NRZ Line Coding: Advantages and Disadvantages

This page explores the advantages and disadvantages of Non-Return-to-Zero (NRZ) pulse shapes used in line coding. It highlights the benefits and drawbacks of NRZ encoding.

Introduction

Digital signals are sequences of discrete, discontinuous voltage pulses. They’re used to map data bits using various encoding techniques such as NRZ, RZ, and biphase.

What is NRZ Encoding?

In NRZ encoding, binary data is represented by the voltage level of a pulse signal. The voltage level remains constant during the entire bit period (Tb). In NRZ coding, a binary ‘1’ is represented by a high voltage level, and a binary ‘0’ is represented by a low voltage level.

There are two variants of NRZ encoding:

• NRZ-Level (NRZ-L)
• NRZ-Inverted (NRZ-I)

NRZ-L (NRZ-Level)

In this type, the signal level depends on the state of the bit. A positive voltage maps to a binary ‘1’, and a negative voltage maps to a binary ‘0’, or vice versa. In NRZ-Level encoding, the polarity of the signal changes when the incoming signal changes from 1 to 0 or from 0 to 1. NRZ-L is similar to NRZ, except for the first data bit. NRZ doesn’t consider the 1st data bit as a polarity change, whereas NRZ-L encoding does.

NRZ-I (NRZ-Inverted)

In this type, a voltage inversion represents a binary ‘1’, and no voltage change represents a binary ‘0’. Data isn’t represented by any specific voltage level but by a transition between two polar voltages. A transition at the beginning of the bit interval is considered a binary ‘1’, whereas no transition is considered a binary ‘0’, or vice versa. This is also called differential encoding. NRZ-I has an advantage compared to NRZ-L.

Using unipolar NRZ encoding, a binary ‘1’ is mapped using a pulse of some amplitude ‘V’ for the entire bit period, and a binary ‘0’ is mapped by the absence of a pulse for the entire bit period.

Example of Unipolar NRZ coding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• OUTPUT: [+ve pulse, DC, +ve pulse, DC, DC, +ve pulse, +ve pulse, +ve pulse, DC]

Using polar NRZ encoding, a binary ‘1’ is mapped using a positive pulse of amplitude ‘V’, and a binary ‘0’ is mapped using a negative pulse of amplitude ‘V’.

Example of Polar NRZ coding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• OUTPUT: [+ve pulse, -ve pulse, +ve pulse, -ve pulse, -ve pulse, +ve pulse, +ve pulse, +ve pulse, -ve pulse]

Using Bipolar NRZ encoding, a binary ‘1’ is mapped using a positive pulse and a negative pulse of amplitude ‘V’ based on its alternate positions, and a binary ‘0’ is mapped by the absence of a pulse.

Example of Bipolar NRZ coding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• OUTPUT: [+ve pulse, DC, -ve pulse, DC, DC, +ve pulse, -ve pulse, +ve pulse, DC]

Benefits or Advantages of NRZ Encoding

The following are the benefits of NRZ encoding:

• It’s a simpler line coding technique than RZ, as the pulse doesn’t return to zero while mapping binary data (1s and 0s).
• Less bandwidth is needed for unipolar NRZ signaling.
• In polar and bipolar NRZ, there are no low-frequency components in the signaling waveforms after mapping.

Drawbacks or Disadvantages of NRZ Line Coding

The following are the drawbacks of NRZ line coding:

• The presence of low frequencies can cause droop in the signal waveforms.
• No error correction is performed.
• Long strings of ones and zeros can lead to a loss of synchronization between the clocks of the transmitter and receiver. A separate clock line is required between the transmitter and receiver to eliminate this problem.
• No clock is available within the signal itself.