This document discusses various digital encoding and modulation techniques used for transmitting digital and analog data over transmission channels. It describes: - Digital signaling, where digital data is encoded into a digital signal using techniques like NRZ-L, NRZI, etc. to minimize bandwidth and errors. - Analog signaling, where analog or digital data modulates an analog carrier signal using techniques like ASK, FSK, PSK to transmit over analog lines. - Specific digital modulation techniques like BPSK, QPSK, MFSK that encode digital data onto signal properties like phase, frequency or amplitude to maximize bandwidth efficiency and minimize errors. - How analog modulation techniques like AM, FM, PM encode analog data onto an

1. Pass Band Transmission

2. Encoding and Modulation Techniques

3. Digital Signaling Versus Analog Signaling
• Digital signaling
• Digital or analog data is encoded into a digital signal
• Encoding may be chosen to conserve bandwidth or to minimize
error
• Analog Signaling
• Digital or analog data modulates analog carrier signal
• The frequency of the carrier fc is chosen to be compatible with
the transmission medium used
• Modulation: the amplitude, frequency or phase of the carrier
signal is varied in accordance with the modulating data signal
• by using different carrier frequencies, multiple data signals
(users) can share the same transmission medium

4. Digital Signaling
• Digital data, digital signal
• Simplest encoding scheme: assign one voltage level to binary one
and another voltage level to binary zero
• More complex encoding schemes: are used to improve
performance (reduce transmission bandwidth and minimize
errors).
• Examples are NRZ-L, NRZI, Manchester, etc.
• Analog data, Digital signal
• Analog data, such as voice and video
• Often digitized to be able to use digital transmission facility
• Example: Pulse Code Modulation (PCM), which involves
sampling the analog data periodically and quantizing the samples

5. 5/45
Analog Signaling
• Digital data, Analog Signal
• A modem converts digital data to an analog signal so that it can
be transmitted over an analog line
• The digital data modulates the amplitude, frequency, or phase of a
carrier analog signal
• Examples: Amplitude Shift Keying (ASK), Frequency Shift
Keying (FSK), Phase Shift Keying (PSK)
• Analog data, Analog Signal
• Analog data, such as voice and video modulate the amplitude,
frequency, or phase of a carrier signal to produce an analog signal
in a different frequency band
• Examples: Amplitude Modulation (AM), Frequency Modulation
(FM), Phase Modulation (PM)

6. 6/5
Digital Data, Digital Signal
• Digital signal
• discrete, discontinuous voltage pulses
• each pulse is a signal element
• binary data encoded into signal elements

7. Periodic signals
 Data element: a single binary 1 or 0
 Signal element: a voltage pulse of constant amplitude
 Unipolar: All signal elements have the same sign
 Polar: One logic state represented by positive voltage the other by negative
voltage
 Data rate: Rate of data (R) transmission in bits per second
 Duration or length of a bit: Time taken for transmitter to emit the bit
(Tb=1/R)
 Modulation rate: Rate at which the signal level changes, measured in baud
= signal elements per second. Depends on type of digital encoding used.

8. Interpreting Signals
• Need to know
• timing of bits: when they start and end
• signal levels: high or low
• factors affecting signal interpretation
• Data rate: increase data rate increases Bit Error Rate (BER)
• Signal to Noise Ratio (SNR): increase SNR decrease BER
• Bandwidth: increase bandwidth increase data rate
• encoding scheme: mapping from data bits to signal elements

9. 9/45
Comparison of Encoding Schemes
• Signal Spectrum
• Lack of high frequencies reduces required bandwidth,
• lack of dc component allows ac coupling via transformer, providing isolation,
• should concentrate power in the middle of the bandwidth
• Clocking
• synchronizing transmitter and receiver with a sync mechanism based on
suitable encoding
• Error Detection
• useful if can be built in to signal encoding
• signal interference and noise immunity
• cost and complexity: increases when increases data rate

10. 28/45
Modulation Techniques
Amplitude Shift Keying
(ASK)
Binary Frequency Shift
Keying (BFSK)
Binary Phase Shift Keying
(BPSK)

11. 29/45
Amplitude Shift Keying (ASK)
• In ASK, the two binary values are represented by to different
amplitudes of the carrier frequency
• The resulting modulated signal for one bit time is
• Susceptible to noise
• Inefficient modulation technique
• used for
• up to 1200bps on voice grade lines
• very high speeds over optical fiber




0,0
1),2cos(
)(
binary
binarytfA
ts c

12. 30/45
Binary Frequency Shift Keying (BFSK)
• The most common form of FSK is Binary FSK (BFSK)
• Two binary values represented by two different frequencies ( f1 and
f2 )
• less susceptible to noise than ASK
• used for
• up to 1200bps on voice grade lines
• high frequency radio (3 to 30MHz)
• even higher frequency on LANs using coaxial cable




0),2cos(
1),2cos(
)(
2
1
binarytfA
binarytfA
ts


0 0 1 1 0 1 0 0 0 1 0
f2 f2 f1 f1 f2 f1 f2 f2 f2 f1 f2

13. 31/45
Full-Duplex BFSK Transmission on
a Voice-Grade line
• Voice grade lines will pass voice frequencies in the range 300 to
3400Hz
• Full duplex means that signals are transmitted in both directions at
the same time
f1 f 3 f4f2

