‫دانشکده مهندس ی کامپیوتر‬
‫ارتباطات داده (‪)40-883‬‬
‫انتقال دیجیتال باندمیانی‬
‫نیمسال ّ‬
‫دوم ‪92-93‬‬
‫افشین ّ‬
‫همتیار‬
‫‪1‬‬
Passband Digital Transmission
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Introduction
Passband Transmission Model
Coherent Phase-Shift Keying
Hybrid Amplitude/Phase Modulation Schemes
Coherent frequency-Shift Keying
Detection of Signals with Unknown Phase
Noncoherent Orthogonal Modulation
Noncoherent Binary Frequency-Shift Keying
Differential Phase-Shift Keying
Comparison of Digital Modulation Schemes Using a Single Carrier
Voiceband Modems
Multichannel Modulation
Discrete Multitone
Synchronization
2
Introduction (1)
• Amplitude Shift Keying
• Phase Shift Keying
• Frequency Shift Keying
(with continuous phase)
3
Introduction (2)
Digital Modulation Techniques:
• Coherent
• Non-coherent
M-ary Signaling Schemes:
• M-ary ASK
• M-ary PSK
• M-ary FSK
• M-ary Amplitude-Phase Shift Keying (APK)
(Special case: Quadrature Amplitude Modulation (QAM)
4
Introduction (3)
Power Spectra:
Passband Signal
Baseband Signal (Complex)
Power Spectral Density
Bandwidth Efficiency:
5
Passband Transmission Model
Two assumptions for Channel:
• Channel is linear with wide enough bandwidth  no distortion
• Channel noise is zero-mean white Gaussian
6
Coherent Phase-Shift Keying (1)
(BPSK: Constellation)
7
Coherent Phase-Shift Keying (2)
(BPSK: Signal-Space Diagram)
8
Coherent Phase-Shift Keying (3)
(BPSK: Error Probability)
9
Coherent Phase-Shift Keying (4)
(BPSK: Generation and Coherent Detection)
Binary PSK Transmitter
Coherent Binary PSK Receiver
10
Coherent Phase-Shift Keying (5)
(BPSK: Power Spectra)
Pulse Shaping Function:
Baseband Power Spectrum
11
Coherent Phase-Shift Keying (6)
(QPSK: Constellation)
12
Coherent Phase-Shift Keying (7)
(QPSK: Signal-Space Diagram)
13
Coherent Phase-Shift Keying (8)
(QPSK: Example 6.1)
14
Coherent Phase-Shift Keying (9)
(QPSK: Error Probability)
for each
channel
of QPSK
15
Coherent Phase-Shift Keying (10)
(QPSK: Error Probability)
another
way
nearest
and
16
Coherent Phase-Shift Keying (11)
(QPSK: Generation and Coherent Detection)
QPSK Transmitter
Coherent QPSK Receiver
17
Coherent Phase-Shift Keying (12)
(QPSK: Power Spectra)
Pulse Shaping Function:
Baseband Power Spectrum
18
Coherent Phase-Shift Keying (13)
(Offset QPSK)
only
±90°
phase
transitions
but twice as
frequently
±180°
&
±90°
phase
transitions
QPSK
Offset QPSK
Possible paths for switching between message points
Basis
Functions
Same
probability of error
19
Coherent Phase-Shift Keying (14)
(π/4 Shifted QPSK)
QPSK
π/4 Shifted QPSK
Possible paths for switching between message points
20
Coherent Phase-Shift Keying (15)
(π/4 Shifted DQPSK)
Noncoherent Detector
21
Coherent Phase-Shift Keying (16)
(π/4 Shifted DQPSK: Example 6.2)
22
Coherent Phase-Shift Keying (17)
(M-ary PSK: Signal-Space Diagram)
23
Coherent Phase-Shift Keying (18)
(M-ary PSK: Error Probability)
(M-ary PSK: Power Spectra)
Baseband Power Spectrum
24
Coherent Phase-Shift Keying (19)
(M-ary PSK: Bandwidth Efficiency)
25
Hybrid Amplitude/Phase Modulation Schemes (1)
M-ary QAM is a two-dimensional generalization of M-ary PAM.
