Understanding Bandwidth Part (BWP) in 5G NR

1. Introduction

The fifth generation of mobile networks (5G NR - New Radio) has introduced a wide array of technological advancements. One of the pivotal features is the concept of Bandwidth Part (BWP), which enhances flexibility and efficiency in spectrum usage. Unlike 4G LTE, where UEs (User Equipment) have to monitor the full bandwidth, BWP allows UEs to operate on narrower segments of the spectrum, reducing power consumption and improving performance.

Imagine a shopping mall (total bandwidth). Each shop (BWP) is dedicated to specific products (services like URLLC, eMBB). Instead of scanning the entire mall, a shopper (UE) visits only the relevant shop (active BWP), saving time and energy.

2. What is Bandwidth Part (BWP)?

A bandwidth part is characterized by a numerology (subcarrier spacing and cyclic prefix) and a set of consecutive resource blocks in the numerology of the BWP, starting at a certain common resource block.

3. Why was BWP introduced in 5G NR?

4G LTE forced UEs to operate over the full carrier bandwidth, which was inefficient, especially as spectrum usage grew. 5G introduced wider bandwidths (up to 400 MHz), and forcing UEs to constantly monitor such wide ranges would lead to excessive power consumption. BWP was designed to:

• Reduce power consumption

• Improve spectral efficiency

• Support diverse use cases like eMBB, URLLC, and mMTC

  Below picture represents different use case of BWP in 5G and Future technologies.

Real-Time Use Cases of BWP

• Power Saving in IoT Devices

  • IoT sensors typically need small, infrequent data transfers. Assigning them narrow BWPs ensures longer battery life.

• Enhanced Mobile Broadband (eMBB)

  • Video streaming or VR applications benefit from wide BWPs, which are activated when high throughput is required.

• Ultra-Reliable Low Latency Communication (URLLC)

  • Critical applications like autonomous driving require fast, reliable communication. URLLC-specific BWPs ensure minimal latency.

• Network Slicing

  • Different BWPs can be assigned to different network slices, aligning with SLA requirements for enterprises, public safety, etc.

4. Types of BWP in 5G NR

BWP is a subset of contiguous CRBs for a given numerology (note that different numerologies may be used in different BWPs) on a given RF carrier.

Below are the different types of BWPs in 5G NR, explained with practical insights and UE log information elements that appear during RRC signaling:

4.1 Initial BWP

The initial BWP is used during the initial connection setup phase and is defined in the RRC signaling (e.g., RRCReconfiguration). UE is provided by higher layer parameter initial-UL-BWP an initial uplink BWP to perform random-access procedure.

RRCConnectionReconfiguration:

  radioBearerConfig:

  secondaryCellGroup:

    initialDownlinkBWP:

      bwp-Id = 0

      genericParameters:

        locationAndBandwidth = 6300 (RB start = 100, size = 50 RBs)

        subcarrierSpacing = kHz30

        cyclicPrefix = normal

This shows that the initial BWP is set to use 50 RBs starting at RB index 100, with 30 kHz SCS.

4.2 Active BWP

Active BWP refers to the currently used BWP for ongoing transmission and reception. The UE switches to the active BWP based on RRC signaling or MAC-level commands.

The BWP switching for a serving cell is used to activate an inactive BWP or to deactivate an active BWP at any given time. The BWP switching is controlled by the PDCCH indicating a downlink assignment or an uplink grant, by the bandwidthPartInactivityTimer, or by the MAC entity itself upon initiation of random-access procedure.

ActiveBWP: 1

This indicates that the UE is currently using BWP ID 1 for data transmission.

4.3 Default BWP

The default BWP is used when there’s no ongoing data activity or specific requirement. It acts as a fallback.

defaultDownlinkBWP-Id = 0

4.4 Configured BWPs

These are additional BWPs configured by the gNB, allowing the UE to switch between them based on traffic, power efficiency, or service requirements.

bwp-ToAddModList:

  - bwp-Id = 1

    genericParameters:

      locationAndBandwidth = 6500 (RB start = 200, size = 100 RBs)

      subcarrierSpacing = kHz60

This defines a wider BWP (100 RBs) configured with a different numerology (60 kHz) for high throughput applications like eMBB.

5. Comparison of BWP in 5G NR vs Carrier Aggregation in 4G LTE

In 4G LTE, Carrier Aggregation (CA) was introduced to boost peak data rates by allowing UEs to use multiple component carriers (CCs) simultaneously. However, one major drawback is that the UE must monitor all configured carriers, even when data activity is low. This results in higher power consumption.

In contrast, 5G NR introduces Bandwidth Part (BWP), which divides the entire bandwidth into smaller parts. The UE only needs to monitor and operate within the active BWP, thereby improving energy efficiency and allowing flexible adaptation to different services (e.g., URLLC, eMBB).

