L11_energy_noise

L11_energy_noise - Energy and Noise Signal-to Noise...

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1 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 1 Energy and Noise Signal-to Noise Ratio (SNR) Constellation diagrams and SNR Bit error rate versus SNR Copyright © 2007 by M.H. Perrott & C. G. Sodini All rights reserved. 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 2 Review of Digital Modulation Transmitter sends discrete-valued signals over an analog communication channel Receiver samples recovered baseband signal Noise and ISI corrupt received signal Key techniques Properly design transmit and receive filters for low ISI Sample and slice received signals to detect symbols
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2 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 3 Definition of Signal-to-Noise Ratio Signal-to-Noise ratio (SNR) indicates impact of noise on system performance (Ratio of Signal power to Noise power) We often like to use units of dB to express SNR: A signal[n] n n noise[n] rx[n] 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 4 A signal[n] n n noise[n] rx[n] n n n SNR = 20.4 dB SNR = 10.7 dB SNR = 0.4 dB SNR Example Scaling the gain factor A leads to different SNR values Lower results in lower SNR Signal quality steadily degrades with lower SNR
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3 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 5 Impact of SNR on Receiver Constellation SNR influenced by transmitted power, distance between transmitter and receiver, and noise 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 6 Impact of Increased Signal on Constellation Increase in received signal power leads to increased separation between symbols SNR is improved if noise level unchanged
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4 6.02 Spring 2008 Lecture 11 Energy and Noise Slide 7 n noise[n] (Trial = 1) sample value Histogram of 100 samples Histogram of 1,000 samples Histogram of 10,000 samples Histogram of 1,000,000 samples sample value sample value sample value Experiment to see Statistical Distribution Create histograms of sample values from trials of increasing lengths Assumption of independence and stationarity implies histogram should converge to a shape known as a probability density function (PDF) 6.02 Spring 2008 Lecture 11
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L11_energy_noise - Energy and Noise Signal-to Noise...

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