Week 5 - · PDF file3/19/2013 Muhammad Ali Jinnah University, Islamabad Digital...
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3/19/2013 Muhammad Ali Jinnah University, Islamabad Digital Communications EE3723 1
EE4723 : Digital Communications II
Week 5:
Line coding schemes
(Details and Characteristics)
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Line coding and decoding
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Signal element versus data element
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Synchronization
Receiver’s clock Setting must match the sender’s one
Effect of lack of synchronization
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Considerations for PCM Waveforms
DC components
Transmission bandwidth
Power efficiency
Error detection and correction capability
Favorable power spectral density
Adequate timing content – Self Synchronization
Noise and Interference Immunity
Cost and Complexity
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Polar NRZ-L and NRZ-I schemes
• In NRZ-L, the level of the voltage determines the value of the
bit: RS232
• In NRZ-I (-M or –S), the inversion or the lack of inversion
determines the value of the bit. USB, CD, and Fast-Ethernet
• NRZ-L and NRZ-I both have an average signal rate of N/2
Bd.
NRZ-L and NRZ-I both have a DC component problem.
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Example
A system is using NRZ-I to transfer 1-Mbps data. What
are the average signal rate and minimum bandwidth?
Solution
– The average signaling rate is S = N/2 = 500 kbaud. The
minimum bandwidth for this average baud rate is Bmin = S
= 500 kHz.
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RZ scheme
Return to zero
Self clocking
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Polar biphase: Manchester and differential Manchester
schemes In Manchester and differential Manchester encoding, the
transition at the middle of the bit is used for synchronization.
The minimum bandwidth of Manchester and differential
Manchester is 2 times that of NRZ. 802.3 token bus and 802.4
Ethernet
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Bipolar schemes: AMI and pseudoternary
In bipolar encoding, we use three levels: positive, zero, and
negative.
Pseudoternary:
– 1 represented by absence of line signal
– 0 represented by alternating positive and negative
DS1, E1
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PSD of various line codes
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HDB3 (High Density Bipolar of order 3 code)
Replacing series of four bits that are equal to "0" with a code word
"000V" or "B00V", where "V" is a pulse that violates the AMI law
of alternate polarity and is rectangular or some other shape. The
rules for using "000V" or "B00V" are as follows:
– "B00V" is used when up to the previous pulse, the coded signal
presents a DC component that is not null (the number of
positive pulses is not compensated for by the number of
negative pulses).
– "000V" is used under the same conditions as above when up to
the previous pulse the DC component is null.
– The pulse "B" ("B" for balancing), which respects the AMI
alternancy rule, has positive or negative polarity, ensuring that
two successive V pulses will have different polarity.
Used in E1
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HDB3
The timing information is preserved by embedding it in
the line signal even when long sequences of zeros are
transmitted, which allows the clock to be recovered
properly on reception.
The DC component of a signal that is coded in HDB3 is
null.
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Bipolar 8-Zero Substitution (B8ZS)
Adds synchronization for long strings of 0s
North American system
Same working principle as AMI except for eight consecutive 0s
Evaluation
– Adds synchronization without changing the DC balance
– Error detection possible
Used in T1
Amplitude
Time
0 0 0 0 0 0 0 0 1 0 1
Violation Violation
10000000001 +000+-0-+01 in general 00000000000V(-V)0(-V)V
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Coded Mark Inversion (CMI)
Another modification from AMI: Binary 0 is represented by a half period of
negative voltage followed by a half period of positive voltage
Advantages:
– good clock recovery and no d.c. offset
– simple circuitry for encoder and decoder compared with HDB3
Disadvantages: high bandwidth
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Multilevel: 2B1Q scheme
Integrated Services Digital Network ISDN
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Multitransition: MLT-3 scheme
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Summary of line coding schemes
Plus HDB3 and B8ZS
/Bipolar