From Noise-Shaped Coding to Energy Efficiency - One bit at the time

Sammanfattning: Three parameters that drive the research and development of future RF transmitter technologies for high speed wireless communication today are energy efficiency, flexibility and reduction of the physical footprint. This thesis treats the use of single-bit quantization in conjunction with a method called Noise-Shaped Coding (NSC), as an enabler for these parameters, foremost in terms of energy efficiency. The first part of the thesis provides a short introduction to the common Radio Frequency Power Amplifier (RFPA) power efficiency enhancement techniques. The pulsed RF transmitter is introduced in which the RFPA is used as a switch, modulated by a single-bit quantized signal which allows it to operate solely at its two most efficient states. The second part of the thesis provides an introduction to the concept of NSC and the underlying idea of how high signal quality can be achieved with one bit quantization of the signal amplitude. A particular method of implementing NSC, namely the ΣΔ-modulator, is introduced and some common methods for design and analysis are discussed. An optimization-based approach to ΣΔ-modulator design is proposed and benchmarked against conventional methods in terms of its ability to shape the power spectral density of the quantization noise according to a given reconstruction filter response, minimizing the reconstructed error metric. The third and final part of the thesis focuses specifically on the application of ΣΔ-modulation in a pulsed RF transmitter context. The concepts of band-pass and baseband ΣΔ-modulation are introduced. A few important challenges related to the use of ΣΔ-modulation in a pulsed RF-transmitter context are identified. A ΣΔ-modulator topology which handles a complex input signal is investigated in great detail and advantages compared to conventional methods for using ΣΔ-modulation are unveiled by means of theoretical analysis and simulations. A method for suppressing the quantization noise within a frequency band surrounding the modulated RF carrier, enabling the use of more wideband reconstruction filter and moderate pulse-rates, is also presented. A detailed theoretical analysis reveals how optimized Noise-Shaped Coding, as provided by the optimization method introduced in the second part, can be deployed in order to improve the system performance. Finally, the method is validated by experimental measurements on two different high efficiency RFPAs at 1 and 3.5 GHz respectively, showing promising results.