Dormant Bandwidth Part

Description

The Dormant Bandwidth Part (BWP) is a concept introduced in 3GPP Release 16 for 5G New Radio (NR) to enhance network energy efficiency and reduce UE power consumption. In carrier aggregation (CA) scenarios, a UE may be configured with multiple serving cells, including a Special Cell (SpCell) and several Secondary Cells (SCells). Traditionally, SCells could be activated or deactivated. When deactivated, the UE stops monitoring the Physical Downlink Control Channel (PDCCH) and ceases most activities, but it also stops reporting Channel State Information (CSI), which can delay reactivation. The dormant BWP provides an intermediate state: the UE stops monitoring PDCCH on the SCell but continues to perform CSI measurements, Automatic Gain Control (AGC), and beam management. This allows the network to quickly reactivate the SCell when needed, as the UE maintains up-to-date channel information.

The dormant BWP is one of the downlink BWPs configured by the network via dedicated Radio Resource Control (RRC) signalling. For each serving cell other than the SpCell or PUCCH SCell, the network may configure one BWP as a dormant BWP. The UE is not required to monitor PDCCH on the dormant BWP, but it must perform CSI measurements and report them periodically or aperiodically, depending on configuration. The UE also performs AGC to maintain receiver gain and beam management if operating in mmWave frequencies. This ensures that when the network decides to activate the SCell, the transition is seamless and fast, as the UE already has the necessary channel state information.

The dormant BWP is configured using RRC parameters such as `dormant-BWP-Id` within the `SCellConfig` information element. The network can switch the UE to the dormant BWP using a PDCCH order or a MAC Control Element (CE). The UE transitions to the dormant BWP when it receives a specific indication, such as a `DormantBWP` field in the MAC CE. The UE then stops monitoring PDCCH on the SCell but continues CSI measurements. The network can later activate the SCell by sending a PDCCH order to switch to an active BWP. The dormant BWP concept is particularly beneficial in scenarios with bursty traffic, where SCells are frequently activated and deactivated. By keeping the UE in a dormant state, the network can reduce signaling overhead and latency associated with SCell activation.

The dormant BWP is specified in 3GPP TS 38.321 (MAC protocol) and TS 38.331 (RRC protocol). It is part of the broader framework for bandwidth part operation in NR, which allows flexible spectrum usage and power saving. The concept has been further enhanced in later releases, with Release 17 introducing additional optimizations for dormant BWP operation, such as enhanced CSI reporting and beam management. The dormant BWP is a key enabler for network energy saving and UE power efficiency, aligning with the goals of 5G to support diverse use cases with varying traffic patterns.

In summary, the dormant BWP is a specialized BWP that allows the UE to save power while maintaining measurement readiness. It is configured per SCell and enables fast activation by keeping CSI, AGC, and beam management active. This makes it an essential tool for efficient carrier aggregation operation in 5G NR.

Purpose & Motivation

The dormant BWP was introduced to address the trade-off between power saving and fast SCell activation in carrier aggregation. In earlier releases, SCells could be activated or deactivated. Deactivation saved power but required the UE to perform a lengthy activation procedure, including CSI reporting and beam management, before data could be transmitted. This latency was detrimental for bursty traffic. The dormant BWP provides a middle ground: the UE stops monitoring PDCCH, saving power, but continues CSI measurements and beam management, so that when the SCell is activated, the network already has the necessary channel information to schedule data immediately. This reduces latency and improves user experience.

Another motivation was network energy efficiency. By allowing SCells to remain in a dormant state, the network can reduce the number of active cells and thus save energy, while still maintaining the ability to quickly ramp up capacity when needed. This is particularly important for 5G networks, which are expected to support high data rates and diverse services with varying traffic patterns. The dormant BWP concept aligns with the broader industry push for energy-efficient networks and green communications.

The dormant BWP also simplifies UE implementation by providing a clear state with defined behaviors. It avoids the complexity of frequent activation/deactivation cycles and reduces signaling overhead. The concept has been standardized in Release 16 and refined in subsequent releases, demonstrating its importance in the 5G ecosystem.