Digital Transmitters

Abstract

Over the last few decades, mobile data traffic has increased exponentially, and this growth is expected to continue with the further rollout of 5G and the anticipated rise of 6G wireless networks. To support these upcoming networks in a sustainable and economically viable manner, their costs and energy usage need to be dramatically lowered. In view of this, driven by the ever-increasing speed and functionality of digital CMOS, the interest in fully digital transmitter (DTX) solutions has sharply risen, as it can offer higher system integration and functionality at lower costs. However, most DTX research reported so far has focused on single-chip solutions, which are limited in their RF output power due to the low breakdown voltage of the CMOS technologies they rely on. This blocks DTX implementations from achieving the power and efficiency levels that are required for the next generation massive-Multi-Input-Multi-Output (mMIMO) base stations, which will be employed in upcoming 5G and 6G wireless networks.

The work described in this dissertation is part is the research projects DIPLOMAT and DRASTIC, which target the implementation of high-power digital transmitters. As such, they introduced a new dual-chip DTX approach that features a CMOS controller, which is high-density flip-chip interconnected to an RF-power LDMOS MMIC. This LDMOS MMIC contains switch banks with hundreds of gate segments which can be individually controlled by the CMOS controller. The resulting combination enables high-resolution RF power DTX operation, which is demonstrated in this dissertation. Following this approach, higher DTX system efficiencies and modulation bandwidths come within reach. Although being part of a bigger research activity, the prime and unique focus of this dissertation is the signal processing aspects of the digital transmitter, with the main research question:

“How can we control a segmented digital transmitter output stage such that the optimum RF waveform is created that is capable of supporting wideband modulation, with high spectral purity and efficiency?”

To answer this question, this dissertation provides a comprehensive overview and analysis of the various DTX architectures in the literature. Existing shortcomings of DTX approaches are identified, and where needed, new (DTX signal processing) techniques are proposed to diminish or overcome them. In conclusion, a totally new DTX upconversion technique is proposed, which allows full control over the dc, fundamental, and harmonic content of the RF output waveform, as such enabling the selection of the optimum trade-off between energy efficiency and wideband spectral purity for a given application.

This dissertation is divided in three parts; the first part focusses on digital RF current waveforms, their power utilization and efficiency, the second part discusses the dynamic behaviour of RF-mixing-DACs, and the third part introduces the aforementioned new DTX upconversion technique.