15.4.2.9 On-demand SIB1

To facilitate reducing gNB downlink transmissions, instead of always periodically transmitting SIB1, the gNB can provide on-demand SIB1, i.e., upon receiving an OD-SIB1 request from a UE supporting OD-SIB1. OD-SIB1 is supported for UEs in RRC_IDLE, RRC_INACTIVE and RRC_CONNECTED when T311 is running. A request for SIB1 triggers a random access procedure, where MSG1 is used for indicating OD-SIB1 request and the gNB acknowledges the request in MSG2. OD-SIB1 request configurations of one or more cells which support OD-SIB1 are included in SIB26, which can be broadcasted in any cell, including cell’s own OD-SIB1 request configuration. UE may request SIB1 based on the OD-SIB1 request configuration from SIB26 in order to determine the suitability of a cell during and after cell reselection as specified in TS 38.331 [12].

A gNB may request neighbour gNB(s) over the Xn interface to transmit or stop transmitting the OD-SIB1 configuration for a cell supporting OD-SIB1. The neighbour gNB(s) deciding to stop the OD-SIB1 configuration transmission informs the requesting gNB over Xn. The inter-gNB coordination procedures are defined in TS 38.401 [4].