Detect And Avoid

Description

Detect And Avoid (DAA) is a critical feature for New Radio in Unlicensed spectrum (NR-U) operation, mandated to ensure harmonious coexistence with incumbent systems like Wi-Fi (IEEE 802.11) and radar in shared frequency bands such as 5 GHz and 6 GHz. The mechanism operates through a continuous two-phase process: detection and subsequent avoidance. In the detection phase, the gNB (Next Generation NodeB) or UE (User Equipment) performs energy detection or specific waveform sensing (e.g., for radar pulses) on the target channel before and during transmission opportunities. This sensing is defined by regulatory requirements (e.g., FCC, ETSI) and involves listening for signals above a predefined threshold over a minimum channel occupancy time. The gNB coordinates this sensing, potentially using UE-assisted measurements, to build a comprehensive view of the channel environment.

Upon detecting an incumbent signal, the system enters the avoidance phase. The core principle is that the NR-U transmission must immediately vacate the channel or significantly reduce its interference potential. This is achieved through mechanisms like channel switching, where the gNB schedules transmissions on an alternative clear channel identified during prior sensing. Alternatively, the gNB can employ dynamic power control, reducing its transmission power below the interference threshold for the incumbent. For radar avoidance, a more stringent 'move away' protocol is often required, where the system must not use the channel for a specified quiet period. The gNB manages these procedures via RRC (Radio Resource Control) signaling, configuring UEs with measurement gaps for sensing and updating scheduling decisions based on sensing results.

Architecturally, DAA functionality is integrated within the gNB's MAC (Medium Access Control) and PHY (Physical Layer), with support from higher-layer RRM (Radio Resource Management). The gNB acts as the central controller, instructing UEs when to perform Listen-Before-Talk (LBT) and DAA measurements. These measurements are reported back to the gNB, which consolidates the data to make channel occupancy decisions. The system supports both wideband and narrowband sensing to accommodate different incumbent signal characteristics. DAA's role is foundational for NR-U, transforming unlicensed spectrum from a 'best-effort' arena into a managed resource where 5G can operate predictably while respecting legal mandates and promoting spectral efficiency through intelligent, reactive sharing.

Purpose & Motivation

DAA was created to address the fundamental challenge of deploying 5G New Radio in globally available unlicensed and shared spectrum bands. The primary motivation was to expand cellular network capacity and support high-throughput, low-latency services without exclusive reliance on licensed spectrum, which is scarce and expensive. However, bands like 5 GHz and 6 GHz are already populated by entrenched technologies, primarily Wi-Fi for consumer and enterprise use, and by radar systems for weather, military, and satellite applications. Regulatory bodies worldwide impose strict 'coexistence' or 'incumbent protection' rules on new entrants to these bands. Without DAA, NR-U transmissions could cause harmful interference, degrading Wi-Fi performance or disrupting critical radar operations, leading to regulatory non-compliance and service failure.

The technology solves the problem of how a scheduled, synchronous cellular system like 5G NR can fairly share spectrum with contention-based, asynchronous systems like Wi-Fi and protected radar services. Previous approaches in LTE-LAA (License Assisted Access) introduced basic Listen-Before-Talk (LBT), but this was primarily designed for coexistence with other LBT-based systems. DAA extends this concept with more sophisticated detection capabilities tailored to specific incumbent signatures (e.g., radar pulse patterns) and more deterministic avoidance protocols mandated by regulators. It addresses the limitation of simple LBT, which might not reliably detect non-LBT signals like certain radar types, by incorporating feature detection and longer measurement periods. The creation of DAA in 3GPP Release-18 was driven by the need to standardize a unified, globally applicable framework that enables NR-U equipment to meet diverse regional regulatory requirements, thereby ensuring device interoperability and successful market deployment of 5G in shared spectrum.