8.0 Communication Systems
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Transcript of 8.0 Communication Systems
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8.0 Communication Systems Modulation: embedding an information-bearing
signal into a second signale.g. x(t) : information-bearing signal c(t) : carrier signal y(t) = x(t)c(t) : modulated signal
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Date Transmission (p.111 of 2.0)
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Communication
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8.1 Amplitude Modulation (AM) and Frequency-Division Multiplexing (FDM)
ModulationComplex Exponential Carrier
c
cc
tj
ctj
jjXjY
jC
etxty
tx
etc
cc
cc
0 ,2
:
frequencycarrier : , information-bearing signal
Demodulation
txetytz cctj See Fig. 8.1, p.584 of text
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Modulation
Sinusoidal Carrier
cc
ccc
cc
cc
jjXjjXjY
jC
ttxty
ttc
21
0 ,
cos
cos
See Fig. 8.4, p.586 of text
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Demodulation
Sinusoidal Carrier
ttx
ttxtz
ttytz
c
cc
cc
2cos121
0 , cos
cos2
See Fig. 8.6, 8.8, p.588, 589 of text
– Synchronous demodulation (detection)
A lowpass filter gives x(t)
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Demodulation
Sinusoidal Carrier
ccccc
cccc
ttx
tttxtz
2coscos21
coscos
– Synchronous demodulation (detection)
If the demodulating carrier is not in phase with the carrier
output signal reduced by synchronization required.phase-locked loops.
cc cos
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Demodulation
Sinusoidal Carrier
– Asynchronous demodulation (envelope detection)
envelope (the smooth curve connecting the peaks) carries the information, can be extracted in some other ways
the carrier component consumes energy but carries no information
See Fig. 8.10, 8.11, p.591, 592 of text
ccttxAty cos
always positive
See Fig. 8.14, p.593 of text
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Double-sideband (DSB)/Single-sideband (SSB)
Sinusoidal Carrier
double-sideband modulation uses twice the bandwidth
DSB/WC (with carrier) , DSB/SC (suppressed carrier), upper-sideband, lower-sideband
See Figs. 8.19, 8.20, 8.21, 8.22, p.598-601 of text
– a 90°phase-shift network can be used
0 ,
0 ,
j
jjH
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Vestigial Sideband
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Phase Shift Network
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Frequency-Division Multiplexing
(FDM)each signal allocated with a frequency slot. Many signals transmitted simultaneously over a single wideband channel using a single set of transmission facilities
See Figs. 8.15, 8.16, 8.17, p.594-596 of textSee Fig. 4.27, p.326 of text
Signals mixed in time domain but separated in frequency domain.
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Frequency Division Multiplexing(p.62 of 4.0)
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FDM with SSB
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(p.61 of 4.0)
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(p.60 of 4.0)
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8.2 Pulse Modulation andTime-Division Multiplexing
Amplitude Modulation with a pulse train carrier
kck
ck
ck
ck
kjXajY
kk
a
TkajC
tctxty
2sin
2 ,2
See Figs. 8.23, 8.24, p.602, 603 of text
A lowpass filter gives x(t) if sampling theorem is satisfied, ωc > 2 ωM
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Practical Sampling (p.20 of 7.0)
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– This remains true as long as c(t) is periodic, represented by a sequence ak. Sinusoidal AM is a special case here. Impulse train sampling is the case ∆ → 0.
Amplitude Modulation with a pulse train carrier
Pulse-Amplitude Modulation
– pulse amplitudes corresponds to the sample values
example : rectangular pulses (sample-and-hold)
See Fig. 8.26, p.606 of text
Sampling theorem applies.
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Practical Sampling (p.21 of 7.0)
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Time-Division Multiplexing (TDM)
Each signal allocated with a time slot in a period T.
Many signals transmitted simultaneously over a single channel using a single set of facilities
See Figs. 8.25, 8.27, p.605, 606 of text
Signals mixed in frequency domain but separated in time domain.
