Communications of the ACM
3D Localization for Subcentimeter-Sized Devices
The multiband ?Locate prototype includes two button cell batteries, a 2.4-GHz and 900-MHz dual band chip antenna, and a 5-GHz chip antenna.
Credit: University of Washington
The vision of tracking small IoT devices runs into the reality of localization technologies—today it is difficult to continuously track objects through walls in homes and warehouses on a coin cell battery. Although Wi-Fi and ultra-wideband radios can provide tracking through walls, they do not last more than a month on small coin and button cell batteries because they consume tens of milliwatts of power. We present the first localization system that consumes microwatts of power at a mobile device and can be localized across multiple rooms in settings such as homes and hospitals. To this end, we introduce a multiband backscatter prototype that operates across 900 MHz, 2.4 GHz, and 5 GHz and can extract the backscatter phase information from signals that are below the noise floor. We build subcentimeter-sized prototypes that consume 93 μW and could last five to ten years on button cell batteries. We achieved ranges of up to 60 m away from the AP and accuracies of 2, 12, 50, and 145 cm at 1, 5, 30, and 60 m, respectively. To demonstrate the potential of our design, we deploy it in two real-world scenarios: five homes in a metropolitan area and the surgery wing of a hospital in patient pre-op and post-op rooms as well as storage facilities.
1. Introduction
Recent years have seen significant advances in wireless localization.21, 19 However, existing solutions do not meet the requirements for size-constrained IoT applications. Figure 1 shows battery life of common radio technologies such as BLE, LoRa, ultra-wideband (UWB), and Wi-Fi, each running at a 1% duty cycle with small coin and button cell batteries for equal comparison. The shorter battery life limits the adoption of tracking solutions based on these radio technologies by making them inconvenient for consumer applications and infeasible for large-scale commercial deployments. Requiring large batteries on the other hand prevents scaling down the size of IoT devices. Although RFID tags are attractive from a power and size perspective, they have a limited range and do not work consistently through walls and other barriers. Consumers often deploy devices in rooms throughout homes, and similarly commercial deployments in settings such as hospitals require covering multiple patient rooms with a variety of obstructions and walls. Achieving localization in these scenarios would therefore require readers in every room, which significantly increases deployment cost.
Figure 1. Radio localization battery life. Battery life estimates for different technologies operating at 3 V from coin and button cell batteries running at 1% duty cycle.
This paper presents μLocate, the first wireless localization system that consumes microwatts of power at the mobile IoT devices and can be localized through walls in settings such as homes and hospitals. Our design can achieve 3D localization capabilities while supporting IoT devices that can be scaled to subcentimeter form factor. To achieve this, we design a backscatter-based solution that satisfies all of the above requirements. Specifically, we make the following hardware and systems contributions:
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