How long does BWP switching take in 5G NR?

BWP switching delay depends on the UE capability and the type of switch. 3GPP TS 38.133 defines two types: Type 1 (no frequency change within the carrier) requires 1-3 slots depending on the subcarrier spacing (1 slot at 15 kHz SCS = 1 ms, 1 slot at 120 kHz SCS = 0.125 ms), while Type 2 (center frequency change) requires 2-5 slots. During the switching gap, the UE cannot transmit or receive on the affected carrier. For FR2 mmWave deployments at 120 kHz SCS, the switching delay is as low as 0.25 ms, making rapid BWP adaptation practical for beam management scenarios.

What triggers a BWP switch in 5G NR?

BWP switches are triggered by three mechanisms: DCI-based activation (the gNB sends downlink control information in PDCCH indicating a new BWP), timer-based fallback (the bwp-InactivityTimer expires and the UE reverts to the default BWP), and RRC reconfiguration (the network explicitly reconfigures the active BWP set). DCI-based switching is the fastest, occurring within one scheduling interval. Timer-based fallback is the primary power-saving mechanism, automatically narrowing the UE's bandwidth during idle periods to reduce ADC sampling rate and RF front-end power consumption by 30-50%.

Why does 5G NR use BWP switching instead of always using the full carrier bandwidth?

Using the full carrier bandwidth (up to 400 MHz in FR2) requires wideband ADCs, DACs, and RF chains that consume significant power. A UE streaming 20 Mbps video does not need 400 MHz of bandwidth. BWP switching allows the UE to activate a narrow BWP (e.g., 20 MHz) for low-rate traffic and only widen to 100-400 MHz for peak throughput bursts. This reduces UE power consumption by 30-60% compared to always-on wideband operation. BWP switching also supports UEs with limited RF bandwidth capability, allowing devices with 50 MHz front-end bandwidth to operate on a 100 MHz carrier by accessing different portions at different times.

What is BWP Switching? | 5G NR Bandwidth Part Switching

Definition & Mechanism

Bandwidth Part (BWP) switching is a 5G NR radio resource management mechanism defined in 3GPP Release 15 (TS 38.211, 38.213, 38.133) that allows a User Equipment (UE) to dynamically change which portion of a component carrier's frequency bandwidth it actively monitors and uses. Each BWP is a contiguous set of Physical Resource Blocks (PRBs) with a specific subcarrier spacing and cyclic prefix length. A UE can be configured with up to four BWPs per component carrier on both uplink and downlink, but only one BWP is active at any given time on each link direction.

The switching process involves the UE retuning its ADC/DAC sampling rate, adjusting RF front-end filter bandwidth, and resynchronizing to the CORESET and search spaces configured for the target BWP. The gNB orchestrates BWP switches through DCI signaling in the PDCCH, using a bandwidth part indicator field to direct the UE to a new active BWP. Alternatively, the bwp-InactivityTimer triggers automatic fallback to a narrower default BWP after a period of inactivity, reducing power consumption without network signaling overhead.

Key Formulas

BWP Switching Delay (Type 1):

T_switch = N_slot × T_slot

At 30 kHz SCS: T_slot = 0.5 ms, N_slot = 2 → T_switch = 1.0 ms

Power Savings from BWP Narrowing:

P_save ≈ 1 − (BW_narrow / BW_wide)^0.7

100 MHz → 20 MHz: P_save ≈ 1 − (0.2)^0.7 ≈ 70% ADC power reduction

Maximum PRBs per BWP:

N_PRB = floor(BW_BWP / (12 × Δf))

100 MHz at 30 kHz SCS: N_PRB = 273

BWP Configuration Comparison

Parameter	Narrow BWP	Medium BWP	Wide BWP	Full Carrier
Bandwidth	5-20 MHz	40-60 MHz	80-100 MHz	100-400 MHz
PRBs (30 kHz)	11-51	106-162	217-273	273 (FR1 max)
Peak Throughput	50-200 Mbps	400-600 Mbps	800+ Mbps	1-4 Gbps
UE Power	Low (baseline)	Moderate	High	Maximum
ADC Rate	30-60 MSPS	120-180 MSPS	240-300 MSPS	600-1200 MSPS
Typical Use	Idle monitoring, VoNR	Video streaming	File download	Speed test, AR/VR

Practical Application

In a commercial 5G NR n78 (3.5 GHz) deployment with 100 MHz carrier bandwidth, a smartphone is configured with three BWPs: BWP 0 (default, 20 MHz, 30 kHz SCS) for paging and low-rate signaling, BWP 1 (50 MHz) for moderate traffic, and BWP 2 (100 MHz) for peak throughput. When the user launches a large file download, the gNB sends a DCI with BWP indicator = 2, switching the UE to the full 100 MHz BWP within 1 ms. After the download completes and the bwp-InactivityTimer (set to 100 ms) expires, the UE automatically falls back to BWP 0, reducing baseband power consumption by approximately 65% while maintaining reachability for incoming data or calls.

Frequently Asked Questions

How long does BWP switching take?

1-3 slots for Type 1 (same center frequency), 2-5 slots for Type 2 (center frequency change). At 120 kHz SCS for FR2, switching completes in as little as 0.25 ms. During the gap, the UE cannot transmit or receive on that carrier.

What triggers a BWP switch?

Three mechanisms: DCI-based activation (gNB signals new BWP via PDCCH), timer-based fallback (bwp-InactivityTimer expiry reverts to default BWP), and RRC reconfiguration. DCI switching is fastest; timer fallback is the primary power-saving tool, reducing UE power by 30-50%.

Why not always use full carrier bandwidth?

Full bandwidth (up to 400 MHz in FR2) requires wideband ADCs and RF chains that consume heavy power. A 20 Mbps video stream doesn't need 400 MHz. BWP switching lets the UE narrow to 20 MHz for low-rate traffic and widen only for throughput bursts, cutting power consumption 30-60%.

BWP Switching