Viterbi Faculty of Electrical Engineering, Technion
EFFICIENCY ENHANCEMENT OF DIGITAL BEAMFORMING PHASED ARRAYS
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 and polar digital transmitters. These techniques show promising solutions at low frequency and moderate data rates of tens of MHz, but become ineffective at mm-wave and for Gb/s data rates. Another well-known technique is the Doherty power amplifier, which provides improved efficiency at power back-offs. However, its implementation in CMOS is challenging, especially at mm-wave frequencies, due to the insertion losses of integrated passive components needed for combining the main and auxiliary amplifiers. A derivative of the Doherty PA is the Sequential PA, which uses a broadband combining network, but at the cost of additional efficiency degradation.
Several attempts to increase the transmitter efficiency at mm-wave have been reported. A prevalent approach is to combine the different data streams over-the-air instead of on-die, 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 (UELA).
This thesis presents a new phased-array transmitter for improving the transmit efficiency of broadband signals with high PAPR, operating at sub 6 GHz and the 28 GHz, which are employed as part of the fifth-generation mobile network bands. The proposed phased array utilizes sequential over-the-air combining of weighted power amplifiers to increase the system efficiency without any die 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 the PA class being selected. Furthermore, we include 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. The phased array's theoretical and experimental results advance the state-of-the-art in the research of over-the-air combining techniques and is a competitive candidate for an integrated CMOS solution in next-generation new-radio communication systems.
* Ph.D. Under the supervision of Prof. Emanuel Cohen.
Zoom link: https://technion.zoom.us/j/92754087402
Wed 24 Feb 2021
Start Time: 11:00
End Time: 12:00
zoom meeting | Electrical Eng. Building