System-Level Design of All-Digital LTE / LTE-A Transmitter Hardware

This thesis presents a detailed system-level analysis of an all-digital transmitter hardware based on the Direct-Digital RF-Modulator (DDRM). The purpose of the presented analysis is to evaluate whether this particular transmitter architecture is suitable to be used in LTE / LTE-A mobile phones. The DDRM architecture is based on the Radio Frequency Digital-to-Analog Converter (RF-DAC), whose system-level characteristics are investigated in this work through mathematical analysis and MATLAB simulations. In particular, a new analytical model for the timing error in the distributed upconversion is developed and verified. Moreover, this thesis reviews the LTE and LTE-A standards, and describes how a baseband environment for signal generation/demodulation can be implemented in MATLAB. The presented system enables much more flexibility with respect to current commercial softwares like Agilent Signal Studio. Simulation results show that the most challenging specification to meet is the out-of-band noise floor, because of the stringent linearity and timing requirements posed on the RF-DAC. This suggests that new means of reducing the out-of-band noise in all-digital transmitters should be researched, in order not to make their design more complicated than for their analog counterpart

La distorsione di memoria negli amplificatori

La “memory distortion” è un fenomeno che si verifica nei circuiti audio, in modo particolare negli amplificatori, consistente nella distorsione del segnale a causa di effetti di memoria che si verificano all'interno del circuito. Questo testo mette in luce le cause del fenomeno, tenta di spiegare in che modo esso può influire sulla qualità di un amplificatore audio e, infine, suggerisce un possibile metodo di misura della distorsione di memoria e alcune soluzioni circuitali che la riducon

Integrated High-Speed DSP for All-Digital RF Transmitters

During the recent years, the design of integrated RF transceivers has been shifting towards the digital domain. There are two main motivations behind this change. First, the reconfigurability needed in 4G and 5G wireless communication calls for A/D and D/A conversion as close to the antenna as possible. Second, the advance of deep-submicron CMOS processes poses new design challenges for traditional analog topologies, whereas digital circuits typically benefit from decreasing linewidth and supply voltage.  This dissertation presents advances related to all-digital RF transmitters, with special focus on the cartesian and outphasing architectures. Specifically, this work attempts to extend the share of transmitter functionality that is implemented using integrated digital signal processing (DSP). The main motivation in the research of DSP-based solutions is that, besides exploiting all the benefits of nanoscale CMOS, they also take advantage of highly automated standard design methodologies, thus enabling straightforward design reusability. The research work is demonstrated with two integrated circuit (IC) implementations and seven scientific publications.  In the context of all-digital cartesian transmitters, this dissertation focuses on the replacement of the traditional analog filters for D/A reconstruction and out-of-band emission attenuation by means of DSP circuits. The D/A reconstruction filter is replaced by a programmable interpolation chain, which is specifically optimized for 4G mobile transmitters. Furthermore, a new DSP technique based on delta-sigma modulation and mismatch-shaping is proposed for receive band noise attenuation. The latter technique is experimentally verified for a prototype 4G transmitter IC fabricated in 28nm CMOS, with measurement results demonstrating up to 20 dB noise attenuation at a programmable 30-400 MHz duplex distance.  All-digital outphasing transmitters push the D/A conversion even closer to the antenna, by utilizing time-domain processing of rail-to-rail signals up to the power amplifiers. This dissertation presents a new delay-line phase modulator architecture, which improves the modulation linearity by performing DSP-intensive first-order hold phase interpolation. Measurement results on a prototype multilevel outphasing transmitter IC, fabricated in 28nm FDSOI CMOS, demonstrate that this concept enables up to 400 MHz instantaneous RF bandwidth, which is a 10x improvement compared with the state-of-art

The synthesis of noise transfer functions for bandpass delta-sigma modulators with tunable center frequency

This paper presents a method to synthesize the noise transfer function (NTF) for tunable bandpass delta-sigma modulators, where the quantization noise stopband can be programmed over the whole Nyquist range. Instead of relying on traditional filter design theory, the proposed method allows to create NTFs of arbitrary order by directly placing the zeros and poles on the z-plane. The advantage is that the NTF can be re-calculated for each center frequency by using simple closed form expressions, thus avoiding the need of large lookup tables to store multiple pre-computed coefficient sets. Extensive system-level simulations show that our method yields equal performance as the Chebyshev-II design method. As an example, the synthesis of a binary 12th-order tunable bandpass delta-sigma modulator is demonstrated, and its stability is proven for any choice of the center frequency.Peer reviewe

