58 research outputs found
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Channel equalization to achieve high bit rates in discrete multitone systems
textMulticarrier modulation (MCM) techniques such as orthogonal frequency division
multiplexing (OFDM) and discrete multi-tone (DMT) modulation are attractive
for high-speed data communications due to the ease with which MCM can combat
channel dispersion. With all the benefits MCM could give, DMT modulation has an
extra ability to perform dynamic bit loading, which has the potential to exploit fully
the available bandwidth in a slowly time-varying channel. In broadband wireline
communications, DMT modulation is standardized for asymmetric digital subscribe
line (ADSL) and very-high-bit-rate digital subscriber line (VDSL) modems. ADSL
and VDSL standards are used by telephone companies to provide high speed data
service to residences and offices.
In an ADSL receiver, an equalizer is required to compensate for the channel’s
dispersion in the time domain and the channel’s distortion in the frequency domain
of the transmitted waveform. This dissertation proposes design methods for linear
equalizers to increase the bit rate of the connection. The methods are amenable
to implementation on programmable fixed-point digital signal processors, which are
employed in ADSL/VDSL transceivers.
A conventional ADSL equalizer consists of a time-domain equalizer, a fast
Fourier transform, and a frequency domain equalizer. The time domain equalizer
(TEQ) is a finite impulse response filter that when coupled with a discretized channel
produces an equivalent channel whose impulse response is shorter than that of
the discretized channel. This channel shortening is required by the ADSL standards.
In this dissertation, I first propose a linear phase TEQ design that exploits symmetry
in existing eigen-filter approaches such as minimum mean square error(MMSE),
maximum shortening signal to noise ratio (MSSNR) and minimum intersymbol interference
(Min-ISI) equalizers. TEQs with symmetric coefficients can reach the
same performance as non-symmetric ones with much lower training complexity.
Second, I improve Min-ISI design. I reformulate the cost function to make
long TEQs design feasible. I remove the dependency of transmission delay in order
to reduce the complexity associated with delay optimization. The quantized
weighting is introduced to further lower the complexity. I also propose an iterative
optimization procedure of Min-ISI that completely avoids Cholesky decomposition
hence is better suited for a fixed-point implementation.
Finally I propose a dual-path TEQ structure, which designs a standard singleFIR
TEQ to achieve good bit rate over the entire transmission bandwidth, and
designs another FIR TEQ to improve the bit rate over a subset of subcarriers. Dualpath
TEQ can be viewed as a special case of a complex valued filter bank structure
that delivers the best bit rate of existing DMT equalizers. However, dual-path
TEQ provides a very good tradeoff between achievable bit rate vs. implementation
complexity on a programmable digital signal processor.Electrical and Computer Engineerin
Spectrum optimization in multi-user multi-carrier systems with iterative convex and nonconvex approximation methods
Several practical multi-user multi-carrier communication systems are
characterized by a multi-carrier interference channel system model where the
interference is treated as noise. For these systems, spectrum optimization is a
promising means to mitigate interference. This however corresponds to a
challenging nonconvex optimization problem. Existing iterative convex
approximation (ICA) methods consist in solving a series of improving convex
approximations and are typically implemented in a per-user iterative approach.
However they do not take this typical iterative implementation into account in
their design. This paper proposes a novel class of iterative approximation
methods that focuses explicitly on the per-user iterative implementation, which
allows to relax the problem significantly, dropping joint convexity and even
convexity requirements for the approximations. A systematic design framework is
proposed to construct instances of this novel class, where several new
iterative approximation methods are developed with improved per-user convex and
nonconvex approximations that are both tighter and simpler to solve (in
closed-form). As a result, these novel methods display a much faster
convergence speed and require a significantly lower computational cost.
Furthermore, a majority of the proposed methods can tackle the issue of getting
stuck in bad locally optimal solutions, and hence improve solution quality
compared to existing ICA methods.Comment: 33 pages, 7 figures. This work has been submitted for possible
publicatio
Dynamic Resource Allocation in Cognitive Radio Networks: A Convex Optimization Perspective
This article provides an overview of the state-of-art results on
communication resource allocation over space, time, and frequency for emerging
cognitive radio (CR) wireless networks. Focusing on the
interference-power/interference-temperature (IT) constraint approach for CRs to
protect primary radio transmissions, many new and challenging problems
regarding the design of CR systems are formulated, and some of the
corresponding solutions are shown to be obtainable by restructuring some
classic results known for traditional (non-CR) wireless networks. It is
demonstrated that convex optimization plays an essential role in solving these
problems, in a both rigorous and efficient way. Promising research directions
on interference management for CR and other related multiuser communication
systems are discussed.Comment: to appear in IEEE Signal Processing Magazine, special issue on convex
optimization for signal processin
Physical Layer Techniques for High Frequency Wireline Broadband Systems
This thesis collects contributions to wireline and wireless communication systems with an emphasis on multiuser and multicarrier physical layer technology. To deliver increased capacity, modern wireline access systems such as G.fast extend the signal bandwidth up from tens to hundreds of MHz. This ambitious development revealed a number of unforeseen hurdles such as the impact of impedance changes in various forms. Impedance changes have a strong effect on the performance of multi-user crosstalk mitigation techniques such as vectoring. The first part of the thesis presents papers covering the identification of one of these problems, a model describing why it occurs and a method to mitigate its effects, improving line stability for G.fast systems.A second part of the thesis deals with the effects of temperature changes on wireline channels. When a vectored (MIMO) wireline system is initialized, channel estimates need to be obtained. This thesis presents contributions on the feasibility of re-using channel coefficients to speed up the vectoring startup procedures, even after the correct coefficients have changed, e.g., due to temperature changes. We also present extensive measurement results showing the effects of temperature changes on copper channels using a temperature chamber and British cables. The last part of the thesis presents three papers on the convergence of physical layer technologies, more specifically the deployment of OFDM-based radio systems using twisted pairs in different ways. In one proposed scenario, the idea of using the access copper lines to deploy small cells inside users' homes is explored. The feasibility of the concept, the design of radio-heads and a practical scheme for crosstalk mitigation are presented in three contributions
Analysis of the impact of impulse noise in digital subscriber line systems
In recent years, Digital subscriber line (DSL) technology has been gaining popularity as a high speed
network access technology, capable of the delivery of multimedia services. A major impairment for
DSL is impulse noise in the telephone line. However, evaluating the data errors caused by this noise is
not trivial due to its complex statistical nature, which until recently had not been well understood, and
the complicated error mitigation and framing techniques used in DSL systems. This thesis presents a
novel analysis of the impact of impulse noise and the DSL framing parameters on transmission errors,
building on a recently proposed impulse noise model. It focuses on errors at higher protocol layers, such
as asynchronous transfer mode (ATM), in the most widely used DSL version, namely Asymmetric DSL
(ADSL).
