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

Ambient IoT, which integrates backscatter communication and energy harvesting techniques, has emerged as a promising technology for massive connectivity in 6G. Symbiotic radio (SR), a novel backscatter communication paradigm, has shown great potential in advancing ambient IoT. Nevertheless, existing multiple access schemes for SR are limited by low transmission rates when enabling massive ambient IoT transmissions, primarily due to time-division access or inter-device interference. Additionally, the impact of energy harvesting on the multiple access system is rarely investigated. In this paper, we propose a code division multiple access (CDMA)-based symbiotic radio system to support interference-free simultaneous access for massive IoT devices. In the proposed system, multiple IoT devices first harvest energy from cellular signals, modulate their information onto the incident signal using unique spreading codes, and then transmit the modulated signals to the receiver. Two decoding schemes are proposed to retrieve the information from the IoT devices, and the throughput is analyzed accordingly. Moreover, we derive a closedform expression of the ergodic sum throughput, and reveal that there exists an optimal energy harvesting time that maximizes overall IoT transmission throughput. Finally, simulation results validate the performance of the proposed system and demonstrate the tradeoff between energy harvesting and data transmission.