EEE353 Lec9 Handouts Dcs

8/7/2019 EEE353 Lec9 Handouts Dcs

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Digital CommunicationSystemsDr. Shurjeel Wyne

Lecture 9CH 4 -Bandpass Modulation& Demodulation

Last time we talked about: Another source of error due to filtering effectof the system: Inter-symbol interference (lSI)

The techniques to reduce lSI Pulse shaping to have zero lSI at the samplingtime Equalization to combat the filtering effect of thechannel


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Today, we are going to talkabout: Some bandpass modulation schemes usedin DeS for transmitting information overchannel M-PAM, M-PSK, M-FSK, M-QAM

How to detect the transmitted information atthe receiver Coherent detection Non-coherent detection

Block diagram of a DeS

~ Source-~ encode-

Digital demodulationc==1_ Source

-~ decode- DetectDemod.Sample


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Bandpass modulation Bandpass modulation: The process of converting atasignal to a sinusoidal waveform whose amplitude, phase,

frequency, or a combination of them, is varied in accordancewith the transmitted data. Bandpass signal:

Si(t) = gr(t)~ C O S(U Jct+ ( - l )~UJt + ( A (t)) 0 ~ t ~ Twhere gr(t) is the baseband pulse shape with energyE .g We assume here (unless stated otherwise): gr (t) is a rectangular pulse shape with unit energy. Gray coding is used for mapping bits to symbols. 1 M E, denotes average symbol energy given byE, = M Ii~, 5

Bandpass Modulation Schemes

One dimensional waveforms Amplitude Shift Keying (ASK)

On-Off Keying M-ary Pulse Amplitude Modulation (M-PAM)

Two dimensional waveforms M-ary Phase Shift Keying (M-PSK) M-ary Quadrature Amplitude Modulation (M-QAM) Multidimensional waveforms M-ary Frequency Shift Keying (M-FSK)


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Demodulation and detection Demodulation: The receiver signal is converted tobaseband, filtered and sampled. Detection: Sampled values are used for detection using adecision rule such as ML detection rule.

' 1 / 1 (t )

z Decision "-= Z f--------o circuits -- m(ML detector)

Coherent and Non-coherentdetection Coherent detection

requires carrier phase recovery at the receiverand hence, circuits to estimate carrier phase

Possible sources of carrier-phase mismatch atthe receiver: Propagation delay causes carrier-phase offset inthe received signal. The oscillators at the receiver which generate thecarrier signal, are not usually phased locked to thetransmitted carrier.


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Coherent and non-coherentdetection ... Circuits such as Phase-Locked-Loop (PLL) areimplemented at the receiver for carrier phaseestimation (a ~ a ). t r a n C h J

Hcos(av+&) }L...---r--.....I Used by

1-------- 90 deg.~ correlatorsL . . . . - _ . . . . . . #Sin(OJ/+&ts- ________ Non-coherent detection L ~ b r a n c h J does not require carrier phase recovery (useseither differentially encoded modulation orenergy detectors. It has lesser complexity thancoherent detection, but at the price of a highererror rate.

One dimensional modulation,demodulation and detection Amplitude Shift Keying (ASK) modulation:

On-off keying (M=2):S j ( t ) = a j ' l / ) (t ) i = 1 , . .. ,M' 1 / ) (t ) =# cos(mct + )aj = . J E :

"0 " "1"

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One dimensional mod., ... M-ary Pulse Amplitude modulation (M-PAM)

Si(t) = a i# cos(UJct)Si(t ) = a ilf/l (t ) i=l, ... ,Mv. (t ) = H cos(wct)a i = (2i -1- M) . J E :Ei = l l s J =Eg(2i-1-MYE = (M2 -1)E

s 3 g

4-PAM:"00" "01" "II"

S1 S S3 . 2 I - 3 . . [ E ; - . . [ E ; 0 . . [ E ;E =_1 " M E.s M L..i~1 1

"10"S4 I 'lfJ/)3 . . [ E ;

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One dimensional mod., ...-cont'd Coherent detection of M-PAM

If/l (t )r(t) J' " rT ZI ML detector "-- - - - -+ - - - - - -- - J o r - - - - - - + (Compare with M-l thresholds) ---+m

-

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Two dimensional modulation, I : : : : .demodulation and detection (M-PSK :::: M-ary Phase Shift Keying (M-PSK)

Si( t )= ail!f/l(t)+a i2 !f/2(t) i= l, ... ,M!f/l(t)= H cos(m c t) !f/2(t)= HSil l (mct)

tt: ( 2 m ) tt:.( 2 m )il ="';i'Js cos M ai2 =v=. SIll Mt:=Ei = l l s J

Two dimensional mod, ... (MPSK)BPSK (M=2) Exceptional case: can be represented by single dimensio

