Energy-Efficient Communication for IoT Devices
In the era of smart cities, hundreds and thousands of Internet-of-Things (IoT) devices will be connected to the Internet. The growing number of IoT devices increase the demands for low-power communication technologies. Existing communication paradigms, such as Wi-Fi, are ill-suited for the coming waves of power-constrained IoT devices. To enable the ubiquitous connectivity of massive IoT devices, we need a rethink of the communication architecture.
Our research group works toward ubiquitous IoT connectivity by exploring two directions: enabling energy-efficient Wi-Fi support for IoT devices and exploring low-power passive radios such as backscatter communication.
Energy-efficient Wi-Fi Support for IoT Devices
Wi-Fi is the dominant wireless LAN technology with wide deployments of hotspots. Growing numbers of IoT devices on the market have enabled Wi-Fi connectivity to the Internet. For example, Apple Watch comes with built-in Wi-Fi chipsets. However, the current crop of Wi-Fi designs are primarily designed for high-end devices, which turns out to be an overwhelming energy burden for low-end devices with light workloads. Based on current Wi-Fi architecture, we enable energy-efficient Wi-Fi support for IoT devices by proposing Wi-Fi Teeter-Totter and Sampleless Wi-Fi.
Wi-Fi Teeter-Totter: Overclocking OFDM for Internet of Things
We argue that the fundamental hurdle for energy efficient uplink transmission lies in the transceiver design. Existing Wi-Fi transceivers are originally designed for symmetric nodes with equal hardware capabilities and power constraints. This assumption no longer stands for IoT applications, where a Wi-Fi AP is much more powerful and almost energy-unconstrained compared to IoT devices. APs are equipped with Wi-Fi chipsets that can support up to 160 MHz bandwidth, while IoT devices normally support only 1-2MHz bandwidth. Our design rationale is that we trade the computing and energy resources of APs for the TX power of IoT devices in a teeter-totter fashion, that is, we allow IoT devices to transmit uplink packets using the lowest power while pushing all the decoding burdens to the AP side, as illustrated in Figure 1. IoT devices reap benefits from such an asymmetric design by configuring TX power lower than the minimum power required for decoding. Such an asymmetric design trades resources that are cheap to AP for power consumptions that are expensive to IoT devices. Such a design completely conforms to Wi-Fi protocols and thus can be readily integrated with existing standards. The only changes are standalone updates of computational logics and RF settings at APs.
Sampleless Wi-Fi: Bringing Low Power to Wi-Fi Communications
On one hand, Wi-Fi needs to support high data rate services such as high-definition video streaming and bulky file transfer; on the other hand, IoT devices are low-end devices with light workloads. We need to find the sweet spot given the disparity between different types of traffic types.
We propose Sampleless Wi-Fi, which aims to provide reliable communications between legacy Wi-Fi APs and clients at various sampling rates. Sampleless Wi-Fi is inspired by the wisdom of rateless codes in that an undecodable packet in a single transmission can be recovered by combining multiple transmissions. In Sampleless Wi-Fi, in a same data transmission, packet reception can be achieved at different energy consumption for receivers with different sampling rates. The concept of Sampleless Wi-Fi is illustrated in Figure 2, where versatile devices with various workloads access the Internet via a Wi-Fi AP. A laptop communicates with the AP at the Nyquist sampling rate (40 MHz) to meet heavy traffic demands; while for less bandwidth-hungry mobile devices that use sub-Nyquist sampling rates (20 MHz or 10 MHz) for energy saving, the AP incrementally adds redundancy by sending extra packets until successful reception. As a result, the AP sends legacy packets without PHY modifications, and energy-constrained devices can downclock their sampling rates without requiring specific channel conditions or PHY redundancy.
Ultra-low Power Backscatter Communication
Backscatter communication is an ultra-low power communication paradigm. It works by modulating existing RF signals in the air. The backscatter transmitter will switch its antenna load in two states. In one state, the incident signals will be reflected due to the impedance mismatch. In the other state, the incident signals are mainly absorbed as the antenna impedance and load impedance are matched. By switching between these two states, the tag can transmit ‘0, 1’ sequence. As the transmitter does not actively generate radio signals, it consumes almost zero power.
With backscatter, we could enable on-body devices to communicate with each other at zero power consumption. We have built a pair of energy harvesting shoes that can support battery-free sensing with backscatter communication techniques. We also enable backscatter communication on commodity devices without any hardware modification.
Battery-free Sensing Platform with On-body Backscatter Communication
Although there are existing works on designing energy harvesting shoes, they mainly focus on harvesting the energy during foot strike on one foot. However, during walking or running, our two feet can generate equal amounts of energy. But there is no existing feasible solution to combine these two parts of available energy to support a complete sensing system. Now, with ambient backscatter techniques, we can build a communication channel between two feet at almost zero cost and coordinate these two distributed parts. In our design, we allocate separate tasks to each foot. One foot is equipped with sensing hardware (e.g., accelerometer, pulse sensor, temperature sensor). It senses user activity or ambient environment, and informs the other foot of the results by ambient backscatter. The other foot is equipped with Bluetooth radio and can transmit this information wirelessly to users’ smartphone. In this way, we fully utilize the harvested energy on both feet and enable a battery-free sensing platform with end-to-end functionality. As the moving of two feet will change the channel condition among two feet, we borrow the idea from rate-less transmission, which can adapt to unpredictable channel conditions with high flexibility. The transmitter will continuously transmit encoded packets until the receiver has received enough number of packets for decoding. We test the performance of rate-less transmission in both indoor and outdoor environments. Results show that the performance is robust in various scenarios.
NICScatter: Backscatter Communication on Commodity IoT Devices
Backscatter communication usually requires dedicated hardware circuits, which is yet not available on commodity devices on the market. We achieve backscatter communication on commodity devices using the wireless communication module, which is available in every IoT device. In the communication module, the antenna is connected to the network interface card (NIC). When we change the working states of the NIC, we are actually changing the load impedance of the antenna. So, we can use the ON/OFF states of WiFi NICs to emulate the two states on a backscatter tag. In this way, we can modulate wireless signals by changing the working states of NIC. It becomes a way for the device to transmit information. We call it NICScatter. We test the performance of NICScatter on an HP laptop and a Samsung smartphone. The effective communication distance is 2 m and 20 cm, respectively.
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