Improving LPWANs for the Internet of Things
Michael Alba posted on September 21, 2017 | 3136 views
The experimental setup used to test Choir. (Image courtesy of Eletreby et al.)
The experimental setup used to test Choir. (Image courtesy of Eletreby et al.)

A team of researchers has developed a new system, Choir, that can overcome some of the drawbacks of low-power wide-area networks (LPWANs). LPWANs are a promising communication option for the Internet of Things (IoT), which requires low-power devices that can nonetheless send data at regular intervals.However, LPWANs are difficult to deploy in dense urban environments because of increased interference and attenuation of signals.

Exploiting Hardware Offsets

Choir, which was presented in a paper at SIGCOMM 2017 in Los Angeles, is a system designed to work around these difficulties. Choir is fully implemented at the LPWAN base station, so it doesn’t require any hardware modifications to LPWAN nodes (i.e. IoT devices). In fact, Choir depends on exploiting LPWAN hardware imperfections inherent in each node.

Each LPWAN node will deviate slightly from the prescribed time, frequency and phase of LPWAN transmissions, due to minor imperfections in the hardware. These deviations, called offsets, can be used to filter interfering transmissions. Choir uses a series of algorithms that exploit these offsets and allow the system to reconstruct data packets coming from separate transmitters.

“You can think about it this way: An ideal radio should be able to transmit and receive signals at 900 megahertz,” said Swarun Kumar, one of the Choir researchers from Carnegie Mellon University (CMU). “But in practice, because these radios are imperfect, there will be small differences. If you ask five or six different radios to transmit at 900 megahertz, they will all transmit at slightly different sets of frequencies. Some might transmit at 900.01. Others might transmit at 900.05. Our system, Choir, exploits these differences and filters out the received signal to essentially disentangle the transmissions.”

The benefit of this method is that it increases the LPWAN throughput in dense urban environments and can also be used to improve LPWAN range. At the same time, Choir improves the latency and battery life of the LPWAN nodes, because of the decreased need for retransmission.

The bottom line? In a test setup covering 10 square kilometers of the CMU campus, the researchers demonstrated that Choir can achieve a throughput gain of 6.84x and a range gain of 2.65x (compared to standard LoRaWAN, a widely-used LPWAN protocol). Furthermore, Choir achieves a 4.88x reduction in latency and a 4.54x reduction in the number of transmissions for LPWAN nodes.

Pushing the LP in LPWAN

Though still in the early stages, IoT engineers should take note of Choir as a means of reducing LPWAN power consumption. Battery life is a critical aspect of many IoT devices, especially those designed with LPWANs, and improving this one parameter is a big step towards improving your IoT devices.So whether it’s Choir or some other method, always be on the lookout for power-saving opportunities.

To learn more about Choir, you can read the SIGCOMM 2017 paper here. To learn more about LPWANs, read “IoT Trends: Low-Power Wide-Area Networks and Standard Consolidation.”

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