Only by adding a few cents’ worth of components (similar to what would be found in a passive RFID tag) to an existing Bluetooth Low Energy radio, the researchers were able to expand a connected device’s Bluetooth radio to three modes of operation: active, passive, and backscatter.
Presented in a paper “Braidio: An Integrated Active-Passive Radio for Mobile Devices with Asymmetric Energy Budgets” at the Association for Computing Machinery’s special interest group on data communication (SIGCOMM) conference, the new radio architecture allows a device to switches between the three modes of operation to only consume power in proportion to the energy availability detected between the two talking devices.
Their initial research started with an analysis of commercial RFID readers (backscatter readers used to read passive tags), accepting some trade-offs in sensitivity to drastically simplify their architecture and raise their energy efficiency.
At reasonably close distances (under one meter), the backscatter mode could sustain 1Mbps data rates. As the range increased, bit rates supported by the backscatter mode of the receiver decreased to 100kbps and 10kbps (at 1.8m and 2.4m, respectively).
For longer distances, the modified Bluetooth-enabled IoT device could switch to a passive receiver mode, operating at up to 3.9m at 1Mbps, or up to 4.2m at 100kbps. Then for distances over 6 meters, the regular BLE active mode of the two devices would take over.
It is by multiplexing these three modes that the two devices constantly adapt their communication scheme to achieve the best bit rate at the lowest power consumption for the device with the least battery capacity.
Associate Professor in the Department of Computer Science at UMASS Amherst, Deepak Ganesan gave this interleaved multi-mode communication scheme the name Braidio as for “braided radios”.
The different modes enable two devices to communicate on asymmetric power modes, dynamically splitting the power burden of communication between the transmitter and receiver, with the smallest device typically calling for the best power budget (although the larger device could make that call if its larger battery was running lower).
The power-proportional low-power radio pair described in the paper is able to shift through power consumption ratios from 1:2546 to 3546:1 between a transmitter and a receiver, a huge dynamic range (seven orders of magnitude) that addresses a wide range of energy budgets between different devices.
The team also gathered the data of performance gains obtained using Braidio over regular symmetric load BLE transmission, across multiple devices from differing battery capacities. The figures are striking!
The “braided radio” was shown to consume between 16uW and 129mW across the different modes, increasing the total bits transmitted by several orders of magnitude when compared with Bluetooth.
The benefits are particularly appealing when there is significant asymmetry in battery levels, making the most energy-constrained device last much longer than it would if operating in full active mode.
“We are in the process of securing patents and contemplating IP licensing”, said Ganesan, though the researchers remain uncertain about future business plans.
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