RZ vs NRZ: Understanding the Differences in Line Coding Techniques

This article compares RZ (Return-to-Zero) and NRZ (Non-Return-to-Zero) line coding techniques, highlighting the differences between their pulse shapes. These methods are commonly used in digital communication and optical Duobinary transmission systems.

Introduction

Digital signals are represented using binary zeros and ones. Line coding techniques, including unipolar, polar, and bipolar, are employed to transmit this digital data using digital signals. RZ and NRZ pulse shapes are used within these techniques to minimize Inter-Symbol Interference (ISI) by reducing distortion and preventing overlapping of consecutive pulses.

What is Line Coding?

Line coding is a technique that represents digital data as digital signals. It maps a sequence of bits into a specific digital signal format.

Figure-1 : Line Encoding and Decoding

As shown in Figure 1, the line encoder performs line coding at the sender side, and the line decoder reverses the process at the receiver side.

Line coding offers several advantages:

• Bandwidth Reduction: Transmits multiple bits using a single signal pulse.
• Power Efficiency: Enhances power efficiency for a given bandwidth.
• Error Reduction: Decreases the probability of errors.
• Avoids Long Strings: Prevents long sequences of ones and zeros.
• Error Correction: Some techniques, like bipolar coding, offer error correction capabilities.

Let’s explore Unipolar, Polar, and Bipolar signaling types:

• Unipolar: Binary ‘1’ is represented by the presence of a pulse, while binary ‘0’ is represented by the absence of a pulse (also known as “ON-OFF Keying”). It is categorized into Unipolar NRZ and Unipolar RZ.
• Polar: Binary ‘1’ is represented by a positive pulse, and binary ‘0’ is represented by a negative pulse. It is categorized into Polar NRZ and Polar RZ.
• Bipolar: Uses three voltage levels: positive, negative, and zero. Binary ‘0’ is encoded as a neutral zero voltage. Binary ‘1’ is encoded as either a positive or negative pulse, alternating between them (also known as Alternate Mark Inversion or AMI). It is categorized into Bipolar NRZ and Bipolar RZ.

NRZ Pulse Shape | Non Return to Zero

In NRZ line coding, binary ‘1’ is represented by a positive voltage that remains constant throughout the bit period (T0), and binary ‘0’ is represented by zero voltage.

Unipolar NRZ

Figure-2 : Unipolar NRZ coding

In unipolar NRZ, binary ‘1’ is represented by a pulse with amplitude ‘V’, and binary ‘0’ is represented by the absence of a pulse.

Polar NRZ

Figure-3 : Polar NRZ coding

In polar NRZ, binary ‘1’ is represented by a positive pulse with amplitude ‘V’, and binary ‘0’ is represented by a negative pulse with amplitude ‘V’.

Bipolar NRZ

Figure-4 : Bipolar NRZ coding

In bipolar NRZ, binary ‘0’ is represented by zero voltage. Alternate binary ‘1’s are represented by a sequence of positive and negative pulses with the same amplitude ‘V’. The pulse duration equals the symbol bit duration.

Advantages of NRZ Line Coding

• Simpler than RZ, as the pulse does not return to zero while mapping binary data.
• Unipolar NRZ requires less bandwidth.
• Polar and bipolar NRZ do not have low-frequency components in the signaling waveforms after mapping.

Disadvantages of NRZ Line Coding

• Low frequencies can cause droop in the signal waveforms.
• No inherent error correction.
• Long strings of ones and zeros can lead to loss of synchronization.
• No explicit clock signal.

RZ Pulse Shape | Return to Zero

In RZ line coding, the pulse representing a binary signal returns to zero or ground potential after half of the bit period.

Unipolar RZ

Figure-5 : Unipolar RZ coding

Binary ‘1’ is represented by a pulse with a high-to-low transition. The pulse is high during the first half of the bit period and returns to zero in the second half. Binary zero is represented by the absence of a pulse.

Polar RZ

Figure-6 : Polar RZ coding

Binary ‘1’ is mapped by a pulse with a positive-to-negative transition, and binary ‘0’ is mapped by a pulse with a negative-to-positive transition. For binary ‘1’, the pulse is initially high for the first half of the bit period and low for the second half. For binary ‘0’, the pulse is initially low for the first half and high for the second half.

Bipolar RZ

Figure-7 : Bipolar RZ coding

Binary ‘0’ is represented as zero voltage. Alternate binary ones are represented by positive and negative pulses with a transition in the center from High->Low and Low->High, respectively. For example, given the binary data pattern 101001110:

• The first binary ‘1’ is represented by a pulse with positive voltage in the first half bit period and returns to zero in the next half bit period.
• The next is binary ‘zero’, represented by no voltage for the entire bit duration.
• The next is binary ‘one’, represented by a pulse with negative voltage in the first half and returns to zero voltage in the next half bit period.

This mapping continues for alternate ones in the digital data. The pulse duration is half of the symbol bit duration, compared to the Bipolar NRZ type.

Advantages of RZ Line Coding

• Simple line coding technique.
• Polar and bipolar RZ do not have 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.

Disadvantages of RZ Line Coding

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