14. 32/45
Multiple FSK (MFSK)
• More than two frequencies (M frequencies) are used
• More bandwidth efficient compared to BFSK
• More susceptible to noise compared to BFSK
• MFSK signal:
elementsignalperbitsofnumberL
elementssignaldifferentofnumberM
frequencydifferencethef
frequencycarrierthef
fMiff
where
MitfAts
L
d
c
dci
ii






2
)12(
1),2cos()( 

15. 33/45
Multiple FSK (MFSK)
 MFSK signal:
 Period of signal element
 Minimum frequency separation
 MFSK signal bandwidth:
elementsignalperbitsofnumberL
elementssignaldifferentofnumberM
fMiff
where
MitfAts
L
dci
ii




2
)12(
1),2cos()( 
ddd MffMW 2)2( 
periodbitTperiodelementsignalTLTT bsbs ::,
)(2/12)/(12/1 ratebitLfTfLTfT dbdbds 

16. 34/45
Example
 With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following
frequency assignment for each of the 8 possible 3-bit data combinations:
 This scheme can support a data rate of:
KHzMfWbandwidth
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
ds 4002
425111
375110
325101
275100
225011
175010
125001
75000
8
7
6
5
4
3
2
1






















KbpsHzbitsLfT db 150)25)(3(22/1 
dci fMiff )12( 

17. 35/45
Example
• The following figure shows an example of MFSK with M=4. An
input bit stream of 20 bits is encoded 2bits at a time, with each of the
possible 2-bit combinations transmitted as a different frequency.
dc
dc
dc
dc
dci
fffi
fffi
fffi
fffi
fMiff
3411
310
201
3100
)12(
4
3
2
1






18. 36/45
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent data
• Binary PSK (BPSK): two phases represent two binary digits
0 0 1 1 0 1 0 0 0 1 0
π π 0 0 π 0 π π π 0 π
1)(),2cos()(
0),2cos(
1),2cos(
0),2cos(
1),2cos(
)(











tdtftAd
binarytfA
binarytfA
binarytfA
binarytfA
ts
c
c
c
c
c






19. 37/45
Differential PSK (DPSK)
• In DPSK, the phase shift is with reference to the previous bit
transmitted rather than to some constant reference signal
• Binary 0:signal burst with the same phase as the previous one
• Binary 1:signal burst of opposite phase to the preceding one

20. 38/45
Four-level PSK: Quadrature PSK (QPSK)
















10)
4
2cos(
00)
4
3
2cos(
01)
4
3
2cos(
11)
4
2cos(
)(








tfA
tfA
tfA
tfA
ts
c
c
c
c
• More efficient use of bandwidth if each signal element
represents more than one bit
• eg. shifts of /2 (90o)
• each signal element represents two bits
• split input data stream in two & modulate onto the phase of the
carrier
• can use 8 phase angles & more than one amplitude
• 9600bps modem uses 12 phase angles, four of which have two
amplitudes: this gives a total of 16 different signal elements

21. 39/45
QPSK and Offset QPSK (OQPSK)
Modulators
)2sin()(
2
1
)2cos()(
2
1
)(:
)2sin()(
2
1
)2cos()(
2
1
)(:
tfTtQtftItsOQPSK
tftQtftItsQPSK
cbc
cc





22. 40/45
Example of QPSK and OQPSK Waveforms
4
1101
4
3
1100
4
3
1110
4
1111
:










QPSKfor

23. 41/45
Performance of ASK, FSK, MFSK, PSK and
MPSK
• Bandwidth Efficiency
• ASK/PSK:
• MPSK:
• MFSK:
10,
1
1


 r
rB
R
bandwidthontransmissi
ratedata
T
elementssignaldifferentofnumberM
r
M
B
R
T
:,
1
log2


Mr
M
B
R
T )1(
log2


• Bit Error Rate (BER)
• bit error rate of PSK and QPSK are about 3dB superior to
ASK and FSK (see Fig. 5.4)
• for MFSK & MPSK have tradeoff between bandwidth
efficiency and error performance

24. 42/45
Performance of MFSK and MPSK
• MFSK: increasing M decreases BER and decreases bandwidth Efficiency
• MPSK: Increasing M increases BER and increases bandwidth efficiency

25. 43/45
Quadrature Amplitude Modulation (QAM)
• QAM used on asymmetric digital subscriber line
(ADSL) and some wireless standards
• combination of ASK and PSK
• logical extension of QPSK
• send two different signals simultaneously on same
carrier frequency
• use two copies of carrier, one shifted by 90°
• each carrier is ASK modulated

26. 44/45
QAM modulator
    
ASK
c
ASK
c tftdtftdtsQAM )2sin()()2cos()()(: 21  

27. 45/45
QAM Variants
• Two level ASK (two different amplitude levels)
• each of two streams in one of two states
• four state system
• essentially QPSK
• Four level ASK (four different amplitude levels)
• combined stream in one of 16 states
• Have 64 and 256 state systems
• Improved data rate for given bandwidth
• but increased potential error rate