Energy Harvesting-Data Transmission Tradeoff in Symbiotic Radios for Ambient IoT

Ambient Internet of Things (IoT), which integrates backscatter communication and energy harvesting, is a promising technology for massive connectivity in 6G. Symbiotic radio (SR), a novel backscatter communication paradigm, has shown great potential in enabling cellular ambient IoT. This paper proposes an energy harvesting-based code division multiple access (CDMA)-SR system to support interference-free access for massive ambient IoT devices. In the proposed system, the IoT devices first harvest energy from ambient cellular signals. In the subsequent data transmission phase, the IoT devices that have accumulated sufficient energy transmit information to the receiver using backscatter communication, with each device employing a distinct spreading code. The decoding scheme is proposed to recover both the cellular information and the information from the active IoT devices, and the throughput is analyzed accordingly. Additionally, closed-form expressions of the ergodic throughput for both cellular and IoT transmissions are derived. Using these expressions, we reveal an optimal energy harvesting time that maximizes overall IoT throughput and provide a method to determine this optimal time. Numerical results are presented to verify the derived throughput expressions and demonstrate the advantages of the proposed system. The results show that the proposed CDMA-SR system achieves superior IoT transmission performance, and the tradeoff between energy harvesting and data transmission is also validated.