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EE 551/451, Fall, 2006 Communication Systems

EE 551/451, Fall, 2006 Communication Systems. Zhu Han Department of Electrical and Computer Engineering Class 14 Oct. 5 th , 2006. Outline. Moon cake tasting Pulse Modulation in Presence of Noise Optimum Pre-emphasis and De-emphasis Pulse Modulation Demo. Mid-Autumn Festival 中秋節.

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EE 551/451, Fall, 2006 Communication Systems

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  1. EE 551/451, Fall, 2006Communication Systems Zhu Han Department of Electrical and Computer Engineering Class 14 Oct. 5th, 2006

  2. Outline • Moon cake tasting • Pulse Modulation in Presence of Noise • Optimum Pre-emphasis and De-emphasis • Pulse Modulation Demo EE 541/451 Fall 2006

  3. Mid-Autumn Festival 中秋節 • August Fifteenth (Lunar Calendar) is a popular Asian celebration of abundance and togetherness, dating back over 3,000 years. Chinese Thanksgiving. • At this time, the moon is at its fullest and brightest, marking an ideal time to celebrate the abundance of the summer's harvest. The traditional food of this festival is the moon cake, of which there are many different varieties. • Story of the Festival: Archer, Hou Yi, and Chang’e, the Moon Goddess EE 541/451 Fall 2006

  4. Two Types of Errors • Round off error • Detection error • Variance of sum of the independent random variables is equal to the sum of the variances of the independent random variables. • The final error energy is equal to the sum of error energy for two types of errors (12.56-12.59) • Round off error in PCM • Example 10.17 Page 467 EE 541/451 Fall 2006

  5. Mean Square Error in PCM • If transmit 1101 (13), but receive 0101 (5), error is 8 • Error in different location produces different MSE • Overall error probability • Page 469 • Gray coding: if one bit occur, the error is minimized. EE 541/451 Fall 2006

  6. Bit Errors in PCM Systems Simplest case is Additive White Gaussian Noise for baseband PCM scheme -- see the analysis for this case. For signal levels of +A and -A we get pe = Q(A/s) • Notes • Q(A/s) represents the area under one tail of the normal pdf • (A/s)2 represents the Signal to Noise (SNR) ratio • Our analysis has neglected the effects of transmit and receive filters - it can be shown that the same results apply when filters with the correct response are used. EE 541/451 Fall 2006

  7. Q Function • For Q function: • The remain of cdf of Gaussian distribution • Physical meaning • Equation • Matlab: erfc • y = Q(x) • y = 0.5*erfc(x/sqrt(2)); • Note how rapidly Q(x) decreases as x increases - this leads to the threshold characteristic of digital communication systems EE 541/451 Fall 2006

  8. SNR vs.  • Figure 12.14: Exam, Exam and Exam • Threshold • Saturation • Equation 12.64C, slightly better than ADC • Example 12.8 • Exchange of SNR for bandwidth is much more efficient than in angle modulation • Repeaters EE 541/451 Fall 2006

  9. Companded PCM • Problem of • Without compand, 12.67 • With compand, 12.68 • Final SNR 12.72b • Saturation 12.72C • Small input signal 12.73 • Figure 12.17 for  law • Even the input signal is very small, the output SNR is still reasonable. EE 541/451 Fall 2006

  10. Optimal Pre-emphasis and De-emphasis • System model: Figure 12.18 • Distortion-less condition • 12.76a for amplitude and 12.76b for phase • SNR expression 12.78 • Maximize SNR s.t. distortionless • Lagrange multiplier • Optimal Hp and Hd, 12.83a and 12.83b • SNR improvement, 12.83c • Example 12.11 EE 541/451 Fall 2006

  11. Pre-emphasis and De-emphasis in AM and FM • AM • Figure 12.19 • No that effective • FM • Noise is larger when frequency is high • Optimal pre-emphasis and de-emphasis, 12.88d • Only simple suboptimum is used in commercial FM for historical and practical reasons EE 541/451 Fall 2006

  12. Homework • 6.2.5 • 6.2.7 • 6.4.1 • 12.4.3 • 12.5.1 (graduate) • Due 10/10/06, no homework next week EE 541/451 Fall 2006

  13. Questions? EE 541/451 Fall 2006

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