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For Best Results, Send Molecular Messages Using MIMO

MIMO nearly doubles the data rate of digital communications by molecular means

5 min read
For Best Results, Send Molecular Messages Using MIMO
Researchers used this experimental set-up to test MIMO for molecular communications. It consists of two mechanized spray nozzles (left) and two breathalyzer sensors (right).
Photo: Changmin Lee and Nariman Farsad

Just as cell phones propagate radio waves to connect users, it’s also possible to transmit messages by emitting molecules. A sender can compose a message by diffusing bursts of a specific type of molecule, so long as a recipient can detect that molecule and interpret the pattern. Using this method, nanodevices could create a digital signaling system within the body, a locale where radio waves are quickly absorbed and where there is little space for bulky antennas.

Neurons and other cells in the body already communicate through the transfer of neurotransmitters, hormones, and other signaling molecules. And researchers have shown in early experiments that molecular digital communications through the air works but only with a fairly low data rate. Now, a team from Toronto’s York University and Yonsei University in Seoul, South Korea found that they can nearly double that data rate by applying multiple-input multiple-output (MIMO) technology.

MIMO is a technique more commonly applied to radio antennas. It boosts data rates by using at least two transmitters and two receivers to exchange messages, rather than a single pair. Using the traditional setup of just one transmitter and one receiver—placed 1 meter apart— the Yonsei-York group transmitted data at a rate of 0.2 bits per second (bps). But with two sets, they achieved a rate of 0.34 bps, spelling “YONSEI” in 88 seconds. For comparison, today’s LTE networks can reach peak download speeds of 50 Mbps.

“MIMO is very timely. It's new and it's challenging and it's part of the roadmap” for molecular communications, says Eduard Alarcon, an electrical engineer at Polytechnic University of Catalonia in Barcelona who was not involved in the research.

This group was working with alcohol molecules instead of radio waves, so they used motorized spray nozzles as transmitters, and cheap breathalyzer sensors as receivers. A tank of compressed air provided the force required to expel alcohol molecules from both nozzles at once, while a microcontroller coordinated the timing of these spritzes over 4 second intervals.  As the alcohol molecules from both sprayers spread through the air, the sensors recorded their presence after each timed spritz. If there were no alcohol molecules in the air, the sensors recorded a pause. As the system spritzed away, two more microcontrollers hooked to the sensors fed these logs to a computer for processing.

The entire set-up would cost around $400 to build from scratch, says Andrew Eckford, a co-author and electrical engineer at York University. He was part of a team that used molecular communications to transmit “O CANADA” in 2013 in one of the first live demonstrations of the technology.

In any molecular communications system, the receiver uses the concentration of alcohol molecules to interpret messages based on a code similar to Morse Code. Each letter in the alphabet is represented by a five-bit number sequence. For example, an “a,” is 11000. Each spritz of molecules represents a “1” and each non-spritz represents a “0”. By spritzing twice and then pausing for three beats, a sender can transmit an “a.”

It may seem like a laborious way to text a friend, but Chan-Byoung Chae, a co-author and communications researcher at Yonsei University, says that process wouldn’t be so painful for tiny sensors that merely need to communicate basic concepts, such as whether or not a heart monitor was working properly. With a single spritz or pause, a nanodevice placed inside the body  could alert physicians to a problem if it were somehow connected to the outside world. 

There are other potential uses, too, all of which are admittedly pretty far down the road. If an airplane deployed a fleet of molecular messengers into the ocean as it crashed, it might be easier to find. Eckford is even working on a project to enable robot rescuers to leave behind a molecular signal so other robots know an area has already been searched.

Still, the most successful examples of molecular messaging today are those that occur naturally. Neurons and cells rely on chemical messengers to carry information and trigger reactions throughout the body. Animals emit pheromones to attract or warn one another, and bacterial cells use chemical signals to count themselves and decide when they’ve reached a critical mass through a process called quorum sensing.

These naturally-occurring examples also provide further support for the idea that MIMO is a smart way to go for molecular communications research to go. “Most of the actual biological systems using molecular communications have multiple transmitters and multiple receivers,” points out Alarcon from Barcelona.

Back in the lab, the higher data rate the Yonsei-York researchers achieved was still not close to what radio waves deliver. But Weisi Guo, a molecular communications researcher at University of Warwick, thinks molecules could eventually offer data rates competitive to radio waves if researchers figure out how to build systems that release many types of molecules instead of just one.

However, this method would require a way to rapidly manufacture and disperse many chemicals at once or in quick succession. For now, “there is not magical machine that can suddenly produce these chemical compounds,” he says.

Rather than sending messages one spritz at a time, Guo also hopes to someday be able to securely encode a molecular message in DNA. Doing so would allow researchers to more quickly transmit complex messages by air to a recipient, who would decrypt it using biological markers matched to a shared code. That DNA encoding would be similar to the way the amplitude or frequency of radio waves are modulated to carry information, and could greatly hasten the communication process. Switching from spritzes to encoding information onto molecules is analogous to the jump from Morse code, which uses pulses of waves, to directly modulating radio waves to carry much more information.

In some cases, molecular communications may even outperform radio waves. Airborne molecules can easily maneuver through mazes, over mountains, and around buildings while waves are often absorbed. As Guo points out, he has strategically placed five routers throughout his home to ensure a reliable wireless signal, but the minute he begins to cook dinner—everyone in the house can smell it.

Alarcon says one of the next logical steps for researchers is to deploy molecules through massive MIMO, a technology in which many transmitters and receivers exchange molecules at once. But even with that possibility, he says this method will likely remain a low bandwidth form of communication for the near future.

A key challenge for the future will be building a networking layer that could  connect nanodevices in the human bloodstream to the external equipment used by a  physician to monitor a patient , Alarcon says. Chae, one of the original authors, says his next step is to try to achieve the equivalent of full duplex, or to be able to transmit and receive spritzes of molecules at once. Researchers will also have to drum up enough interest from commercial partners to continue this work.

Already, though, Guo says the technology is advanced enough to begin to build some basic prototypes. “If you weren't interested in data rates, if you just wanted to build a few nanorobots that can talk to one another using chemicals, that's probably doable,” Guo says.

The concept of molecular communications was first proposed by Satoshi Hiyama of NTT DOCOMO in 2008, and the technology’s broader potential is still uncertain. At the end of the day, molecules will likely find their place as a complimentary technology, rather than a competitive one. “It's not to challenge 5G or 4G,” Guo says. “It’s to open up, what some people would say, a new Internet of bio- or nano-things.”

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