On-Demand Synchronization Signal Block

Description

On-Demand Synchronization Signal Block (OD-SSB) is a Release 19 enhancement to 5G NR that enables the network to transmit SSB (Synchronization Signal Block) on demand rather than continuously. The SSB is a fundamental downlink signal in NR that carries the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH), used by UEs for cell search, synchronization, and measurement. Traditionally, SSB is broadcast periodically (e.g., every 20 ms) to ensure UEs can discover the cell. However, continuous transmission consumes energy and resources, especially in low-traffic scenarios or for cells that are mostly dormant. OD-SSB addresses this by allowing the network to activate SSB transmission only when there is a need, such as when a UE requests access or when measurements are required.

Architecturally, OD-SSB is specified in 3GPP TS 38.300 (NR overall description) and TS 38.331 (RRC protocol). The feature introduces mechanisms for the network to signal the availability of on-demand SSB and for UEs to request it. For example, a UE in idle/inactive mode may use a wake-up signal (WUS) or a dedicated random-access preamble to trigger SSB transmission. The network can also configure periodic SSB occasions but skip actual transmission if no UE is expected, using a form of DTX (discontinuous transmission). The UE behavior includes monitoring for SSB presence and performing measurements accordingly, with relaxed requirements when SSB is not continuously available.

Key components of OD-SSB include: (1) a trigger mechanism, where the UE or network initiates SSB transmission; (2) configuration signaling, via SIB or RRC, indicating when SSB may be transmitted on demand; (3) measurement handling, ensuring that UEs can still perform cell reselection and mobility without continuous SSB; and (4) energy-saving coordination with other features like cell DTX/DRX. The feature is particularly beneficial for network energy saving (NES) in 5G-Advanced, as it allows cells to reduce periodic transmissions when not needed, aligning with 3GPP's broader Rel-18/19 energy efficiency goals.

OD-SSB interacts with other NR procedures such as initial access, beam management, and mobility. For initial access, a UE must first detect SSB to acquire system information; with OD-SSB, the UE may need to trigger SSB transmission via a pre-configured resource. For connected-mode UEs, the network can schedule SSB on demand for measurements, potentially reducing overhead. The feature also supports multi-beam operation, where individual beams may have on-demand SSB, though this adds complexity.

In summary, OD-SSB is a Rel-19 RAN feature that optimizes SSB transmission by making it on-demand, thereby saving energy and resources while preserving essential synchronization and measurement functions. It is a key enabler for energy-efficient 5G-Advanced networks.

Purpose & Motivation

OD-SSB was created to address the energy consumption and resource overhead associated with continuous SSB transmission in 5G NR. In traditional NR, SSB is broadcast periodically even when no UEs are present, which is wasteful in terms of power and spectrum, especially for small cells, mmWave deployments, and cells with low traffic. As networks densify and energy costs rise, operators need mechanisms to reduce always-on transmissions without compromising coverage or accessibility. OD-SSB solves this by allowing the network to transmit SSB only when required, such as when a UE attempts access or when measurements are needed.

Previous approaches, such as increasing SSB periodicity or using cell DTX, offered some savings but were limited: longer periodicity could delay cell discovery, and cell DTX often applied to all signals. OD-SSB provides a more flexible, on-demand mechanism that can be triggered by UEs, ensuring that synchronization signals are available when needed while enabling deep sleep states otherwise. This aligns with 3GPP's Rel-18/19 Network Energy Saving (NES) study and requirements, and it supports the goal of reducing network operational expenditure (OPEX) and environmental impact. The feature is specified in 38.300 and 38.331, ensuring standardized UE behavior and interoperability.