Dynamic element matching in digital-to-analog converters with code-dependent output resistance

This paper evaluates the pertormance ot dynamic element matching (DEM) in digital-to-analog-converters, when the unit conversion cells of the converter have finite output resistance. DEM is already known to be effective against the static amplitude, timing and pulse shaped mismatches. However, the effect of output resistance and its mismatches has not been studied. A comprehensive code-dependent output resistance model for the current-steering DAC is presented. System level simulations show that the non-linearity caused by the output resistance, in the absence of mismatches, is not shaped by the DEM encoder since the output resistance is same for all the conversion cells. In this paper we demonstrate that, in the presence of mismatches, the DEM encoder is able to shape the non-linearity they cause since the output resistance now varies among different conversion cells.Peer reviewe

RX-Band Noise Reduction in All-Digital Transmitters With Configurable Spectral Shaping of Quantization and Mismatch Errors

Peer reviewe

Tri-Phasing Modulation for Efficient and Wideband Radio Transmitters

| openaire: EC/H2020/704947/EU//ADVANTAG5In this paper, we show that amplitude transitions that are inherent to the multilevel outphasing radio transmitter architecture distort the transmitted signal due to time-domain discontinuities. In order to address this challenge, we propose a new transmitter architecture called tri-phasing which avoids discontinuities in signal waveforms and thus achieves significantly better linearity than multilevel outphasing. The output waveform in tri-phasing can be made continuous by representing the baseband signal with three components. One of the three components is amplified by discrete amplitude steps, whereas the other two are used to compensate the instantaneous shift in the output waveform due to the discrete amplitude step and to provide fine amplitude resolution. An implementation of the tri-phasing transmitter requires three phase modulators and additional digital signal processing. The system-level simulations performed in this paper demonstrate that the ACLR of a multilevel outphasing transmitter with 4 amplitude levels and 10-bit phase resolution is limited to -48 dBc, when simulated with a 100 MHz carrier-aggregated LTE downlink signal at 2.46 GHz carrier frequency. The proposed tri-phasing transmitter achieves -58 dBc ACLR with the same simulation parameters, indicating that continuous amplitude transitions can significantly improve the transmitter linearity.Peer reviewe

Digital Interpolating Phase Modulator for Wideband Outphasing Transmitters

Radio transmitters are evolving towards digital-intensive solutions to exploit reconfigurability and benefit from CMOS process scaling. Outphasing has been identified as a suitable candidate for digital wideband transmitters. However, with recent digital-intensive outphasing transmitters the achieved performance in terms of adjacent channel leakage ratio (ACLR) has been limited. This paper identifies the sampling images of the modulating phase signal as the main factor limiting the ACLR of digital outphasing transmitters. We present a new digital interpolating phase modulator architecture, capable of providing significantly better sampling image attenuation. When evaluated in outphasing configuration with a 100 MHz OFDM signal at the carrier frequency of 2.46 GHz, and 10-bit phase resolution, the proposed solution achieves an ACLR of −59 dBc, compared to −43 dBc achievable with the phase modulator architecture utilized in state-of-the-art digital outphasing transmitters. The proposed digital interpolating phase modulator is also capable of custom carrier generation, a straightforward method for generating an arbitrary carrier frequency up to 1.25 times the phase modulator sampling rate.Peer reviewe

Spectral Effects of Discrete-Time Amplitude Levels in Digital-Intensive Wideband Radio Transmitters

This paper examines one source of spectral degradation in polar and multilevel outphasing transmitters. The degradation is caused by the amplitude signal appearing at the transmitter output as a baseband component, in addition to the desired RF signal. This baseband component contains sampling images and quantization noise across the spectrum. Thus, it adds noise at the signal band where it cannot be filtered and limits the achievable ACLR, particularly in wideband LTE and 5G systems. We analyze the origin of this phenomenon and related effects of system and signal parameters, and propose three design solutions for eliminating or alleviating the problem. Our analysis and simulations demonstrate that using a voltage-subtracting power combiner cancels the described degradation, potentially leading to significant improvement in spectral performance.Peer reviewe