213 research outputs found
DSP compensation for distortion in RF filters
There is a growing demand for the high quality TV programs such as High Definition TV (HDTV). The CATV network is often a suitable solution to address this demand using a CATV modem delivering high data rate digital signals in a cost effective manner, thereby, utilizing a complex digital modulation scheme is inevitable. Exploiting complex modulation schemes, entails a more sophisticated modulator and distribution system with much tighter tolerances. However, there are always distortions introduced to the modulated signal in the modulator degrading signal quality.
In this research, the effect of distortions introduced by the RF band pass filter in the modulator will be considered which cause degradations on the quality of the output Quadrature Amplitude Modulated (QAM) signal. Since the RF filter's amplitude/group delay distortions are not symmetrical in the frequency domain, once translated into the base band they have a complex effect on the QAM signal. Using Matlab, the degradation effects of these distortions on the QAM signal such as Bit Error Rate (BER) is investigated.
In order to compensate for the effects of the RF filter distortions, two different methods are proposed. In the first method, a complex base band compensation filter is placed after the pulse shaping filter (SRRC). The coefficients of this complex filter are determined using an optimization algorithm developed during this research. The second approach, uses a pre-equalizer in the form of a Feed Forward FIR structure placed before the pulse shaping filter (SRRC). The coefficients of this pre-equalizer are determined using the equalization algorithm employed in a test receiver, with its tap weights generating the inverse response of the RF filter. The compensation of RF filter distortions in base band, in turn, improves the QAM signal parameters such as Modulation Error Ratio (MER). Finally, the MER of the modulated QAM signal before and after the base band compensation is compared between the two methods, showing a significant enhancement in the RF modulator performance
Compensation of fibre impairments in coherent optical systems
Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 201
Carrier Synchronization in High Bit-Rate Optical Transmission Systems
In this dissertation, design of optical transmission systems with differential detection and coherent detection is briefly described. More over, algorithms for carrier synchronization and phase estimation with their implementation in high bit-rate optical transmission systems are proposed
Digital electronic predistortion for optical communications
The distortion of optical signals has long been an issue limiting the performance of
communication systems. With the increase of transmission speeds the effects of distortion
are becoming more prominent. Because of this, the use of methods known from digital
signal processing (DSP) are being introduced to compensate for them.
Applying DSP to improve optical signals has been limited by a discrepancy in digital signal
processing speeds and optical transmission speeds. However high speed Field
Programmable Gate Arrays (FPGA) which are sufficiently fast have now become available
making DSP experiments without costly ASIC implementation possible for optical
transmission experiments.
This thesis focuses on Look Up Table (LUT) based digital Electronic Predistortion (EPD) for
optical transmission. Because it is only one out of many possible implementations of EPD,
it has to be placed in context with other EPD techniques and other distortion combating
techniques in general, especially since it is possible to combine the different techniques.
Building an actual transmitter means that compromises and decisions have to be made in
the design and implementation of an EPD based system. These are based on balancing the
desire to achieve optimal performance with technological and economic limitations. This
is partly done using optical simulations to asses the performance.
This thesis describes a novel experimental transmitter that has been built as part of this
research applying LUT based EPD to an optical signal. The experimental transmitter
consists of a digital design (using a hardware description language) for a pair of FPGAs and
an analogue optical/electronic setup including two standard DAC integrated circuits. The
DSP in the transmitter compensated for both chromatic dispersion and self phase
modulation.
We achieved transmission of 10.7 Gb/s non-return-to-zero (NRZ) signals with a +4 dBm
launch power over 450 km keeping the required optical-signal-to-noise-ratio (OSNR) for a
bit-error-rate of 2x10^{-3} below 11 dB. In doing so we showed experimentally, for the first
time, that nonlinear effects can be compensated with this approach and that the
combination of FPGA-DAC is a viable approach for an experimental setup
Design of a Moderate-Resolution Dual-Slope ADC using Noise-Shaping Techniques and a Double Speed Quantizer
Being the slowest Analog-to-Digital Converter, the Dual-Slope quantizer is often used in
sigma-delta ADC or SAR converter architectures, and in measurement instruments, due
to its high accuracy. Despite the utility of the quantizer and the existent techniques to
increase the accuracy and the conversion speed, the usability of this converter is still very
limited by the its slow conversion rate.
The main interest of the Dual-Slope Quantizer lies in the high accuracy from the
quantization technique used. To convert the input value, the value is integrated in the
charge phase, by an integrator circuit, to be quantized, in the discharging phase using
a digital block. Other benefits of the Dual-Slope Quantizers are the small size when
implemented in a system on a chip (SOC) and the low power consumption.
By reducing the the conversion time of this ADC, while maintaining the high accuracy
it will be possible to increase the converters utility, such as in IoT devices, or even mobile
devices, benefiting all from the high accuracy and low power consumption of this circuit.
Nowadays, many techniques are being used in the Dual-Slope converters, such as,
the addition of bi-directional capabilities, to increase the conversion speed, the addition
of an half LSB compensation, to increase the accuracy, and the use of Noise-Shaping
capabilities originated from the quantization error from each discharge phase. All of this
techniques are presented and used in this research.
For the proposed solution, a Double-Speed Quantizer composed of two additional
comparators will be added to grant the conversion speed increase, which will increase
the power consumption and will lead to a redesigning of the digital block to receive more
inputs.
