Bandwidth Part (BWP)

A CORESET defines the time-frequency resources where a UE searches for the Physical Downlink Control Channel (PDCCH). In the context of BWP, each configured downlink BWP can have up to 3 CORESETs. When a BWP switch occurs, the UE must monitor the CORESETs configured for the newly active BWP to receive scheduling grants. The CORESET configuration is BWP-specific, meaning the frequency domain allocation and symbol duration of the control region can be optimized independently for each bandwidth part.

A Search Space Set dictates the PDCCH monitoring occasions—specifically the aggregation levels, slot periodicity, and symbol offset—that a UE uses to blind-decode DCI formats within a CORESET. Each BWP can be configured with up to 10 Search Space Sets. BWP switching implicitly deactivates the Search Space Sets of the old BWP and activates those of the new BWP. This tight coupling ensures that the UE's blind decoding effort scales with the active BWP's bandwidth, directly enabling the power-saving benefits of BWP adaptation.

The network triggers a BWP switch via the Bandwidth part indicator field in Downlink Control Information (DCI) formats 1_1 and 0_1. This mechanism allows dynamic, slot-level adaptation of the UE's active BWP. Key constraints include:

• The DCI must be received in a slot where the UE is actively monitoring PDCCH on the current BWP.
• The switch takes effect after a defined BWP switch delay (measured in slots), dependent on the UE capability and subcarrier spacing.
• Cross-BWP scheduling is not supported; the scheduling DCI and the scheduled PDSCH/PUSCH must be on the same BWP.

The bwp-InactivityTimer is a UE-specific RRC-configured timer that enables autonomous fallback to a default BWP. The timer starts or restarts whenever the UE successfully decodes a DCI scheduling a unicast PDSCH or PUSCH on the active BWP. Upon expiry, the UE automatically switches to the defaultDownlinkBWP (if configured) or the initial BWP. This mechanism is critical for power saving: it prevents a UE from remaining on a wide, high-throughput BWP when data activity ceases, without requiring explicit DCI signaling.

A Subcarrier Spacing (SCS)-Specific Carrier is an RRC information element that defines the frequency domain location and bandwidth of a carrier for a given numerology. Each BWP is defined relative to an SCS-Specific Carrier via a locationAndBandwidth field, which is a Resource Indicator Value (RIV) encoding the starting PRB and length. This hierarchical structure allows a UE to be configured with multiple BWPs that share the same underlying carrier definition but occupy different contiguous subsets of the resource blocks.

The initial BWP is the default bandwidth part used for initial access procedures before any UE-specific BWP configuration. It is configured via the System Information Block 1 (SIB1) for downlink and uplink. Key characteristics:

• It contains the CORESET#0 and the search space for RMSI scheduling.
• It serves as the fallback BWP if no defaultDownlinkBWP is explicitly configured.
• For RedCap (Reduced Capability) UEs, the initial BWP can be configured with a narrower bandwidth to support devices with limited RF capabilities.