In the LTE system, handovers are classified into premature handovers, delayed handovers, and ping-pong handovers. Due to the introduction of RRC connection reestablishment, premature handovers can trigger RRC connection reestablishment to the original cell. Therefore, call drops will not occur. Delayed handovers and ping-pong handovers are more likely to cause call drops.
According to the signal change trend, delayed handovers have the following symptoms:
Corner effect: The RSPR/SINR of the source cell decreases sharply, and the RSRP/SINR of the target cell increases sharply (that is, the RSRP/SINR of the target cell suddenly appears in the neighboring cell list as a very high value).
Needle-tip effect: The RSPR/SINR of the source cell decreases rapidly and then increases for a period of time. The RSPR/SINR of the target cell increases sharply and then decreases immediately.
If a delayed handover occurs, the target cell may not have the UE context. In versions earlier than eRAN2.2SPC230, RRC connection reestablishment without UE context is not implemented. As a result, the reestablishment may fail, causing service drops.
According to the signaling procedure, the UE reports the A3 measurement report of the neighboring cell before the call drop, and the eNodeB receives the measurement report and delivers the handover command. However, the UE cannot receive the handover command. In this case, if the target cell has the UE context and the RRC connection can be successfully reestablished, the call drop does not occur.
Ping-pong handovers have the following characteristics in terms of signal strength:
1. The primary serving cell changes rapidly. Two or more cells alternately become the primary serving cell. The primary serving cell has a good RSRP and SINR, and each cell becomes the primary serving cell in a short time.
2) No best cell: There are multiple cells, the RSRP is normal and has little difference between them, and the SINR of each cell is low.
According to the signaling procedure, the UE sends a new measurement report and initiates another handover immediately after a handover is complete. After the handover, many reconfiguration messages (such as CQI reporting mode and sounding) are delivered. In ping-pong areas, these commands may fail due to timeout, causing call drops.
To solve call drops caused by delayed handovers, adjust antenna positions, modify handover parameters, or configure CIOs so that handovers can be performed in advance. To solve the call drop problem caused by ping-pong handovers, adjust the antenna position to improve the RF performance and ensure that a stable best cell is available in the area.
Before an inter-frequency or inter-RAT handover, the gap-assisted measurement must be started to measure inter-frequency or inter-RAT frequencies. Therefore, A2 parameters must be properly configured to ensure that the gap-assisted measurement is started in time. This prevents service drops caused by delayed signal quality measurement in the target cell. In addition, the threshold for the target cell must be properly configured.
For details about handover-related problems, see the handover optimization guide.
