Acquiring System Information

While IDLE, a UE needs to acquire system information prior to getting connected. This process of system information acquisition also takes place at other instances, and in both IDLE and CONNECTED states. Such instances include reselection, recovery from link failure, both intra- and inter-RAT handoffs, and when the UE receives notification of change in system information.

System information is divided into information blocks. The standard identifies a MasterInformationBlock (MIB) and several types SystemInformationBlocks (SIBs). The MIB defines the most essential physical layer information of a cell required to receive further system information. Type 1 SIB (i.e., SystemInformationBlockTYpe1) contains information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling of other SIBs. Type 2 SIB contains common and shared channel information, while Type 3 SIB contains cell re-selection information, mainly related to the serving cell. Other types of SIB carry are used to relay other specific information such availability of other EUTRA frequencies, neighboring RATs (UTRA, GERAN, CDMA2000), HNBID, ETWS, CMAS and MBMS-related information.

The MIB is transferred over the BCH. SIBs other than Type 1 SIB are carried in SystemInformation (SI) messages. Type 1 SIB also carries scheduling information for other SIB types. All SIB are transmitted on DL-SCH. Note, however, that MIB and SIB follow a fixed schedule, MIB with a periodicity of 40 ms and Type 1 SIB of 80 ms.

Apart from ETWS and CMAS, change of system information occurs per a concept of a modification period. Within a modification period system information may be transmitted a number of times with the same content. When some of or all of the system information changes, the network notifies the UE any time during a modification period, and the updated system information is transmitted in the next modification period. Until notified of and acquired new system information, a UE applies the system information it has already acquired.

The TS 36.331 describes in detail the UE's response to receiving the MIB and various SIBs. For example, upon receiving a MIB, the UE shall apply the MIB's resource configuration. Whether IDLE or connected, it should also apply the received values of downlink and uplink bandwidth allocations until SIB Type 2 is received. When Receiving type 1 SIB, the UE considers whether it supports the indicated frequency bands. If the UE does not, it would consider the cell barred. Otherwise, the UE would forward received cell identity and tracking area code for upper layers. A UE cannot initiate an RRC connection establishment procedure until it has a valid version of MIB, and SIB types 1 and 2.