Fourier-domain data-converters : new concepts for high data rate wireless transmitter systems

Hanay, Oner; Negra, Renato (Thesis advisor); Ascheid, Gerd (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

The burgeoning demand on large modulation bandwidths pushes the recent mobile communication standards towards mmW frequencies. Even at these frequencies a high spectral efficiency is crucial in order to fully exploit the available channel capacity. State-of-the-art transmitter (Tx) topologies face challenges when employed for large modulation bandwidths in the range of several gigahertz. Because of the serialized high-speed data stream, the digital signal processing requires extremely high sampling rates for spectral shaping by oversampling and FIR filtering which sets a practical technological limitation. Besides, the digital-to-analogue conversion also takes place at this high sampling rate. In order to address these fundamental challenges, more advanced and expensive CMOS processes are required. Moreover, high sampling rates are associated with large power consumption and more complex circuits. Since 5G and beyond communication is aiming at a substantially increased data rate of up to 1Tbit/s, evolution cannot rely primarily on technological advances, also because CMOS scaling is coming to an end. Innovation on both conceptual as well as architectural level are thus necessary to meet the ambitious goals. The goal of this thesis is to introduce a distinct data-converter concept, the Fourier-domain digital-to-analogue converter (FDDAC), demonstrated in a novel Tx architecture to overcome the limitations and bottlenecks in the analogue and the digital domain in typical Txs. The FDDAC concept exploits the relation between the discrete-time and inverse continuous-time Fourier transform to generate a wide modulation bandwidth at virtually any frequency while reducing the sampling rate of the DSP and the data converters by up to two orders of magnitude. At the same time, without additional digital and analogue filtering, the FDDAC provides inherent spectral shaping. In the proposed FDDAC-based Tx the entire signal processing and analogue/mixed-signal circuits operate far from today’s technological limitations while providing very wide modulation bandwidths, i.e. in the range of multiple gigahertz. This makes the proposed concept an enabling technology for extremely high data-rate future wired and wireless communication. The proposed data conversion approach is thoroughly analysed and employed in a transmitter prototype which is modelled in order to understand its capabilities and limitations. A first lab validation prototype is implemented based on commercially available off-the-shelf discrete components that demonstrated a modulation bandwidth of up to 100MHz at a maximum sampling rate of 25MSps achieving 400Mbit/s. Based on the gained practical experience, three integrated transmitters in a 65nm CMOS technology are implemented. Due to the architectural advances introduced by the FDDAC technique, a modulation bandwidth of up to 2GHz is targeted while the highest sampling rate in the Tx is 250MSps. Thus, the complete DSP including pseudo-random bit generation, modulation, FFT calculations and spectral shaping is cointegrated with the I/Q transmit cores and phasor tone synthesisers on a single die. The I/Q transmit cores are implemented based on 8bit I/Q RFDACs in the first two design iterations whereas the third design contains power efficient 9bit I/Q DACs, passive mixers and output amplifier buffers allowing a 18dB higher output power while reducing the power consumption and increasing the signal quality. The implemented Txs iteratively improve the demonstrated data rate from 2 to 8 Gbit/s while reducing the power consumption by a factor of 2 each iteration. The measured EVM values are 7.2% and 11.8% for a 1 and 2GHz QPSK signal, respectively, whereas they are 9.6% and 13.9% for the 1 and 2GHz-wide 16QAM signal. The generated transmit signal fully complies with the spectral mask defined in the IEEE WiGig standard while surpassing the defined modulation bandwidth by 14%. Furthermore, the same hardware is used to replace a multitude of simultaneously operating conventional Txs which is demonstrated by generating 16 31.25MHz modulated signals with a constant frequency spacing. The measured EVM values are as low as 2.2% and 2.5% for QPSK and 16QAM modulated signals, respectively. Finally, two mmW upconversion mixers are implemented in the same 65nm CMOS technology demonstrating an outstanding conversion gain and operation bandwidth allowing to shift the output of the implemented transmitters to the 5G 28GHz and 60GHz ISM bands. The proposed transmitter architecture has the potential to replace a multitude of transmitters in mobile communication devices while allowing substantially higher data rates at significantly reduced power consumption. Thus, the cost and complexity of handheld communication devices can be reduced remarkably by allowing the implementation of true multistandard transmitters. The developed concept will enable new consumer experience in applications such as wireless virtual reality, beyond 5G wireless communications, and extremely low latency high data rate applications. The demonstrated advances in combination with the presented vision have the potential to form beyond 5G communication standards and change the entire class of mobile communication devices.

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