Bandwidth Part

(BWP) in 5G NR

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Introduction to Bandwidth
Part (BWP)
With the evolution from LTE to 5G NR, wireless networks have gained the
ability to support extremely wide channel bandwidths—sometimes up to 100
MHz in sub-6 GHz bands (FR1) and up to 400 MHz in mmWave bands (FR2).
While this flexibility unlocks unprecedented peak data rates, it also creates
new challenges for both network operators and UE. Not every UE is capable,
or needs, to always process the entire bandwidth.

This is where the concept of the Bandwidth Part (BWP) comes in. In 5G NR, a
BWP is a mechanism that allows the network to split the overall carrier
bandwidth into smaller, more manageable frequency segments. Each
segment can be independently configured and assigned to the UE, enabling
the network to optimize resource allocation, power consumption, and device
complexity based on the requirements of different services.

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Why Was BWP Introduced in 5G?
The introduction of BWPs is a direct response to the unique needs and challenges
of 5G:
Wider Bandwidths: 5G NR supports much larger channel bandwidths than LTE,
but low-end UEs (such as IoT devices) don’t need or can’t handle these wide
channels.
Device Diversity: 5G must serve both high-end smartphones and low-power
sensors on the same network.
Service Flexibility: 5G offers eMBB, URLLC, and mMTC—each with different
bandwidth and power requirements.

By configuring and switching between BWPs, the network can ensure:

High-end devices use wide BWPs for peak throughput.
Low-power devices use narrow BWPs to conserve energy.
The same UE can switch between BWPs dynamically as its service or
application changes.

Example:
When a smartphone is idle or receiving small control messages, it can operate on
Profit
a narrow BWP to save power. When streaming a 4K video, it can switch to a wide
BWP for maximum throughput.

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What is BWP (Bandwidth Part)
According to the specification TS 38.211, a Bandwidth Part is a “subset of
contiguous common resource blocks (CRBs) for a given numerology
(subcarrier spacing), defined within the carrier bandwidth.”

Each BWP has a defined start (the CRB index where it begins) and a size
(number of CRBs).
BWPs can overlap in frequency, or be separate, and each can have
different subcarrier spacings (numerologies).

Channel Bandwidth vs.

Bandwidth Part
Channel Bandwidth: This is the total frequency width allocated by the
network for a 5G NR carrier. For example, the operator may deploy a 100
MHz wide NR carrier.
Profit New Client
Bandwidth Part (BWP): Each BWP is a subset of the total channel
bandwidth. It is defined by its start position and size (both in terms of
resource blocks). The %
BWP may span a much
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smaller portion%than
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the
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entire channel.
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Reference: A Primer on Bandwidth Parts in 5G New Radio, Xingqin Lin, Dongsheng Yu, Henning Wiemann

Points to be considered
5G NR divides the available spectrum into operating bands, categorized
as FR1 (sub-6 GHz) and FR2 (mmWave).
New Client
Each mobile operator is allocated a specific portion of the operating band
for their use.
Within the operator’s spectrum, a carrier operates
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over a defined channel
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bandwidth.
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A reference point (Point A) is established for all resource block numbering
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and bandwidth definitions.
The cell-specific channel bandwidth is configured with an offset from
Point A, determining the bandwidth available to each cell.
Each UE can be assigned its own channel bandwidth, which may also be
offset within the cell-specific allocation.
Bandwidth Part (BWP) is a subset of the UE-specific bandwidth, defined
by its starting resource block and size.
The minimum size of a BWP must be at least as large as the SSB
(Synchronization Signal Block) bandwidth, to ensure initial access and
broadcast are always possible.
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The maximum BWP size is limited by the configured carrier bandwidth and
the UE’s capability.
The SSB defines the minimum BWP requirement—every BWP must fully
contain the SSB region so the UE can receive system information.
A UE can be configured with up to four BWPs per direction
(downlink/uplink) on a given carrier, but only one BWP can be active at a
time in each direction.
In the downlink, the bandwidth of each BWP should be equal to or greater
than the SSB bandwidth, but it may or may not actually contain the SSB.
For downlink, the UE is not expected to receive PDSCH, PDCCH, CSI-RS, or
TRS outside the active BWP.
Each downlink BWP includes at least one CORESET with UE-Specific Search
Space (USS).
In the primary carrier, at least one of the configured downlink BWPs must
include a CORESET with Common Search Space (CSS).
For uplink, the UE shall not transmit PUSCH or PUCCH outside the active
BWP.
If a UE is configured with a supplementary uplink, it can be configured with
up to four BWPsProfit
in the supplementary uplink, butNew Client
only one BWP can be
active at a time.
At any given moment, only one BWP per direction (UL/DL) is active.
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The network controls which BWP is active via signalling or configuration.
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Switching between BWPs can be triggered by control messages, timer
expiration,Trends
or UE activity. Trends

