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Title:Vector modulators for analog-beamforming receivers
Author(s):Tseng, Richard Y.
Director of Research:Chiu, Yun
Doctoral Committee Chair(s):Chiu, Yun
Doctoral Committee Member(s):Poon, Ada S.; Feng, Milton; Schutt-Ainé, José E.; Shanbhag, Naresh R.
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Vector modulator
radio frequency
sigma delta
Abstract:This work focuses on new architectures for analog-beamforming receivers. In recent years, the drive for wireless communication techniques that can accomodate more users, provide higher data rates, and facilitate more reliable connections has generated a great amount of interest in multiantenna beamforming receivers. The high component count and stringent circuit block dynamic range (DR) requirements required in digital-beamforming implementations make analog approaches particularly attractive. Analog-beamformers use vector modulators (VMs) to phase-shift and scale the amplitudes of the signals from each antenna before signal combining. These VMs require accurate phase and gain control and high DR to achieve accurate beamforming and avoid corruption of the desired signal by blockers. At the same time, degrading transistor amplification characteristics and low supply voltages at deeply scaled CMOS nodes limit the complexity of analog circuit blocks, making the simultaneous achievement of all of these objectives extremely challenging. In this work, we address these issues by shifting the burden of synthesizing complex gains to the architecture level, thereby allowing for the simplification of core analog blocks in the receive path. We demonstrate this concept with the phase-oversampling and outphasing VM architectures. The phase-oversampling VM applies the mathematical concept of overcomplete expansion and coarse quantization to replace the complex radio-frequency variable-gain amplifiers used in conventional VMs with simple switches to linearize the receive path. To demonstrate the feasibility of this architecture, a 4-GHz, four-channel, analog-beamforming direct-conversion down-converter fabricated in 90-nm CMOS is presented. A bank of passive mixers driven by a multiphase local oscillator signal in each VM performs accurate phase-shifting with minimal signal distortion, and transimpedance amplifiers combine the mixer outputs to perform beamforming weighting and combining. Each individual channel achieves accurate complex gain control, enabling the beamformer to achieve precise beamforming and blocker cancellation. This work concludes with a discussion of the outphasing VM architecture, which adapts the phase-based modulation architecture from outphasing power amplifiers to beamforming VMs. System level analysis shows that this architecture achieves greater complex gain accuracy and gain range than its component phase-shifters while removing circuit complexity from the signal path, thereby maximizing receiver DR.
Issue Date:2010-05-14
Rights Information:Copyright 2010 Richard Y. Tseng
Date Available in IDEALS:2010-05-14
Date Deposited:May 2010

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