signal encoding techniques - national tsing hua...
TRANSCRIPT
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Signal Encoding Techniques
Chapter 6
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Reasons for ChoosingEncoding Techniques
Digital data, digital signal Equipment less complex and expensive than
digital-to-analog modulation equipment
Analog data, digital signal Permits use of modern digital transmission and
switching equipment
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Reasons for ChoosingEncoding Techniques
Digital data, analog signal Some transmission media will only propagate
analog signals E.g., optical fiber and unguided media
Analog data, analog signal Analog data in electrical form can be
transmitted easily and cheaply Done with voice transmission over voice-grade
lines
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Signal Encoding Criteria
What determines how successful a receiver will be ininterpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth
An increase in data rate increases bit error rate (SeeFig. 2.9)
An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data
rate
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Factors Used to CompareEncoding Schemes
Signal spectrum With lack of high-frequency components, less
bandwidth required With no dc component, ac coupling via
transformer possible Transfer function of a channel is worse near band
edges Clocking
Ease of determining beginning and end of eachbit position
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Factors Used to CompareEncoding Schemes
Signal interference and noise immunity Performance in the presence of noise
Cost and complexity The higher the signal rate to achieve a given data
rate, the greater the cost
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Basic Encoding Techniques
Digital data to analog signal Amplitude-shift keying (ASK)
Amplitude difference of carrier frequency
Frequency-shift keying (FSK) Frequency difference near carrier frequency
Phase-shift keying (PSK) Phase of carrier signal shifted
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Basic Encoding Techniques
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Amplitude-Shift Keying
One binary digit represented by presence of carrier,at constant amplitude
Other binary digit represented by absence ofcarrier
where the carrier signal is Acos(2πfct)
ts tfA c2cos0
1binary0binary
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Amplitude-Shift Keying
Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical
fiber
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Binary Frequency-ShiftKeying (BFSK)
Two binary digits represented by two differentfrequencies near the carrier frequency
where f1 and f2 are offset from carrier frequency fc byequal but opposite amounts
ts tfA 12cos tfA 22cos
1binary0binary
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fc fc
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Binary Frequency-ShiftKeying (BFSK)
Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz)
radio transmission Can be used at higher frequencies on LANs
that use coaxial cable
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Multiple Frequency-ShiftKeying (MFSK)
More than two frequencies are used More bandwidth efficient but more susceptible to
error
f i = f c + (2i –1 –M)f d
f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L
L = number of bits per signal element
tfAts ii 2cos Mi 1
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Multiple Frequency-ShiftKeying (MFSK)
To match data rate of input bit stream,each output signal element is held for:
Ts=LT secondswhere T is the bit period (data rate = 1/T)
So, one signal element encodes L bits
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Multiple Frequency-ShiftKeying (MFSK)
Total bandwidth required 2Mfd
Minimum frequency separation required 2fd = 1/Ts
Therefore, modulator requires a bandwidthof Wd = 2L/LT = M/Ts
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Multiple Frequency-ShiftKeying (MFSK)
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Phase-Shift Keying (PSK)
Two-level PSK (BPSK) Uses two phases to represent binary digits
ts tfA c2cos tfA c2cos
1binary0binary
tfA c2cos tfA c2cos
1binary0binary
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Phase-Shift Keying (PSK)
Differential PSK (DPSK) Phase shift with reference to previous bit
Binary 0 –signal burst of same phase as previoussignal burst
Binary 1 –signal burst of opposite phase to previoussignal burst
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Phase-Shift Keying (PSK)
Four-level PSK (QPSK) Each element represents more than one bit
ts
42cos
tfA c 11
43
2cos tfA c
43
2cos tfA c
42cos
tfA c
01
00
10
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Phase-Shift Keying (PSK)
Multilevel PSK Using multiple phase angles with each angle having
more than one amplitude, multiple signals elementscan be achieved
D = modulation rate (symbol rate), baud R = data rate, bps M = number of different signal elements = 2L
L = number of bits per signal element
MR
LR
D2log
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Performance
Bandwidth of modulated signal (BT) ASK, PSK BT = (1 + r) R FSK BT = 2F + (1 + r) R R = bit rate 0 < r < 1; related to how technique is used to
filtered the signal F = f2 - fc = fc - f1
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Performance
