In order to solve the problem of limited capacity in traditional MIMO system

the 3D MIMO system model is introduced in this paper.The multicell coordinated transmission technology is applied to the 3D MIMO system to improve the system capacity.According to the Shannon theory

we derive the expression of signal to interference plus noise ratio in single-cell transmission and multi-cell joint transmission

respectively.Then we adopt the Zero Forcing（ZF）precoding algorithm

and analyze theinfluences of the base station antenna down tilt

the distance between the user and the base station

the number of users for the system transmission rate.Simulation results show that appropriate adjustments to the base station antenna tilt can improve the transmission rate of the system.It is also verified the multicell coordinated transmission has better performance in increasing the throughput of the system

whencompared to the single-cell transmission.