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Electrical Communications Systems ECE.09.331 Spring 2010

Electrical Communications Systems ECE.09.331 Spring 2010. Lecture 13a April 20, 2010. Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring10/ecomms/. Plan. Performance of Comm. Systems corrupted by Noise Performance Measures: Digital and Analog

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Electrical Communications Systems ECE.09.331 Spring 2010

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  1. Electrical Communications SystemsECE.09.331Spring 2010 Lecture 13aApril 20, 2010 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring10/ecomms/

  2. Plan • Performance of Comm. Systems corrupted by Noise • Performance Measures: Digital and Analog • Thermal (Johnson) Noise • Amplitude: Recall Random Variables: lab1.ppt • Power Spectral Density • Autocorrelation function • Wiener-Khintchine Theorem • Bit Error Rates (BERs) for Digital Comm. Sys. • General Procedure • M-ary PSK and M-ary QAM • Output SNR for Analog Comm. Sys. • AM, DSB-SC, SSB and FM

  3. ECOMMS: Topics

  4. Performance of Communications Systems Corrupted by Noise Analog Output SNR Digital Bit Error Rate (BER)

  5. Noise • A random, unwanted fluctuation in signal amplitude • Thermal (Johnson) Noise • Amplitude vs. time: Gaussian PDF Model • See Lab1 Pre-lab Lecture: lab1.ppt • We also want to know how much noise power there is per Hz – why?

  6. |W(f)| |W(f)| 0 fc -fc f Bandpass 0 2. Demodulation 1. Modulation f Baseband |W(f)| 3. Demodulation -fIF fIF 0 f Bandpass Why?

  7. Power Spectral Density (PSD) • Normalized power of a waveform in the frequency domain • Used for measuring signal/noise power loss/transfer in communications system blocks

  8. F Rw(t) Pw(f) Rx(t) Slowly fluctuating signal rapidly fluctuating signal t (time delay) 0 Autocorrelation Function • Measure of • similarity of a waveform observed at times t seconds apart • how rapidly a random waveform fluctuates with time Matlab Demo: autocorr.m Wiener-Khintchine Theorem

  9. -21 PSD of thermal noise x 10 2 1.5 PSD, W/Hz 1 0.5 0 0 0.5 1 1.5 2 2.5 3 frequency, Hz 12 x 10 Thermal (Johnson) Noise Matlab script: psd_noise.m

  10. Digital Communications System Noisy Receiver Model + Sample & Hold Threshold Processing Modulated Signal s(t) S + Noise w(t) Digital Output

  11. Symbol Error in M-ary PSK Systems Matlab script: dignoise.m

  12. Symbol Error in M-ary QAM Systems Matlab script: dignoise.m

  13. Analog Communications Systems Baseband Model Baseband Output (Signal+ Noise) + Message Signal m(t) LPF S + Noise n(t) Noisy Receiver Model Output (Signal+ Noise) + Modulated Signal s(t) Demodulator BPF S + Noise n(t)

  14. Output SNR in Analog Comm. Systems Matlab script: analognoise.m

  15. Summary

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