Symbiotic radios for future ambient IoT

In recent years, there has been an extensive deployment of the Internet of Things (IoT) across various vertical industries. However, this progress comes with the challenges of accommodating billions of IoT devices, leading to significant issues in power consumption, spectrum allocation, and device costs. Ambient IoT, empowered by energy harvesting and low-power communication techniques such as backscatter communications, stands out as a promising technology supporting massive net-zero connections. Notably, both 3GPP and IEEE 802.11 have initiated standardization research in this area, through which ambient IoT devices have been well identified with their advantages in terms of low power consumption and low cost. Despite these benefits, ambient IoT still faces challenges in integrating with existing legitimate communication networks, particularly regarding the spectrum issue and the ambient signal utilization issue. Fortunately, symbiotic radio (SR) offers a promising solution by introducing cooperation between the legitimate transmission and the IoT transmission. In this article, we provide a comprehensive tutorial on SR-enabled future ambient IoT. Specifically, we first present the fundamentals of ambient IoT and then introduce SR within this context, as well as highlighting its advantages in ambient IoT. Following that, we focus on addressing three important issues for SR in ambient IoT: cooperative modulation design, multiple access design, and resource allocation from the perspective of communication design. Moreover, we highlight future research directions in this emerging field.