A Fast Carrier Scheduling Algorithm for Battery-free Sensor Tags in Commodity Wireless Networks

Abstract

New battery-free sensor tags that interoperate with unmodified standard IoT devices and protocols can extend a sensor network's capabilities in a scalable and cost-effective manner. The tags achieve battery-free operation through backscatterrelated techniques, while the standard IoT devices avoid additional dedicated infrastructure by providing the unmodulated carrier that tags need to communicate. However, this approach requires coordination between devices transmitting, receiving and generating carrier, adds extra latency and energy consumption to already constrained devices, and increases interference and contention in the shared spectrum. We present a scheduling mechanism that optimizes the use of carrier generators, minimizing any disruptions to the regular nodes. We employ time slots to coordinate the unmodulated carrier while minimizing latency, energy consumption and overhead radio emissions. We propose an efficient scheduling algorithm that parallelizes communications with battery-free tags when possible and shares carriers among multiple tags concurrently. In our evaluation we demonstrate the feasibility and reliability of our approach in testbed experiments. We find that we can significantly reduce the excess latency and energy consumption caused by the addition of sensor tags when compared to sequential interrogation. We show that the gains tend to improve with the network size and that our solution is close to optimal on average.

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“…Moreover, logistics should be made fully traceable and transparent. Current promising research directions include the development of sustainable tracking tags and ultra-low-power tracing solutions [ 217 ], and the adoption of blockchain to achieve trustworthy and long-term preservation of product histories [ 218 ].…”

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“…They are able to provide an unmodulated carrier (by using their radio test mode [9]) and employ a time-slotted medium access mechanism. The duration of a timeslot is sufficient for an IoT device to interrogate one tag by transmitting a request to the desired tag and receiving the response a short interval after while a second IoT node provides an unmodulated carrier, as demonstrated by Pérez-Penichet et al [10], [11]. Note that both IEEE 802.15.4 and Bluetooth specify a time-slotted access mechanism with the necessary characteristics in their respective standards [13], [14].…”

“…Note that all parameters from the Inference Module listed (Eqs. [4][5][6][7][8][9][10][11][12][13][14][15] are shared across timeslots s j .…”

“…The number of timeslots in the schedule is the secondary objective in the tag scheduling problem; this is because, while the duration of the schedule can impact latency and other performance metrics in the network, the number of carriers directly impacts energy and spectral efficiency. Note that reducing the number of carrier slots, can potentially also shorten the schedule owing to carrier reuse [11]. We compute the timeslots savings in a manner analogous to the carrier savings and compare with the other schedules in a similar way as depicted in Fig.…”

“…Computing the optimal schedule is an NP-hard combinatorial optimization problem. The only known scalable solution is a heuristic with suboptimal performance [11], which results in wasted energy and spectral resources. We will refer to this suboptimal solution as the TagAlong scheduler [10] hereafter.…”