Pulse Position Modulation (PPM) and Variants

This document describes PPM (Pulse Position Modulation) and its variants, namely Multilevel PPM, Differential PPM, and Differential Amplitude PPM (DAPPM).

Introduction

Pulse Position Modulation (PPM) is a form of signal modulation where the information is encoded in the position of a pulse within a frame. Instead of varying the amplitude or frequency of the carrier wave, PPM varies the timing of the pulse.

This article explores different PPM variants, including:

• Multilevel PPM
• Differential PPM
• Differential Amplitude PPM (DAPPM)

Multilevel PPM

In standard PPM, each pulse position represents a specific symbol or bit pattern. Multilevel PPM expands upon this by allowing multiple distinct pulse positions within a single time slot. This increases the number of symbols that can be transmitted in a given timeframe, thereby boosting the data rate. Essentially, it’s like having more “slots” for each pulse to occupy, allowing for finer granularity in representing information.

Differential PPM

Differential PPM (DPPM) encodes information based on the difference in pulse position between successive pulses. Instead of the absolute position of a pulse conveying information, it’s the change in position relative to the previous pulse. This approach offers potential advantages in scenarios where the absolute timing reference is unstable or difficult to maintain. DPPM is less susceptible to errors caused by timing drift because it relies on relative changes rather than absolute values.

Differential Amplitude PPM (DAPPM)

Differential Amplitude PPM (DAPPM) combines the concepts of differential encoding with amplitude modulation alongside pulse position. In DAPPM, the information is encoded in the difference in pulse position and the amplitude of the pulse, both relative to the previous pulse. This allows for even greater data density compared to standard PPM or DPPM alone, as both pulse position difference and amplitude are used to represent different symbols. The receiver must therefore be able to decode both timing and amplitude information to correctly interpret the transmitted data.