LTE system Information (3)-System Information changes

1. Why you need to add system information change mechanism

From the LTE System Information (2)-sib Cycle Schedule We already know that the most of the system information required by UE is contained in different SIB blocks, which are broadcast to UE by SIB1 messages and SI messages, respectively. These parameters will not change in general, but there is no absolute, considering that some parameters may need to be modified in some specific cases, such as modifying the Rach parameter in SIB1, or modifying the Ac-barringinfo parameter in SIB2, so There is a need to add a mechanism to allow the SIB parameters to change when the UE can get the updated system information in a timely manner . This mechanism is the "System Information change process" that is to be described in this article (changes of systemsinformation).

Perhaps some students will ask, UE will not be in every system information cycle time to decode it? UE is clearly aware of all the information of each system to send the moment, ENB is also a periodic broadcast, so long as the UE is willing to read, is able to timely access to the latest system information, then why does UE do not do it? Because in many cases the system information will not change, if the UE in the case has obtained the system information, and still every dozens of MS uninterrupted to read the system information, it is relatively cost-electricity . The power consumption problem is a very important problem that affects the overall performance of UE, other such as paging, CDRX mechanism is also based on similar starting point.

Since the UE does not read every time the system information is sent, when does the UE need to read it? When one of the following scenarios occurs, the UE needs to obtain or regain system information:

(1)the cell selection process after boot。 (2)when you're ready to re-elect to another neighborhood。 (3)after the switchover process is complete。 (4)after entering the LTE format from other formats。 (5)return from Lost overwrite。 The above 5 scenarios can be summed up when the UE enters a new service cell, need to obtain the system information of the cell. (6)Receive System Information change Instructions。 (7)receive ETWS message indication。 Whether it appears in the pager messageetws-indicationfield is determined. (8)receive CMAs message indication。 Whether the pager message appearscmas-indicationfield is determined. Above these 3 kinds of scenarios can be summed up in the service cell broadcast parameters have changed, need UE re-acquisition. (9)receive CDMA2000 upper instruction。 At this point, the UE needs to retrieve the CDMA2000 parametersSIB8。 (10)Maximum validity of system information exceeded。 If the UE has already read the system information, the UE-stored system information is only3-hour validity, you need to regain system information for more than 3 hours.

2. System Information Change process

Changes to system information can only occur in specific wireless frames, which are also referred to as "Change cycles" (modification period, MP). The boundary of the change period or the frame position of the starting point should match the condition (SFN mod m = 0). where m represents the change period, is determined by the Modificationperiodcoeff parameter and the defaultpagingcycle parameter in the SIB2 block, as shown in 1. For example, MODIFICATIONPERIODCOEFF=N2,DEFAULTPAGINGCYCLE=RF32, then change the period m=2x32=64, Unit is the number of wireless frames.

(Figure 1 The parameters related to the change cycle in SIB2)

When the network needs to modify system information, the following 2 procedures are performed:

(1) Send system Information change instructions . ENB sends a system information change instruction to the UE, informing the UE that it will need to re-read the system information during the next system change cycle. Once the UE receives the System Information change instruction, it will start acquiring new system information at the beginning of the next change cycle. The UE needs to continue to use the old system information parameters before decoding to the new system information.

(2) in the next change cycle, the network side broadcasts the modified system Information .

As shown in 2, different color blocks represent different system information. In the system change Cycle N, UE received a system change instructions, but at this time the system information is still the old system information, that is, the red block in the diagram, in the next system change cycle (N+1), the network side began to broadcast a new system information block, that is, the pink content in the picture. The yellow System Information block has not changed in this process.

(Figure 2 System Information change process)

fields and SIB1 systeminfovaluetag field. If you are modifying the In addition to SIB10/11/12 three kinds of system information, you need to add the message in the pager. except mib/ System information for outside of SIB1/SIB10/SIB11/SIB12, you need to modify systeminfomodification field, The MIB and SIB1 messages need to be re-read during the same system change cycle to see if an SI message has been modified.

The blog "Drx discontinuous receive (2)-Paging paging" has been said in the pager message structure has an optional field systeminfomodification, which represents the addition of sib10/sib11/ System Information other than SIB12 is changed, as shown in Figure 3 below. If the UE decodes the paging information when it is found to contain the field, it is considered that the network side will modify the system information during the next system change cycle, the UE needs to perform the system information change process, regardless of whether the current UE is in Rrc-idle state or in connected state. The System Information change instruction that is carried in the pager message is only a mark bit, can only tell UE "System Information has modified" This simple information, cannot tell the UE specific is which system information wants to modify.

