line coding and decoding - · pdf filehdb3 (high density bipolar of order 3 code) replacing...
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Line coding and decoding
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Signal element versus data element
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Self-synchronization
� Receiver Setting the clock matching the sender’s
� Effect of lack of synchronization
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Other properties
� DC components
– Eliminating DC components from the signal power spectrum, the system to be AC coupled
– Magnetic recording system, and system using transformer coupling , have little sensitivity to low frequencies.
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� Transmission bandwidth
– Some schemes, such as multilevel codes, increase the efficiency of bandwidth by allowing a reduction in required bandwidth for a given data.
� Power efficiency
– Some codes utilize low power and are power efficient.
� Error detection and correction capability
– Detecting data errors without introducing additional error-detection bits, e.g. duobinarydetection bits, e.g. duobinary
� Self-clocking
– Some PCM codes have inherent synchronizing bits that aid in the recovery of the clock signal. e.g. Manchester codes.
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� Transparency
– Some codes are transparent, i.e. they have no long zero components.
� Noise Immunity
� NRZ better than RZ unipolar waveforms� NRZ better than RZ unipolar waveforms
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Line coding schemes
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Unipolar NRZ scheme
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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, the inversion or the lack of inversion determines the value of the bit. USB, Compact CD, and Fast-Ethernet.
• NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.
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� NRZ-L and NRZ-I both have a DC component problem.
Example� A system is using NRZ-I to transfer 1-Mbps data. What
are the average signal rate and minimum bandwidth?
� Solution
– The average signal rate is S = N/2 = 500 kbaud. The minimum bandwidth for this average baud rate is Bmin = S
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minimum bandwidth for this average baud rate is Bmin = S = 500 kHz.
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
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– 0 represented by alternating positive and negative
� DS1, E1
PSD of various line codes
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HDB3 (High Density Bipolar of order 3 code)
� Replacing series of four bits that are to 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
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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.
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
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null.
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
Amplitude0 0 0 0 0 0 0 01 0 1
10000000001 ���� +000+-0-+01 in general 00000000����000V(-V)0(-V)V
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� Evaluation
– Adds synchronization without changing the DC balance
– Error detection possible
� Used in T1/DS1
Time
Violation Violation
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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Multilevel: 4D-PAM5 scheme
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Multitransition: MLT-3 scheme
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Summary of line coding schemes
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Plus HDB3 and B8ZS