Chapter 4
Baseband Data Transmission
&
Digital Modulation Techniques
Chapter Overview
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Baseband transmission
Line coding
Intersymbol Interference (ISI)
Nyquist Wave
Digital Modulation techniques
Baseband Transmission
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The original band of frequencies of a signal before it is
modulated for transmission at a higher frequency.
A type of data transmission in which digital or analog
data is sent over a single unmultiplexed channel, such as
an Ethernet LAN.
Baseband transmission use TDM to send simultaneous
bits of data along the full bandwidth of the transmission
channel.
Cont’d...

Baseband Center Point Detection

The detection of digital signals involve two processes:
1.
2.
Reduction of each received voltage pulse
(i.e. symbol) to a single numerical value.
Comparison of this value with a reference
voltage (or for multisymbol signaling, a set
of reference voltages) to determine which
symbol was transmitted.
Line Coding

Pulse modulation applied to binary symbol, the
resulting binary waveform is called PCM
waveform.
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
In telephony applications : line codes
Pulse modulation is applied to nonbinary
symbol, the resulting waveform called M-ary
pulse modulation waveform.
Some basic nomenclature
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Information source: can be either analog or digital
binary sequence: sequence of {1, 0} to describe information
source
Bit
symbol: a symbol represents k bits (M=2k)
Symbol alphabet: s0,s1,…,sM-1 (alphabet size M)
symbol stream: sequence of symbols selected from the
alphabet
Data rate
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Bit rate Rb in bits/sec (bps).
Symbol rate Rs in symbols/sec (sps)
Bit interval Tb: duration of a bit (sec/bit)
Symbol interval Ts: duration of a symbol (sec/symbol)
Important relation
1
(1) Rb 
,
Tb
1
(2) Rs 
Ts
Ts
(3) Rb  kRs , (4) Tb 
k
(5) M  2
k
Illustration and example
Cont’d...

Line codes
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
Converting standard logic level to a form more
suitable to telephone line transmission.
Four main groups
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Non return to zero (NRZ)
Return to Zero (RZ)
Phase encoded
Multilevel binary
Cont’d...
1.
Non Return To Zero (NRZ)
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Subgroups :
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NRZ-L
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NRZ-M
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Used extensively in digital logic circuit
Binary 1 one represented by one voltage level
Binary 0 is represented by another voltage level.
Used in magnetic tape recording.
The 1 (mark) is represented by change in level
The 0 (space) is represented by no change in level
Differential encoding
NRZ-S
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Complement of NRZ-M
1 is represented by no change in level
0 is represented by a change in level
Cont’d...
2.
Return To Zero (RZ)
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Subgroups:
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Unipolar RZ
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Bipolar RZ
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1 is represented by a half bit wide pulse.
0 is represented by the absence of pulse.
1 & 0 are represented by opposite level pulses that are one
half bit wide.
Pulse present in each bit interval.
RZ-AMI
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1 is represented by equal amplitude alternating pulses.
0 is represented by the absences of pulse.
Cont’d...
3.
Phase Encoded
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Subgroups:
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4.
Bi-phase-level @ Manchester coding
Bi-phase-mark
Bi-phase-space
Delay modulation @ Miller coding
Multilevel Binary
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Used three levels to encode the binary data.
Dicode and duo binary
Cont’d...
Parameters :
•
•
•
•
•
•
DC components
Self-clocking
Error detection
Bandwidth Compression
Differential encoding
Noise immunity
Cont’d...

PCM Word size
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Magnitude of the quantization
distortion error

L = number of quantization
levels, large enough
|e| < pVpp
|emax|
=
=
q/2
Vpp
Number of bits/sample,
2ι = L ≥ (1/2p)
levels
ι ≥ log 2 (1/2p)
bits
2(L-1)
=
Vpp
2L
ι
Cont’d...

M-ary Pulse Modulation waveforms
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Three basic ways to modulate information on to a
sequence of pulse
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PAM
PPM
PWM @ pulse duration modulation (PDM)
M-ary pulse modulation
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When the information samples are first quantized,
yielding symbols from an M-ary alphabet set, then
modulated on to pulses, the resulting pulse modulation is
digital.
Cont’d...
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M-ary PAM
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M-ary PPM
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one of M allowable amplitude levels are assigned
to each of the M possible symbol values.
modulation is effected by delaying or advancing a
pulse occurrence.
M-ary PWM

modulation is effected by varying the pulse width
by an amount that corresponds to the value of the
symbols.
M-ary pulse modulation blockdiagram
Binary
PCM
sequence
k-bit
tuple
Symbol
Sequence
M-ary Pulse
Modulator
M-ary
Pulse-modulation
Waveform
Illustration of PCM signals
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Bandwidth efficiency
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How much date rate can be supported by the
system with each unit frequency band
The higher bandwidth efficiency, the better.
Rb
bit rate
Bandwidth Efficiency=

Signal bandwdith W
bits/sec/Hz
Intersymbol Interference (ISI)
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Tx – the information symbols characterized as
impulse or voltage levels, modulate pulses that are
then filtered to comply with some bandwidth
constraint.
Baseband system – the channel has distributed
reactance that distorts the pulses.
Bandpass system – characterized by fading channels
that behave like undesirable filters manifesting signal
distortion.
Rx – equalizing filter is configured to compensate for
the distortion caused by Tx & channel.
Cont’d...

