Unipolar vs. Polar vs. Bipolar Line Coding: A Detailed Comparison

This page explores the differences between unipolar, polar, and bipolar line coding techniques commonly used in digital communication. We’ll delve into each signaling type, outlining their advantages and disadvantages.

Introduction

In the world of signals, we primarily deal with two types: digital and analog. Digital signals represent information as binary zeros and ones. Line coding techniques, including unipolar, polar, and bipolar, are the methods we use to transmit this digital data using digital signals.

To minimize Inter-Symbol Interference (ISI) by preventing distortion and overlap of successive pulses, these techniques leverage pulse shapes like Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ).

What is Line Coding?

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

Figure-1 : Line Encoding and Decoding

As illustrated in Figure 1, a line encoder performs line coding at the sender. The resulting digital signal is then decoded by a line decoder at the receiver. The line encoder converts digital data into a digital signal format, and the line decoder reverses this process.

Unipolar Signaling

Figure-2/3 : Unipolar signaling

In unipolar signaling, a binary ‘1’ is represented by the presence of a pulse, while a binary ‘0’ is represented by the absence of a pulse. This is often referred to as “ON-OFF Keying.” It’s further divided into two categories: unipolar NRZ and unipolar RZ.

Advantages of Unipolar Signaling

• Simplicity: It’s a very straightforward line coding technique.
• Bandwidth Efficiency (NRZ): Unipolar NRZ requires minimal bandwidth.
• Clock Recovery (RZ): The spectral line present at the symbol rate in unipolar RZ can be used for clock synchronization.

Disadvantages of Unipolar Signaling

• Clocking Issues (NRZ): Unipolar NRZ lacks inherent clock information.
• Signal Droop: Low-frequency components in the signal waveforms can cause signal droop, especially in Unipolar NRZ. In Unipolar RZ, signal droop occurs where the signal is non-zero at 0 Hz.
• No Error Correction: Neither unipolar NRZ nor unipolar RZ provides error correction capabilities.
• Synchronization Loss: Long strings of ones or zeros can lead to a loss of synchronization (Unipolar NRZ).
• Bandwidth Consumption (RZ): Unipolar RZ occupies twice the bandwidth compared to unipolar NRZ.

Polar Signaling

Figure-4/5 : Polar signaling

In polar signaling, a binary ‘1’ is encoded as a positive pulse, while a binary ‘0’ is encoded as a negative pulse. Similar to unipolar, it’s categorized into polar NRZ and polar RZ types.

Advantages of Polar Signaling

• Simplicity: It’s a relatively simple technique.
• Reduced Low-Frequency Components: Low-frequency components are generally absent, mitigating signal droop issues.

Disadvantages of Polar Signaling

• No Error Correction: Polar signaling lacks error correction.
• No Clock Recovery: It doesn’t inherently provide clock information.
• Signal Droop: Signal droop can still occur at locations where the signal is non-zero at 0 Hz.
• Bandwidth Consumption (RZ): Polar RZ occupies twice the bandwidth compared to polar NRZ.

Bipolar Signaling

Figure-6/7 : Bipolar signaling

Bipolar signaling uses three voltage levels: positive, negative, and zero. A binary ‘0’ is encoded as a neutral zero voltage. A binary ‘1’ is encoded as either a positive or negative pulse, alternating between the two for consecutive ‘1’s. This alternation is known as Alternate Mark Inversion (AMI). Like the others, it comes in NRZ and RZ variations.

Advantages of Bipolar Signaling

• Simplicity: It’s a simple line coding type, similar to unipolar and polar.
• Reduced Low-Frequency Components: Low-frequency components are minimized.
• Bandwidth Efficiency: Bipolar signals generally occupy less bandwidth compared to unipolar and polar NRZ types.
• Signal Droop Mitigation: Signal droop is less of a concern compared to unipolar and polar signaling.
• Error Detection: Single-error detection is possible.

Disadvantages of Bipolar Signaling

• No Clock Recovery: It lacks inherent clock information.
• Synchronization Loss: Long strings of 1’s and 0’s can lead to a loss of synchronization.