Energy-saving SIB1 stays locked to the standard frequency raster

Network energy savings for NR hinges on letting a base station skip broadcasting SIB1 and send it only when a phone asks for it via an uplink wake-up signal. The catch is which beam carries the reply: Ericsson, Samsung, LG and Fujitsu wanted a 1-bit flag letting the network answer only on the beam the phone used, cutting transmissions elsewhere. ZTE, Huawei, Apple, Google and OPPO opposed it three separate times across RAN1#120-bis and RAN1#121, arguing it could leave other phones in the area unable to hear SIB1 at all. The flag eventually made it into the specification as an optional behavior, but only after the moderator, MediaTek, reworked the proposal twice.

Companies did agree to keep the SSB itself on the standard synchronization raster — the fixed frequency grid phones already scan for 5G cells. A push to let energy-saving cells sit off-raster was voted down four times, with only CATT dissenting each time; Nokia, Google, Samsung and Apple all backed the status quo. That keeps idle and inactive phones from needing extra power-hungry frequency searches, which matters directly for standby battery life on every device using the feature.

Timing got two firm rules. The MIB, refreshed every 80 ms, can't change the K_SSB parameter faster than that period, so a network can't flip a cell between broadcast and on-demand SIB1 mode more often — proposed by Nokia and backed by CMCC, Google and Qualcomm. Separately, RAN1 fixed the K_SSB field itself at 5 bits for sub-7 GHz spectrum and 4 bits for millimeter wave, with no changes to the existing broadcast channel structure, and confirmed the SIB1 reception window can start at the reference point or later, never earlier.

Google tried to force phones to always pick the beam with the lowest index number when sending their wake-up signal, arguing it would cut both network and device power use. CMCC, vivo, Fujitsu, Qualcomm, Sharp, Sony, LG, NEC, Ofinno and Tejas rejected it twice, preferring to keep the legacy 5G rule that lets a phone pick any beam above the signal threshold — one of the clearer defeats of the quarter, ten companies against one.

Two questions stayed open without even a proposal reaching a vote: whether a phone should listen for an existing SIB1 broadcast before bothering to send a wake-up signal (LG, Ericsson and MediaTek want it to check; Apple doesn't), and whether the feature should be tested against RedCap devices and unlicensed-spectrum operation, where Google, Qualcomm and Futurewei argue the work is out of scope. Both return to RAN1's next meeting, and until the wake-up-signal question is settled, chipset vendors can't finalize how much standby power the feature actually saves.