(Figure 3 The System Information change indication in the paging message)

The SIB1 contains a systeminfovaluetag field that can be used to determine if the current system information other than MIB/SIB1/SIB10/SIB11/SIB12 is still valid, as shown in Figure 4 below. The value range of the Systeminfovaluetag field is 0~31, each time the network executes 1 system Information change process, the field increment 1,ue side through whether the value changes to determine whether the need to perform system information change process.

(Figure 4 Parameters related to message changes in SIB1)

if the UE does not receive a pager message during a change cycle, the UE may assume that no changes to the system information will occur during the next change cycle . Therefore, if the network side to modify the system information parameters (in addition to Etws and CMAs), you should send a pager message, and add the systeminfomodification field in the pager message.

As you can see from the description above, whether it's a systeminfomodification field in a pager message or a systeminfovaluetag field in SIB1, Does not indicate whether the ETWS or CMAs message has changed. The ETWS and CMAs messages that are carried by SIB10/11/12 need to be determined by whether the pager message contains etws-indication and cmas-indication fields.

3. Evolution of System Information Change scheme

How to get UE to know the changes of system information in time, was first presented by Motorola in a 3GPP conference in Korea in 2007. Moto proposed: First, you can add a mib_value_tag field in the MIB to indicate whether the SIB1 or other SIB blocks have changed, and bind a sib_value_tag for each other SIB block in SIB1. fields, which are used to indicate whether a specific SIB block has changed. With this design, ENB can control the behavior of the UE read system information by modifying the Mib_value_tag field in the MIB and the Sib_value_tag set field in the SIB1. Students familiar with the GSM-RR/RRC protocol will find that the original scheme is similar to the design of the Bcch_change_mark and si_change_field fields of SI-TYPE13 messages in GSM. Interested students can take a look at 44018 and 44060 of these two agreements.

A closer look will find that the only way to achieve the system information change process is not possible, why? Because the UE does not always read the system information (MIB and SIB), UE can not know exactly when the network side to modify the system information parameters, so here you need to bind a timer to UE (such as using 30s timer in GSM). When the timer expires, each UE will actively re-read the MIB message, detect the mib_value_tag field, once the mib_value_tag field has changed to read the SIB1 message again, detect the Sib_ Value_tag value to determine if the other SIB blocks have changed.

Isn't it all right to be here? The same problem. Since when to open this timer is fastidious. Service cell Broadcast system parameters once changed, then all the UE in this community should be at the same time to obtain updated system information, if different UE timer Opening time is inconsistent, will cause some UE has received a new system information parameters, and some UE is still old system information parameters. As a rule: once the UE is decoded to the MIB, the timer is turned on. In this way, all the UE in the same community can keep the System Information update process synchronized.

This was later optimized by Infineon: The most basic information is carried in the MIB, and the modified field of system information should not be value_tag in the MIB, we can put the Value_tag field in the pager message. Because all the UE in the same service community can decode the paging message p-rnti scrambling, so that every UE can receive the same value_tag. To facilitate the difference, the Value_tag placed in the pager message is temporarily recorded as Paging_value_tag. Mib/sib1/sib2/is represented by the Paging_value_tag field. /sib9/sib13 is modified, and the Sib1_value_tag field in SIB1 is used to represent sib2/sib3/. /sib9/sib13 whether there are any modifications.

"Drx discontinuous receive (2)-Paging paging" has been analyzed, for the same service area of the different UE, paging messages are sent at the moment is related to IMSI, different UE corresponding paging time is not the same. In order to ensure that all UE can read the updated system information at the same time, all UE can only read the system information from the unified "specified moment". The different UE should not rush to read the new system information after receiving the Value_tag, but only at the "designated moment" to read the system information. This "specified moment" is distinguished by the "system change Cycle" described earlier.

After this optimization, theUE can determine whether the system information needs to be re-read by reading the Paging_value_tag field in the pager message and the Sib_value_tag field in the SIB1 .

Resources:
(1) 3GPP TS 36.331 V9.18.0 (2014-06) Radio Resource Control (RRC)

LTE system Information (3)-System Information changes