Talking When No One is Listening: Piggybacking City-scale IoT Control Signals Over LTE.

This paper presents FreeIoT, a control plane paradigm that allows fine grained signaling for city-scale IoT deployments without installing any additional infrastructure. FreeIoT overlays control/wake-up information for sensors over existing standards compliant LTE through the following contributions: First, we develop a novel encoding scheme that changes the spatial positioning of Almost Blank Subframes (ABS) within a standard LTE frame to convey control information. ABS was originally defined in the standard to allow coexistence between the macro-cell eNB and nearby small cells, which FreeIoT leverages as a side channel for IoT signaling. Our approach works with any number of ABS settings chosen by the LTE eNB, and accordingly adjusts the encoding of control messages at maximum possible transmission rates. Second, a session management protocol is introduced to maintain contextual information of the control signaling. This allows FreeIoT to handle situations where the control message may span multiple frames, or when the LTE operator temporarily reduces the number of ABS. FreeIoT also incorporates an error detection and correction mechanism to counter channel and fading errors. Finally, we implement a proof of concept testbed to validate the operation of FreeIoT using a software defined LTE eNB and custom-designed RF energy harvesting circuit interfaced with off-the-shelf sensors.