Chapter 8. Impact of Noise - PowerPoint Presentation

Slide1

Chapter 8. Impact of Noise

Husheng Li

The University of TennesseeSlide2

White Noise

When the noise spectrum is flat, we call it white noise.

The spectral density is given bySlide3

Filtered (Colored) Noise

When passed through a LTI filter with transfer function H(f), we have

Example: noise passed through RC networkSlide4

Noise Equivalent Bandwidth

Average noise power:

Noise equivalent bandwidth:

The filtered noise is

What about the RC circuit?Slide5

Illustration of Equivalent BandwidthSlide6

Bandpass Noise

Bandpass

noise results when white noise passes through a

bandpass

filter.Slide7

SNR

The

predetection

signal-to-noise ratio is given by

We also define a system parameter (W is the low pass filter bandwidth)

Destination SNRSlide8

Quadrature Components

The

bandpass

noise can be written as

The power spectral densities are identical lowpass functions related to G_n(f):Slide9

Impact on AM (Synchronous Detection)

For DSB, the detected signal is given by

Then, the destination SNR is given bySlide10

Impact on AM (Synchronous Detection)

For generic AM, we have

For SSB, we have

For VSB, we haveSlide11

Summary

The message and noise are additive at the output if they are additive at the input.

If the

predetection

noise spectrum is reasonably flat over the transmission band, then the destination noise spectrum is essentially constant over the message band.Relative to (S/N)_D, SSB has no particular advantage over DSB.Making due allowance for the wasted power in unsuppressed-carrier systems, all types of linear modulation have the same performance as baseband transmission on the basis of average transmitted power and fixed

noise density.Slide12

Envelop Detection

When envelop detector is used for the demodulation of AM, the noise can affect the amplitude.Slide13

Two Extreme Cases

When the SNR is high, we have

When the SNR is low, then the signal modulates the noise. Slide14

Threshold Effect

There is some value of SNR above which message corruption is negligible and below which system performance rapidly deteriorates.

We define the threshold level as that value of SNR_R for which

A_c

>A_n with probability 0.99.The threshold effect is usually not a serious limitation for AM broadcasting.Slide15

Angle Modulation with Noise

Now (S/N)_R is often called carrier-to-noise ratio (CNR).

The

phasor

construction shows Slide16

Noise Spectrum in PM and FM

When the signal is 0, the noise is given by

PM

FMSlide17

SNR Gain of PM and FM

Both PM and FM give SNR gains over the base band transmissions:Slide18

Threshold Effect in FM

When the system is operating near the threshold, small variations of received signal power cause sizable changes in the output signal --- one moment it is there and the next moment it is gone.Slide19

Comparison of Continuous Waveform ModulationsSlide20

Review for Final Exam

Nyquist

criterion

Aliasing

Flat top sampling and aperture effectPAM, PPM and PDM (how to generate them? How to recover the original signal?What is the superhet principle? What are the frequency conversion procedure? How to determine the image frequency?

Specifications of frequencies

Tradeoff in spectrum analyzer

What types of multiple access schemes do we have? What are their major concerns?Slide21

Review

How to derive the dynamics of phase locked loop? How to analyze the steady state of phase locked loop? Need to write down the details

What if there is no carrier in the signal (say, DSB) for phase locked loop?

What are the SNR properties of AM, FM and PM (just need to remember the qualitative conclusions)?

What happens to the noise when envelop detection is used for demodulating AM signals?What are the noise spectrum shapes of FM and PM?Slide22

Review