Millimeter wave (mmWave) experiences a high path-loss in free space. To overcome this weakness, antenna array is used to obtain a high eamforming gain. However, due to high complexity of digital beamforming systems, the hybrid structure consisting of baseband and analog precoder has been proposed. We consider the well known block diagonalization (BD) precoding for transmission technique. Because of the practical constraints such as channel estimation error, the limited number of radio frequency (RF) chains and the quantized analog phase shifters, the BD precoder cannot provide accurate interferencefree effective channel. In this paper, we propose the extended BD precoding method which extends the null space of interference channels to reduce the effect of practical constraints. Simulation results show that the proposed method provides much improved sum rate and error performances than original BD precoder.