A Bandwidth Part (BWP) is a designated portion extracted from the overall carrier bandwidth. It represents a subset of the total available spectrum in the carrier.

Within this BWP, a contiguous grouping of Common Resource Blocks (CRBs) is formed. This set of CRBs is situated within the larger carrier bandwidth and spans across a sequence of adjacent CRBs.

Each individual BWP has its own defined numerology, which encompasses parameters such as Subcarrier Spacing (SCS) and Cyclic Prefix (CP). This distinction ensures specific communication characteristics within the BWP. User Equipment (UE) can be configured to work with up to four separate downlink BWPs and up to four uplink BWPs for each serving cell. In scenarios involving Supplementary Uplink (SUL), an additional four uplink BWPs can exist on the SUL carrier. This configuration structure offers flexibility in managing different communication streams and optimizing spectrum usage.

• BWP operates within NR’s spectrum management hierarchy.
• NR defines frequency ranges (FRs) with operating bands, each tied to specific RF requirements.
• NR supports channel bandwidths from 5 – 400 MHz, accommodating various spectrum scenarios.
• BWP is a set of contiguous resource blocks (RBs) in a channel bandwidth.
• Minimum bandwidth of a BWP should be equal or larger than SSB Bandwidth
• BWP’s size can be equal to or smaller than the cell bandwidth.
• It is not mandatory for every BWP should transmit SSB
• Max number of BWP that can configured is 4, but only one of them can be active at a specific time
• BWP serves purposes like UE bandwidth adaptation and supporting devices with different capabilities.
• BWP is a crucial concept in 3GPP specs, essential for understanding NR.

Reference Xingqin Lin, Dongsheng Yu, Henning Wiemann -5G NR spectrum management and configuration

• NR employs scalable OFDM numerologies with subcarrier spacing (SCS) of 2𝜇⋅15 kHz (𝜇 = 0, 1, …, 4) for efficient communication.
• A Resource Block (RB) includes 12 consecutive subcarriers in the frequency domain, using “Point A” as a reference point for RB grids.
• A Bandwidth Part (BWP) begins at a specific common RB and encompasses contiguous RBs with a given numerology on a carrier.
• For a UE’s serving cell, at least one downlink (DL) BWP is configured, with the option for up to four DL BWPs, but only one active at a time.
• Similar to DL, for cells with uplink (UL), at least one UL BWP is configured, with potential for up to four, but only one active at a time.
• Supplementary UL (SUL) is supported, allowing UL BWP configuration similar to normal UL.
• In frequency division duplex (FDD) scenarios, DL and UL BWPs are configured separately; in time division duplex (TDD), paired DL and UL BWPs share indices.
• Paired DL and UL BWPs in TDD share the same centre frequency while potentially having different bandwidths.
• UE typically receives PDSCH, PDCCH, or CSIRS within an active DL BWP; RRM measurements occur outside via measurement gaps.
• UE transmits PUSCH or PUCCH within an active UL BWP; for an active serving cell, SRS isn’t transmitted outside an active UL BWP

Why BWP came in to picture?

• LTE’s maximum carrier bandwidth is much smaller than that of NR, with LTE at 20 MHz and NR at 400 MHz.
• Scanning the entire NR carrier bandwidth (e.g., 400 MHz) in NR UE would consume excessive power.
• NR supports various UE types and capabilities; not all devices can handle the full carrier bandwidth.
• BWPs offer the advantage of reducing UE power consumption, especially for UEs capable of receiving the full carrier bandwidth.

• Resource Efficiency: 5G networks are designed to support a wide range of services with varying bandwidth requirements. BWP enables efficient utilization of the available spectrum by allowing dynamic allocation of smaller portions of spectrum, catering to the specific needs of different services and devices. This efficient use of resources enhances overall network capacity and performance.
• Spectrum Flexibility: Different parts of the available frequency spectrum might have different characteristics and regulatory constraints. BWP allows for the creation of customized subsets of the spectrum, enabling operators to adapt to regulatory requirements and optimize usage for specific services.
• Service Differentiation: 5G serves diverse use cases such as enhanced mobile broadband, massive machine-type communications, and ultra-reliable low-latency communications. BWP permits operators to allocate appropriate portions of spectrum to each service, tailoring the network to meet the distinct requirements of each application.
• Bandwidth Adaptation: BWP provides the capability for User Equipment (UE) to adjust its bandwidth dynamically. This adaptation is particularly useful to conserve power consumption. For instance, a UE can utilize wider bandwidth when high data rates are necessary and switch to narrower bandwidth during periods of lower activity, reducing energy usage.
• Interference Management: BWP allows for focused allocation of resources within a cell’s bandwidth. This minimizes interference with neighbouring cells, leading to improved network performance and better user experiences.
• Device Compatibility: With the diversity of devices in the 5G ecosystem, not all devices support the same bandwidths. BWP enables devices with varying bandwidth capabilities to coexist within the network by allocating resources that match the devices’ capabilities.
• Dynamic Traffic Handling: BWP facilitates dynamic changes in resource allocation based on real-time traffic demands. For instance, during periods of high network congestion, BWPs can be adjusted to prioritize critical services or specific areas with increased demand.