M-ary QAM Square Constellation: (M=L2)
Cartesian Product of L-ary PAM constellation
PAM
26
Hybrid Amplitude/Phase Modulation Schemes (2)
(M-ary QAM: Probability of Error)
Probability of correct detection:
(M=L2)
Probability of symbol error for L-ary PAM
Probability of symbol error for M-ary QAM:
27
Hybrid Amplitude/Phase Modulation Schemes (3)
(M-ary QAM: Cross Constellation)
(for odd number of bits per symbol)
2n-1 + 4x(2n-3) = 2n
28
Hybrid Amplitude/Phase Modulation Schemes (4)
(CAP: Carrierless Amplitude/Phase Modulation)
29
Hybrid Amplitude/Phase Modulation Schemes (5)
(CAP: Presentation)
• The transmitted signal s(t) appears to be carrierless.
• The transmitted signal s(t) represents a symbol-time-invariant
realization of hybrid amplitude and phase modulation.
Ignoring rotation 
30
Hybrid Amplitude/Phase Modulation Schemes (6)
(CAP: Properties)
31
Hybrid Amplitude/Phase Modulation Schemes (7)
(CAP: Example)
32
Hybrid Amplitude/Phase Modulation Schemes (8)
(CAP: Basic Structure)
33
Hybrid Amplitude/Phase Modulation Schemes (9)
(CAP: Basic Structure)
34
Hybrid Amplitude/Phase Modulation Schemes (10)
(CAP: Digital Implementation of CAP Receiver)
35
Coherent frequency-Shift Keying (1)
(BFSK: Constellation)
(Continuous Phase FSK)
36
Coherent frequency-Shift Keying (2)
(BFSK: Signal-Space Presentation)
37
Coherent frequency-Shift Keying (3)
(BFSK: Probability of Error)
Same for p01
38
Coherent frequency-Shift Keying (4)
(BFSK: Generation and Coherent Detection)
Transmitter
Coherent
Receiver
39
Coherent frequency-Shift Keying (5)
(BFSK: Power Spectra)
Baseband Power Spectrum
40
Coherent frequency-Shift Keying (6)
(MSK: Minimum Shift Keying)
CPFSK
Angle Modulated Signal
Center Frequency
Deviation Ratio
41
Coherent frequency-Shift Keying (7)
(MSK: Phase Trellis)
h=1
h=1  πh is same as –πh  No memory
(minimum value for orthogonal basis)
h=0.5  ±π/2 at odd multiples of Tb
and
0 , π at even multiples of Tb
Phase Trellis:
Boldfaced path represents the sequence
1101000.
Phase Tree
h=0.5
42
Coherent frequency-Shift Keying (8)
(MSK: Signal-Space Diagram)
Half Cycle
Cosine Pulse:
Half Cycle
Sine Pulse:
43
Coherent frequency-Shift Keying (9)
(MSK: Signal-Space Diagram)
44
Coherent frequency-Shift Keying (10)
(MSK: Signal-Space Diagram)
45
Coherent frequency-Shift Keying (11)
(MSK: Signal-Space Diagram)
46
Coherent frequency-Shift Keying (12)
(MSK: Example)
47
Coherent frequency-Shift Keying (13)
(MSK: Probability of Error)
Same as BPSK and QPSK(BER)
This good performance is the result of the detection of the MSK signal
being performed in the receiver on the observation over 2Tb interval.
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Coherent frequency-Shift Keying (14)
(MSK: Generation and Coherent Detection)
Transmitter
Coherent
Receiver
49
Coherent frequency-Shift Keying (15)
(MSK: Power Spectra)
3. In-phase and quadrature components are
statistically independent. Hence the
baseband power spectral density is given by:
50
Coherent frequency-Shift Keying (16)
(Gaussian-Filtered MSK: Necessity)
Desired specifications of MSK:
• Constant envelope
• Relatively narrow bandwidth
• Coherent detection performance equivalent to that of QPSK
Need to improve:
Although the out-of-band spectral characteristics of MSK signals
are good, but they do not satisfy the stringent requirements of
certain applications such as wireless communications.
51
Coherent frequency-Shift Keying (17)
(GMSK: Conditions )
Premodulation lowpass filter or baseband pulse-shaping filter:
1) Frequency response with narrow bandwidth and sharp cutoff
characteristics.
Is needed to suppress the high-frequency components of the
transmitted signal.
2) Impulse response with relatively low overshoot.
Avoids excessive deviation in the instantaneous frequency of the FM
signal.
