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).