Bandwidth of modulated signal (BT)
MPSK
MFSK
L = number of bits encoded per signal element M = number of different signal elements
RMr
RL
rBT
2log
11
R
MMr
BT
2log1
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Quadrature AmplitudeModulation
QAM is a combination of ASK and PSK Two different signals sent simultaneously on
the same carrier frequency
If two-level ASK is used, QAM is equal toQPSK
tftdtftdts cc 2sin2cos 21
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Quadrature AmplitudeModulation
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Reasons for AnalogModulation
Modulation of digital signals When only analog transmission facilities are available,
digital to analog conversion required
Modulation of analog signals A higher frequency may be needed for effective
transmission For unguided transmission, it is impossible to transmit
baseband signal Modulation permits frequency division multiplexing
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Basic Encoding Techniques
Analog data to analog signal Amplitude modulation (AM) Angle modulation
Frequency modulation (FM) Phase modulation (PM)
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Amplitude Modulation
tftxnts ca 2cos1 Amplitude Modulation
cos2fct = carrier x(t) = input signalna = modulation index <= 1
Ratio of amplitude of input signal to carrier
a.k.a double sideband transmitted carrier(DSBTC)
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Spectrum of AM signal
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Amplitude Modulation
Transmitted power
Pt = total transmitted power in s(t) Pc = transmitted power in carrier
21
2a
ctn
PP
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Single Sideband (SSB)
Variant of AM is single sideband (SSB) Sends only one sideband Eliminates other sideband and carrier
Advantages Only half the bandwidth is required Less power is required
Disadvantages Suppressed carrier can’t be used for synchronization
purposes
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Angle Modulation
Frequency modulation (FM) and phasemodulation (PM) are special cases of anglemodulation:
Phase modulation Phase is proportional to modulating signal
np = phase modulation index
ttfAts cc 2cos
tmnt p
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Angle Modulation
Frequency modulation Derivative of the phase is proportional to
modulating signal
nf = frequency modulation index
tmnt f'
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Angle Modulation
Compared to AM, FM and PM result in asignal whose bandwidth: is also centered at fc
but has a magnitude that is much different Angle modulation includes cos((t)) which
produces a wide range of frequencies
Thus, FM and PM require greaterbandwidth than AM
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Angle Modulation
For AM, BT = 2B Carson’s rule
where
The formula for FM becomes
BBT 12
BFBT 22
FMforPMfor
2
B
An
BF
An
mf
mp
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Basic Encoding Techniques
Analog data to digital signal Pulse code modulation (PCM) Delta modulation (DM)
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Analog Data to Digital Signal
Once analog data have been converted todigital signals, the digital data: can be transmitted using NRZ-L (non return to
zero) A negative voltage represents binary 1 and a
positive signal represents binary 0 can be encoded as a digital signal using a code
other than NRZ-L can be converted to an analog signal, using
previously discussed techniques
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Pulse Code Modulation
Based on the sampling theorem If a signal f(t) is sampled at a rate higher than twice the
highest signal frequency, then the samples contain allthe information of the original signal
Each analog sample is assigned a binary code Analog samples are referred to as pulse amplitude
modulation (PAM) samples
The digital signal consists of block of n bits,where each n-bit number is the amplitude of aPCM pulse
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Pulse Code Modulation
By quantizing the PAM pulse, originalsignal is only approximated
Leads to quantizing noise Signal-to-noise ratio for quantizing noise
Thus, each additional bit increases SNR by6 dB, or a factor of 4
dB76.102.6dB76.12log20SNR dB nn
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Delta Modulation
Analog input is approximated by staircasefunction Moves up or down by one quantization level ()
at each sampling interval
The bit stream approximates derivative ofanalog signal (rather than amplitude) 1 is generated if function goes up 0 otherwise
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Delta Modulation
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Delta Modulation
Two important parameters Size of step assigned to each binary digit () Sampling rate
Accuracy improved by increasing samplingrate However, this increases the data rate
Advantage of DM over PCM is thesimplicity of its implementation
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Reasons for Growth ofDigital Techniques
Growth in popularity of digital techniquesfor sending analog data Repeaters are used instead of amplifiers
No additive noise
TDM is used instead of FDM No intermodulation noise
Conversion to digital signaling allows use ofmore efficient digital switching techniques
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Exercises
1 –10, 12