Digital Predistortion Concepts for Linearization of mmW Phased Array Transmitters

Abstract

Linearization of millimeter-wave (mmW) phased arrays is one of the key enablers for improving the system performance in terms of power, efficiency and linearity. However, phased array transceiver topologies that have multiple parallel nonlinear components with a shared digital input challenge the standard digital predistortion techniques. In addition, different analogue beamforming techniques complicate the linearization even further due to the fact that the signal nonlinearity has to be observed or modelled over-the-air (OTA) together with the impacts of antennas and even the directive mmW radio channel. The best linearization strategy depends on the system level targets of linearity such as error vector magnitude and adjacent channel power ratio which have slightly different nature when observed in the radiated far-field. In this paper, we present our view and the status of the literature on the topic of phased array digital predistortion. We highlight that the nonlinear distortion have a beam shape which may be different from the linear part of the beam. We also review the antenna array figures of merit describing the nonlinearity. Finally, we show an experimental example of OTA linearization of a 28 GHz phased array transmitter.

Digital Predistortion of Phased-Array Transmitter With Shared Feedback and Far-Field Calibration

“…[4]- [7], [9]- [11]), and over-the-air (OTA) (e.g. [3], [8], [12]- [17]) FB methods. The OTA methods provide low-complexity ways to measure the radiated response.…”

“…In an antenna array, the nonlinear distortion can also be beamformed and it may even have different beam pattern compared to the linear part of the signal as discussed in [4], [11]. This is due to the fact that the nonlinear signal radiated by each antenna is different due to the varying PA characteristics [3], [11], beamforming [4], active load pulling from the neighbouring elements due to the finite antenna isolation [23], [24], and antenna impedance variations [25]. The frequency domain waveform samples radiated by the antenna array to each spatial direction can be characterized as…”

“…High-order modulations and orthogonal frequency division multiplexing (OFDM) waveform set stringent linearity requirements for the phased array transmitter (Tx) in order to achieve low error vector magnitude (EVM). On the contrary, the adjacent channel power ratio (ACPR) specifications for 5G FR2 Tx are relatively loose [1], [3]. One way to improve the Tx linearity is to use digital predistortion (DPD).…”

Digital Predistortion of Millimeter-Wave Phased Array Transmitter With Over-the-air Calibrated Simplified Conductive Feedback Architecture

“…[4]- [9], [10]- [11]), and over-the-air (OTA) (e.g. [3], [8], [12]- [14]) FB methods. OTA methods give low-complexity ways of measuring the radiated response.…”

“…High-order modulations and orthogonal frequency division multiplexing (OFDM) waveform set high linearity requirements for the phased array transmitters (Txs) in order to achieve low error vector magnitude (EVM). On the contrary, the adjacent channel power ratio (ACPR) specifications for 5G FR2 Tx are relatively loose [1], [3]. One way to improve the Tx linearity is to use digital predistortion (DPD).…”