Concepts for radiated nonlinear distortion and spatial linearization in millimeter-wave phased arrays

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Auditorium L6, Linnanmaa

Topic of the dissertation

Concepts for radiated nonlinear distortion and spatial linearization in millimeter-wave phased arrays

Doctoral candidate

Master of Science (Tech) Nuutti Tervo

Faculty and unit

University of Oulu Graduate School, Faculty of Information Technology and Electrical Engineering, Centre for Wireless Communications - Radio Tehnologies

Subject of study

Radio Engineering

Opponent

Professor Christian Fager, Chalmers University of Technology

Custos

Professor Aarno Pärssinen, University of Oulu

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Concepts for radiated nonlinear distortion and spatial linearization in millimeter-wave phased arrays

This thesis presents concepts for understanding and utilizing the spatial behavior of nonlinear distortion in a millimeter-wave multi-antenna transmitter. The nonlinear distortion caused by radio frequency power amplifiers (PAs) is studied in phased array systems that use analog beamforming. In particular, the focus is on the scenarios in which also the amplitudes of individual paths are controlled to shape the beam pattern. One of the main findings is that the differences in the nonlinear behavior of the individual paths make the beams of the signal and the distortion differ from each other. This observation is one of the keys for understanding how the phased arrays can be linearized: In phased array systems, multiple PAs share the same digital input and hence the PA linearization by digital predistortion (DPD) turns into an array linearization problem.

The linearization of phased array transmitter can be performed by modeling, emulating, or measuring the transmitter radiated response in various ways. This thesis discusses different conductive and radiated feedback receiver approaches and proposes procedures for training the DPD objective based on the measurements. It is shown that linearizing the array in the main lobe does not mean linearization of individual PAs if the transmit paths differ from each other. In such scenarios, it is shown that linearizing the signal in the main lobe creates a notch in the training direction that is a result of over-the-air combining different nonlinear components. This phenomenon can be beneficial when improving linearity in a single direction, but the problem is that it also spreads the distortion to other directions that may be harmful for systems operating in adjacent frequency bands. It is proposed that such a problem can be tackled by modifying the nonlinearity of individual paths by PA bias control to make the distortion of the PAs more similar. This is shown to simplify the DPD trained in a certain steering angle to be effective also in other directions while achieving decent power efficiency.

In addition to the nonlinear distortion analysis, the thesis work discusses also briefly other radio frequency nonidealities and present and approach how they could be better considered in the system design. Also, the nonlinear distortion of the receiver array is briefly analyzed, but the main emphasis is on the transmitter side.


Last updated: 9.2.2022