COMMUNICATION SYSTEM EEEB453Chapter 5 (Part V)

DIGITAL TRANSMISSION-LINE ENCODING

Intan Shafinaz MustafaDept of Electrical Engineering

Universiti Tenaga Nasionalhttp://metalab.uniten.edu.my/~shafinaz

Line Encoding

The PCM signal is indeed digital, but it is not ready to be transmitted along the channel.

Line coding is the process of converting binary data (i.e a sequence of bits) to a digital signal.

Line Encoding

Line coding is use to overcome some typical problems like:  The frequency range of a baseband signal is very low

(near to zero, including DC). Such a frequency range isn’t suitable for transitions.

Many applications require synchronization, therefore the signal should imply when a bit (or a block of bits) starts and ends.

Some line coding scheme leaves a residual direct-current (DC) component (zero frequency).

This component is undesirable for 2 reason:i. Some system does not allow the passage of DC component,

the signal is distorted and may create errors in the output.ii. Create extra energy residing on the line and is useless.

a) The positive voltage are not canceled by the negative voltages.

b) The positive voltages are canceled by any negative voltage.

Lack of synchronization To correctly interpret the signals received from the sender, the

receiver’s bit intervals must correspond exactly to the sender’s bit intervals.

If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might interpret the signal differently than the sender intended.

Figure shows a situation in which the receiver has a shorter bit duration.

The sender send 10110001, while the receiver receives 110111000011

Line Coding Schemes

Unipolar encoding uses only one voltage level

Polar encoding uses two voltage levels (+ve and –ve)

Bipolar encoding uses 3 voltage levels (+ve, 0, -ve )

Unipolar Encoding

It has two major defects: first, it has a DC component, meaning that its average voltage is not 0 but some positive constant.

Some electrical components (e.g. capacitor) need constant change in voltage, and in case we have a sequence of ones, it won’t be the case.

Second, it has the inability to carry synchronization information. Again, if we have a series of ones, we won’t be able to know how many we got.

Types of Polar Encoding

NRZ-Level

This code is similar to the previous one. It handles the DC component issue, meaning the average voltage level is 0. It still has the synchronization problem.

NonReturn to Zero (NRZ) In NRZ-L the level of the signal is dependent upon the state of the bit.

In NRZ-I the signal is inverted if a 1 is encountered.

Return to Zero (RZ)

The signal changes not between bits but during bit.

Disadvantage – it requires 2 signal changes to encode 1 bit i.e more BW

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

Manchester Encoding

Differential Manchester Encoding

In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization.

The bit representation is defined by the inversion or noninversion at the beginning of the bit.

Bipolar AMI (Alternate Mark Inversion) Encoding

Word Mark means 1. So AMI means alternate 1 inversion.

A neutral, zero voltage represents binary 0.

Binary 1s are represented by alternating positive and negative voltages.

Here the problem of DC component (average not 0) was solved by introducing negative voltage level.

The code is not sensitive for polarity but we can lose synchronization on a long sequence of zeroes.

Bipolar RZ-AMI “0” represented by no line signal (0 Volt) “1” represented by half-positive or half-negative pulse Half-Pulses alternates in polarity (AMI) Second half-bit: zero Volt (RZ)

Here the problem of DC component (average not 0) was solved by introducing negative voltage level. The code is not sensitive for polarity but we can lose synchronization on a long sequence of zeroes.

Example – Encode the bit stream of 0001110101 usinga. Unipolarb. NRZ-Lc. NRZ-Id. Manchestere. Differential Manchesterf. AMI