As result the conversion speed will double in comparison to the existent 4 bit dual
slope quantizer, being needed 8 clock cycles to quantize a input value, instead of 16
Linear Operation of Switch-Mode Outphasing Power Amplifiers
Radio transceivers are playing an increasingly important role in modern society. The
”connected” lifestyle has been enabled by modern wireless communications. The demand
that has been placed on current wireless and cellular infrastructure requires increased spectral
efficiency however this has come at the cost of power efficiency. This work investigates
methods of improving wireless transceiver efficiency by enabling more efficient power
amplifier architectures, specifically examining the role of switch-mode power amplifiers in
macro cell scenarios. Our research focuses on the mechanisms within outphasing power
amplifiers which prevent linear amplification. From the analysis it was clear that high power
non-linear effects are correctable with currently available techniques however non-linear effects
around the zero crossing point are not. As a result signal processing techniques for suppressing
and avoiding non-linear operation in low power regions are explored. A novel method of digital
pre-distortion is presented, and conventional techniques for linearisation are adapted for the
particular needs of the outphasing power amplifier. More unconventional signal processing
techniques are presented to aid linearisation of the outphasing power amplifier, both zero
crossing and bandwidth expansion reduction methods are designed to avoid operation in nonlinear
regions of the amplifiers. In combination with digital pre-distortion the techniques
will improve linearisation efforts on outphasing systems with dynamic range and bandwidth
constraints respectively.
Our collaboration with NXP provided access to a digital outphasing power amplifier,
enabling empirical analysis of non-linear behaviour and comparative analysis of behavioural
modelling and linearisation efforts. The collaboration resulted in a bench mark for linear
wideband operation of a digital outphasing power amplifier. The complimentary linearisation
techniques, bandwidth expansion reduction and zero crossing reduction have been evaluated in
both simulated and practical outphasing test benches. Initial results are promising and indicate
that the benefits they provide are not limited to the outphasing amplifier architecture alone.
Overall this thesis presents innovative analysis of the distortion mechanisms of the
outphasing power amplifier, highlighting the sensitivity of the system to environmental effects.
Practical and novel linearisation techniques are presented, with a focus on enabling wide band
operation for modern communications standards
THE APPLICATION OF REAL-TIME SOFTWARE IN THE IMPLEMENTATION OF LOW-COST SATELLITE RETURN LINKS
Digital Signal Processors (DSPs) have evolved to a level where it is feasible
for digital modems with relatively low data rates to be implemented entirely with
software algorithms. With current technology it is still necessary for analogue
processing between the RF input and a low frequency IF but, as DSP technology
advances, it will become possible to shift the interface between analogue and digital
domains ever closer towards the RF input. The software radio concept is a long-term
goal which aims to realise software-based digital modems which are completely
flexible in terms of operating frequency, bandwidth, modulation format and source
coding. The ideal software radio cannot be realised until DSP, Analogue to Digital
(A/D) and Digital to Analogue (D/A) technology has advanced sufficiently. Until
these advances have been made, it is often necessary to sacrifice optimum
performance in order to achieve real-time operation. This Thesis investigates practical
real-time algorithms for carrier frequency synchronisation, symbol timing
synchronisation, modulation, demodulation and FEC. Included in this work are novel
software-based transceivers for continuous-mode transmission, burst-mode
transmission, frequency modulation, phase modulation and orthogonal frequency
division multiplexing (OFDM).
Ideal applications for this work combine the requirement for flexible baseband
signal processing and a relatively low data rate. Suitable applications for this work
were identified in low-cost satellite return links, and specifically in asymmetric
satellite Internet delivery systems. These systems employ a high-speed (>>2Mbps)
DVB channel from service provider to customer and a low-cost, low-speed (32-128
kbps) return channel. This Thesis also discusses asymmetric satellite Internet delivery
systems, practical considerations for their implementation and the techniques that are
required to map TCP/IP traffic to low-cost satellite return links
RF applications in digital signal processing
Ever higher demands for stability, accuracy, reproducibility, and monitoring capability are being placed on Low-Level Radio Frequency (LLRF) systems of particle accelerators. Meanwhile, continuing rapid advances in digital signal processing technology are being exploited to meet these demands, thus leading to development of digital LLRF systems. The rst part of this course will begin by focusing on some of the important building-blocks of RF signal processing including mixer theory and down-conversion, I/Q (amplitude and phase) detection, digital down-conversion (DDC) and decimation, concluding with a survey of I/Q modulators. The second part of the course will introduce basic concepts of feedback systems, including examples of digital cavity eld and phase control, followed by radial loop architectures. Adaptive feed-forward systems used for the suppression of repetitive beam disturbances will be examined. Finally, applications and principles of system identi cation approaches will be summarized
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Digital Signal Processing for Coherent Transceivers Employing Multilevel Formats
Digital coherent transceivers have revolutionized optical fiber communications due to their superior performance offered compared to intensity modulation and direct detection based alternatives. As systems employing digital coherent transceivers seek to approach their information theoretic capacity, the use of multilevel modulation formats combined with appropriate forward error correction becomes essential. Given this context, in this tutorial paper, we therefore explore the digital signal processing (DSP) utilized in a coherent transceiver with a focus on multilevel modulation formats. By way of an introduction, we open by discussing the photonic technology required to realize a coherent transceiver. After discussing this interface between the analog optical channel and the digital domain, the rest of the paper is focused on DSP. We begin by discussing algorithms that correct for imperfections in the optical to digital conversion, including IQ imbalance and timing skew. Next, we discuss channel equalization including means for their realization for both quasi-static and dynamic channel impairments. Synchronization algorithms that correct for the difference between the transmitter and receiver oscillators both optical and electrical are then discussed and issues associated with symbol decoding highlighted. For most of the cases, we start with polarization division multiplexed quadrature phase-shift keying (PDM-QPSK) format as a basis and then discuss the extension to allow for high order multilevel formats. Finally, we conclude by discussing some of the open research challenges in the field.This work was supported in part by the EU project ICONE (608099) and EPSRC through INSIGHT (EP/L026155/2) and UNLOC (EP/J017582/1)
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