Digital Communications (by B. Sklar)

Transcription

1 Digial Commuicaios by B. Sklar Sug Ho Cho Voice Cellular E mail: drago@hayag.ac.kr

2 Coes 1 Sigals ad Specra Formaig ad Basebad Trasmissio Badpass Modulaio ad Demodulaio Chael Codig Modulaio ad Codig Trade offs Spread Specrum Techiques

3 2 Sigals ad Specra

4 Aalog & Digial Commuicaios 3 Aalog Commuicaios Aalog Iformaio Modulaio Chael Demodulaio Aalog Iformaio Waveguide Digial Commuicaios Digial Iformaio Modulaio DAC Chael ADC Demodulaio Digial Iformaio Waveguide Wire Free space

5 Why Digial? 4 More reliable More flexible Simpler Example: A Regeeraive Repeaer

6 A Typical Digial Commuicaio Sysem 5 Iformaio Source Source Bis Chael Bis From Oher Sources Forma Source Ecode Ecryp Chael Ecode Muliplex Modulae Frequecy Spread Muliple Access XMT Make up Digial Ipu Digial Oupu Weigh Corol Proecio Error Correcio Bi Sream Compaio Adapaio Sychroizaio Privacy Digial Waveform Sociey Chael Carrier Forma Source Decode Decryp Chael Decode Demuliplex Demodulae Frequecy Despread Muliple Access RCV Iformaio Sik Source Bis Chael Bis To Oher Desiaios

7 Brief Descripio of Digial Commuicaio Sysems 6

8 Noaios 1/3 7 Iformaio Source: Aalog Discree Texual Message: A sequece of characers Ex Characer: A member of a alphabe or se of symbols ASCII, EBCDIC, Morse Ex Biary Digi Bi: The fudameal iformaio ui 1 ad/or

9 Noaios 2/3 8 Bi sream: A sequece of biary digis Ofe called a basebad sigal Ex 7 bi ASCII Symbol: A digial message A group of k bis M 2 k M size of he alphabe Baud symbol rae Ex

10 Noaios 3/3 9 Digial Waveform: A volage of curre waveform ha represes a digial symbol Ex Daa Rae: Bi rae R b : R b 1/T b bis/s T b Bi duraio Symbol rae Baud R : R 1/T symbols/s T Symbol duraio Relaioship bewee R b ad R: R R b / k or R b kr R log 2 M Performace Crieria Figure of Meri: Aalog commuicaios: SNR, Perce disorio, Mea square error Digial Commuicaios: BER, SEP, Badwidh efficiecy

11 Classificaio of Sigals 1/3 1 Deermiisic Sigal vs. Radom Sigal Periodic Sigal vs. Noperiodic Sigal Aalog Sigal vs. Discree Sigal Eergy Sigal vs. Power Sigal Eergy sigal: Fiie eergy, bu zero average power Power sigal: Fiie power, bu ifiie eergy

12 11 Wireless everywhere! Classificaio of Sigals 2/3 Impulse Fucios Dirac Dela Fucio: Kroecker Dela Fucio Ui Sep Fucios 1 1 d < 1 d d u τ τ τ τ < 1 k k u k k u u

13 12 Wireless everywhere! Classificaio of Sigals 3/3 Some Mahemaics: For ay x or x u d d u d d α α 1 u u x x x x τ τ τ d x x k k x x k τ τ k k

14 Specral Desiy 13 Eergy Specral Desiy ESD: Toal Eergy: Power Specral Desiy PSD: Fourier series coefficie For periodic sigals For operiodic sigals Trucaed versio Average Power:

15 Auocorrelaio 14 For eergy sigals: For power sigals:

16 Radom Variables 15 Radom Variable X: A fucioal relaioship bewee a radom eve ad a real umber Disribuio Fucio: Probabiliy Desiy Fucio pdf: Compuaio of Probabiliy Usig pdf:

17 Esemble Averages 16 Mea Value or Expeced Value: Mome: Variace:

18 Radom Processes 17 X: A represeaio of he radom variable as a fucio of ime Esemble of he Radom Processes: Time Average Esemble Average Mea of he Radom Process: Auocorrelaio Fucio:

19 Saioariy 18 Sric Sese Saioary SSS: All he saisics do o chage wih ime. Wide Sese Saioary WSS: Auocorrelaio of a WSS Process:

20 Time Averagig ad Ergodiciy 19 Ergodic Process: Time Average Saisical Average Ergodic i he Mea: Ergodic i he Auocorrelaio Fucio:

21 PSD ad Auocorrelaio 1/3 2

22 PSD ad Auocorrelaio 2/3 21 Example 1:

23 PSD ad Auocorrelaio 3/3 22 Example 2:

24 Noise i Commuicaio Sysems 23 A zero mea Gaussia radom process :

25 Whie Noise 24 PDF: Auocorrelaio: Average Power: