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Backscatter presents an emerging ultralow-power wireless communication paradigm. The ability to offer submilliwatt power consumption makes it a competitive core technology for Internet of Things (IoT) applications. In this article, we provide a tutorial of backscatter communication from the signal processing perspective as well as a survey of the recent research activities in this domain, primarily focusing on bistatic backscatter systems. We also discuss the unique real-world applications empowered by backscatter communication and identify open questions in this domain. We believe this article will shed light on the low-power wireless connectivity design toward building and deploying IoT services in the wild.
Overview of backscatter communication
The vision of the IoT promises a world where sensors and actuators are ubiquitous and interconnected so that we can better understand and control the surrounding world. One critical challenge toward this vision is to build such devices that can be easily deployed and run autonomously for a lengthy duration. Backscatter communication, an emerging microwatt-level wireless communication paradigm, is gaining popularity as a suitable solution to fulfill such a need.
The principle of backscatter communication is similar to that of the heliograph shown in Figure 1. People have been using mirrors to reflect sunlight for communication for a long time, and this method is especially important when there is no source of energy like a campfire or a flashlight. By flipping the mirror, the sender can signal the remote target by controlling the presence of reflected light using Morse code. For backscatter communication, the same reflecting while manipulating process is applied on radio-frequency (RF) signals. At a high level, the system model of backscatter communication is shown in Figure 2. A special device called a backscatter tag reflects the incoming excitation signal emitted by a nearby (carrier) transmitter. At the same time, it selectively changes the amplitude, frequency, and/or phase of the signal for modulation. The backscattered signal is then captured by a receiver and piped through a signal processing engine to extract information injected by the backscatter tag. Note that the transmitter and the receiver were previously integrated in the conventional or monostatic backscatter system [e.g., RF identification (RFID) reader ] but are separated in bistatic backscatter system designs. Specifically, the transmitters can be available ambient RF sources [e.g., TV or frequency modulation (FM) radio towers, cellular base stations, and Wi-Fi access points (APs)] from anywhere. This new modular design introduces the following intrinsic properties for performance enhancement.