20 research outputs found
Asymmetrical digital subscriber line (ADSL) an in-depth study
Asymmetrical Digital Subscriber Line (ADSL) is one member of a group of broadband access technologies that uses the existing copper-based local loop of the analog PSTN for high-speed digital data transmission. One feature of ADSL is that it permits analog voice POTS transmissions to continue uninterrupted over the same wiring. Specifically, POTS continues to use the 0 to 4 KHz frequency range of the copper wiring, while ADSL uses bandwidth starting at 25 KHz and extending up to approximately 1.1 MHz for data transmission. The term asymmetrical refers to the fact that data rates downstream (to the user) and upstream (from the user) are not the same. Typical ADSL data rates range from 1.536 to 6.144 Mbps downstream and from 16 to 640 Kbps upstream. Local loop length, wire size, and the presence of devices to improve voice communication such as bridged taps and loading coils all affect ADSL data rates. Digital data is coded by one of two methods: Discrete Multitone Modulation (DMT) or Carrierless Amplitude and Phase Modulation (CAP). Echo control is also accomplished by one of two methods: Frequency Division Multiplexing (FDM) or echo cancellation. This paper consists of four sections: 1) A technical review and comparison of the CAP and DMT line encoding technologies. 2) A market review of the presence of CAP and DMT technologies in customer premise equipment (CPE) such as modems and routers. 3) A review of the POTS physical layer that exists between the ADSL subscriber and the Telco CO, and its impact on ADSL availability and quality of service (QOS). 4) A technical review of the newer, splitterless, G.Lite technolog
FFT and FIR Filter implementations for the DSL MODEMS
Broad band digital communication that operates over a standard copper wires. It requires
the DSL modems which splits the transmissions into 2 frequency bands. The lower frequencies for
voice and the higher frequencies for digital data (internet) in order to transmit the data to larger
distances through a copper cable we need modulation techniques. Generally in this DSL modems
modulation used is QAM technique. The output of the QAM is complex data this complex data we
cannot transfer directly through a copper cable because the data should be in time domain or
otherwise the phase of the data which is in frequency domain can be lost, in copper cable so this
data should be converted in time domain by using IDFT technique. As IDFT requires more
number of complex multiplications and more number of complex additions in comparison to IFFT
so to reduce the additions and multiplications IFFT technique is used. At the receiver side we can
retrieve the same data by using FFT technique. In this section the implemented FFT architecture is
fully efficient and this architecture will require less area. And before we have to transmit through
the copper line we have to do interpolation or decimation by using the Filtering operation. The
implemented poly phase architecture for the filtering is fully efficient, symmetrical and it requires
less number of multipliers
Determination of ADSL capacity in a generic exchange environment
Please read the abstract in the front matter this documentDissertation (M Eng (Electronic Engineering))--University of Pretoria, 2006.Electrical, Electronic and Computer Engineeringunrestricte
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
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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
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An Examination of the Evolution of Broadband Technologies in the UK
The aim of this thesis is to examine the reasons due to which Digital Subscriber Line (DSL) became the most widely used technology to deliver broadband connectivity in the United Kingdom (UK). The research examines the outcome starting with events in 1960s when broadband as it is defined today did not exist. The research shows that a combination of factors involving regulatory decisions, changing market conditions, and unexpected technological breakthroughs contributed to the current day mix of broadband technologies in the last mile access in the UK.
To interpret the events that have shaped the development, deployment, and adoption of broadband technologies in the UK, the thesis draws from various theoretical ideas related to Science and Technology Studies (STS) to understand and analyse the events. In order to discover and establish the historical context, the thesis employs original, unpublished interviews along with the extensive use of archival material and secondary sources. Influenced by some of the core ideas of social constructionist studies, this research combines concepts from economic studies of technological change along with themes involving maintenance of technology, path dependence, and the role of bandwagon effect.
