Chapter 8. Impact of Noise
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Transcript of Chapter 8. Impact of Noise
Chapter 8. Impact of NoiseHusheng LiThe University of Tennessee
White NoiseWhen the noise spectrum is flat, we call it
white noise.The spectral density is given by
Filtered (Colored) NoiseWhen passed through a LTI filter with transfer
function H(f), we have
Example: noise passed through RC network
Noise Equivalent BandwidthAverage noise power:
Noise equivalent bandwidth:
The filtered noise is
What about the RC circuit?
Illustration of Equivalent Bandwidth
Bandpass NoiseBandpass noise results when white noise
passes through a bandpass filter.
SNRThe predetection signal-to-noise ratio is given
by
We also define a system parameter (W is the low pass filter bandwidth)
Destination SNR
Quadrature ComponentsThe bandpass noise can be
written as
The power spectral densities are identical lowpass functions related to G_n(f):
Impact on AM (Synchronous Detection)For DSB, the detected signal is given by
Then, the destination SNR is given by
Impact on AM (Synchronous Detection)For generic AM, we have
For SSB, we have
For VSB, we have
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.
Envelop DetectionWhen envelop detector is used for the
demodulation of AM, the noise can affect the amplitude.
Two Extreme CasesWhen the SNR is high, we have
When the SNR is low, then the signal modulates the noise.
Threshold EffectThere 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.
Angle Modulation with NoiseNow (S/N)_R is often called carrier-to-noise
ratio (CNR).The phasor construction shows
Noise Spectrum in PM and FMWhen the signal is 0, the noise is given by
PM FM
SNR Gain of PM and FMBoth PM and FM give SNR gains over the base
band transmissions:
Threshold Effect in FMWhen 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.
Comparison of Continuous Waveform Modulations
Review for Final Exam Nyquist criterion Aliasing Flat top sampling and aperture effect PAM, 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?
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?
Review