Digital Signal Design and Nonlinear Distortions in Antenna Array Beamforming

Antenna array beamforming reduces multipath fading components, and decreases the complexity of equalizer in a single carrier receiver. Nonlinear analog parts cause, however, impairments in the beamforming. In this paper we show that when the separate antenna elements include nonlinear amplifiers with different amplitude modulation to amplitude modulation (AM-AM) and amplitude modulation to phase modulation (AM- PM) conversions, there are errors in the level of main beam and in the direction of the main beam, respectively. There are no impairments in the beamforming if there is phase noise at the input of the antenna array. Anyhow, the phase noise may dramatically distort the signal constellation. A common method to avoid nonlinear distortions is to use modulation which has constant envelope. This is not enough if the nonlinear amplifier has memory.