<< Chapter < Page Chapter >> Page >

Multipath interference

In some situations, an electromagnetic wave can propagate directly from one place to another. For instance, when aradio signal from a spacecraft is transmitted back to Earth, the vacuum of space guarantees that the wave will arrivemore or less intact (though greatly attenuated by distance). Often, however, the wave reflects, refracts, or diffracts,and the signal arriving is quite different from the one that was sent.

These distortions can be thought of as a combination of scaled and delayed reflections of the transmitted signal, which occurwhen there are different propagation paths from the transmitter to the receiver. Between two transmission towers, for instance, the paths may include one alongthe line-of-sight, reflections from the atmosphere, reflections from nearby hills, and bounces from a field or lake betweenthe towers. For indoor digital TV reception, there are many (local)time-varying reflectors, including people in the receiving room, nearby vehicles, and the buildings of an urban environment. [link] , for instance, shows multiple reflections that arrive after bouncing off a cloud, after bouncing offa mountain, and others that are scattered by multiple bounces from nearby buildings.

The received signal may be a combination of several copies of the original transmitted signal, each with a different attenuation and delay.
The received signal may be a combination of several copies of the original transmitted signal, eachwith a different attenuation and delay.

The strength of the reflections depends on the physical properties of the reflecting surface, while the delay of thereflections is primarily determined by the length of the transmission path. Let s ( t ) be the transmitted signal. If N delays are represented by Δ 1 , Δ 2 , ... , Δ N , and the strengths of the reflections are h 1 , h 2 , ... , h N , then the received signal r ( t ) is

r ( t ) = h 1 s ( t - Δ 1 ) + h 2 s ( t - Δ 2 ) + ... + h N s ( t - Δ N ) .

As will become clear in "Convolution in Time: It's What Linear Systems Do" , this model of the channel has the form of a linear filter(since the expression on the right hand side is a convolution of the transmitted signal and the h i 's). This is shown in part (a) of [link] . Since this channel model is a linear filter,it can also be viewed in the frequency domain, and part (b) shows its frequency response.When this is combined with the BPF and the spectrum of the signal (shown in (c)), the result is the distorted spectrumshown in (d).

(a) The channel model Equation 4 as a filter. (b) The frequency response of the filter. (c) An idealized BPF and the spectrum of the signal. The product of (b) and (c) gives (d), the distorted spectrum at the receiver.
(a) The channel model [link] as a filter. (b) The frequency response of the filter. (c) An idealized BPF andthe spectrum of the signal. The product of (b) and (c) gives (d), the distorted spectrum at the receiver.

What can be done?

If the kinds of distortions introduced by the channel are known (or can somehow be determined), then thebandpass filter at the receiver can be modified in order to undo the effects of the channel. This can beseen most clearly in the frequency domain, as in [link] . Observe that the BPF is shaped (part (d)) to approximately invert the debilitatingeffects of the channel (part (a)) in the frequency band of the signal and to remove all the out-of-band frequencies.The resulting received signal spectrum (part (e)) is again a close copy of the transmitted signal spectrum, in stark contrast tothe received signal spectrum in [link] where no shaping was attempted.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
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?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
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
Devang Reply
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
Abhijith Reply
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?
s. Reply
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 ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
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
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Software receiver design' conversation and receive update notifications?

Ask