Digital Communication EcE 4034 B.Tech. Second Year for EcE

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Digital Communication EcE 4034B.Tech. Second
Year for EcE

• Date 14.3.08
• Dr. Kyawt Khin
• Professor and Head
• Department of Electronic Engineering
• and Information Technology
• Yangon Technological University

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Chapter 12Digital Communication Concepts

• 12.1 Digital Information
• Bit
• Coding
• Coding Efficiency
• One bit can define 2 objects
• 2 bit can define 2 of 2 2 . 2 22 4 object
• 3 bit can define 2 of 2 of 2 2 . 2 . 2 23 8
  object
• 4 bit can define 2 of 2 of 2 of 2 2.2.2.2 24
  16 object

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• 
  2n
  M
• the number of required bits n
• different things or levels M

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12.2 Information Transfer rate (fi)

• Unit ?bit/ sec or bps
• e.g Serial digital word 101001 (6 bits)
• Time taken 6 ms

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12.3 Signaling (BAUD) Rate (fb)
Signal level (V)
1
t (ms)
0
1
6
8
9
3
4
5
7
0
Tb 1 ms fb 1/Tb 1 k baudNote In a
purely binary system the bit rate the baud
rateFig 12.1 Binary transmission
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e.g.Binary message 1 0
1 0 0 1 1 1Quaternary transmission
2V 2V 1V 3V
fi 1 kbps
fb 1 k baud (bit/sec)
Volts
Fig 12.2 Four level transmission of a binary
message
4
3
2
t (ms)
1
0
1
4
5
6
2
3
fi (transfer rate) 8 bits/4ms 2 kbps
fb (band rate) 4 symbols/4ms 1 k baud
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12.4 System Capacity (OR) Imformation
Capacity (C) C information x ( 1/Tm) (1/Tb)
where Tm is the message time 1/Tb is the
signaling rate log2M is the number of bits
(OR) Hartly Law C a B X T Where C
information capacity B
bandwidth , T transmission
time
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• 12.5 Bandwidth Considerations
• the minimum possible bandwidth required for a
  given pulse rate
• how pulses can be shaped to minimize the
  bandwidth and distortion of the data pulses
• fcmim cut off ? (1/2Tb) ½ fb
• Eg. If 1000 bit/s are transmitted NRZ,
• fcmim cut off ½ fb ½ x 1000 500 Hz

Fig 11.17 Squarewave and fundamental frequency
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• Continued
• Tb 1/ fb
• f 1/T 1/ 2Tb ½ fb
• BWmin ½ fb
• fb the transmission line bit-rate (baud rate)

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The pulse repetition rate is f 1/T (symbols/sec)
Volts
Amplitude (Volts)
A
t
T
Time domain description
f (Hz)
1/T 2/T
0
f 2/T
f 1/T
Frequency domain description
Figure 12-5 Time and frequency description of a
rectangular pulse train
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12.6 Power in Digital Signal

• Compare the power of an NRZ square wave to NRZ-
  bipolar

1
0
1
v
NRZ
t
A
0
1
0
1
v/2
NRZ-B
t
v
B
-v/2
Fig 12.2 Comparison of NRZ and NRZ-bipolar
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• Comparison of NRZ and NRZ- bipolar power in an
  NRZ signal NRZ signal
• PNRZ v2m /2R
• PNRZ-B 2(V/2)2/ 2R V2 / 4R
• It is seen that the on/off NRZ signal has twice
  the power of the NRZ-bipolar signal.
• Also, the instantaneous (peak) power for
• NRZ is V2/R and NRZ-B V2/ 4R,
• For a 41 difference in peak power dc power for
  rectangular RZ and NRZ signal.

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Digital Transmission Formats 1. NRZ Non-return
to zero 2. NRZ-B NRZ-Bipolar 3. RZ Return to
zero ( 50 duty cycle) 4. Biphase (Bi- ),
also called Manchester code 5. AMI Alternate
mark inversion
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Digital sequence
1
1
1
1
0
1
0
0
A. NRZ Nonreturn to zero
B. NRZ-B NRZ - Bipolar
C. RZ Return to zero(50 duty cycle)
D. Biphase (Bi- ) Also called Manchester
code
E. AMI Alternate mark inversion
Figure 12-10 A few digital transmission formats
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• Continued
• TTL (Transistor-Transistor Logic) Level Signal
  Format
• 01.3 volts for a logic 0
• 3.65 volts for a logic 1
• current level less than 16 mA

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12.7 PCM System Analysis

• Sampling f s gt 2 fA(max)
• fs sampling frequency
• fAmax input max frequency
• Quantization
• Encoding
• Quantization is the process of approximating
  sample levels into their closed fixed value.

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Digital clock
Analog input
Serial PCM output
A(t)
7
110
5
100
1
2
3
4
3
t
010
11 0
10 1
011
10 0
1
Digital signal
000
0
1
2
3
4
t
Ts
Sampling pulses
Figure 11.14 A 3-bit PCM system showing analog
to 3-bit digital
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Dynamic Range and Resolution Dynamic range is
the ratio of largest to smallest analog
signal.Resolution is the smallest analog input
voltage change that can be distinguished by A/D
converter. q V Fs /
2nwhere q resolution n number of
bits in the digital code word VFs
full-scale voltage range for the analog signal
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Dynamic Range( DR)

• ADC parameters V Fs / q
• 2n M
• DR Vmax/ Vmin 2n
• DR (dB) 20 log Vmax/ Vmin
• 20 log 2n 20n log 2 6.02n
• or DR(dB) ? 6n
• For linearly encoded PCM system
• DR(dB) 6 dB/ bit

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Signal to Quantization Noise Ratio (SQR)

• For input signal minimum amplitude
• SQR minimum voltage / quantization noise
• For input signal maximum amplitude
• SQR maximum voltage / quantization noise
• Linear quantizng in PCM systems has two major
  drawbacks.(i)
• Companding
• Companding is the process of compressing, then
  expanding.
• Or nonlinear encoding/decoding, called companding

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Companding

• Linear quantizng in PCM systems has two major
  drawbacks.
• The uniform step size means that weak analog
  signals will have a much poorer S/Nq than the
  strong signals.
• Systems of wide dynamic range require many ending
  bits and consequently wide system bandwidth.
• Companding
• Companding is the process of compressing, then
  expanding.
• Or nonlinear encoding/decoding, called companding

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A
B. Quantum uncertainty or quantization noise,
q/2
Fig 12.15Linear ADC characteristic and
quantization noise.
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References

• Advanced Electronic Communication Systems by
  WAYNE TOMASI, Mesa Community College, Second
  Edition
• Principles of Digital and Analog Communications
  by Jerry D. Gibson, Texas A M University
• 3. Electronic Communication Techniques by Paul
  H..Young, P.E. , Arizona State University
• Advanced Electronic Communication Systems by
  WAYNE TOMASI, Fifth Edition
• 5. Introduction to Digital and Data
  Communication