BWP Switching and Types of Active BWPs in 5G NR:

BWP Switching Overview:

• Switching between an active and an inactive BWP is termed “BWP switching.”
• BWP switching prevents the deactivation of all BWPs or activating more than one simultaneously.
• In paired spectrum (FDD), DL and UL BWPs can be switched separately.
• For unpaired spectrum (TDD), paired DL and UL BWPs switch together.

Types of Active BWPs:

Initial DL/UL BWP:

• Used for initial access before Radio Resource Control (RRC) connection establishment.
• An initial BWP, denoted as BWP #0, is configured for this purpose.
• UE performs cell search using synchronization signals (SSBs) including PSS, SSS, and PBCH.
• System Information Block 1 (SIB1) carries key info, including initial BWP configuration.
• UE follows SIB1 to perform random access for RRC connection.
• Initial DL BWP aligns with CORESET #0’s frequency range initially.

First Active DL/UL BWP:

• Configured for Special Cell (SpCell) or secondary cell (SCell).
• In master cell group (MCG), SpCell is the primary connection cell.
• In secondary cell group (SCG), SpCell is the primary SCell for random access.
• SCell provides additional resources in a cell group.
• First active DL and UL BWPs apply after RRC (re-)configuration or SCell activation.

                                              Figure reference: http://www.sharetechnote.com

Default BWP:

• Configurable for a serving cell, UE may switch to it due to inactivity.
• UE shifts to default BWP after a timer indicates no scheduled transmission/reception.
• Default DL BWP is configurable; else, initial DL BWP is used.
• In unpaired spectrum, switching DL BWP triggers UL BWP switch, common for TDD.

How does a UE use BWP in RRC idle mode and RRC connected mode?

From above picture, these BWP types from a UE perspective.

below is summary:

• UE network access initiates by capturing Synchronization Signal Block (SSB): includes PSS, SSS, and PBCH, spanning 4 OFDM symbols and 20 RBs, containing Master Information Block (MIB).
• MIB holds CORESET#0 config, used by UE to deduce initial Downlink Bandwidth Part (DL BWP).
• UE decodes CORESET#0, holding System Information Block 1 (SIB1) that sets initial BWP for DL and UL; termed BWP#0. DL BWP#0 includes CORESET#0.
• Random Access Channel (RACH) employs UL BWP#0. Network uses DL BWP#0 until Radio Resource Control (RRC) connection.
• After RRC connection, UE can be configured with personalized UE-specific BWPs.
• Example figure illustrates BWP#0 (24 RBs), BWP#1 (270 RBs), and default DL BWP#2 (52 RBs).
• In Frequency Division Duplex (FDD), DL switches to BWP#2, UL stays at BWP#1. In Time Division Duplex (TDD), DL and UL BWP switching occurs simultaneously.

5G NR Reference Point A

“Point A” is a fundamental concept in 5G wireless networks that serves as a reference point within the frequency domain. It is a fixed reference point used to establish the structure of frequency resources, including subcarriers and resource blocks. The placement of “Point A” significantly influences how various communication components are positioned within the spectrum.

Key Characteristics of “Point A”:

• Positioning of Reference Point A:

• “Point A” is positioned at the starting point of the frequency grid for resource allocation.
• It is associated with subcarrier 0 of common resource block 0, regardless of the subcarrier spacing used in the system.
• Importantly, “Point A” can be located outside the actual carrier frequency range, serving as an abstract reference.
• Role as a Reference:

• “Point A” acts as a critical reference from which the entire frequency structure is described.
• It helps establish a consistent framework for allocating subcarriers and resource blocks across different communication scenarios.
• Locating “Point A”:

• Devices, such as user equipment (UE) or base stations, locate “Point A” based on system information.
• The discovery of “Point A” is facilitated by decoding the broadcasted System Information Block 1 (SIB1).
• Once SIB1 is decoded, the device knows the precise location of “Point A” within the frequency spectrum.

