Turbo Codes Supercharge Internet of Things

Leti’s Turbo-FSK is a low-power, long-range solution for IoT

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Internet of Things networks are different beasts than traditional communication networks. Many, though not all, IoT networks are characterized by devices that communicate very little for most of the time, only to burst into action when they need to send—or receive—a relatively large amount of data in a very short amount of time.

That’s why researchers at Leti, the Laboratoire d’électronique des technologies de l’information in Grenoble, France, developed a new modulation scheme to improve IoT communication. What’s more, they have developed a system to integrate that scheme, and the waveforms it produces, alongside existing schemes into a more effective communication system for IoT devices. The system conserves power and ensures that IoT devices can send and receive signals over a reasonable area.

“The low-power, wide-area approach depends on two fundamental aspects,” says Vincent Berg, the head of Leti’s Smart Object Communication Laboratory. “First off is flexibility. The second aspect is low power, long range.”

But providing a long-range signal, especially if that signal is data-heavy, is difficult to do without it becoming a large power suck. That’s where the flexibility comes into play.

Leti’s system, Berg explains, allows a device to switch among three distinct waveforms, depending on its communication needs in the moment.

Take the example of an alarm system that consists of a smart camera pointed at the front door of a house. The vast majority of the time, that camera is in stand-by mode. It is only in the rare moment when someone is entering the house—or worse, trying to break in—that the camera would need to perk up, snap a couple pictures or a short video, and send that off while sounding the alarm, chewing through data and transmission power in the process.

Ideally, the smart camera would communicate via single-carrier frequency-division multiplexing (SCFDM), a modulation scheme that produces waveforms that excel at high data rates while being relatively low power. Berg says Leti performed field trials with a 25-milliwatt transmission signal. For SCFDM, a 196.9-kilohertz frequency waveform propagated 7.4 kilometers with a 122.6 kilobit per second data rate, while a 525-kHz waveform traveled 3.6 km with a 327 kb/s data rate.

But the moment a burglar appears at the door trying to break in, the camera needs to send out an alert, and pictures and video, as soon as possible. The best way to do that would be to switch over to another waveform, orthogonal frequency division multiplexing (OFDM), which can handle far higher data rates, at the cost of woefully high and inefficient power consumption. When Leti tested OFDM in the field, the OFDM waveforms transmitted at 25 mW required higher frequencies and didn’t travel quite as far. Berg says a 525-kHz waveform traveled 4.1 km and transmitted data at 327 kb/s, while a 1.18-MHz waveform traveled only 1.56 km while transmitting data at 1.47 Mb/s.

However, Leti researchers have developed a third modulation scheme, should the hypothetical camera need a more efficient waveform to communicate in noisy environments—like the home of a paranoiac with dozens of cameras watching every door and window. Leti’s Turbo Frequency Shift Keying (FSK), produces a waveform that operates very close to the Shannon limit—the maximum rate at which data can be transmitted through a noisy channel. Ideal for an environment with tons of smart devices operating in close proximity, Turbo-FSK could be the waveform of choice for a device to communicate quietly in the background, until it needs to leap into action. In fact, when Leti tested the scheme, the 25-mw signal produced a 65.6-kHz waveform that transmitted 3.8 kb/s up to 19.4 km, perfect for low-level background communication.

Turbo codes have been used in wireless communications for more than a decade to drastically improve the data rate for a given transmission power. Nowadays, they’re a commonplace addition to linear modulations to improve efficiency. Frequency-shift keying, on the other hand, is an orthogonal modulation, which switches between multiple frequencies to transmit data.

Berg says that Leti’s Turbo-FSK isn’t simply smashing turbo codes together with traditional frequency-shift keying. Instead, it requires exploiting the FSK’s orthogonal properties for efficiency using a set of codes distinct from those used to achieve efficiency with traditional turbo codes. The upshot is that Turbo-FSK achieves a longer-range, lower-sensitivity data transmission without much loss than most turbo codes.

Berg explains that Leti has developed a system that can easily toggle among the three waveforms depending on the ideal conditions at the time, as well as the individual device’s own needs. After developing the waveform algorithmically, Leti validated it using a programmable circuit on a standard pin grid array platform. The center is now looking for partners who are interested in developing the technology for integrated chips for new IoT devices.

Leti’s system, if successful, could improve the efficiency and effectiveness of IoT networks. Regardless, their researchers’ efforts prove the importance of finding new communication methods for these very different systems.

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