RZ Line Coding: Advantages and Disadvantages

This page explores the advantages and disadvantages of using Return-to-Zero (RZ) pulse shaping in line coding. We’ll cover the benefits and drawbacks of RZ encoding in detail.

Introduction

Digital signals are sequences of discrete voltage pulses. Various encoding techniques, such as NRZ, RZ, and biphase, are used to map data bits.

What is RZ Encoding?

In RZ encoding, binary data is represented by pulses that have a transition, rather than maintaining a constant voltage level. The transition in RZ occurs precisely at the center of the bit period, hence the name ‘Return to Zero’. Unipolar, polar, and bipolar signaling rules can be applied before encoding the binary data using RZ.

Unipolar RZ

Using unipolar RZ encoding, a binary ‘1’ is mapped to a positive pulse with amplitude ‘V’ for half the bit period and then returns to zero for the next half. A binary ‘0’ is represented by the absence of a pulse for the entire bit period.

Example of Unipolar RZ encoding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• Bit period: Tb
• OUTPUT: [+ve pulse (over Tb/2 time duration) DC (Tb/2), DC (over Tb duration), +ve pulse (Tb/2) DC (Tb/2), DC (Tb) , DC (Tb), +ve pulse (Tb/2) DC (Tb/2) , +ve pulse (Tb/2) DC (Tb/2) , +ve pulse (Tb/2) DC (Tb/2) , DC (Tb) ]

Polar RZ

In polar RZ encoding, a binary ‘1’ is mapped to a positive pulse with amplitude ‘V’ in the first half of the bit period and returns to zero in the second half. A binary ‘0’ is mapped to a negative pulse with amplitude ‘V’ in the first half of the bit period and returns to zero in the next half.

Example of Polar RZ coding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• OUTPUT: [+ve pulse (Tb/2) DC (Tb/2), -ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2) , -ve pulse (Tb/2) DC (Tb/2), -ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC(Tb/2), -ve pulse(Tb/2) DC (Tb/2) ]

Bipolar RZ

In bipolar RZ encoding, a binary ‘1’ is mapped to a positive or negative pulse of amplitude ‘V’ based on its alternating position during the first half of the bit period. The polarity alternates for consecutive 1s. The signal remains at zero DC level for the next half-bit period. A binary ‘0’ is mapped to the absence of a pulse for the entire bit period.

Example of Bipolar RZ line coding:

• INPUT: [1 0 1 0 0 1 1 1 0]
• OUTPUT: [+ve pulse (over ‘Tb/2’) DC (‘Tb/2’), DC (over ‘Tb’), -ve pulse (‘Tb/2’) DC (‘Tb/2’), DC (‘Tb’), DC (‘Tb’), +ve pulse (‘Tb/2’) DC (‘Tb/2’) , -ve pulse (‘Tb/2’) DC (‘Tb/2’) , +ve pulse (‘Tb/2’) DC (‘Tb/2’), DC (‘Tb’)]

Benefits or Advantages of RZ Encoding

Here are the advantages of using RZ encoding:

• It is a simple line coding technique.
• In polar RZ and bipolar RZ, there are no low-frequency components.
• Bipolar NRZ/RZ signaling waveforms occupy lower bandwidth than unipolar NRZ and polar NRZ waveforms.
• Signal drooping does not occur in Bipolar coding, making it suitable for data transmission over AC-coupled lines.
• Single error detection is possible with this line coding technique.

Drawbacks or Disadvantages of RZ Line Coding

Here are the disadvantages of using RZ encoding:

• Signal droop occurs when the signal is non-zero at 0 Hertz.
• Unipolar/Polar RZ occupies twice the bandwidth compared to Unipolar/polar NRZ, respectively.
• No error correction capability.
• No inherent clock signal is available.
• Loss of synchronization can occur due to long strings of ones and zeros in the binary data.