Pulse-position modulation

Lore & Background

An ancient use of pulse-position modulation was the Greek hydraulic semaphore system invented by Aeneas Stymphalus around 350 B.C. that used the water clock principle to time signals. In this system, the draining of water acts as the timing device, and torches are used to signal the pulses. The system used identical water-filled containers whose drain could be turned on and off, and a float with a rod marked with various predetermined codes that represented military messages. The operators would place the containers on hills so they could be seen from each other at a distance. To send a message, the operators would use torches to signal the beginning and ending of the draining of the water, and the marking on the rod attached to the float would indicate the message.

In modern times, pulse-position modulation has origins in telegraph time-division multiplexing, which dates back to 1853, and evolved alongside pulse-code modulation and pulse-width modulation. In the early 1960s, Don Mathers and Doug Spreng of NASA invented pulse-position modulation used in radio-control (R/C) systems. PPM is currently being used in fiber-optic communications, deep-space communications, and continues to be used in R/C systems.

One of the key difficulties of implementing this technique is that the receiver must be properly synchronized to align the local clock with the beginning of each symbol. Therefore, it is often implemented differentially as differential pulse-position modulation, whereby each pulse position is encoded relative to the previous, such that the receiver must only measure the difference in the arrival time of successive pulses. It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of affecting all successive measurements.

Reader's Guide

Pulse-position modulation is notable for its ability to be implemented non-coherently, making it suitable for optical communications where coherent phase modulation is difficult and expensive. It is one of only two common M-ary non-coherent modulation techniques, the other being M-ary frequency-shift keying (M-FSK). PPM and M-FSK systems with the same bandwidth, average power, and transmission rate have identical performance in an additive white Gaussian noise channel, but their performance differs in fading channels: frequency-selective fading disrupts PPM severely while affecting only some M-FSK shifts, whereas frequency-flat fading impairs all M-FSK shifts but only a few PPM time-shifts due to the short pulse duration.

Narrowband RF channels with low power and long wavelengths are affected primarily by flat fading, making PPM better suited than M-FSK in these scenarios. A common application is radio control of model aircraft, boats and cars, first used in the early 1960s. The advantage is that the electronics required to decode the signal are extremely simple, leading to small, lightweight receiver/decoder units. A complete PPM frame is about 22.5 ms, with a start frame and up to 8 channels encoded by pulse timing. More sophisticated systems now often use pulse-code modulation, and the advent of 2.4 GHz band FHSS systems changed this further. PPM is also used for communication with ISO/IEC 15693 contactless smart cards and in the HF implementation of the Electronic Product Code Class 1 protocol for RFID tags.

Did You Know?

• Don Mathers and Doug Spreng of NASA invented PPM for radio-control systems in the early 1960s.
• PPM is inherently sensitive to multipath interference because information is encoded in the time of arrival of pulses.