Pulse Interval Modulation (PIM): Basics, Advantages, and Disadvantages

This article explores the basics of Digital Pulse Interval Modulation (PIM), its advantages, disadvantages, and a comparison with other modulation techniques like PCM and PPM. PIM, along with other pulse modulation schemes, is often used in Free Space Optics (FSO) communication systems to improve average power efficiency, even if it means using more bandwidth.

The following pulse modulation schemes are employed which can provide higher average power efficiency at the expense of higher bandwidth requirement:

On-Off Keying (OOK)
Pulse Position Modulation (PPM)
Pulse Interval Modulation (PIM)

OOK Modulation

OOK modulation

OOK, or On-Off Keying, is a simple modulation scheme based on intensity modulation. In this scheme, a ‘0’ is represented by zero intensity, and a ‘1’ is represented by a positive intensity. For more details, refer to the difference between OOK vs ASK vs FSK.

PPM Modulation

PPM modulation

PPM stands for Pulse Position Modulation. In PPM, the relative position of a pulse is proportional to the amplitude of the modulating signal. This is similar to phase modulation (PM). See advantages and disadvantages of pulse modulation schemes such as PAM vs PWM vs PPM for a comprehensive view.

What is Pulse Interval Modulation (PIM)?

In PIM, data bits (typically in pairs) are encoded as the number of time intervals between successive pulses, as shown in the figure below.

PIM , Pulse Interval Modulation. Image Courtesy: slideplayer.com

The figure above illustrates a digital PIM waveform. It also presents a table showing how digital bit pairs are represented by different pulse interval durations.

Benefits (Advantages) of Pulse Interval Modulation (PIM)

The following are the advantages of Pulse Interval Modulation (PIM):

High Average Power Efficiency (APE): PIM offers higher APE compared to OOK modulation.
No Symbol-Level Synchronization Required: This simplifies the receiver design.
High Information Capacity: PIM can transmit more data within a given timeframe.
Simple Receiver Architecture: PIM receivers can be relatively simple to implement.

Drawbacks (Disadvantages) of Pulse Interval Modulation (PIM)

The main disadvantage of Pulse Interval Modulation (PIM) is:

Lower APE than PPM: PIM has a lower Average Power Efficiency (APE) compared to PPM modulation.

Comparison Between PIM, PCM, and PPM

The following points summarize the comparison between PIM, PCM, and PPM modulation techniques:

PCM (Pulse Code Modulation): In PCM, the analog signal (e.g., speech) is sampled and converted directly into a multi-bit digital code using an Analog-to-Digital converter (ADC). Each sample is represented by a digital binary value. For more information, read more on advantages and disadvantages of PCM modulation.
PPM (Pulse Position Modulation): In PPM, the position of the pulse relative to a reference point represents the amplitude of the modulating signal.
PIM (Pulse Interval Modulation): In PIM, data bits (usually pairs) are encoded as the number of time intervals between successive pulses, as shown in the table earlier.