BWP

Bandwidth parts solved a practical problem: NR carriers can be huge (up to 100 MHz in FR1, 400 MHz in FR2), but a UE doesn't need to keep its receiver wide open across all of it all the time. A BWP lets the network configure a UE to operate in a slice of that carrier — with its own numerology, control resources, and bandwidth — and switch between BWPs as needs change.

Each UE can be configured with up to four downlink and four uplink BWPs, but only one is active at a time. The power-saving angle is the main draw: park an idle-ish device on a narrow BWP and its RF front-end burns far less energy, then switch it to a wide one when a big transfer arrives. Switching is fast — triggered by DCI, a timer, or RRC — but it isn't free; there's a switching delay during which the UE can't transmit or receive, so schedulers don't flip BWPs casually. A gotcha: the initial BWP used during access is derived from CORESET#0, not something you pick freely.