# 0.1 A telecommunication system  (Page 8/15)

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Even within the frequency range of the message signal, the medium can affect different frequencies in different ways. (These are called frequency selective effects .) For example, a signal may arrive at the receiver, and a momentlater a copy of the same signal might arrive after having bounced off a mountain or a nearby building.This is called multipath interference , and it can be viewed as a sum of weighted and delayed versions

of the transmitted signal. This may be familiar to the (analog broadcast) TV vieweras “ghosts,” misty copies of the original signal that are shifted and superimposed over the main image.In the simple case of a sinusoid, a delay corresponds to a phase shift,making it more difficult to reassemble the original message. A special filter called the equalizer is often added to the receiver to help improve the situation.An equalizer is a kind of “deghosting” circuit, We refrain from calling these ghost busters. and equalization is addressed in detail in [link] .

Because of the proliferation of inexpensive and capable digital processors, receivers often contain chipsthat are essentially special purpose computers. In such receivers, many of the functions that are traditionally handled bydiscrete components (such as analog oscillators and filters) can be handled digitally.Of course, this requires that the analog received signal be turned into digital information (a series of numbers) that acomputer can process. This analog-to-digital conversion (A/D) is known as sampling .

Sampling measures the amplitude of the waveform at regular intervals, and thenstores these measurements in memory. Two of the chief design issues in a digital receiverare the following:

• Where should the signal be sampled?
• How often should the sampling be done?

The answers to these questions are intimately related to each other.

When taking samples of a signal, they must be taken fast enough so that important information is not lost.Suppose that a signal has no frequency content above ${f}^{*}$ Hz. The widely known Nyquist reconstruction principle (see [link] ) says that if sampling occurs at a rate greater than $2{f}^{*}$ samples per second, it is possible to reconstruct the original signal fromthe samples alone. Thus, as long as the samples aretaken rapidly enough, no information is lost. On the other hand, when samples are taken too slowly,the signal cannot be reconstructed exactly from the samples, and the resulting distortion is called aliasing .

Accordingly, in the receiver, it is necessary to sample at least twice as fast as the highest frequency present in the analog signal being sampledin order to avoid aliasing. Because the receiver contains modulators that change thefrequencies of the signals, different parts of the system have different highest frequencies.Hence the answer to the question of how fast to sample is dependent on where the samples will be taken.

The sampling

1. could be done at the input to the receiver at a rate proportional to the carrier frequency,
2. could be done after the downconversion, at a rate proportional to the rate of the symbols, or
3. could be done at some intermediate rate.

anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
what's the easiest and fastest way to the synthesize AgNP?
China
Cied
types of nano material
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
how did you get the value of 2000N.What calculations are needed to arrive at it
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