-dimensional space freedom, bring more power interference control capabilities, and improve system performance [12]. At present, the research on the codebook design scheme for 3D precoding has achieved certain results, but most studies are based on the discrete Fourier transform (DFT) codebook [13; 14]. However, since the DFT codebook has strong beam directivity and uniform distribution, it is suitable for horizontal dimension codebooks using a uniform linear array (ULA). However, due to the narrow and nonuniform distribution of the up-and-down inclination of the vertical direction, it is not applicable to the vertical dimension codebook using the uniform planar array (UPA) [15]. The improved codebook study still uses the DFT codebook in the horizontal direction and uses the distribution characteristics of the vertical down tilt in the vertical direction. Several discrete quantization angles are extracted in the vertical down-tilt angle interval and constructed by the corresponding beam steering vectors. The vertical codebook [16], or the codeword phase determined by the specific position of the antenna array element, is used to construct the vertical codebook [17], or on the basis of analyzing the correlation of the 3D-MIMO channel, for the transmit correlation and receive correlation matrix in the decomposition of the horizontal and vertical dimensions, a new 3D codebook is constructed combining the DFT codebook and the Grassmannian codebook in the 2D-MIMO channel [18]. However, these codebook designs do not consider the launch angle in the horizontal direction of the antenna array elements. Although the horizontal launch angle can be adjusted in a small range, if it is regarded as a fixed angle, the accuracy of the beam pointing will be reduced, causing inter-user interference, which seriously affect system performance.  
Based on the quantization error and statistical characteristics of the random distribution of 3D-MIMO systems, the impact of stochastic geometry analysis feedback overhead on system performance was studied. Based on the deployment of UPA antenna at the base station, a horizontal launch angle and vertical downcast angle and Kronecker were used. We construct a design scheme for the 3D-MIMO multiuser codebook and simulate and analyze the designed codebook scheme. The proposed scheme has obvious performance enhancement as compared with the traditional schemes.