Difference between Unipolar Polar Bipolar Line Coding

This page compares Unipolar vs Polar vs Bipolar line coding and mentions difference between Unipolar, Polar and Bipolar signaling types with their advantages and disadvantages. These line coding techniques are used in digital communication.

Introduction:
There are two types of signals viz. digital and analog. Digital signals are represented in the form of binary zeros and ones. Line coding techniques such as unipolar, polar and bipolar are used to transmit digital data using digital signals. The RZ and NRZ pulse shapes are used in these techniques to minimize ISI (Inter Symbol Interference) by avoiding distortion and overlapping of consecutive pulses.

What is Line Coding

Line coding is a technique which represents digital data into digital signals. Using this coding, it is possible to map sequence of bits into digital signal.

Figure-1 : Line Encoding and Decoding

Line encoder does line coding at the sender side as shown in figure-1. The same digital signal is decoded at the receiver side by line decoder. Line encoder converts digital data to digital signal format and line decoder does the reverse.

Unipolar Signaling

Figure-2/3 : Unipolar signaling

In the unipolar signaling type, binary one ('1') is encoded as presence of pulse where as binary zero ('0') is encoded as absence of pulse. Hence it is known as "ON-OFF Keying". It is categorized into unipolar NRZ and unipolar RZ types as described below.

Advantages of Unipolar Signaling

Following are the benefits or advantages of Unipolar Signaling.
➨It is very simple line coding technique.
➨Unipolar NRZ requires very less bandwidth (BW).
➨Spectral line present at symbol rate in unipolar RZ can be used as clock.

Disadvantages of Unipolar Signaling

Following are the drawbacks or disadvantages of Unipolar Signaling.
➨No clock is present in unipolar NRZ.
➨Low frequency components in the signal waveforms may cause signal droop (Unipolar NRZ). In Unipolar RZ signal droop is caused at places where signal is non-zero at 0 hertz.
➨No error correction is done (in both unipolar NRZ and unipolar RZ).
➨When long string of ones and zeros are present in the data, it leads to loss of synchronization (Unipolar NRZ).
➨Unipolar RZ occupies twice bandwidth as unipolar NRZ.

Polar Signaling

Figure-4/5 : Polar signaling

In the polar signaling type, one ('1') is encoded as positive pulse where as zero ('0') is encoded as negative pulse. It is further categorized into polar NRZ and polar RZ types.

Advantages of Polar Signaling

Following are the benefits or advantages of Polar Signaling.
➨It is simple technique.
➨Low frequency components are not present.

Disadvantages of Polar Signaling

Following are the drawbacks or disadvantages of Polar Signaling.
➨No error correction.
➨No clock is present.
➨Droop in signal is caused at locations where signal is non zero at 0 hertz.
➨Polar RZ signal occupies twice bandwidth than polar NRZ.

Bipolar Signaling

Figure-6/7 : Bipolar signaling

In the Bipolar signaling type, there are three voltage levels viz. positive, negative and zero. Binary '0' is encoded as neutral zero voltage. Binary '1' is encoded either as positive pulse or as negative pulse based on its alternate positions. Binary ones are mapped by alternatine positive and negative voltages. Hence it is called as Alternate Mark Inversion (AMI). It is further categorized into NRZ and RZ.

Advantages of Bipolar Signaling

Following are the benefits or advantages of Bipolar Signaling.
➨It is simple line coding type like unipolar and polar.
➨Low frequency components are not present.
➨Bipolar signal occupies low bandwidth than unipolar and polar NRZ types.
➨Signal droop does not occure in bipolar type unlike unipolar and polar.
➨Single error detection is possible.

Disadvantages of Bipolar Signaling

Following are the drawbacks or disadvantages of Bipolar Signaling.
➨Clock is not present.
➨Long string of 1's and 0's cause loss of synchronization.

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