4.3 A Multiphase Interpolating Digital Power Amplifier for TX Beamforming in 65nm CMOS

Future 5G communications will heavily leverage beamforming and MIMO techniques, owing to the improvement in data transmission capacity. In a transmit (TX) beamformer, the phase and amplitude of an array of TXs can be arbitrarily adjusted to spatially steer a transmitted beam toward a user or multiple users. The amplitude and direction of the beam is adjusted by controlling the gain and phase of each TX in the array. Gain is typically adjusted with a variable-gain amplifier in the driver chain. In recent beamforming TXs, four primary means to control the output beam angle have been leveraged. Passive RF phase shifting is bulky and lossy, owing to high losses in the passive elements, and is difficult to design to provide phase control through 360 ° . LO phase shifting often uses a single passive phase shifter or a multiphase ring oscillator, but these devices typically provide lower phase resolution. Digitally controlled delay lines provide a wider bandwidth response but suffer from reduced phase resolution as frequency is increased and can consume high power [1]. Digital phase-shifting techniques have recently shown promise in polar systems [2, 3], but the quadrature modulators, typically used for phase shifting, incur high loss if placed directly at the output stage.