Pulse Modulation-Communication Systems-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Recall that analog signals can be represented by a sequence of discrete samples (output of sampler) Pulse Modulation results when some characteristic of the pulse (amplitude, width or position) is varied in correspondence with the data signal Two Types: Pulse Amplitude Modulation (PAM) • The amplitude of the periodic pulse train is varied in proportion to the sample values of the analog signal Pulse Time Modulation • Encodes the sample values into the time axis of the digital signal • Pulse Width Modulation (PWM)  Constant amplitude, width varied in proportion to the signal • Pulse Position Modulation (PPM)  sample values of the analog waveform are used in determining the location of the pulse signal docsity.com iL iL i tL Pulse Beacon (PPM) docsity.com Goals of Line Coding (qualities to look for) A line code is designed to meet one or more of the following goals: Self-synchronization • The ability to recover timing from the signal itself  That is, self-clocking (self-synchronization) - ease of clock lock or signal recovery for symbol synchronization • Long series of ones and zeros could cause a problem Low probability of bit error • Receiver needs to be able to distinguish the waveform associated with a mark from the waveform associated with a space • BER performance  relative immunity to noise • Error detection capability  enhances low probability of error docsity.com Spectrum Suitable for the channel • Spectrum matching of the channel  e.g. presence or absence of DC level • In some cases DC components should be avoided • The transmission bandwidth should be minimized Power Spectral Density • Particularly its value at zero  PSD of code should be negligible at the frequency near zero Transmission Bandwidth • Should be as small as possible Transparency • The property that any arbitrary symbol or bit pattern can be transmitted and received, i.e., all possible data sequence should be faithfully reproducible docsity.com Line Coder The input to the line encoder is the output of the A/D converter or a sequence of values an that is a function of the data bit The output of the line encoder is a waveform: where f(t) is the pulse shape and Tb is the bit period (Tb=Ts/n for n bit quantizer)  This means that each line code is described by a symbol mapping function an and pulse shape f(t)  Details of this operation are set by the type of line code that is being used ( ) ( )n b n s t a f t nT     docsity.com Bipolar RZ • A unipolar line code, except now we alternate between positive and negative pulses to send a ‘1’ • Alternating like this eliminates the DC component  This is desirable for many channels that cannot transmit the DC components Generalized Grouping Non-Return-to-Zero: NRZ-L, NRZ-M NRZ-S Return-to-Zero: Unipolar, Bipolar, AMI Phase-Coded: bi-f-L, bi-f-M, bi-f-S, Miller, Delay Modulation Multilevel Binary: dicode, doubinary Note:There are many other variations of line codes (see Fig. 2.22, page 80 for more) docsity.com Commonly Used Line CodesPolar line codes use the antipodal mapping Polar NRZ uses NRZ pulse shape Polar RZ uses RZ pulse shape , 1 , 0 n n n A w h en X a A w hen X      docsity.com Unipolar NRZ Line Code Unipolar non-return-to-zero (NRZ) line code is defined by unipolar mapping In addition, the pulse shape for unipolar NRZ is: where Tb is the bit period , 1 0, 0 n n n A when X a when X      Where Xn is the nth data bit ( ) , NRZ Pulse Shape b tf t T        docsity.com Summary of Line Codes NRZ-L NRZ-M : NRE Space NRZES| "1" represented by no change in "0" represented by a change in level Unipolar Unipolar RZ "1" represented by a 1/2-bit wide pulse "0" represented by no pulse condition Bipolar Bipolar RZ RZ "0's" & "1's" represented by opposite level polar pulses that are half bit wide RZ-AMI RZAMI "0 represented by no signal: successive "I's" represented by equal amplitude alternating pulses Bi-phase Level (Manchester IT + 180) "1" represented by "10" *0" represented by a" -phase Mark (Manchester 1) A transition at beginning of every bit period "1" represented by a 2nd transition 1/2 bit period later "0 represeured by no 2nd transition Bi-phase Spuce A transition ac beginning of every bit period "1" represented by a no 2nd transition *0" represented by a 2nd transition one-hal? bit period Jer Delay Modulation "1" to "0" of "0" to"1" changes polarity; otherwise a zero is sent Decode NRZ AI to "0" oF” 1° tansition produces a half duration polarity change: otherwise 2 zero is seat or A"1" represented by a transition atthe midpoint of a bit interval: a "0" is represented By no transition unless itis followed by another zero; In this ease, a Oller se oily 410T 1A4T, -tuusitionis pleced atthe end ofthebitperiod docsity.com e— 3Tbleck —>| : : ; : i | The binary code and some line codes eo s AML HDB S and CMI are CCITT standards = docsity.com Comparison of Line Codes Self-synchronization Manchester codes have built in timing information because they always have a zero crossing in the center of the pulse Polar RZ codes tend to be good because the signal level always goes to zero for the second half of the pulse NRZ signals are not good for self-synchronization Error probability Polar codes perform better (are more energy efficient) than Unipolar or Bipolar codes Channel characteristics We need to find the power spectral density (PSD) of the line codes to compare the line codes in terms of the channel characteristics docsity.com