DELTA SIGMA MODULATION BASED DIGITAL TRANSMITTERS FOR UPCOMING WIRELESS COMMUNICATION

Abstract

This thesis aims to demonstrate the energy-efficient, frequency-agile advanced multi-band digital transmitter design using the delta-sigma modulation (DSM) technique. In this context, the thesis focused on four main directions. The first work of this thesis is to improve the coding efficiency (CE) of low pass delta-sigma modulation (LP-DSM) for single-band transmission without affecting the efficiency at the amplification stage. For this, an optimized multi-level delta-sigma modulation (OML-DSM) technique is investigated, which will improve the CE compared to conventional multi-level DSM. In OML-DSM, the position of quantization levels and their selection window range are analyzed as per the probability distribution of quantizer input. The proposed scheme significantly improves the CE of multi-level DSM. In order to maintain the efficiency at the amplification stage, a time-interleaved level splitting technique is proposed. This will convert the multi-level DSM output into bi-level signal streams so that the amplification stage always operates in the saturation region and gives high efficiency. In this work, a 10 MHz longterm evolution (LTE) signal is divided into 6-levels OML-DSM, and the level splitter splits these levels into three bi-level signal streams with different magnitude and power levels. Class-E power amplifiers (PAs) are used to amplify these bi-level signal streams. The PA outputs are combined using an unequal power combiner. The CE is improved by 18.7%, and channel efficiency is improved by 11.4% compared to conventional 6-level DSM. The second work of this thesis is the design of reconfigurable multi-band transmitter architecture for increasing the aggregated transmitted bandwidth. In this, the band pass delta-sigma modulation (BP-DSM) architecture is investigated. The zeros of the DSM noise transfer function (NTF) are reconfigured to change the positions of different carriers in the radio frequency (RF). The proposed scheme has been validated in the experiment for aggregating up to four 15 MHz LTE signals with an overall aggregated bandwidth of 60 MHz. The proposed scheme offers reconfiguration of all four bands to any frequency within the tuning range of ± fDSM /2 around the carrier frequency set by the quadrature upconverter. A complex DSM is also investigated for reconfigurable multi-band transmission. The proposed scheme is implemented for concurrent triple-band transmission of 5 MHz LTE signals. These IF frequencies can be reconfigured to any frequency in the frequency range 0 to fDSM/2, where fDSM is the sampling frequency of the DSM stage. A single RF chain is used in such multi-band transmitters, which will reduce the required hardware resources. The third work of this thesis is the development of a noise shaping technique to reduce the outof- band quantization noise for multi-band transmission. This work aims to improve the CE of the DSM technique without affecting the efficiency at the amplification stage so that the overall transmitter efficiency can be improved. For this, an augmented noise shaping (ANS) technique is investigated for reducing the out-of-band quantization noise for multi-band transmission. The proposed technique inserts multiple notches between the carriers and outside the bands. This will decrease the quantization noise level, and thus, CE improves. CE is enhanced by 10-25% and 15- 35 dB improvement in noise level near the operating frequency band by using the proposed scheme. The corresponding improvement of 8% in the overall efficiency is observed. The polar quantizer is also investigated for reducing the in-band and out-of-band quantization noise. The polar quantizer quantizes the magnitude of the signal and passes the phase quantization, which will help in reducing the generated quantization noise. The proposed scheme shows up to 12.4 dB improvement in signal to noise distortion ratio (SNDR) and 12.3% improvement in CE compared to the standard cartesian DSM architecture. The fourth work of this thesis is the implementation of the software-defined radio (SDR) based transmitter test-bed, which is implemented on a system on chip (SoC) platform. The implemented transmitter test-bed can be used for multi-standard wireless applications. For achieving appropriate linearity, a memory polynomial-based real-time adaptive digital predistortion (ADPD) system is designed. A 20 MHz LTE signal is used for measurement purposes. The LTE signal is amplified using a gallium nitride (GaN) based power amplifier in measurement. The proposed ADPD system improves the normalized mean square error (NMSE) from -5.65dB to -30.29dB. The left and right adjacent channel power ratio (ACPR) improvement is 13.8 and 13.9 dBc.