IQ-aware precoding for atomic MIMO receivers

Leveraging the strong atom-light interaction, Rydberg atomic receivers significantly enhance the sensitivity of electromagnetic signal measurements, outperforming traditional antennas. Existing research primarily focuses on improving the architecture and signal detection algorithms of atomic receivers, while established signal processing schemes at the transmitter end have remained constant. However, these schemes fail to maximize the throughput of atomic receivers due to the nonlinearity of transmission model. To address this issue, we propose to design transmitter precoding in multiple-input multiple-output systems to achieve the capacity of atomic receivers. Initially, we harness a strong reference approximation to convert the nonlinear magnitude-detection model of atomic receivers into a linear real-part detector. Based on this approximation, we prove that the degree of freedom is min{Nr/2,Nt} for a MIMO system comprising an Nr-antenna atomic receiver and an Nt-antenna classic transmitter. To achieve the system capacity, we propose an IQ-aware fully digital precoding method. Unlike traditional complex-valued digital precoders that jointly manipulate the inphase and quadrature (IQ) symbols, our method employs four real matrices to independently precode the IQ baseband symbols, which is shown to be optimal for atomic receivers. Then, to eliminate the reliance on fully digital precoding architecture, we further explore IQ-aware hybrid precoding techniques. Our design incorporates a low-dimensional IQ-aware digital precoder and a high-dimensional complex analog precoder. Alternating minimization algorithms are proposed to produce IQ-aware hybrid precoders, with the objective of approaching the optimal IQ-aware fully digital precoder. Simulation results validate the superiority of proposed IQ-aware precoding methods over existing techniques in atomic MIMO communications.