# 6.37 Information communication problems  (Page 2/8)

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## Digital amplitude modulation

Two ELEC 241 students disagree about a homework problem. The issue concerns the discrete-time signal $s(n)\cos (2\pi {f}_{0}n)$ , where the signal $s(n)$ has no special characteristics and the modulation frequency ${f}_{0}$ is known. Sammy says that he can recover $s(n)$ from its amplitude-modulated version by the same approach used in analog communications. Samanthasays that approach won't work.

1. What is the spectrum of the modulated signal?
2. Who is correct? Why?
3. The teaching assistant does not want to take sides. He tells them that if $s(n)\cos (2\pi {f}_{0}n)$ and $s(n)\sin (2\pi {f}_{0}n)$ were both available, $s(n)$ can be recovered. What does he have in mind?

## Anti-jamming

One way for someone to keep people from receiving an AM transmission is to transmit noise at the same carrierfrequency. Thus, if the carrier frequency is ${f}_{c}$ so that the transmitted signal is ${A}_{T}(1+m(t))\sin (2\pi {f}_{c}t)$ the jammer would transmit ${A}_{J}n(t)\sin (2\pi {f}_{c}t+\phi )$ . The noise $n(t)$ has a constant power density spectrum over the bandwidth of the message $m(t)$ . The channel adds white noise of spectral height $\frac{{N}_{0}}{2}$ .

1. What would be the output of a traditional AM receiver tuned to the carrier frequency ${f}_{c}$ ?
2. RU Electronics proposes to counteract jamming by using a different modulation scheme. The scheme'stransmitted signal has the form ${A}_{T}(1+m(t))c(t)$ where $c(t)$ is a periodic carrier signal (period $\frac{1}{{f}_{c}}$ ) having the indicated waveform . What is the spectrum of the transmitted signal with the proposed scheme?Assume the message bandwidth $W$ is much less than the fundamental carrier frequency ${f}_{c}$ .
3. The jammer, unaware of the change, is transmitting with a carrier frequency of ${f}_{c}$ , while the receiver tunes a standard AM receiver to aharmonic of the carrier frequency. What is the signal-to-noise ratio of the receiver tuned to theharmonic having the largest power that does not contain the jammer?

## Secret comunications

A system for hiding AM transmissions has the transmitter randomly switching between two carrier frequencies ${f}_{1}$ and ${f}_{2}$ . "Random switching" means that one carrier frequency is used for some period of time, switches tothe other for some other period of time, back to the first, etc. The receiver knows what the carrierfrequencies are but not when carrier frequency switches occur. Consequently, the receiver must be designed toreceive the transmissions regardless of which carrier frequency is used. Assume the message signal hasbandwidth $W$ . The channel adds white noise of spectral height $\frac{{N}_{0}}{2}$ .

1. How different should the carrier frequencies be sothat the message could be received?
2. What receiver would you design?
3. What signal-to-noise ratio for the demodulated signal does your receiver yield?

## Am stereo

Stereophonic radio transmits two signals simultaneously that correspond to what comes out of the left and rightspeakers of the receiving radio. While FM stereo is commonplace, AM stereo is not, but is much simpler tounderstand and analyze. An amazing aspect of AM stereo is that both signals are transmitted within the samebandwidth as used to transmit just one. Assume the left and right signals are bandlimited to $W$ Hz. $x(t)=A(1+{m}_{l}(t))\cos (2\pi {f}_{c}t)+A{m}_{r}(t)\sin (2\pi {f}_{c}t)$

1. Find the Fourier transform of $x(t)$ . What is the transmission bandwidth and how does it compare with that of standard AM?
2. Let us use a coherent demodulator as the receiver, shown in [link] . Show that this receiver indeed works: It produces the left and right signalsseparately.
3. Assume the channel adds white noise to the transmitted signal. Find the signal-to-noise ratioof each signal.

#### Questions & Answers

where we get a research paper on Nano chemistry....?
what are the products of Nano chemistry?
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
I only see partial conversation and what's the question here!
what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe