# A Novel Multi-User Codebook Design for 5G in 3D-MIMO Heterogeneous Networks
## 6 Simulation Results
It can be seen from the simulation results that the 3D DFT precoding scheme can effectively improve the system throughput compared to the 2D DFT precoding scheme because it fully utilizes the vertical dimension of the UPA array. The improved 3D precoding scheme proposed in this paper jointly considers the horizontal adjustable range and the vertical down-tilt adjustable range, and considers the specific location of the antenna array elements rather than the UPA array as a whole, so it is more effective. The proposed codebook scheme controls inter-user interference at the edge of the cell to significantly increase system throughput. Figure 10 compares the spectral efficiency (SE) versus SNR of the proposed 3D codebook scheme with other state-of-the-art codebook schemes. As can be seen from Figure 10 that the proposed 3D codebook scheme outperforms the other codebook schemes which show its superior performance over other codebooks schemes. Moreover, the rate gap between the proposed scheme and other schemes increases with increasing SNR which means that the proposed 3D codebook scheme shows better SE performance in low as well as high SNR level. Such results cover the real-world aspect of the MIMO system deployment where the SNR changes due to certain channel impairments and there is a need for a flexible and robust codebook scheme which should provide better service in wireless communications systems. Figure 11 shows the impact of the inter-cell interference on the system using different codebook schemes. As can be seen from the Figure 11, when the number of users increases, the interference from different cells increases which causes communication interruption. Moreover, the proposed 3D codebook scheme shows have lowest Inter-Cell Interference (ICI) level which is much better as  
Figure 8: The relationship between the BER and SNR for the proposed precoding scheme and other schemes. compared with those of previous papers [24; 25] and 2D DFT schemes. Therefore, the proposed 3D codebook scheme also shows better ICI performance which obviously makes it an attractive candidate to deploy in real-world application scenarios. Figure 10: Spectral efficiency comparison versus SNR for the proposed 3D codebook scheme with other state-of-the-art schemes. Figure 9: The CDF of different precoding schemes versus throughput. Figure 12 compares a number of feedback bits required versus the number of users for the proposed 3D MIMO codebook and other state-of-the-art schemes. It is clear from the results that the proposed scheme shows efficient performance as it requires a smaller number of feedback bits for channel estimation whereas the other codebook schemes require large number feedback bits which indicates that such schemes have large feedback overhead. Therefore, such comparison proves that the proposed 3D MIMO codebook scheme is also cost-effective as we know that feedback bits directly impact the system implementation cost, thus the proposed scheme is time-efficient and cost-efficient. Figure 11: Comparison of the inter-cell interference versus a number of users for the proposed 3D codebook and other state-of-the-art schemes. Figure 12: Comparison of feedback bits versus the number of users for the proposed 3D codebook scheme and other state-of-the-art schemes. Figure 13 compares the spectral efficiency of the proposed 3D MIMO codebook scheme against the number antennas (\(N_{v}\)) and compares it with other schemes. As can be seen from Figure 13, when the number of antennas increases, the spectral efficiency increases for all codebook schemes. Furthermore, the proposed 3D DFT codebook scheme shows better performance than 2D and 3D DFT [11] codebook schemes. It is worth notable from the results that the proposed codebook scheme shows nearly optimal SE performance as its curve is closer to the ideal CSI performance. Therefore, such results show that the proposed codebook scheme have efficient performance in case of a large number of antennas.