Direction-of-Arrival Estimation Against Direct Wave Interference Based on Space-Time Modulated Metasurface in Code Domain

Space-time modulated metasurface (STMM)-based Direction-of-Arrival (DoA) estimation methods has emerged as a prominent research area in recent years, owing to its remarkable flexibility and low-complexity. However, the receiving antenna is typically positioned in the far-field region of the STMM, thereby having potential risks of direct wave interference (DWI). In this article, we propose a DoA estimation method based on STMM in the code domain to mitigate the effects of DWI. The wideband DWI shares the same center frequency with the incident waves under estimation, leading to frequency spectrum overlap and making conventional frequency-domain methods ineffective. In contrast, through the design of pseudo-random modulation signals, our proposed method effectively suppresses specific types of wideband DWI within the code domain. As a result, the DoA can be accurately estimated using the interference-suppressed spectrum in the code domain. The novelty of the proposed method lies in its unique approach to DoA estimation within the code domain, along with its ability to effectively counter in-band DWI. To validate the proposed DoA estimation method, we design and fabricate a reconfigurable metasurface operating at the X-band. Experimental results show that the DoA estimation error remains below 2° even in the presence of a linear frequency modulation (LFM) DWI, demonstrating the effectiveness and accuracy of the proposed method.