These research threads are combined to understand the way development, deployment, and adoption of broadband technologies took place in the UK. The research is intended to contribute to the understanding of technology in a constantly changing regulatory and socio-economic environment and how it is shaped by multiple factors. The targeted readership is researchers, analysts, and decision makers working with broadband technology, telecommunications policy, and STS. Further research is suggested in the form of studies of wireless broadband technologies and the role of regulatory policies in the development of the UK communications market
A Hybrid voice/text electronic mail system: an application of the integrated services digital network
The objective of this thesis is to present a useful application for the Integrated Services Digital Network (ISDN) that is expected to one day replace the analog phone system in use today. ISDN itself and its continuing evolution are detailed. The system developed as a part of this thesis involved the creation of an inexpensive phone terminal that can serve as an ISDN terminal and also as a bridge to a Local Area Network (LAN). The phone terminal provides a hybrid electronic mail system that allows the attachment of speech to text within a message. Messages created with this phone terminal could theoretically be sent locally using the LAN interface and globally using ISDN to other users with either phone terminals or multimedia personal computers. For this project, the two phone terminals created were interconnected via an Ethernet and using an 80486 PC to act as a Central Office System. This Central Office System provides speech/message storage for the phone terminals. It makes use of speech compression techniques to minimize the storage requirements. The speech compression techniques used as well as the field of speech coding in general are discussed
Impulsive noise cancellation and channel estimation in power line communication systems
Power line communication (PLC) is considered as the most viable enabler of the smart grid. PLC exploits the power line infrastructure for data transmission and provides an economical communication backbone to support the requirements of smart grid applications. Though PLC brings a lot of benefits to the smart grid implementation, impairments such as frequency selective attenuation of the high-frequency communication signal, the presence of impulsive noise (IN) and the narrowband interference (NBI) from closely operating wireless communication systems, make the power line a hostile environament for reliable data transmission. Hence, the main objective of this dissertation is to design signal processing algorithms that are specifically tailored to overcome the inevitable impairments in the power line environment.
First, we propose a novel IN mitigation scheme for PLC systems. The proposed scheme actively estimates the locations of IN samples and eliminates the effect of IN only from the contaminated samples of the received signal. By doing so, the typical problem encountered while mitigating the IN is avoided by using passive IN power suppression algorithms, where samples besides the ones containing the IN are also affected creating additional distortion in the received signal.
Apart from the IN, the PLC transmission is also impaired by NBI. Exploiting the duality of the problem where the IN is impulsive in the time domain and the NBI is impulsive in the frequency domain, an extended IN mitigation algorithm is proposed in order to accurately estimate and effectively cancel both impairments from the received signal. The numerical validation of the proposed schemes shows improved BER performance of PLC systems in the presence of IN and NBI.
Secondly, we pay attention to the problem of channel estimation in the power line environment. The presence of IN makes channel estimation challenging for PLC systems. To accurately estimate the channel, two maximumlikelihood (ML) channel estimators for PLC systems are proposed in this thesis.
Both ML estimators exploit the estimated IN samples to determine the channel coefficients. Among the proposed channel estimators, one treats the estimated IN as a deterministic quantity, and the other assumes that the estimated IN is a random quantity. The performance of both estimators is analyzed and numerically evaluated to show the superiority of the proposed estimators in comparison to conventional channel estimation strategies in the presence of IN. Furthermore, between the two proposed estimators, the one that is based on the random approach outperforms the deterministic one in all typical PLC scenarios. However, the deterministic approach based estimator can perform consistent channel estimation regardless of the IN behavior with less computational effort and becomes an efficient channel estimation strategy in situations where high computational complexity cannot be afforded.
Finally, we propose two ML algorithms to perform a precise IN support detection. The proposed algorithms perform a greedy search of the samples in the received signal that are contaminated by IN. To design such algorithms, statistics defined for deterministic and random ML channel estimators are exploited and two multiple hypothesis tests are built according to Bonferroni and Benjamini and Hochberg design criteria. Among the proposed estimators, the random ML-based approach outperforms the deterministic ML-based approach while detecting the IN support in typical power line environment.