'lf2(t)"0" "1" 8PSK(M=8)~1 l2

- . J E : . J E : 'lfJt)

QPSK(M=4)

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Two dimensional mod, ... (MPSK) Coherent detection of MPSK

' 1 / 1 (t )

,-------, "-Choose msmallest

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~

. . .Two dimensional mod, ... (M-QAM) :::: M-ary Quadrature Amplitude Mod. (M-QA ).:

Si(t) = ~2:i cos(UJct + ((Ji)Si(t) =a i [ lf /[ ( t) + a i21 f /2 ( t) i=l, ...Mv,(t) =H cos (a Jct ) 1 f/2(t) =HSin(aJct)where ai l and a i2 are PAM symbols

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Two dimensional mod, ... (M-QAM)16-QAM

'1/, (I)"0000" "0001" "0011" "0010"~1 '.2.... ~ ~.1 ' 4

"1000" "1001" "101~" "1010"$5 $6 ~7 $8 . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . . . .. . .. . .. . .. . . 1 " " . . .. . .. . . . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .- 6 - ~ ~ i3

_ _ _ , 1 " - 9 - - - - - ~ + !)-0- -+ _-l-_,..~-1.-- - - \ ! " " i 12-+'1/1(I)"1~00" "l1Q1" "11~1" -mn-L ~14 ~15 ~16 .. .. .. .. .. .. .. . .. .. .. .. 3 . . .. . .. .. .. .. .. .. .. .. ."0100" "0101" "0111" "0110"

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Two dimensional mod, ... (M-QAM) Coherent detection of M-QAM

If/j (t )

"-mML detector

(Compare with .J M -1 thresholds)Parallel-to-serial

converter

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ML detector(Compare with .J M -1 thresholds)


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Multi-dimentional modulation,demodulation & detection M-ary Frequency Shift keying (M-FSK)

Si(t) = ~ cos(m J)= ~ 2 : sco s (mc t + (i - l )~mt )I\r = ~m = _1_ Hz (f .. d t t rth Iij requency spacing require 0 guaran ee 0 ogona2Jr 2T FSK waveforms with coherent detection at receiver)

MsJt) =L>ijV/t) i =1,..., M

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a ..= { . J E : i = j'i 0 i * ' jVi (t ) =H c o s ( c u , t )

E, =E, = I I S i 1 1 2

"-m

Multi-dimensional mod, ... (M-FSK)

' 1 / 1 (t )ML detector:

ChooseZ the largest elementin the observed vector

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=Z


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Non-coherent detection Non-coherent detection: No need for a reference in-phase with the

received carrier Less complexity as compared to coherentdetection at the price of higher error rate.

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Differentially coherent detection Differential PSK (DPSK)

The information bits and previous symbol, determinethe phase of the current symbol.

Types of non-coherent detection


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Non-coherent detection Differential coherent detection

Differential encoding of the message The symbol phase changes if the current bit isdifferent from the previous bit.Si(t) =Jcos(av+ Bi(t)), 0 5, t 5, T, i =1,...,M

Bk(nT) =Bk((n -l)T) + ( A (nT)

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0 1 2 3 4 5 6 71 1 0 1 0 1 1

1 1 1 0 0 1 1 17r 7r 7r 0 07r 7r 7r

Non-coherent detection ... Coherent detection for diff encoded mod.

Symbol index: kData bits: mkDiff. encoded bitsSymbol phase: O k o

assumes slow variation in carrier-phase mismatch duringtwo symbol intervals. correlates the received signal with basis functions uses the phase difference between the current receivedvector and previously estimated symbol

ret) =J cos(av+ Bi(t) +a)+ net), 05, t 5, T(Bi(nT) +a )-(Bj((n -l)T) +a)= Bi(nT) - Bj((n -l)T) = (A(nT)'l/2(t)

(a,,~/,q)24'1 /1 (t )


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Non-coherent detection Optimum differentially coherent detector

' 1 / 1 (t )r(t) ~-----,----------

Sub-optimum differentially coherent detector

Performance degradation about 3 dB by using sub-optimum detector

Non-coherent detection ... E nergy d etection

Non-coherent detection for orthogonal signals (e.g.M-FSK) Carrier-phase offset causes partial correlation between Iand Q braches for each candidate signal.

The received energy corresponding to each candidatesignal is used for detection.

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Non-coherent detection ... Non-coherent detection of BFSK

r ( t ) + z(T) Decision stage:!-t-l--+if z(T) > 0, m =1ifz(T)

EEE353 Lec9 Handouts Dcs

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