Multiuser Detection for Low-Activity CDMA-Based Symbiotic Radios

Symbiotic radio (SR) has emerged as a promising technology that enables ambient Internet of Things (IoT) devices to passively access wireless networks for data transmission. To support massive low-activity IoT devices, this paper proposes a novel low-activity code-division multiple access (LACDMA) based SR system, where a large number of low-activity backscatter devices (BDs) simultaneously transmit information in a CDMA manner by backscattering signals from a primary transmitter (PTx). To jointly detect the signals from both the PTx and the low-activity BDs, we first develop an optimal sparsity-aware maximum a posteriori (S-MAP) detector, which performs an exhaustive search over all candidates in the transmit alphabet. To address the prohibitive complexity of this approach, we further propose a low-complexity sparsity-aware iterative successive interference cancellation (S-SIC) detector. In this design, the PTx's signal is first decoded by treating the backscatter signals as interference. Then, the BDs' signals are recovered by exploiting their sparse activity patterns with the estimated PTx signal. Subsequently, the PTx's signal is reestimated by regarding the backscatter link as a multi-path with the estimated BDs' signal. Simulation results are provided to evaluate the proposed detectors. It is demonstrated that the developed detectors can realize the multipath gain contributed by the BDs to enhance the primary transmission and the proposed S-SIC detector can closely approach the performance of the SMAP detector, particularly in scenarios with a large number of receive antennas.