Hence, this thesis studies the power line environment for reliable data transmission to support smart grid. The proposed signal processing schemes are
robust and allow PLC systems to effectively overcome the major impairments in an active electrical network.The efficient mitigation of IN and NBI and accurate estimation of channel enhances the applicability of PLC to support critical applications that are envisioned for the future electrical power grid.La comunicación a través de líneas de transmisión eléctricas (PLC) se considera uno de los habilitadores principales de la red eléctrica inteligente (smart grid). PLC explota la infraestructura de la red eléctrica para la transmisión de datos y proporciona una red troncal de comunicación económica para poder cumplir con los requisitos de las aplicaciones para smart grids. Si bien la tecnología PLC aporta muchos beneficios a la implementación de la smart grid, los impedimentos, como la atenuación selectiva en frecuencia de la señal de comunicación, la presencia de ruido impulsivo (IN) y las interferencias de banda estrecha (NBI) de los sistemas de comunicación inalámbrica de operación cercana, hacen que la red eléctrica sea un entorno hostil para la transmisión fiable de datos. En este contexto, el objetivo principal de esta tesis es diseñar algoritmos de procesado de señal que estén específicamente diseñados para superar los impedimentos inevitables en el entorno de la red eléctrica como son IN y NBI. Primeramente, proponemos un nuevo esquema de mitigación de IN en sistemas PLC. El esquema propuesto estima activamente las ubicaciones de las muestras de IN y elimina el efecto de IN solo en las muestras contaminadas de la señal recibida. Al hacerlo, el problema típico que se encuentra al mitigar el IN con técnicas tradicionales (donde también se ven afectadas otras muestras que contienen la IN, creando una distorsión adicional en la señal recibida) se puede evitar con la consiguiente mejora del rendimiento. Aparte de IN, los sistemas PLC también se ven afectados por el NBI. Aprovechando la dualidad del problema (el IN es impulsivo en el dominio del tiempo y el NBI es impulsivo en el dominio de la frecuencia), se propone un algoritmo de mitigación de IN ampliado para estimar con precisión y cancelar efectivamente ambas degradaciones de la señal recibida. La validación numérica de los esquemas propuestos muestra un mejor rendimiento en términos de tasa de error de bit (BER) en sistemas PLC con presencia de IN y NBI. En segundo lugar, prestamos atención al problema de la estimación de canal en entornos PLC. La presencia de IN hace que la estimación de canal sea un desafío para los sistemas PLC futuros. En esta tesis, se proponen dos estimadores de canal para sistemas PLC de máxima verosimilitud (ML) para sistemas PLC. Ambos estimadores ML explotan las muestras IN estimadas para determinar los coeficientes del canal. Entre los estimadores de canal propuestos, uno trata la IN estimada como una cantidad determinista, y la otra asume que la IN estimada es una cantidad aleatoria. El rendimiento de ambos estimadores se analiza y se evalúa numéricamente para mostrar la superioridad de los estimadores propuestos en comparación con las estrategias de estimación de canales convencionales en presencia de IN. Además, entre los dos estimadores propuestos, el que se basa en el enfoque aleatorio supera el determinista en escenarios PLC típicos. Sin embargo, el estimador basado en el enfoque determinista puede llevar a cabo una estimación de canal consistente independientemente del comportamiento de la IN con menos esfuerzo computacional y se convierte en una estrategia de estimación de canal eficiente en situaciones donde no es posible disponer de una alta complejidad computacionalPostprint (published version
Cost-effective Information and Communication Technology (ICT) infrastructure for Tanziania
The research conducted an Information and Communication Technology (ICT) field
survey, the results revealed that Tanzania is still lagging behind in the ICT sector due to
the lack of an internationally connected terrestrial ICT infrastructure; Internet connectivity
to the rest of the world is via expensive satellite links, thus leaving the majority of the
population unable to access the Internet services due to its high cost. Therefore, an ICT
backbone infrastructure is designed that exploits optical DWDM network technology,
which un-locks bandwidth bottlenecks and provides higher capacity which will provide
ICT services such as Internet, voice, videos and other multimedia interactions at an
affordable cost to the majority of the people who live in the urban and rural areas of
Tanzania. The research analyses and compares the performance, and system impairments, in a DWDM system at data transmission rates of 2.5 Gb/s and 10 Gb/s per wavelength channel. The simulation results show that a data transmission rate of 2.5 Gb/s can be successfully transmitted over a greater distance than 10 Gb/s with minimum system impairments. Also operating at the lower data rate delivers a good system performance for the required ICT services. A forty-channel DWDM system will provide a bandwidth of 100 Gb/s.
A cost analysis demonstrates the economic worth of incorporating existing optical fibre
installations into an optical DWDM network for the creation of an affordable ICT
backbone infrastructure; this approach is compared with building a completely new optical
fibre DWDM network or a SONET/SDH network. The results show that the ICT backbone
infrastructure built with existing SSMF DWDM network technology is a good investment,
in terms of profitability, even if the Internet charges are reduced to half current rates. The
case for building a completely new optical fibre DWDM network or a SONET/SDH
network is difficult to justify using current financial data