Comparison Table

Feature	4G LTE Carrier Aggregation	5G NR Bandwidth Part (BWP)
Number of Active Regions	Multiple CCs simultaneously	Single active BWP at a time
Energy Consumption	Higher	Lower due to selective monitoring
Flexibility	Static carrier assignment	Dynamic BWP switching supported
Numerology Flexibility	Fixed SCS per carrier	Different SCS per BWP supported
Use Case Adaptability	Limited	High (e.g., IoT, URLLC, eMBB)
RRC Complexity	Moderate	Higher due to dynamic BWP management

Example: UE Log Comparison

4G LTE Log (Carrier Aggregation)

RRCConnectionReconfiguration:

  carrierAggregationConfig:

    secondaryCellList:

      - cellId = 1

        dl-Bandwidth = 50RB

      - cellId = 2

        dl-Bandwidth = 20RB

Here, the UE is configured with two additional carriers, and it must monitor both regardless of traffic load.

5G NR Log (Bandwidth Part)

RRCConnectionReconfiguration:

  secondaryCellGroup:

    initialDownlinkBWP:

      bwp-Id = 0

      locationAndBandwidth = 6300 (RB start = 100, size = 50 RBs)

    bwp-ToAddModList:

      - bwp-Id = 1

        locationAndBandwidth = 6500 (RB start = 200, size = 100 RBs)

        subcarrierSpacing = kHz60

    defaultDownlinkBWP-Id = 0

In this 5G log, multiple BWPs are configured, but only one is active at a time depending on the service or power-saving policy. Subcarrier spacing and bandwidth can differ across BWPs, offering better adaptability.

This comparison illustrates how BWP improves upon Carrier Aggregation by offering better energy efficiency, dynamic control, and use-case adaptability while meeting the demands of diverse 5G services.

In 4G LTE, carrier aggregation was used to combine multiple frequency blocks to increase data rates. However, UEs had to monitor all aggregated carriers, increasing power consumption. BWP allows UEs to focus on a narrow part of the spectrum, reducing monitoring overhead.

6. Key Parameters and Configuration Aspects of BWP

• For each serving cell, up to 4 downlink/uplink BWPs can be configured separately and independently for paired spectrum (FDD); nevertheless, only one BWP can be active at a given time and the UE is not expected to receive downlink/uplink physical signals/channels outside of an active BWP.

• For paired spectrum, a downlink BWP and an uplink BWP are jointly configured as a pair and up to four pairs can be configured.

• One can configure up to four BWPs on a supplemental uplink (SUL) carrier.

• In 5G NR, Bandwidth Part (BWP) configuration involves several key parameters. Understanding these is crucial for interpreting UE logs and optimizing BWP usage. Below is a detailed explanation of each parameter along with its significance and example from actual logs:

6.1 Bandwidth Size

Definition: The number of Resource Blocks (RBs) allocated in a BWP.

Significance: Determines the data rate potential; more RBs allow higher throughput.

locationAndBandwidth = (RBstart << 6) + (RBsize - 1)

RBstart = locationAndBandwidth // 64

RBsize = (locationAndBandwidth % 64) + 1

For ex:

locationAndBandwidth = 6300

→ RB start = 98, size = 29 RBs

This indicates the BWP spans 29 RBs starting at index 98.

6.2 Subcarrier Spacing (SCS)

Definition: Distance between two adjacent subcarriers.

Significance: Impacts latency and spectral efficiency. Higher SCS (e.g., 60kHz) suits low-latency apps; lower SCS (e.g., 15kHz) is power-efficient.

Example:

subcarrierSpacing = kHz60

Suitable for URLLC or high-throughput scenarios.

6.3 Control Resource Set (CORESET)

Definition: Defines frequency/time resources used for PDCCH (control channel).

Significance: Essential for decoding DCI (Downlink Control Information).

Example:

controlResourceSetId = 1

frequencyDomainResources = '0x1FF'

This sets control channel resources over specific subcarriers and symbols.

6.4 Activation/Deactivation Timers

Definition: Timers controlling when a BWP becomes active or inactive.

Significance: Enables dynamic BWP switching based on traffic demand or inactivity.

Example:

bwp-InactivityTimer = ms20

If no data activity is detected for 20ms, the UE switches to default BWP.

These parameters work together to ensure that BWPs are tailored for energy savings, low latency, or high throughput based on the application requirement. Proper interpretation of these elements in UE logs enables optimization for real-world deployments.

7. Summary and Future Outlook

5G aims to be more sustainable. BWP contributes by reducing UE processing needs, especially during idle or low-data conditions. BWP is a foundational element in 5G NR, offering tailored performance, energy efficiency, and service flexibility. Future releases (like Rel-18) are expected to add further enhancements such as AI-driven BWP management.

8. References

• 5G NR: The Next Generation Wireless Access Technology, By Erik Dahlman, Stefan Parkvall, Johan Sköld

• 5G NR Architecture, Technology, Implementation, and Operation of 3GPP New Radio Standards By Sassan Ahmadi

• 3GPP TS 38.331, 38.213, 38.211