What is a BWP?
A Bandwidth Part is a contiguous slice of the carrier (≤ 275 PRBs wide), defined by a BWP start position, width, numerology, and cyclic prefix. The UE monitors one DL BWP and one UL BWP at any time. Up to 4 DL BWPs and 4 UL BWPs can be configured per UE.
Initial BWP vs Default BWP vs Active BWP
Initial BWP: UE acquires this during cell search (defined by the SIB1's offsetToPointA). Default BWP: UE falls back to this on timer expiry. Active BWP: the one the UE is currently monitoring. RRC configures all four; DCI/MAC-CE/timer flips the active one.
Per-BWP Numerology
Each BWP has its own SCS, CP, frequency-domain reference. So the same UE can have a μ=0 narrowband BWP (low latency, narrow allocations) and a μ=1 wideband BWP (high throughput) simultaneously configured — switching between them via DCI.
PRB = CRB - BWPstart
CRB (Common Resource Block) is the cell-wide PRB index from Point A. PRB (Physical Resource Block) is BWP-relative — counted from BWPstart. DCI scheduling allocates by PRB always.
BWP Switching · Three Mechanisms
1) RRC Reconfiguration: slow (>10 ms), reconfigures the BWP pool. 2) DCI Switch: BWP indicator field in DCI 0_1/1_1, takes effect within ~1 slot. 3) Inactivity Timer: bwp-InactivityTimer expires (no DCI received for X ms) → UE falls back to default BWP.
Why BWPs Save UE Battery
A 100 MHz carrier needs full RF front-end + ADC bandwidth. A 20 MHz BWP needs only 20 MHz worth — 5× lower power. The UE sits on a narrowband BWP at idle (DRX); when traffic arrives, gNB sends DCI switching to a wideband BWP; UE pulls the data; bwp-InactivityTimer expires; UE returns to narrowband. This single mechanism saves 30-50% UE battery vs always-on full bandwidth.
LTE Had No BWPs
In LTE, the UE always operated at full carrier bandwidth. Battery was wasted on RF/ADC paths during idle moments. NR's BWP design was directly targeted at the smartphone-battery problem and at scheduler agility.
Frequently Asked Questions
What is a Bandwidth Part (BWP) in 5G NR?
A BWP is a contiguous slice of the carrier (up to 275 PRBs) that a UE actively monitors. NR allows up to 4 DL and 4 UL BWPs configured per UE, with one active at a time.
How many BWPs can be active simultaneously?
Only one DL BWP and one UL BWP can be active per UE at any moment. The other configured BWPs are dormant — they exist in RRC config but are not being monitored.
How does a UE switch BWPs?
Three ways: (1) RRC reconfiguration to change the BWP pool (slow, > 10 ms); (2) DCI 0_1/1_1 BWP indicator field flips active BWP within ~1 slot; (3) bwp-InactivityTimer expires after no DCI activity, UE falls back to default BWP.
Why do BWPs save UE battery?
A narrow BWP needs less RF and ADC bandwidth than the full carrier. UE sits on a 20 MHz BWP at idle, switches to 100 MHz only when traffic arrives, saving 30-50% battery vs always-on full bandwidth.