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CRB and PRB relation

CRBs provide a common reference grid for the entire channel, while PRBs are
UE/BWP-specific slices within that grid—linked by a simple offset.

Common Resource Blocks (CRBs) are numbered from CRB0 (anchored at

“Point A”) across the entire channel bandwidth for a given subcarrier
spacing configuration.
Physical Resource Blocks (PRBs) are defined within a specific Bandwidth
Part (BWP) and are numbered from PRB0 up to the size of the BWP minus
one.
The mapping between a PRB inside a BWP and its location in the overall
channel (as a CRB) is:

CRB index = PRB index + BWP start (N_{start_BWP,μ})

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This means PRB0 of a BWP always aligns with the New Client
starting CRB of that BWP,
and each subsequent PRB aligns sequentially with the next CRB.
This mapping enables the network to flexibly define which section of the
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carrier bandwidth% aChange Previous
UE will use, supporting % Change
spectrum efficiency, device
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capability adaptation, and dynamic resource allocation.
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www.techedgewireless.com
Types of BWPs and RRC
Parameters for configurating
BWP in 5G NR

initialDownlinkBWP: DL BWP#0 configuration for initial access (ID=0)

firstActiveDownlinkBWP-Id: 1: DL BWP#1 becomes active immediately after
attach or reconfiguration
defaultDownlinkBWP-Id: 0: DL BWP#0 is the default/fallback after inactivity
bwp-InactivityTimer: ms40: Sets fallback timer to 40 ms
downlinkBWP-ToAddModList: List of all DL BWPs configured for this cell/UE
initialUplinkBWP: UL BWP#0 configuration for initial UL
firstActiveUplinkBWP-Id: 1: UL BWP#1 is active after attach/reconfiguration
uplinkBWP-ToAddModList: List of all UL BWPs

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 Initial BWP: The bandwidth part configured for the UE during cell access or
initial connection. It is typically used for system information, paging, and
idle/inactive operation. The Initial BWP is always included in the RRC
configuration as the default setup for the UE.
FirstActiveBWP: The bandwidth part that becomes active for the UE
immediately after the RRC connection setup or following a specific
reconfiguration. This is where the UE starts its active communication on the
carrier.
Default BWP: The bandwidth part to which the UE reverts when no specific
service or traffic condition requires another BWP. The Default BWP acts as a
fallback or “baseline” operating mode for ongoing network activity.
Regular BWP: Refers to any bandwidth part (other than Initial, FirstActive, or
Default) that is configured for the UE. Regular BWPs are used to support
specific services, higher throughput, QoS adaptations, or to optimize network
resource allocation. Up to four regular BWPs can be configured in each
direction (uplink and downlink).
Configured BWPs: This is a collective term for all BWPs (Initial, FirstActive,
Default, and Regular) that are set up for the UE in RRC signalling.

BWP Switching Mechanisms

A key innovation of BWPs is the ability to switch between them, allowing the
network and the UE to optimize performance and energy consumption
dynamically.

The selection (or switching) of the active BWP is controlled by several

mechanisms as per 3GPP TS 38.321 Section 5.15:

How is BWP Switching Triggered?

There are ways to switch between active BWPs:

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 Key BWP Switching Mechanisms:

PDCCH (via DCI): The network may activate a specific BWP for the UE using
the bandwidth part indicator field in DCI Format 0_1 (UL grant) or 1_1 (DL
scheduling).
bwp-InactivityTimer: The network sets a timer (bwp-InactivityTimer); if the
UE is inactive on the current BWP and the timer expires, the UE switches
back to the configured default BWP.
RRC Signalling: The network can command a BWP switch at any time via an
RRC Reconfiguration message.
MAC Trigger (Random Access): The MAC layer may switch BWPs during
procedures such as random access, typically reverting to the initial BWP.

