Toward Event-Driven Per-UE xApps in Open RAN Near-RT RIC
The Open Radio Access Network (O-RAN) NearReal-Time RAN Intelligent Controller (Near-RT RIC) hosts xApps that perform per-User-Equipment (UE) closed-loop control over the E2 interface, and proactive control of handover, scheduling, and anomaly response depends on the freshness of the underlying counters. Surveyed per-UE xApps that explicitly describe their E2 binding rely on Periodic Report subscriptions through E2 Service Model for Key Performance Measurement (E2SM-KPM), which forces every measurement to wait for a configured reporting period before reaching the Near-RT RIC. This paper surveys seven per-UE Near-RT RIC xApp works published between 2023 and 2026 and aligns them with the Measurement Event trigger style introduced in E2SM-KPM v7.0 (2025). The new trigger style enables cell-condition-gated per-UE reporting through REPORT Service Styles 3 and 5, while perUE-bound event conditions remain explicitly out of scope and constitute a clean specification-evolution opportunity. Closing the gap between Periodic-only xApp practice and the Measurement Event primitive opens a class of event-driven per-UE xApps that the current control loop cannot support.