High-speed infrared communications with DPPM

Efficient encoding of digital data for transmission over an infrared link


Navy scientists have optimized the use of Differential Pulse Position Modulation (DPPM) to speed infrared communications.

Varying of a signal waveform by DPPM is a technique used in applications that require power efficiency and speed such as optical communications. A single pulse encodes multiple bits of data in the differential time between pulses.

Transmitting four bits of data at a time decreases processing. And, because the only important measurement is the rising edge of each pulse, the pulse itself can be shorter than a full clock cycle, which saves power. Since many devices using infrared transmission rely on batteries and relatively slow processors, these advantages make DPPM encoding well suited for infrared data transmission.

Infrared communications often use a Light-Emitting Diode (LED) as a low-cost transmitter. However, there is a maximum rate at which any given LED can be switched. In order to exceed a certain data rate without custom or high-performance hardware or another transmission method, it is necessary to transmit more than one bit of data through a single pulse.

While it is possible to implement DPPM in the firmware of a microcontroller, it is not a task that a microcontroller can easily process in parallel with other tasks. In order to implement a reasonable speed (Mbps), the difference between pulse positions for a ‘3’ versus a ‘4’ can only be one or two clock cycles. If the microcontroller is engaged in some other task when a pulse is received, it may not be able to handle that data immediately. Even a slight delay in the reception of consecutive symbols could corrupt the data. Now, Navy scientists and engineers have optimized the use of DPPM to speed up IR communications.

The current invention includes an encoder that converts data from a set of parallel digital lines to a single data stream. This data stream is suitable and optimized for transmission using an infrared LED. However, it can also be used with several alternate channels. The optimization involves limiting the amount of time the LED is on to conserve power, as well as allowing a receiver to decode the data, converting it back to parallel digital data, without requiring a synchronized clock.

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