Sub-Nyquist receiver for digital predistortion of RF power amplifiers
- Digital predistortion (DPD) is an essential building block of modern wireless base stations: it linearizes the radio frequency (RF) power amplifier (PA) such that the transmitter can be operated efficiently. A crucial ingredient to a DPD system is the feedback receiver which includes an analog-to-digital converter (ADC) which digitizes the PA output signal. Ever increasing signal bandwidths push conventional DPD receivers sampling at the Nyquist rate to their practical and economical limits. This issue is exacerbated in emerging technologies like 5G and beyond where the trend towards a denser grid of pico- and femto cells demands efficient and inexpensive components. Furthermore, DPD systems employing conventional Nyquist-rate feedback receivers do not allow efficient implementation of transmitter arrays (massive MIMO) due to the enormous bit rates. This thesis presents a framework for nonlinear system identification which allows to obtain the PA model with fewer samples than conventional methods. Furthermore, it is shown that the sampling rate as well as the ADC acquisition bandwidth is irrelevant for obtaining the PA model. To my knowledge, this is the first method which decouples the requirements of the feedback ADC from the input signal bandwidth. The presented framework is then used to implement a feedback receiver prototype for DPD of RF PAs. In contrast to conventional architectures, the receiver samples in the frequency domain. By randomly selecting a frequency bin in each sampling interval, the PA model is identified with substantially lower acquisition bandwidth and sampling rate. The receiver prototype is implemented in 28nm FD-SOI technology and samples at 4MS/s instead of the full Nyquist rate of 168MS/s for a 20MHz Long Term Evolution (LTE) signal with spectral regrowth. The prototype is shown to linearize a 4W small base station power amplifier from an adjacent channel leakage ratio (ACLR) of −35dBc to −47.7dBc after processing only about 250 samples. It is to my knowledge the first sub-Nyquist DPD feedback receiver implemented on an integrated circuit (IC).
|Type of resource
|electronic resource; remote; computer; online resource
|1 online resource.
|Degree committee member
|Degree committee member
|Stanford University, Department of Electrical Engineering.
|Statement of responsibility
|Submitted to the Department of Electrical Engineering.
|Thesis Ph.D. Stanford University 2019.
- © 2019 by Nikolaus Hammler
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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