Illustrative Example:

Let’s consider a practical example involving a 5G base station and a user equipment (UE) operating within a frequency band from 2.8 GHz to 3.5 GHz. Within this band, “Point A” is positioned at 3.0 GHz.

Subcarrier Allocation:

• “Point A” at 3.0 GHz serves as the starting reference point for subcarrier allocation.
• Subcarriers are distributed in both directions from “Point A” at regular intervals, such as every 15 kHz.

Resource Block Structure:

• The base station divides the spectrum into resource blocks, each comprising a certain number of subcarriers.
• For instance, a resource block might consist of 12 consecutive subcarriers.

Channel Allocation:

• Different channels, including control channels and data channels, are allocated based on the positioning of “Point A.”
• These channels are placed relative to the reference point for consistent and organized communication.

UE Discovery and Communication:

• The UE detects synchronization signals (SSBs) broadcasted by the base station.
• By decoding the SIB1 carried in these signals, the UE learns the location of “Point A” (3.0 GHz).

Resource Allocation:

• All physical resource blocks used for transmitting actual signals are positioned relative to “Point A.”
• The base station allocates resources based on the reference point to ensure efficient and controlled communication.

In this example, “Point A” at 3.0 GHz serves as the anchor for establishing the entire frequency structure. It guides how subcarriers, resource blocks, and channels are organized and allocated, leading to well-structured communication within the 5G network.

What is the process of BWP adaptation or switching

• During the shift from idle mode to RRC connected mode, the RRC signaling possesses the capability to configure UE-specific Bandwidth Parts (BWPs).
• In specific scenarios, the RRC configuration or reconfiguration message might single out one of these BWPs for activation, consequently prompting BWP switching.
• However, this process can exhibit a delay due to the inherent processing time of the RRC, potentially occurring within the realm of tens of milliseconds.

Reference: Xingqin Lin, Dongsheng Yu, Henning Wiemann  Understanding the BWP Indicator Field within DCI Format 0_1/1_1 for Switching BWPs Based on DCI

• Once a UE is equipped with multiple BWPs through configuration, the network holds the authority to instruct BWP switching in the UE.
• This is effectively accomplished through the utilization of Downlink Control Information (DCI) within the Physical Downlink Control Channel (PDCCH).
• Employing DCI format 1_1 for downlink assignment and format 0_1 for uplink grant, these formats incorporate a BWP indicator variable that can take either 1 or 2 bits.
• In instances where more than 2 BWPs are integrated into the system, a 2-bit indicator serves this purpose efficiently.
• A third mechanism for BWP switching materializes when the BWP inactivity timer attains its endpoint, triggering an automatic transition to the default BWP.
• The temporal range of this timer extends from 2 to 2560 milliseconds.
• It’s important to note that the highest value of this timer is correlated with the Discontinuous Reception (DRX) inactivity timer, revealing a harmonious synchronization of timing components.

DCI- and timer-based BWP switch delay requirements

• Network and UE Transmission/Reception Delay:
• Delay exists between network and UE due to DCI-based BWP switch.
• UE must finalize active DL and/or UL BWP switch within set BWP switch delay.
• BWP switch time limits specified in Table 2 for DCI and timer-based BWP switch

• BWP Switch Delay Types:
• Two levels of BWP switch delay: type 1 and type 2 (Table 2).
• TBWPswitchDelay for DCI-based BWP switch:
• Defined as slot difference between DL slot of switch request and first slot for PDSCH reception (DL) or PUSCH transmission (UL) on new BWP.

• UL/DL Signals during Switch Delay:

• UE not obliged to send UL signals or receive DL signals for TBWPswitchDelay during DCI-based BWP switch on serving cell.

• Dependency on SCS:
• BWP switch delay tied to Subcarrier Spacing (SCS).
• If switch spans BWPs of differing SCS values:
• Delay rule follows smaller SCS’s requirement.

FIG : DCI- and timer-based BWP switch delay requirements

Bandwidth Adaptation (BA) introduces a transformative capability for User Equipment (UE), allowing it to independently regulate its reception and transmission bandwidth without being constrained by the cell’s total bandwidth. This versatile feature empowers the UE to conserve energy while dynamically accommodating fluctuating data demands. The responsiveness of bandwidth adjustments becomes evident across a range of scenarios:

Important Points:

Efficient Utilization: BA enables the UE to adopt a narrower bandwidth, making it ideal for tasks like monitoring control channels or receiving moderate data loads. This strategic move optimizes power consumption.

Seamless Scaling: In scenarios demanding data-intensive transfers, the UE seamlessly transitions to a broader or full bandwidth to fulfil the escalated requirements.

