Clipping effect on radiation pattern in downtilt beamforming

In this paper, we investigate the features of radiation pattern in downtilt beamforming (a user-specific vertical beamforming) when applying the clipping technique at each antenna to reduce the peak-to-average power ratio problem. First, we define and analyze the signal and noise radiation pattern in one-beam downtilt, and then prove that the clipping noise in one-beam downtilt systems has the same radiation pattern as that of the original signal but changes in two-beam downtilt systems. Next, we split the clipping noise into two parts in two-beam downtilt, desired noise and undesired noise, and demonstrate that desired noise produces the same radiation pattern as that of the original signal while undesired noise creates undesired radiation pattern, which causes unexpected noise pattern in two-beam downtilt. Simulation results show that the analysis well matches the numerical results.