Inter-Technology Handover means mobility in connected mode between two different cells and different technology. This topic will be assigned to Event B1 which is used for LTE Inter-Technology handover. This event type is also known as Inter-RAT or Inter-System handover. If the signal level of the UE is below than certain threshold then it starts to make Inter-Technology measurement (gap measurement) and makes handover from LTE to other system such as WCDMA or GSM.
It is recommended to have a look HO Events in LTE before reading this topic.
Details:
• LTE Event A2 is triggered when the serving cell becomes worse than a threshold.
Event A2 triggering (start measurement for other technology):
Mserv + Hyst < Threshold
• LTE Event A1 is triggered when the serving cell becomes better than a threshold.
Event A1 triggering (stop measurement for other technology):
Mserv – Hyst > Threshold
• LTE Event B1 is triggered when a neighboring cell becomes better than a threshold.
Event B1 triggering (start inter-technology handover):
Mneigh + Oneigh,freq – Hyst > Threshold
Mserv: signal level or quality of the serving cell (RSRP or RSRQ).
Mneigh: signal level or quality of the neighbour cell (RSCP, EcNo, RxLev).
Threshold: defined signal threshold and it is different for A1, A2, B1. It is defined in eNodeB.
Hyst: hysteresis, it is defined in eNodeB and used to avoid ping-pong handover.
Oneigh, freq: indicates the frequency offset for the inter-technology neighboring cell.
Time to Trigger: this timer helps to avoid irregular measurement and handover.
* Note: all types of triggers is evaluated during Time to Trigger period and occur if the condition stays stable during the period, otherwise the event will be cancelled without triggering.
Conclusion:
Signal of serving cell is poor and UE starts to make measurement with A2 to find better neighbour cell, if the serving cell signal is getting better and stays stable during Time to Trigger then the UE stops measurement with A1. If it is not getting better during the Time to Trigger then it will make handover with B1 if neighbour cell condition is sufficient.