Tuesday, 5 April 2016

FDD-FL-LTE FDD handover signaling process differences between with and without neighbor relation

Incident Description (Incident Phenomena)
  Analyzing the DT data analysis, found that there were different handover result where the signaling flows were different.
Problem Cause Analysis

1.       network structure

Cell C LTE experimental network eNB provided by ZTE, EPC by HW, the following is the road test eNB distribution process:

2.       Analysis of PCI handover point  

PCI switching points shown above are Handover1 and Handover2, have a look what is their signaling.

We can see that the UE is synchronized to the cell 78_3, corresponding to the second cell of site 4372 (PCI=14), but then RRC reestablishment has occurred in the second cell of site 8434 (PCI=28), and the RRC reestablishment reason is otherFailure rather than handoverFailure, see below:
Why are the handover from PCI=28 to PCI=14 and then back to PCI=28 cell RRC establishment? And the download business had a short break in the PCI handover process, see below:
After the reestablishment reject (Reason is RRC Connection Reestablishment to Reject Before ERAB), UE randomly access to cell PCI=28 (Service Event=Service Request For Call Drop), to continue downloading service, see below:
As can be seen here after PCI handover, RRC Connection Reestablishment was not in the original area, service was not interrupted during handover process, see below:
  What made signaling flow and switch result different between the above two PCI handover points?
1.      check the base station alarm but no hardware exception
2.      replacing terminal but the problem still existed
3.      check the radio parameters, mainly the neighbor relation configuration, found only configuration of the neighbor relationship between site 4003 and 4601, so handover between site 4003 and 4601 were normal, the others with no triggers were not successful, RRC reestablishment was from the source cell, UE searched the cell to attach and initiated the TAU request, to continue the unfinished business.

Summary and Notes
1.      understand the handover process of LTE FDD
2.      to analyze and compare different signaling points
3.      RRC re-establish causes
4.      RRC connection requests

Budi Prasetyo

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