Frequency Domain Agility: BA introduces adaptability in the frequency domain, permitting the UE’s location to shift, along with customizable subcarrier spacing. This adaptability effectively caters to the diverse needs of various services.

BWP Configuration: The realization of BA entails configuring the UE with Bandwidth Parts (BWPs), which act as adjustable segments of bandwidth.

Dynamic Activation: From the array of configured BWPs, the active BWP is explicitly communicated to the UE, allowing instant adaptability.

Scenario Perspective: Illustratively, envision a scenario where three distinct BWPs are configured. In the Primary Cell (PCell), both uplink (UL) and downlink (DL) BWPs are configured, seamlessly accommodating BA. In the context of Carrier Aggregation (CA) involving Secondary Cells (SCells), at least DL BWPs are configured. However, the presence of UL BWPs for SCells hinges on the specific configuration.

Mapping between normalized Channel Resource Blocks (nCRB) and normalized Physical Resource Blocks (nPRB).

Physical resource blocks for subcarrier spacing configuration μ are defined within a bandwidth part and numbered from 0 to

, where i is the number of the bandwidth part.

The relation between the physical resource block

 in bandwidth part i and the common resource block

is given by

Reference: 38.211 v2.0.0 – 4.4.4.4

where ,

is the common resource block where bandwidth part i starts relative to common resource block 0.

When there is no risk for confusion the index μ  may be dropped.

Figure reference: http://www.sharetechnote.com

BWP Activation/Deactivation:

• RRC-Based Adaptation:
• Suited for semi-static services like Voice.
• Resource allocation changes infrequently during the same data session.

• MAC CE Initiation:
• Initiated during Random Access Channel (RACH) procedure.

• DCI-Based:
• Enables rapid BWP switching for low-latency services.
• Requires additional considerations for error handling.
• In case of missed BWP activation in DCI_0 and DCI_1 message.

• Timer-Based Implicit Fallback:
• Designed to counter potential DCI errors.
• If no explicit BWP scheduling post timer expiration, UE transitions to default BWP.

RRC Parameters for BandwidthPart Configuration

Initial Downlink BWP (initialDownlinkBWP):

• Configuration for first downlink bandwidth-part (DL BWP#0).
• If optional elements are configured, UE treats BWP#0 as RRC-configured.
• Otherwise, BWP#0 isn’t considered RRC-configured by UE.
• Network assigns value if no other BWPs configured.
• Changing BWP requires RRC Reconfiguration if dedicated initial UL/DL BWP config is absent.

First Active Downlink BWP (firstActiveDownlinkBWP-Id):

• Identifies DL BWP to activate post initial attach or NR addition.
• For SpCell: Activated after reconfiguration.
• Absence means no BWP switch in RRC reconfiguration.
• For SCell: Used upon MAC-activation.
• Initial bandwidth part referred by BWP-Id = 0.

Default Downlink BWP (defaultDownlinkBWP-Id):

• Automatically switched when no activity in current BWP until bwp-InactivityTimer.
• 0 signifies defaultDownlinkBWP = initialDownlinkBWP.
• BWP-Id used after BWP inactivity timer expires.
• UE-specific field; absent field uses initial BWP as default.

BWP Inactivity Timer (bwp-InactivityTimer):

• Duration (ms) before UE reverts to default Bandwidth Part.
• 0.5 ms for carriers > 6 GHz.
• UE stops timer on network’s timer release, no default BWP switch.

Initial Uplink BWP (initialUplinkBWP):

• For SpCell: Identifies DL BWP after reconfiguration.
• Absence means no BWP switch in RRC reconfiguration.
• For SCell: Identifies uplink bandwidth part after MAC-activation.
• Initial bandwidth part referred by BandwidthPartId = 0.

First Active Uplink BWP (firstActiveUplinkBWP-Id):

• Dedicated config for initial uplink bandwidth-part.

BWP Identifier (BWP-Id):

• ID for a bandwidth part in RRC configuration.
• BWP ID=0 is initial BWP and not used elsewhere.

BWP Switching via DCI:

• NW triggers UE to switch UL/DL BWP using DCI field.
• DCI code points map to RRC-configured BWP-IDs.
• Up to 3 additional BWPs (initial, first dedicated, …).
• If 4 dedicated parts, DCI codes 0 to 3.
• DCI can’t switch to initial BWP if all 4 dedicated configured.
• Corresponds to L1 parameter ‘UL-BWP-index’ / ‘DL-BWP-index’.

Reference: 3gpp 38.211, 38.331, 38.133 , Xingqin Lin, Dongsheng Yu, Henning Wiemann