# 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.

who was the first nanotechnologist
k
Veysel
technologist's thinker father is Richard Feynman but the literature first user scientist Nario Tagunichi.
Veysel
Norio Taniguchi
puvananathan
Interesting
Andr
I need help
Richard
anyone have book of Abdel Salam Hamdy Makhlouf book in pdf Fundamentals of Nanoparticles: Classifications, Synthesis
what happen with The nano material on The deep space.?
It could change the whole space science.
puvananathan
the characteristics of nano materials can be studied by solving which equation?
sibaram
synthesis of nano materials by chemical reaction taking place in aqueous solvents under high temperature and pressure is call?
sibaram
hydrothermal synthesis
ISHFAQ
how can chip be made from sand
is this allso about nanoscale material
Almas
are nano particles real
yeah
Joseph
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
no can't
Lohitha
where is the latest information on a no technology how can I find it
William
currently
William
where we get a research paper on Nano chemistry....?
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
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
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
has a lot of application modern world
Kamaluddeen
yes
narayan
what is variations in raman spectra for nanomaterials
ya I also want to know the raman spectra
Bhagvanji
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
STM - Scanning Tunneling Microscope.
puvananathan

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