Ph.D Thesis


Ph.D StudentSayag Avraham
SubjectEfficiency Improvement of Digital Beam Forming Array
DepartmentDepartment of Electrical and Computer Engineering
Supervisor ASSOCIATE PROF. Emmanuel Cohen


Abstract

The increasing demand for low-cost Gb/s data rates has been steadily pushing the development of high-quality links utilizing compact silicon-based phased-array antennas at millimeter-wave (mm-wave). However, current mm-wave radio transmitters suffer from low energy efficiency, caused mainly by the reduced performance of CMOS power amplifiers (PA) at mm-wave frequencies. In emerging mm-wave systems, the PA efficiency drops even further due to the signals’ high envelope peak-to-average-power-ratio (PAPR) that dictates operation at deeper back-off power needed for linearity.


To address the increasing demand for higher PA efficiency, different techniques have been studied, such as Envelope Tracking, Polar digital transmitters and Doherty PA. These techniques show promising solutions at low frequencies and moderate data rates but become ineffective at mm-wave and for Gb/s data rates. due to the insertion losses and isolation of integrated passive components needed for combining, especially at CMOS implementation.


Several attempts to increase the transmitter efficiency at mm-wave on the system side have been reported. The prevalent approach is to combine the different data streams over-the-air instead of on-chip, taking advantage of the multiple transmitters and small-size antennas of mm-wave systems. Our analysis suggests that these transmitter efficiency improvements come at the expense of the antenna gain. As a result, the overall system performance (transmitter-receiver link) is even poorer compared to the classical uniformly excited array.


This thesis presents a new phased-array transmitter for improving the transmit efficiency of broadband signals with high PAPR, operating at the Ka-band, which is employed as one of the fifth-generation mobile network bands. The proposed phased-array utilizes Doherty (sequential) over-the-air combining of weighted PAs to increase the system efficiency without any chip area nor antenna size penalties. The baseband transmit signal is decomposed into a low duty-cycle signal comprising only the signal peaks and a low PAPR signal containing the remaining envelope levels. The two signals are transmitted through different chains, each optimized for a different power level and recombined over-the-air at the receiver side.


The research includes a comprehensive theoretical study of how to optimize the system, as well as on the impact of the over-the-air technique on the phased-array properties such as the directivity, side-lobe-level, and spectral emission characteristics. The thesis also deals with the implementation of nonidealities such as imbalances between the different channels, the impact of the chain’s transfer function on the system performance, and the implications of PA class being selected. Furthermore, the study includes a comprehensive comparison between other state-of-the-art over-the-air efficiency enhancement techniques based on theoretical analysis and measurements and demonstrate that the proposed technique has the highest efficiency enhancement capabilities and the lowest sensitivity to implementation nonidealities and mismatches.


For the experimental validation, two phased-arrays were designed and fabricated. The first array operates around the 5 GHz band is constructed from discrete PA elements fabricated using TSMS 180 nm. The second array operates around the center frequency of 28 GHz and is fully integrated on chip using TSMC 65 nm. The theoretical and experimental results of both phased-arrays advance the state-of-the-art in the research of over-the-air combining techniques and are therefore competitive candidates for integrated CMOS solutions in next-generation new-radio communication systems.