What are the BWP configuration limits in 5G NR?

3GPP specifies up to 4 BWPs can be configured per serving cell direction (4 DL BWPs + 4 UL BWPs), but only one BWP can be active at a time per direction. Each BWP is defined by: starting PRB position, number of contiguous PRBs, subcarrier spacing (15, 30, 60, or 120 kHz for FR1), and cyclic prefix type (normal or extended). BWP switching is triggered by DCI (Downlink Control Information), RRC reconfiguration, or BWP inactivity timer expiry. The initial BWP is signaled in SIB1 and used during initial access.

5G NR

Bandwidth Part (BWP)

Q: How does BWP save UE power?

BWP enables UE power saving by allowing the device to narrow its RF processing bandwidth when full carrier bandwidth is not needed. During periods of low data demand, the network can switch the UE to a narrow BWP (e.g., 20 MHz instead of 100 MHz). The UE's ADC, FFT processor, and baseband processing scale down accordingly: a 20 MHz BWP uses approximately 5x less ADC power and requires a smaller FFT (512 vs. 4096 points for 100 MHz). This is particularly important for IoT devices and smartphones during idle or low-traffic periods.

Q: How does BWP support mixed numerology?

Different BWPs on the same carrier can use different numerologies (subcarrier spacing values). For example, a 100 MHz carrier might have BWP1 with 15 kHz SCS (for eMBB users) and BWP2 with 60 kHz SCS (for URLLC users requiring lower latency). The shorter symbol duration at 60 kHz SCS enables sub-millisecond latency. Different numerologies cannot coexist in the same time-frequency resource without guard bands, but BWP switching allows the network to dynamically assign the appropriate numerology to each user based on their service requirements.

/BAND-width part/ or /bee-dub-yoo-pee/

A 5G NR mechanism allowing a UE to operate on a contiguous subset of the total carrier bandwidth with a specific numerology (subcarrier spacing and cyclic prefix). Up to 4 BWPs can be configured per cell direction with one active at a time. BWP enables dynamic bandwidth adaptation for UE power saving (narrow BWP during low traffic), mixed-numerology operation (15 kHz eMBB vs. 60 kHz URLLC), and forward compatibility with future NR releases.

Max Configured: 4 per direction

Active: 1 at a time

Switching: DCI, RRC, timer

Understanding Bandwidth Part

In LTE, the UE always processes the full carrier bandwidth (up to 20 MHz). In 5G NR with carriers up to 100 MHz (FR1) or 400 MHz (FR2), requiring every UE to process the full bandwidth at all times would waste enormous power. BWP solves this by allowing the network to dynamically assign a narrower operating bandwidth when the UE does not need full throughput.

The BWP concept also enables mixed numerology on a single carrier. Different users with different latency requirements can be assigned BWPs with different subcarrier spacings. An eMBB user on 15 kHz SCS gets maximum spectral efficiency, while a URLLC user on 60 kHz SCS gets sub-millisecond slot duration for low latency. The network manages switching seamlessly.

BWP Configuration

BWP Parameters:
Starting PRB: position in carrier
Number of PRBs: 24 to 275 (FR1)
SCS: 15, 30, 60, or 120 kHz
CP: Normal or Extended

Power Saving Example:
100 MHz active BWP: ADC at 122.88 MSPS, FFT-4096
20 MHz narrow BWP: ADC at 30.72 MSPS, FFT-512
Power reduction: ~5× in RF/ADC/baseband

Switching Triggers:
DCI (fast, ~1 slot), RRC (slow, ~100 ms)
BWP inactivity timer (configurable)

BWP Use Cases

Scenario	BWP Width	SCS	Purpose
Initial access	~20 MHz	30 kHz	SSB/SIB1 reception
Low traffic	20-40 MHz	15 kHz	Power saving
eMBB burst	100 MHz	30 kHz	Peak throughput
URLLC	40-100 MHz	60 kHz	Low latency

Common Questions

Frequently Asked Questions

How does BWP save UE power?

Narrow BWP (20 MHz vs. 100 MHz) reduces ADC rate, FFT size, and baseband processing. ~5x power reduction in RF/ADC/baseband. Critical for IoT and smartphones during idle/low-traffic. Network switches via DCI or inactivity timer.

How does BWP support mixed numerology?

Different BWPs can have different SCS values. 15 kHz for eMBB (spectral efficiency). 60 kHz for URLLC (low latency). Network dynamically assigns numerology per user based on service requirements. Only one BWP active per direction.

What are BWP limits?

4 configured per direction (4 DL + 4 UL). 1 active at a time. Defined by: start PRB, number of PRBs, SCS, CP type. Switching: DCI (fast, ~1 slot), RRC (slow, ~100 ms), inactivity timer. Initial BWP signaled in SIB1.

Related Terms

← Network Bandwidth Bandwidth (Photodiode) →

5G NR

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