# A Novel Multi-User Codebook Design for 5G in 3D-MIMO Heterogeneous Networks
## 1 Introduction  
Three Dimensional MIMO (3D-MIMO) is a type of Massive MIMO Technology which has the ability to move the beam horizontally as well as vertically. In recent years, with the widespread use of smartphones such as mobile phones and tablets, the volume of mobile data services has exploded, and existing wireless communication systems can no longer meet such huge business demands. Therefore, academia and industry have successfully conducted research on 5G mobile communication. Massive MIMO technology is one of the candidate key technologies for 5G. Compared with the existing 4G technologies, it can significantly improve the spectrum efficiency and energy efficiency of the communication system and has become a research hotspot at home and abroad [1]. The combination of Massive Multi-user MIMO (MU-MIMO) systems can further utilize spatial freedom to significantly increase system throughput and spectrum efficiency [2]. As a key technology of the MU-MIMO system, precoding can effectively suppress multi-user interference and improve the Signal-to-Noise Ratio (SNR) [3]. However, to make full use of the role of precoding, the transmitter must obtain enough channel state information (CSI). For Time Division Duplexing (TDD) systems, CSI can be obtained through the reciprocity of uplink and downlink channels, but susceptible to pilot contamination [4]. In a Frequency Division Duplex (FDD) system, CSI acquisition relies on a feedback link. In an actual system, the feedback link bandwidth is limited. Therefore, in FDD massive MU-MIMO systems, codebook-based Multi-user precoding scheme, in which codebook design and multi-user scheduling strategy, are particularly important.  
In the traditional MIMO system, the base station adopts a uniform linear array with a fixed down-tilt angle, and the wireless channel exhibits two-dimensional (2D) characteristics, that is, only the horizontal dimension freedom is available, which is called 2D-MIMO. However, in massive MIMO systems, base stations usually use 2D or 3D array antennas, which form the so-called 3D-MIMO technology [5; 6]. Due to the expansion of the antenna array structure, 3D-MIMO channels exhibit different spatial correlations [7; 8], and they also include horizontal and vertical dimension channel information. Therefore, 3D beamforming techniques can accurately align different levels of the dimensional sum. Thus, reduce the interference between users.  
This paper proposes a novel codebook design for 3D based MIMO system in heterogeneous network scenarios. According to the quantization error and statistical characteristics of the random distribution of the system, the influence of feedback overhead and base station density on system performance is analyzed using stochastic geometry. An improved 3D precoding scheme based on horizontal emission angle and vertical down-tilt is proposed. Through simulation analysis, the influence of feedback overhead on system throughput is compared, and the effects of traditional DFT precoding methods in 2D and 3D environments and improved 3D precoding schemes on system BER and throughput are compared. The simulation results show that the feedback overhead will reduce the system throughput and improve the system BER, while the improved precoding scheme due to the joint consideration of the horizontal emission angle and vertical down-tilt, thus effectively improving the system throughput and reducing the system BER, optimized system performance.  
The rest of the paper is organized as follows. Section 2 discusses the literature review. Section 3 explains the system model. Section 4 provides system performance analysis based on random geometric finite feedback. Section 5 provides the improved 3D codebook design concepts. Section 6 gives the simulations results and analysis while Section 7 concludes the paper.