Equivalent system transfer function
H(f) = Ht(f) Hc(f) Hr(f)
Where
Ht(f) – transmitting filter
Hc(f) – filtering within the channel
Hr(f) – equalizing filter
Cont’d...
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Due to effect of system filtering, the received
pulses can overlap one another.
Tail of pulse can smear into adjacent symbol
intervals , thereby interfering with the
detection process and degrading the error
performance - Intersymbol interference (ISI)
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effects of filtering
Channel-induced distortion
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ISI measured by eye pattern
Cont’d...
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Nyquist filter is one whose frequency transfer
function can be represented by a rectangular function
convolved with any real even-symmetric frequency
function
Nyquist pulse is one whose shape can be represented
by a sinc (t/T) function multiplied by another time
function
Most popular of Nyquist filter
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Raised-cosine filter
Root-raised cosine filter
Cont’d...
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Pulse Shaping to reduce ISI
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Pulse that spread in time will degrade the system’s
error performance due to increase ISI.
Reduce the required system bandwidth.
Compress the bandwidth of the data impulse to
some reasonably small bandwidth greater than the
Nyquist minimum – pulse shaping with Nyquist
filter.
Zero ISI is only when the sampling is performed at
exactly the correct sampling time when the tails of
pulses are large.
Cont’d...
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Raised-cosine filter
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One frequently used H(f) transfer function
belonging to the Nyquist class (zero ISI at the
sampling time).
It can be expressed as
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Amplitude response of raised-cosine filter with
various roll-off factors
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Impulse response of raised-cosine filter with
various roll-off factors
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Consecutive raised-cosine impulses,
demonstrating zero-ISI property
Cont’d...
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Impulse response for the H(f)
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Minimum required bandwidth
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DSB bandwidth
Cont’d...
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Two types of error-performance degradation
1.
2.
Due to a decrease in received signal power or an
increase in noise or interference power, giving
rise to a loss in signal-to-noise ratio, Eb/No
Due to signal distortion such as ISI
Nyquist’s Wave
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Nyquist pulse
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Impulse response of the raised-cosine filter
Faster transition
Square-root Nyquist pulse
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Impulse response of a root-cosine filter
Does not exhibit zero ISI
Digital Modulation Techniques
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Digital modulation
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The process by which digital symbols are
transformed into waveforms that are compatible
with the characteristic of the channel.
Bandpass modulation
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The shaped pulses modulate a carrier wave

Radio transmission – the carrier is converted to EM
field for propagation to the desired destination
Cont’d...
Carrier Signal
v(t )  V sin( 2ft   )
•If the amplitude, V of the carrier is varied proportional to
the information signal, a digital modulated signal is called
Amplitude Shift Keying (ASK)
•If the frequency, f of the carrier is varied proportional to
the information signal, a digital modulated signal is called
Frequency Shift Keying (FSK)
Cont’d...
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If the phase, θ of the carrier is varied
proportional to the information signal, a
digital modulated signal is called Phase
Shift Keying (PSK)
If both the amplitude,V and the phase, θ of
the carrier are varied proportional to the
information signal, a digital modulated
signal is called Quadrature Amplitude
Modulation (QAM)
Cont’d...
Amplitude Shift Keying (ASK)

A binary information signal directly modulates the amplitude
of an analog carrier.
vask (t )  [1  vm (t )] A2 cos(ct )
Where vask (t) = amplitude shift keying wave
vm(t) = digital information signal (volt)
A/2 = unmodulated carrier amplitude (volt)
ωc = analog carrier radian frequency (rad/s)
 A cos(ct ) for logic '1' , vm (t )  1
vask (t )  
for logic '0' , vm (t )  1
 0
Frequency Shift Keying (FSK)
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
Called as BFSK
The phase shift in carrier frequency (∆f) is proportional to the
amplitude of the binary input signal (vm(t)) and the direction of the
shift is determined by the polarity
v fsk (t )  Vc cos2 [ f c  vm (t )f ]t
Where vfsk(t) = binary FSK waveform
Vc = peak analog carrier amplitude (volt)
fc = analog carrier center frequency (Hz)
∆f = peak shift in analog carrier frequency (Hz)
vm(t) = binary input signal (volt)
Vc cos2 [ f c  f ]t for logic '1' , vm (t )  1
v fsk (t )  
Vc cos2 [ f c  f ]t for logic '0' , vm (t )  1
f 
fm  fs
2
,
where
f  frequency deviation (Hz)
f m  f s  absolute difference between mark & space frequency (Hz)
FSK Bandwidth
B  ( f s  fb )  ( f m  fb )  f s  f m  2 fb  2(f  fb )
FSK Transmitter
Phase Shift Keying (PSK)
BPSK Transmitter
BPSK Receiver
BPSK
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Bit value 1 – sine wave
Bit value 0 – inverted sine wave
Very simple PSK
Low spectral efficiency
Robust , used in satellite system
Output phase vs time relationship
for a BPSK modulator
Binary
input
0
1
0
1
0
1
Time
BPSK
output
Time
QPSK
QPSK
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2 bits coded as one symbol
Symbol determines shift of sine wave
Needs less bandwidth compared to BPSK
More complex
QPSK signal in the time domain
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The binary data that is conveyed by this
waveform is: 1 1 0 0 0 1 1 0.
The odd bits, highlighted here, contribute to
the in-phase component: 1 1 0 0 0 1 1 0
The even bits, highlighted here, contribute to
the quadrature-phase component: 1 1 0 0 0 1 1
0
End of Chapter 4