Understanding BWP Switching in 5G NR

Hello, NR experts!

If a UE switches its Bandwidth Part (BWP) from the first active BWP (e.g., BWP 1) to another BWP (e.g., BWP 2), will the newly activated BWP become the default BWP?

No, it will not.

Alright, so when does the default BWP become active?

The default BWP will be assigned as the initial BWP.

The UE is configured with three main types of BWPs:

  • Initial BWP
  • Dedicated BWP
  • Power Saving BWP

The Initial BWP is sometimes referred to as the Default BWP. When the UE is in RRC Connected mode, all three BWPs are configured, but only one BWP is active at a time.

UL and DL parameters can be aligned accordingly.

A quick question on this - what is the adaptation timer for BWP?

1 Like
  1. The Initial BWP is part of SIB1 and is also present in Connected Mode, with a default ID of 0.
  2. The First Active BWP (DL/UL) is the Initial BWP if no Dedicated BWP is configured.
  3. A Dedicated BWP is configured using the BWP-ToAddModList, with IDs ranging from 1 to 4. In this case, the First Active BWP (DL/UL) is mapped to any ID between 1 and 4.
  4. The UE can switch BWPs through RRC signaling, DCI, or when the BWP Inactivity Timer expires.
  5. The Default BWP has not been observed so far.

Point 5 is simply a name referring to the Initial BWP/Default BWP, as the UE has no choice, and the RAN does not necessarily configure it based on service or data requirements.

All other mentioned points are theoretical assumptions.

A BWP can have any value between 0 to 3, and according to 38.214, it is not mandatory for BWP 0 to be the Initial BWP.

If Point 1 is true, then Point 3 cannot be valid. :yawning_face:

1 Like

Actually, the Initial BWP ID is not explicitly seen in logs, so it is assumed that the default is 0.

Moreover, Point 2 holds true in most cases, which is why the first active DL/UL BWP is generally observed as 0 in the RRC Setup message from the network to the UE.

The first active BWP (DL/UL) will be your default BWP, which is typically assigned during RRC Setup.

The BWP ID can be any value, but BWP ID-X will be the default BWP ID if it falls under the first active BWP DL/UL.

In RRC Setup, the initial BWP does not have an explicit ID, and the first active BWP (DL/UL) is mapped to ID 0.

So, are you saying that the default BWP corresponds to ID 0?

Yes, correct.