3) Evolution of a phase trellis where the carrier phase of the modulated
signal assumes the two values ±π/2 at odd multiples of Tb and the two
values 0 and π at even multiples of Tb as in MSK.
Ensures that the modified FM signal can be coherently detected in the
same way as the MSK signal, or it can be noncoherently detected as a
simple binary FSK signal.
52
Coherent frequency-Shift Keying (18)
(GMSK: Shaping Pulse)
Frequency response
Impulse response
(W is the 3 dB baseband bandwidth)
53
Coherent frequency-Shift Keying (19)
(GMSK: Shaping Pulse)
54
Coherent frequency-Shift Keying (20)
(GMSK: Probability of Error)
(Empirical)
55
Coherent frequency-Shift Keying (21)
(GMSK: Power Spectra)
56
Coherent frequency-Shift Keying (22)
(GMSK: Example)
57
Coherent frequency-Shift Keying (23)
(M-ary FSK)
Constellation
Probability
of Error
58
Coherent frequency-Shift Keying (24)
(M-ary FSK)
Power
Spectra
Bandwidth
Efficiency
59
Detection of Signals with Unknown Phase (1)
(Optimum Quadratic Receiver)
60
Detection of Signals with Unknown Phase (2)
(Optimum Quadratic Receiver)
61
Detection of Signals with Unknown Phase (3)
(Optimum Quadratic Receiver)
Binary
Hypothesis
Test
62
Detection of Signals with Unknown Phase (4)
(Optimum Quadratic Receiver)
Quadrature
receiver
using
correlators
Quadrature
receiver
using
matched
filters
63
Detection of Signals with Unknown Phase (5)
(Optimum Quadratic Receiver)
Noncoherent
matched
filter
Output of
matched
filter for a
rectangular
RF wave
64
Noncoherent Orthogonal Modulation (1)
Sent: s1(t) and s2(t) (two orthogonal signals with equal energy)
Received: g1(t) and g2(t) (phase shifted versions of s1(t) and s2(t))
Generalized binary receiver for noncoherent orthogonal modulation
65
Noncoherent Orthogonal Modulation (2)
(Scaled version of si(t))
Quadrature receiver equivalent to either one of the two matched filters
66
Noncoherent Binary Frequency-Shift Keying
Transmitted FSK Signal
Noncoherent receiver for the detection of binary FSK signal
67
Differential Phase-Shift Keying (1)
Two basic operations: Differential encoding of input binary wave
and Phase Shift Keying  DPSK
Transmitted for “1”
Transmitted for “0”
68
Differential Phase-Shift Keying (2)
DPSK Transmitter
An example
69
Differential Phase-Shift Keying (3)
DPSK Receiver
70
Comparison of Digital Modulation Schemes (1)
71
Comparison of Digital Modulation Schemes (2)
72
Comparison of Digital Modulation Schemes (3)
1) Increasing Eb/N0  Decreasing BER
2) For any Eb/N0 , PSK, QPSK and MSK have smaller BER
3) 3dB less Eb/N0 requirement for PSK and DPSK
than Coh. FSK and Noncoh. FSK
to realize the same BER
4) At high Eb/N0 , DPSK and Noncoh. FSK perform as Coh. FSK
5) In Coherent QPSK two orthogonal carriers are used. Thus two
independent bit streams can be transmitted simultanuously
and subsequently detected in the receiver.
6) In coherent MSK two orthogonal carriers are modulated by the
two antipodal symbol shaping pulses, respectively, over 2Tb
intervals. The receiver uses a coherent phase decoding process
over two successive bit intervals.
7) QPSK has not memory, but MSK has.
73
Comparison of Digital Modulation Schemes (4)
74
Voiceband Modems (1)
MoDem: Modulator-Demodulator
PSTN: Public Switched Telephone Network
ISP:
Internet Service Provider
Symmetric Configuration: data rate downstream = data rate upstream
Asymmetric Configuration: data rate downstream > data rate upstream
75
Voiceband Modems (2)
Signal constellation of V.32 Modem using nonredundant coding
76
Voiceband Modems (3)
Signal constellation of V.32 Modem using trellis coding
77
Multichannel Modulation (1)
(Capacity of AWGN Channel)
Shannon’s
information capacity theorem
Signal to noise ratio gap
Capacity of implementable system
78
Multichannel Modulation (2)
(Continuous-Time Channel Partitioning)
The need for complicated equalization of a wide-band channel is
replaced by the need for multiplexing and de-multiplexing the
transmission of the incoming data stream over a large number
narrowband sub-channels that are contiguous and disjoint.
