Nov 28, 2016A team of Stanford University researchers have developed a small Wi-Fi device that behaves similarly to a radio frequency identification tag, sending a unique identifier that indicates where it is located, but doing so via Wi-Fi access points without impacting the Wi-Fi network where the tag is located. The solution, known as HitchHike, consists of a small tag—which could cost the same as an ultrahigh-frequency (UHF) RFID tag—as well as standard Wi-Fi access points and a smartphone, tablet or laptop. The HitchHike tag captures a transmission from a Wi-Fi access point, and then, in turn, transmits its own Wi-Fi signal to a receiver—a mobile phone, laptop or other Wi-Fi-enabled device. In that way, the tags do not interfere with other traffic on the Wi-Fi network.
The HitchHike tag was developed to enable a low-cost, wireless transmission of data, says Pengyu Zhang, a Stanford University postdoctoral researcher for a research team led by Sachin Katti, an associate professor of electrical engineering and computer science. RFID systems can capture tag ID numbers and sensor data, but they require the installation of fixed or handheld readers. Using the Wi-Fi network to transmit data can create a drag on that network due to the additional traffic. The solution that the Stanford researchers developed is a Wi-Fi tag that uses the network only to energize and prompt a transmission, which the device can then alter and send to a dedicated phone, laptop or tablet.
The tag, approximately the size of a postage stamp, contains an RF switch that reflects the incoming excitation signal to the receiver, a chip and an envelope detector that identifies when the incoming excitation Wi-Fi signal starts. It could also come with a power source connected to it if sensor data were being collected and transmitted.
The chip captures incoming Wi-Fi signals as a series of ones and zeros, then translates that code to its own set of data. To avoid interfering with the original signal from the access points, the chip shifts the new stream of data and forwards it to a different Wi-Fi channel. This prevents collision of data that would otherwise occur in a wide-scale Wi-Fi-enabled device deployment. "The HitchHike tag uses an ultra-low-power circuit to detect the starting point of an ambient Wi-Fi transmission," Zhang says. Once it detects a Wi-Fi signal, such as that from standard access points or routers in businesses or homes, it "reflects" its own ID in another Wi-Fi signal. The frequency-shifted transmission to a receiver could also impact traffic on a Wi-Fi channel, Zhang adds. However, his group is developing a media access control (MAC) layer protocol to solve that problem.
That signal reflection, Zhang explains, uses a technique known as codeword translation, by which it injects its own information by modifying the signal received from the access point. "The phase modification is performed in a way such that the reflected signal is still a valid Wi-Fi signal," he states. "Therefore, we can use commercial Wi-Fi receivers—such as that built into a phone or tablet—to decode the reflected signal."
The tag can be either passive or active, depending on the requirements of a particular use case. If, for instance, a user wanted merely to identify what tags were at a specific location, the tags would not require a battery. Like RFID tags, each HitchHike device is encoded with its own unique identifier, and that can be captured by a phone or a laptop's Wi-Fi device when the tag is energized by a Wi-Fi access point. The HitchHike tag radio consumes 33uW only, enabling it to be energized via a Wi-Fi connection.
On the other hand, in an Internet of Things deployment in which sensor data, such as temperatures or other information, is being collected, the tag would operate with a small battery.
Zhang is currently in conversations with several IoT companies that could potentially adopt the technology for their own solutions. He says the researchers hope to have a product ready for testing and piloting by next year. The deployment would be fairly simple, he adds, since no readers are required. "We already have Wi-Fi access points everywhere," he points out.
In the meantime, the researchers are working to design a MAC layer that would enable tags within a HitchHike system to determine which tag responds first when multiple tags are energized, thereby enabling them to be read in high-density environments (such as a large number of tags in use within the same room, or stored on a shelf). Early testing, Zhang notes, has found that a receiver can capture transmissions from the HitchHike tag at a distance of up to 50 meters (164 feet) if a battery is in use.
Zhang expects early adopters to be solution providers for the health-care industry. For example, he says, some wireless sensors are already commercialized to track the level of glucose in a patient's eye when built into a contact lens. Wearable sensors—biosensors, for instance—could detect health-related information about an individual and forward it to a mobile phone or device via the HitchHike technique.
Once the tag is released commercially sometime during the next year or so, Zhang says, it could be as cheap as an RFID tag. Although it requires similar hardware—a chip and an antenna—to what an RFID tag require, the logic needed on the chip is simpler than that on an RFID tag. He hopes the price could be as low as 1 cent per tag.