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Time Division Multiplexing (TDM)
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Intersymbol interference
pulses distorted during transmission and causing interference to adjacent symbols
See Figs. 8.30, 609 of text
(1) t
TtTtp
11 sin
See Figs. 8.28, 607 of text
,......3 ,2 , ,0 111 TTTttp
Pulses with zero intersymbol interference
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Intersymbol Interference
It is the sample values rather than pulse shapes to be transmitted
Distortionless transmission via distorted channels
intersymbol interference
x[1]x[2]
x[3]
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See Figs. 8.31, 610 of text
111
11 0 , T TjjPTjjP
P1(jw) with odd symmetry about 1Tj
(2)
else , 0
2 ,
,1
111
11
TTjP
T jPjP
,......3 ,2 , ,0 111 TTTttp
Pulses with zero intersymbol interference
Intersymbol interference
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Pulse coded modulation (PCM)
– binary representation of pulse amplitude (sample values) and binary transmission of signals
– much more easier to distinguish between 1’s and 0’s
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Date Transmission (p.111 of 2.0)
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8.3 Angle/Frequency Modulation
Angle Modulation
dt
tdxk
dttd
txkdt
td
txkt
tttty
pc
fc
pc
cc
0
coscos–
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Angle Modulation
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Angle Modulation– instantaneous frequency
dt
tdti
txkt
dttdxkt
fci
pci
frequency modulation
– featuresconstant envelope: transmitter always operates at peak
power
information not carried by amplitudes : amplitude disturbances eliminated to a large extent
Highly nonlinear process See Figs. 8.32, p.612 of text
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Fig. 8.32
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Consider the easiest caseSpectrum of FM Signals
indexmodulaton ,
sincos
,coscos
cos
m
ttty
AkttAkt
tAtx
m
mm
c
fmcmfci
m
Narrowband FM, m << π/2
DSB/WC coscoscos
FM narrowband sinsincos
sinsinsin
1sincos
2 ttmtty
ttmtty
tmtm
tm
cmc
cmc
mm
m
See Figs. 8.33, 8.34, p.615 of text
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Narrowband FM vs. DSB/WC
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Narrowband FM
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Wideband FM , m not small
Spectrum of FM Signals
tmttmtty mcmc sinsinsinsincoscos
See Figs. 8.35, p.616 of text
– cos(m sin ωmt), sin(m sin ωmt)
periodic with fundamental frequency ωm
with spectrum of impulses at multiples of ωm
n-th harmonics considered negligible, | n | > m
AkmB fm 222
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Example : periodic square wave signal
Spectrum of FM Signals
tTtrttrty
t
Aktx
cc
cci
f
cos2
cos
or
,1 wave,-square periodic:
See Figs. 8.36-8.39, p.617, 618 of text
jRejR
jjjRjjjR
jjjRjjjRjY
Tj
cc
cc
2
21
21
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8.4 Discrete-time ModulationComplex exponential carrier
deCeXeY
ncnxny
keC
enc
jjj
kc
j
nj c
21
22
2
See Figs. 8.41, p.620 of text
Complex exponential carrier nnc ccos
See Figs. 8.42, 8.43, p.621, 612 of text
Example : Software Radio
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Discrete-time Realization of
Continuous-time Modulation
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• Problem 8.25, p.633 of text− Frequency inverter as a speech scrambler for secure
speech communication
− Inverse system is itself
Problems
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• Problem 8.34, p.640 of text
− Implementing AM with a nonlinear element (multiplier is difficult to implement)
Problems
removed by filtering
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• Problem 8.39, p.645 of text
− Frequency Shift Keying (FSK) for digital transmission
Problems
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Date Transmission (p.111 of 2.0) (p.2 of 8.0)
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• Problem 8.39, p.645 of textProblems
(b) when T is a common multiple of the periods of
both and
(a)
“orthogonal” but evaluated in a period of T
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• Problem 8.40, p.646 of text
− Quadrature multiplexing
Problems
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• Problem 8.40, p.646 of text
Problems
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Sinusoidals (p.65 of 4.0)
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• Problem 8.40, p.646 of text
Problems
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• Problem 8.44, p.649 of text
− Zero-forcing equalizer for pulse transmission
Problems
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Intersymbol Interference (p.43 of 8.0)
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• Problem 8.44, p.649 of text
Problems
Requirement:
Example:
…