The impulse noise is characterised statistically through its amplitudes, duration, inter-arrival times,
and frequency spectrum, using the British Telecom / University of Edinburgh / Deutsche Telekom
(BT/UE/DT) model. This model is broadband, considers both the time and the frequency domains,
and accounts for the impulse clustering. It is based on recent measurements in two different telephone
networks (the UK and Germany) and therefore is the most complete model available to date and suited
for DSL analysis. A new statistical analysis of impulse noise spectra from DT measurements shows
that impulse spectra can be modelled with three spectral components with similar bandwidth statistical
distributions. Also, a novel distribution of the impulse powers is derived from the impulse amplitude
statistics.
The performance of a generic ADSL modem is investigated in an impulse noise and crosstalk environment
for different bit rates and framing parameters. ATM cell and ADSL frame error rates, and
subjective MPEG2 video quality are used as performance metrics. A new modification of a bit loading
algorithm is developed to enable stable convergence of the algorithm with trellis coding and restricted
subtone constellation size. It is shown that while interleaving brings improvement if set at its maximum
depth, at intermediate depths it actually worsens the performance of all considered metrics in comparison
with no interleaving. No such performance degradation is caused by combining several symbols in a
forward error correction (FEC) codeword, but this burst error mitigation technique is only viable at low
bit rates. Performance improvement can also be achieved by increasing the strength of FEC, especially
if combined with interleaving. In contrast, trellis coding is ineffective against the long impulse noise
error bursts. Alien as opposed to kindred crosstalk degrades the error rates and this is an important issue
in an unbundled network environment. It is also argued that error free data units is a better performance
measure from a user perspective than the commonly used error free seconds.
The impact of impulse noise on the errors in DSL systems has also been considered analytically. A
new Bernoulli-Weibull impulse noise model at symbol level is proposed and it is shown that other models
which assume Gaussian distributed impulse amplitudes or Rayleigh distributed impulse powers give
overly optimistic error estimates in DSL systems. A novel bivariate extension of the Weibull impulse
amplitudes is introduced to enable the analysis of orthogonal signals. Since an exact closed-form expression
for the symbol error probability of multi-carrierQAM assuming Bernoulli-Weibull noise model
does not exist, this problem has been solved numerically. Multi-carrier QAM is shown to perform better
at high signal-to-noise ratio (SNR), but worse at low SNR than single carrier QAM, in both cases because
of the spreading of noise power between subcarriers. Analytical expressions for errors up to frame
level in the specific case of ADSL are then derived from the impulse noise model, with good agreement
with simulation results. The Bernoulli-Weibull model is applied to study the errors in single-pair highspeed
DSL (SHDSL). The performance of ADSL is found to be better when the burst error mitigation
techniques are used, but SHDSL has advantages if low bit error rate and low latency are required
Discrete Wavelet Transforms
The discrete wavelet transform (DWT) algorithms have a firm position in processing of signals in several areas of research and industry. As DWT provides both octave-scale frequency and spatial timing of the analyzed signal, it is constantly used to solve and treat more and more advanced problems. The present book: Discrete Wavelet Transforms: Algorithms and Applications reviews the recent progress in discrete wavelet transform algorithms and applications. The book covers a wide range of methods (e.g. lifting, shift invariance, multi-scale analysis) for constructing DWTs. The book chapters are organized into four major parts. Part I describes the progress in hardware implementations of the DWT algorithms. Applications include multitone modulation for ADSL and equalization techniques, a scalable architecture for FPGA-implementation, lifting based algorithm for VLSI implementation, comparison between DWT and FFT based OFDM and modified SPIHT codec. Part II addresses image processing algorithms such as multiresolution approach for edge detection, low bit rate image compression, low complexity implementation of CQF wavelets and compression of multi-component images. Part III focuses watermaking DWT algorithms. Finally, Part IV describes shift invariant DWTs, DC lossless property, DWT based analysis and estimation of colored noise and an application of the wavelet Galerkin method. The chapters of the present book consist of both tutorial and highly advanced material. Therefore, the book is intended to be a reference text for graduate students and researchers to obtain state-of-the-art knowledge on specific applications
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