Reference: A Primer on Bandwidth Parts in 5G New Radio, Xingqin Lin, Dongsheng Yu, Henning Wiemann

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The above diagram illustrates the dynamic switching of Bandwidth Parts (BWPs)
in 5G NR, where only one downlink and one uplink BWP can be active at any
moment.

Switching between initial, active, and default BWPs is triggered by events such as
RRC configuration, DCI signalling, inactivity timers, or random-access procedures.

Idle Mode:
The UE first synchronizes using SSB and acquires MIB/SIB1.
The initial BWP (DL/UL BWP#0) is configured and used for random access
and early signalling (e.g., paging).
SSB and CORESET#0 are mapped to the smallest region required (20/24
RBs).

UE Enters Connected Mode:

After random access and RRC setup, the first active BWPs (DL BWP#1 / UL
BWP#1) are switched on.
These BWPs have much wider resource allocations (e.g., 270 RBs), supporting
high-throughput data sessions.
BWP switching at this point is typically triggered by RRC signalling
(firstActiveBWP).

Data Transfer Phase:

Only one BWP (DL and UL) is active for actual data transfer.
The network may switch to another BWP (e.g., for QoS, energy savings, or
traffic demand) by sending a DCI with the bandwidth part indicator.

BWP Inactivity or Timer Expiry:

If no traffic is detected for the configured inactivity timer period (e.g., bwp-
InactivityTimer), the UE automatically switches to the default BWP (DL
BWP#2), which typically uses fewer resources (e.g., 52 RBs).
This switch can also occur because of other inactivity or idle triggers.

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 Repeat as Needed:
BWP switching can occur multiple times during a session, based on DCI
commands, RRC messages, inactivity, or MAC triggers.

The Connection Between

CORESET and BWP in 5G NR
CORESET is a fundamental concept in 5G NR, defining a configurable region in
the time-frequency grid where the Physical Downlink Control Channel (PDCCH) is
transmitted. Essentially, a CORESET specifies which resource blocks and OFDM
symbols are used to carry control information such as scheduling grants,
uplink/downlink assignments, and other critical control signalling for the UE.

A CORESET consists of a set of resource blocks (in frequency) and OFDM

symbols (in time), allowing flexible placement and sizing according to
deployment needs.
Multiple CORESETs can be configured per cell, each with its own parameters
(frequency location, duration, mapping, and interleaving).
The PDCCH is only transmitted within a CORESET; therefore, UEs monitor
PDCCH candidates only in the CORESET(s) relevant to them.

Relevance to BWP:
Each Bandwidth Part (BWP) is associated with at least one CORESET in the
downlink.
In fact, a BWP must be configured with at least one UE-specific CORESET (with
UE-Specific Search Space), so the UE knows where to monitor PDCCH within that
BWP.

In the Primary Downlink BWP, there must be at least one CORESET with a
Common Search Space (CSS), used for broadcasted control messages such as
paging or system information.
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 When a BWP is activated, the UE only needs to search for control information
within the CORESET(s) defined for that BWP—this minimizes power
consumption and complexity, especially in wide-band 5G channels.
CORESET configuration within a BWP allows network operators to:
1.Tailor control signalling to device or service needs,
2.Enable device-specific scheduling,
3.Support efficient resource allocation for diverse services (eMBB, URLLC,
mMTC).

CORESET acts as the “control region” for a BWP. It ensures that the UE only
searches for and decodes downlink control information within the part of the
spectrum defined by its current active BWP, supporting both flexible control
signalling and efficient UE operation in 5G NR.

Reference
3GPP TS 38.211, Section 7.3.2: Defines CORESET and its association with BWP in
5G NR.
3GPP TS 38.213, Section 12: Describes BWP operation and CORESET mapping.
3GPP TS 38.331, Section 6.3.2: Covers RRC signalling for BWP and CORESET
configuration.
Dahlman et al., “5G NR: The Next Generation Wireless Access Technology,”
A Primer on Bandwidth Parts in 5G New Radio - Xingqin Lin, Dongsheng Yu,
Henning Wiemann

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