79
Multichannel Modulation (3)
80
Multichannel Modulation (4)
(Properties of pass-band basis functions)
81
Multichannel Modulation (5)
(Properties of pass-band basis functions)
82
Multichannel Modulation (6)
(Geometric Signal to Noise Ratio)
Assuming
is high enough to
ignore the two unity terms
83
Multichannel Modulation (7)
(Loading of the Multichannel Transmission System)
Channel Effect:
Total bit rate:
Constraint:
Constrained optimization problem:
Maximize the rate for the entire multichannel transmission system
through an optimal sharing of the total transmit power P between the N
sub-channels, subject to the constraint the P in maintained constant.
84
Multichannel Modulation (8)
(Loading of the Multichannel Transmission System)
Method of
Lagrange
Multipliers 
Differentiating J with respect to Pn
85
Multichannel Modulation (8)
(Water-Filling Interpretation of the Optimization Problem)
Set of equations to be solved:
K always should to be positive. But some Ps may be negative. They should
to be discarded and the new set of equations should to be solved again.
86
Multichannel Modulation (8)
(Water-Filling Interpretation of the Optimization Problem)
87
Discrete Multitone (1)
88
Discrete Multitone (2)
(Functional Blocks)
Transmitter:
89
Discrete Multitone (3)
(Functional Blocks)
Receiver:
90
Discrete Multitone (4)
(Applications)
91
Discrete Multitone (5)
(Comparison of DSL and Voiceband Modem)
Voiceband Modem
(33.6Kbps Upstream & 56Kbps downstream)
ADSL/VDSL
160Kbps/3Mbps Upstream &
1.54Mbps /26Mbps Downstream
92
Orthogonal Frequency-Division Multiplexing (1)
Another closely related form of multichannel modulation is
orthogonal frequency-division multiplexing (OFDM) that
differs from DMT in areas of application and some aspects of
its design.
OFDM is used for data transmission over radio broadcast
channels and wireless communication channels.
Unlike DMT that uses loading for bit allocation, OFDM uses a
fixed number of bits per subchannel. This restriction is made
necessary by the fact that a broadcast channel involves oneway transmission, and in a wireless communications
environment the channel is varying too rapidly. Accordingly,
in both cases it is not feasible for the transmitter to know the
channel and how to load it.
93
Orthogonal Frequency-Division Multiplexing (2)
The block diagram of DMT applies equally to OFDM except
for the signal constellation encoder does not include a
loading algorithm for bit allocation. In addition, two other
changes have to be made:
 In the transmitter, an upconverter is included after the
digital-to-analog converter to translate frequency, thereby
facilitating the propagation of the transmitted signal over a
radio channel.
 In the receiver, a downconverter is included before the
analog-to-digital converter to undo frequency translation
that was performed by the upconverter in the transmitter.
94
Orthogonal Frequency-Division Multiplexing (3)
Applications:
 Wireless communications
OFDM, combined with coding and interleaving, provides an
effective technique to combat multipath fading that is a
characteristic feature of wireless communication channels.
 Digital audio broadcasting (DAB)
OFDM has been adopted as the standard for digital audio
broadcasting in Europe. Here again the system involves the
combined use of coding and interleaving.
95
Synchronization (1)
 When coherent detection is used, knowledge of both the
frequency and phase of the carrier is necessary. The
estimation of carrier phase and frequency is called carrier
recovery or carrier synchronization.
 To perform demodulation, the receiver has to know the
instants of time at which the modulation can change its state.
It has to know the starting and finishing times of the
individual symbols. So it may determine when to sample and
when to quench the product-integrators. The estimation of
these times is called clock recovery or symbol
synchronization.
96
Synchronization (2)
 Data-aided Synchronization
A preamble, which contains information about the carrier and
symbol timing, is transmitted along with the data bearing signal
in a time-multiplexed manner on a periodic basis.
Its limitations are two-fold:
1) reduced data-throughput efficiency
2) reduced power efficiency
 Nondata-aided Synchronization
The use of a preamble is avoided, and the receiver has the task
of establishing synchronization by extracting the necessary
information from the modulated signal.
Throughput and power efficiency are improved but at the
expense of an increase in the time taken to establish
synchronization.
97