602 research outputs found
The phase-switched screen
Conventional (passive) radar-absorbent materials operate either by phase cancellation or by absorbing incident electromagnetic energy and converting it into heat. This paper examines a new type of active "absorber," called the phase-switched screen (PSS). The PSS operates quite differently from passive absorbers in that it exhibits an apparently low value of reflectivity by redistributing the electromagnetic energy incident upon it over a bandwidth that is wide enough to ensure that little reflected energy falls within the pass-band of the receiver. The discussion considers the basic temporal and spectral properties of several PSS topologies, and includes measured data on both planar and cylindrical PSS structures
25 years of network access technologies: from voice to internet; the changing face of telecommunications
This work contributes to knowledge in the field of semiconductor system architectures, circuit design and implementation, and communications protocols.
The work starts by describing the challenges of interfacing legacy analogue subscriber loops to an electronic circuit contained within the Central Office (Telephone Exchange) building. It then moves on to describe the globalisation of the telecom network, the demand for software programmable devices to enable system customisation cost effectively, and the creation of circuit and system blocks to realise this.
The work culminates in the application challenges of developing a wireless RF front end, including antenna, for an Ultra Wideband communications systems applications.
This thesis illustrates how higher levels of integration over the period of 1981 to 2010 have influenced the realisation of complex system level products, particularly analogue signal processing capabilities for communications applications. There have been many publications illustrating the impact of technology advancement from an economic or technology perspective. The thesis shows how technology advancement has impacted the physical realisation of semiconductor products over the period, at system, circuit, and physical implementation levels
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Investigation into the impedance and communication requirements for the low voltage distribution line in the high frequency spectrum
Power Line Communications is long established for low data rate applications over high- voltage power lines. It is now charting new territory in high speed data transmission to the high frequency band of IMHz and upwards over the low-voltage segment below the distribution transformers.' Since the power line is designed for transmission of power instead of signal transmitting originally; it has many shortages when used as a signal communication channel. The heterogeneous structure of the power line network with numerous branches and impedance mismatcheS causing reflections and attenuations during signal transmission, and thus communication signal cannot be sent out or received completely. From this point of view, the power line impedance is a very important parameter in the design of power line communication (PLC) modem architecture, which is subject to legislations that limit the signals in the line. Variations on the impedance of the power line affect the communications channel performance. For the optimum modem design, power line impedance must be known. Power line impedance changes with time, carrier frequency, load variations, architectures and locations of the lines in city, urban, rural & industrial environment.
The objective of this study is to determine the impedance of power distribution network in a frequency range from IMHz to 30MHz. This is in line with international standard bodies including CENELEC, IEC, ITV and ETSI, which stipulates that for propagation characteristics of power line and EMC regulations, data transmission rate are evolving and . are being extended all the time to data rate up to 100 Mbps.
This thesis covers impedance measurements carried out in college buildings in Somerset, UK together with some residential houses in Somerset and London. The college buildings have both three-phase and single-phase architectures with various laboratories where loads are randomly switched on and off. An impedance analyser is used to carry out the measurements which performs a scan through a programmable frequency limits and acquires impedance parameters in the frequency domain Measurements were monitored using Microsoft Remote Desk Top client application Series of experimental measurements were carried out in the Bridgwater College and residential houses in Bridgwater and also in London.
The first part of the thesis offers detailed introduction to the topics of electricity supply networks, power line communications, modulation techniques and electromagnetic compatibility, noise and transmission line characteristics.
From the experimental results, presented in graphical format, a number of conclusions can be drawn. A wide range of impedances are observed for single phase measurements, within the range of 3 - 584 ⌠for large buildings and residential houses. For three phase measurements impedances varied from 21 - 340 âŚ.
The thesis concludes with a suggestion of how these measurements may be used in PLC modem design. Dynamic output-impedance PLC modems may be designed using a real-time impedance detector of the power line and the adjustable output impedance-power amplifier. Therefore, modem output impedance may be matched to the real time line impedance
The development and implementation of a single-line intelligent digital telephone answering unit on a personal computer
ThesisCommercial telephone answering machines are limited to some extent by
one or more of the following factors:
⢠limited facilities
⢠difficult to upgrade
⢠nonstandard telephone interfacing
⢠expensive
⢠lack of user-friendliness
⢠lack of dialogue and intelligence
The purpose of this study is to design an intelligent digital telephone
system which will overcome as many of the above-mentioned problems
as possible. The following features are proposed and will be discussed:
The use of a commonly available, but powerful, personal computer
processor and memory instead of the elementary and rigid processor and
magnetic tape storage units of the commercial telephone answering
machine . This allows the quick storage and retrieval of digitized
messages, each with its individual name, time and date stamp.
Using the personal computer's hardware and not duplicating the
processor and memory units allows a more cost-effective system
upgrade. Upgrades mainly consist of software changes and minor
hardware changes. This means that an upgrade does not implicate a total
hardware redesign. Standards as prescribed by the local switching network standards and
the Department of Post and Telecommunications, apply to this design
and are applicable for licensing of the product.
It is evident that the cost of this project and design is kept minimal by
not duplicating expensive components like the microprocessor and the
memory units, although these are used in the design. In this respect
upgrades are software orientated to further limit the costs.
The personal computer is equipped with a display which allows the user
to make easy selections in order to execute the required instructions or to
obtain information by using the help functions. This real-time help
function eliminates the need for a user manual.
Dialogue between user and personal computer over the telephone
network offers a simple method of delivering information without the
need for any extra equipment such as modems, keyboards or display
units.
The software used on the personal computer is designed in such a way
that the system is intelligent and capable of decision making.
Communication from the public telephone network is possible by using
the telephone keypad and Dual Tone Multifrequency (DTMF) signalling
Dynamic Capacity Enhancement using a Smart Antenna in Mobile Telecommunications Networks
This work describes an investigation into the performance of antennas for mobile base station applications and techniques for improving the coverage and capacity within a base station cell. The work starts by tracing the development of mobile systems, both in technical and commercial terms, from the earliest analogue systems to present day broadband systems and includes anticipated future developments. This is followed by an outline of how smart antenna systems can be utilised to improve cell coverage and capacity.
A novel smart antenna system incorporating an array of slant Âą 450 dual- polarised stacked patch elements four columns wide excited by a novel multi-beam forming and beam shaping network has been designed, simulated and implemented. It is found that for an ideal smart antenna array, four narrow overlapping beams, one wide âbroadcast channelâ beam and right and left shaped beams can be provided. Results are presented for the simulation of the smart antenna system using CST EM simulation software which inherently includes mutual coupling and the effects of a truncated ground plane on the element patterns. The results show some significant changes to the desired set of coverage patterns and various mutual coupling compensation techniques have been reviewed. An improved design technique has been developed for compensating the performance degrading effects of mutual coupling and finite ground plane dimensions in microstrip antenna arrays. The improved technique utilises combination of two previously known techniques: complex excitation weights compensation by inversion of the array mutual coupling scattering matrix and the incorporation of a WAIM (wide angle impedance matching) sheet. The technique has been applied to a novel multi-beam smart antenna array to demonstrate the efficacy of the technique by electromagnetic simulation. In addition, a demonstrator array has been constructed and tested which has yielded a positive conformation of the simulation results. For the developed demonstrator array which provides seven different beams, beams âfootprintsâ have been predicted both for free space propagation and for urban propagation to evaluate the dynamic capacity performance of the smart antenna in a 3G mobile network. The results indicate that sector capacity can be dynamically tailored to user demand profiles by selection of the appropriate beam patterns provided by the novel smart antenna system
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