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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