PART 0 1
1. Overview of Reconstruction
RRC connection re-establishment (RRC connection re-establishment) is a process in which the UE is in the RRC_CONNECTED state and the connection is interrupted due to some mobility management or underlying link failure. The UE initiates the process of re-establishing the air interface resources to continue the RRC connection of the air interface. Reconstruction is the process of restoring the air interface when the air interface is abnormal when the UE is in the connected state. The prerequisite for successful re-establishment is that the cell receiving the re-establishment request has the context of the UE. The significance of reconstruction is to quickly restore air interface services and improve service continuity.
rebuilding condition
When the UE detects downlink out-of-synchronization, handover failure, and RLC retransmissions reaching the maximum number of reasons, it will initiate the RRC re-establishment process in the new cell in an attempt to quickly re-establish services and improve user experience. The LTE protocol stipulates that the network side can only accept the reestablishment request for the connection with the context, and the request without the context ID will be rejected and the call will be dropped. When the UE re-establishes from base station A to base station B, such re-establishment must fail because the context cannot be obtained. In the live network, the reconstruction failures without top and bottom account for the vast majority of the total number of reconstruction failures. Seriously affected the customer experience.
The context is generally some important information of the UE stored on the eNodeB side, including UE capability, multi-bearer information (bearer ID, QCI level), S1AP_ID, UE security algorithm, etc. For reconstruction without UE context, the target base station must obtain the context of the source station by some means. The protocol stipulates that the source station can bring the context of the UE to the target station through a handover request, so there is a carrier for obtaining the context, but how to notify the source station The station brings the context to the target station through a switch request, which is not specified in the protocol. Therefore, the source site can only be notified through private messages. If the private message goes through the S1 port, it needs to enter the core network, and the core network side also needs to identify the message. The processing is more complicated, so in general, the private message will be processed directly through the X2 port. . After receiving the RRC re-establishment request, the target base station finds that there is no context for the UE, so it sends a private message to the source-side base station through the X2 port to request the source-side base station to send the context. After receiving the reply, it continues to complete the RRC re-establishment according to the normal process process.
cause of rebuilding
According to the agreement, the main reasons for the UE to initiate the reconstruction process are as follows:
1) The upper layer detects the failure of the wireless link notified by the lower layer:
When the UE detects that "radio link failure" is currently detected, it will initiate the middle of the reason value "other", which usually causes RLF to have the following three mechanisms:
ØThe maximum number of uplink RLC retransmissions
ØMAC layer SRI retransmission reaches the maximum number of times
ØUE detects downlink RLF
2) The handover fails and the original cell is rebuilt;
During the handover process, the UE will start T304 after receiving the handover reconfiguration message, but if the UE cannot complete the random access in the target cell before the timeout of T304, it will initiate a reconstruction with the reason value "handover failure"
3) The mobility process in the system or IRAT fails, such as T304 timeout during handover, failure to establish an RRC connection in the target cell, UE does not adapt to the configuration of MobilityFromEUTRACommand, etc.
4) The underlying integrity check failed
The UE cannot configure part of the configuration in the RRC reconfiguration message, and the reconfiguration fails;
In the state where the security mode is activated, if the UE cannot match/compatibility the information elements in the reconfiguration message after receiving the reconfiguration message, it will initiate a reconstruction with the cause value of "reconfiguration failure".
PART02
Reconstruction Scale High Analysis and Optimization Methods
Classification of reasons for reconstruction
According to the agreement, the main reasons for the UE to initiate the reconstruction process are as follows:
Referring to the problem analysis of the common site, the TOP cause of the reconstruction is Other, that is, the radio link failure of the UE occurs. Based on the analysis of live network data, the main causes of reconstruction include the following:
Reconstruction Reason Confirmation
Firstly, analyze the traffic statistics of RRC reconstruction , focusing on:
1. Reconstruction ratio of source and non-source communities;
2. The proportion of reconfiguration failures, handover failures, and other reasons.
Confirmation of the reason for the high reconstruction ratio of the source cell
1) Extract source cell interference, bit error rate, retransmission rate, CQI and other traffic statistics, and confirm whether it is " poor air interface quality" . It can be judged according to the following criteria, and it can be classified as poor air interface quality if it meets one of them:
The average interference during busy hours is greater than -105dBm;
Uplink initial bit error rate is greater than 20% during busy hours;
The downlink initial bit error rate is greater than 20% during busy hours;
The uplink retransmission rate is greater than 40% during busy hours
The downlink retransmission rate is greater than 30% during busy hours
The average CQI is lower than 8 during busy hours
2) Extract the handover statistics of the two-to-two cells of the source cell, and confirm whether the proportion of " premature handover " is high (proportion of premature handover = number of premature handover/number of handover attempts), which can be judged according to the following criteria:
The proportion of premature switching is greater than 5%;
3) Extract Uu/S1/X2 port signaling, analyze the MR signaling measurement report before reconstruction, analyze the last measurement result of UE before reconstruction in the RRC_UE_INFO_RSP signaling after reconstruction, and judge whether there are "neighboring cell missing" and "rapid fading" ”, “weak coverage”, “abnormal terminal” and other phenomena.
In the MR, if the RSRP of the adjacent cell is higher than that of the serving cell and meets the handover conditions, but the handover is not triggered, it is confirmed as "missing adjacent cell configuration";
In the last measurement result, if the downlink RSRP is lower than -115dBm, or the RSRQ is lower than -12, it is confirmed as "weak coverage";
The RSRP of the terminal drops from normal to -115dBm in about 2 seconds, which is confirmed as "fast fading";
There is no missing configuration and weak coverage of adjacent cells, the quality of the air interface is normal, and the terminal frequently initiates reconstruction, which is confirmed as an "abnormal terminal"
4) Extract the CHR log, and use L2_USERCHR_SCH_INFO to judge whether the uplink is " weakly covered " before reconstruction . In addition, it can be combined with L2_USER_ULRLC_STRU to judge whether the cause of reconstruction is the maximum SR retransmission, UE out of sync, or the maximum RLC retransmission (helping the second and third lines).
Uplink Dmrs RSRP is lower than -130dBm;
Uplink Dmrs SINR lower than -3dB
Confirmation of the reasons for the high proportion of reconstruction in non- source communities
1) Extract the Uu/S1/X2 port signaling (or CHR), and analyze the distribution of the source cell where the UE is located before re-establishment in combination with the industrial parameters.
The distribution of the reconstruction source cell can be quickly given through the FMA standard port signaling tool: click diagnosis, check Drop->RRC Reestablishment, and the reconstruction in the standard port can be statistically classified and the corresponding reconstruction source cell PCI. Then determine the cell corresponding to the PCI according to the industrial parameters. In the multi-frequency networking scenario, you need to pay attention to which frequency point the PCI belongs to.
Through the CHR RrcReestabInfo event, you can find out the PCI of the source cell and the cause of the reconstruction.
2) Sort the source cells according to the reconstruction ratio, extract and reconstruct TOP source cell interference, bit error rate, retransmission rate, CQI and other traffic statistics, and confirm whether the "air interface quality is poor" can be judged according to the following criteria, and one of them can be classified The class is empty and poor quality:
The average interference during busy hours is greater than -105dBm;
Uplink initial bit error rate is greater than 20% during busy hours;
The downlink initial bit error rate is greater than 20% during busy hours;
The uplink retransmission rate is greater than 40% during busy hours
The downlink retransmission rate is greater than 30% during busy hours
The average CQI is lower than 8 during busy hours
3) Extract the handover statistics of the local cell and the reconstructed TOP source cell, and confirm whether the proportion of "too early to handover", "too early to handover" and "to the wrong cell" between the local area and the source cell is high (too early to handover Ratio = number of premature switches/number of attempts to switch out), which can be judged according to the following criteria:
The proportion of premature switching is greater than 5%;
The proportion of switching too late is greater than 5%;
The proportion of switching to the wrong cell is greater than 5%.
4) Extract the Uu/S1/X2 port signaling of this cell and the reconstructed TOP source cell, analyze the MR signaling measurement report before the reconstruction, analyze the last measurement result of the UE before the reconstruction in the RRC_UE_INFO_RSP signaling after the reconstruction is completed, and judge whether there is a "neighbor cell Missing configuration" , "rapid fading", " weak coverage " and other phenomena.
5) The problem of non-source cell reconstruction needs to be combined with terminal testing, and Qualcomm QXDM is used to capture UE LOG.
Since the reconstruction trigger action is the behavior of the UE side, in many scenarios, when the log records on the base station side cannot locate the reconstruction problem, then a drive test is required to analyze the UE LOG. The main analysis messages listed in the figure below (more message types can also be used for analysis when performing specific analysis):
PART03
reconstruction optimization method
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