The Unified Synthesis of Flat-Beam Multi-Directional Metasurface Rectennas: A Review for Pervasive Wireless Power Beyond Directional Limits

Battery-free Internet of Things (IoT) systems require wireless power transfer (WPT) receivers that are robust to variations in orientation and polarization. Conventional rectennas, however, exhibit strong directional sensitivity and limited adaptability, which constrains their practical deployment. This review presents a unified synthesis of three emerging rectenna architectures developed to mitigate angular misalignment, flat-beam rectennas, multidirectional rectennas, and metasurface-assisted lens rectennas. A comparative taxonomy is developed to systematically evaluate these approaches in terms of angular coverage, RF–DC conversion efficiency (ηRF-DC), implementation complexity, and scalability. Beyond summarizing recent advances, this paper analyzes persistent research gaps common to all techniques, including narrowband operation, limited system-level validation, and manufacturing constraints. A decision-oriented framework is further proposed to guide architecture selection for application-specific scenarios, such as mobile, wearable, and fixed IoT deployments. Convergent future research directions are outlined, including adaptive and broadband designs, multifunctional integration, and emerging concepts such as dynamic metasurfaces, to support the development of pervasive, orientation-independent wireless power solutions for next-generation IoT networks.