ee354 : communications system i
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EE354 : Communications System I. Lecture 25,26,27: Digital communication Aliazam Abbasfar. Outline. Digital communication Baseband systems Optimum receiver. Digital communication. Transfer of digital messages from source to destination reliably Sometimes called signaling - PowerPoint PPT PresentationTRANSCRIPT
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Lecture 25,26,27: Digital communication
Aliazam Abbasfar
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OutlineDigital communication
Baseband systems
Optimum receiver
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Digital communication Transfer of digital messages from source to destination reliably
Sometimes called signaling
Digital message Sequence of symbols (digits) Symbols are chosen from an alphabet (M symbols)
Binary symbols : bits : alphabet {0,1}
Data rate Symbol/Baud/Signaling rate (symbols per second) (r) bit rate (bits per second) (rb)
Reliability is measured by probability of error Symbol/Bit error rate (BER) Packet error rate (PER)
BER targets Voice : 10-5 Data : 10-6 Video : 10-7
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Digital systems
Digital source Digitized voice/images Data
Source encoder and decoder Data compression
Encryption
Channel encoder and decoder Error detection/correction Example : repetition code
Modulation/demodulation Digital Baseband/bandpass
SourcedecoderChannelSourc
eencoder
messagex(t) y(t)
Digital
Source
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Pulse Amplitude Modulation (PAM) A sequence of pulses with varying amplitudes
y(t) = ak p(t- kT) + n(t) T : symbol time
Inter-symbol interference (ISI) y(kT) = ak p(0) + am p(mT) + n(kT) p(0) = 1; p(mT) = 0; for all m<>0
Rectangular pulse Sinc pulse
Symbols are mapped into pulse amplitudes (ak) M-PAM has M levels unipolar 2-PAM levels: {0, A} Alphabet {0,1} bipolar 2-PAM levels: {-A, A} Alphabet {0,1} bipolar 2-PAM levels: {-A, A} Alphabet {0,1,2} bipolar 3-PAM levels: {-A, 0, A} Alphabet {0,1,2,3} bipolar 4-PAM levels: {-3A, -A, A, 3A}
Data rate Symbol rate : r= 1/T Bit rate : rb = log2(M)/T
Example: binary signaling with rectangular pulse Bipolar 2-PAM RZ and NRZ
T
y(t)
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Performance with noise AWGN with power 2
E[n2(t)] = 2
Sampled signal distribution No ISI and p(0)=1 z = y(kT) = ak + n(kT)
Symbol detection Compare with thresholds Slicer or A/D
Probability of error Pe = Pi Pe|i Pe|i : probability of error for ith symbol
Unipolar binary : Pe = Q(A/2) Bipolar binary : Pe = Q(A/) Bipolar M-PAM : Pe = 2(1-1/M) Q(A/)
= 2(1-1/M) Q(Amax/(M-1))
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Analog vs Digital repeaterDigital (regenerative) repeater detects the
symbols and regenerate them againPem = 1-(1-Pe)m m Pe Accumulate errors
Analog repeater amplifies signal + noise
Accumulate noisem
2 = m 2 Pem = 2(1-1/M) Q(A/m)
Hybrid repeater : A digital repeater after every m analog repeaterPemxk = k Pem
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Pulse detector x(t) = {0 or p(t)} + n(t)
p(t) is time-limited pulse p(t) = 0; t<0 or t> T
AWGN with power spectral density of N0/2 Rn() = N0/2 () Gn(f) = N0/2
Filter x(t) with H(f) and sample at time T Signal amplitude : Noise power :
Maximize A/2 Matched filter
H(f) = P(f)* e-j2fT h(t) = p(T-t)
Amax = Ep = EpN0/2
Probability of error
-
fT j2π dfP(f)H(f)e A
-
202 dfH(f)2
N
0
pmaxe 2N
EQ )
2
AQ( p
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Correlator Matched filter output is the correlation of the signal and the pulse
Detecting one out of two different pulses y(t) = {p0(t) or p1(t)} + n(t) y(t)-p0(t) = {0 or p1(t)-p0(t)} + n(t) Correlate y(t) with p1(t)-p0(t) Decision level : corr( [p1(t)+p0(t)]/2, p(t) )
Error probability
Correlator receiver Correlate y(t) with all pi(t) Detected symbol based on the output of the correlators
If we have a series of pulses, each pulse is detected by correlation
y(t) = ak p(t- kT) + n(t) Correlate y(t) with p(t-kT) ak
T
0 p E dt p(t) x(t) z(T)
0
p0-p1maxe 2N
EQ )
2
AQ( p
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ISI free matched filteringISI free : Matched filter output due to other
pulses = 0 Shifted versions of the pulse are orthogonalcombT(Rp())= Ep() rep1/T(|P(f)|2) = Cte
Folded spectrum is flat
Band-limited pulsesSinc pulseRoot raised cosine
δ[k] E dt kT)-p(t p(t) p
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Power spectrumx(t) = ak p(t- kT) = [ ak (t- kT)] p(t)
Gx(f) = Ga(f) |P(f)|2
Bipolar PAM : Ga(f) = E[ak
2]/T
Gx(f) = E[ak2]/T |P(f)|2
Px = E[ak2] Ep/T = Es/T
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Bandpass modulationsAmplitude shift keying (ASK)
x(t) = ak p(t- kT)
p(t) = cos(wct)
ak = 0 or A
Coherent detectionDown convert unipolar 2-PAM
Envelope detectorSimilar to AM (a strong carrier)
0
b
0
pe N
EQ
2N
EQ p
0
be 2N
Eexp
2
1 p
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PSKPhase shift keying (PSK)
x(t) = p(t- kT) p(t) = cos(wct + k)
BPSK Modulated bipolar 2-PAM x(t) = ak p(t- kT)
ak = -A or A p(t) = cos(wct)
QPSK x(t) = ak p1(t- kT) + bk p2(t- kT)
ak = -A or A p1(t) = cos(wct) p2(t) = sin(wct)
0
be N
2EQ p
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QAMQuadrature amplitude modulation(QAM)
Amplitude and phase modulations
x(t) = ak p1(t- kT) + bk p2(t- kT)
p1(t) = cos(wct)
p2(t) = sin(wct)
2 independent PAM
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FSKFrequency shift keying (FSK)
Two different frequencies fc1 and fc2
x(t) = {A cos(c1t) or A cos(c2t)}
Coherent detectionEp1-p2 = 2K Eb
K=1 when orthogonal pulses
Non-coherent detectionUse frequency detectors
0
b
0
p2-p1e N
EK Q
2N
EQ p
0
be 2N
Eexp
2
1 p
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ReadingCarlson Ch. 11.1, 11.2, 11.3