Near-Field Channel Estimation and Localization: Recent developments, cooperative integration, and future directions

Near-field (NF) signal processing introduces a new epoch in communication and sensing realms, showcasing transformative potential, particularly in extremely large-scale (XL) aperture array (ELAA) systems compared to its far-field (FF) counterpart. The NF spherical wavefront, incorporating the distance/range parameter through amplitude variations and phase differences among antennas, enhances spatial sensing capabilities. Localization, often intertwined with angle estimation, emerges as a direct beneficiary of this phenomenon, commanding substantial research attention. Moreover, the NF effects on spatial channels in ELAA communications mandate the formulation of diverse NF channel estimation (CE) methods. In this vein, our study presents a tutorial review of NF CE and localization, encapsulating fundamental wavefront models and extended advanced scenarios. Recognizing their pivotal roles in integrated sensing and communication (ISAC) systems, we examine their similarities and explore NF-integrated CE and localization (NF-ICEL) at the signal processing level. Additionally, we analyze system-level NF-ICEL under three specific scenarios, comparing them with FF-ICEL and highlighting the unique abilities and potential uses of NF-ICEL in scatterer/environment sensing, high-mobility situations, and unsynchronized systems.