A3 Intra-Frequency Handover in LTE

Intra-Frequency Handover means mobility in connected mode between two different cells, but within same LTE frequency, This topic will be assigned for Event A3 which is used for LTE Intra-Frequency handover. This event type can also be used for Inter-Frequency handover and it is configurable in eNodeB side to choose the event type for Inter-Frequency. It is recommended to have a look HO Events in LTE before reading this topic.

Details:

• LTE Event A3 is triggered when a neighboring cell becomes better than the serving cell by an offset which is shown as “**Tuned Mneigh” in the graph within Time to Trigger period. The event is triggered when the following condition is met. The reason to use TTT is to avoid a ping-pong effect:

Mneigh + Oneigh,freq + Oneigh,cell – Hyst > Mserv + Oserv,freq + Oserv,cell + Offset

Mserv: signal level or quality of the serving cell (RSRP or RSRQ).
Mneigh: signal level or quality of the neighbor cell (RSRP or RSRQ).
Hyst: hysteresis, it is defined in eNodeB and used to avoid ping-pong handover.
Oneigh, cell: cell individual offset for the intra-frequency neighboring cell.
Oserv, cell: cell specific offset for the serving cell.
Offset: if the parameter is set to a large value, an intra-frequency handover is performed only when the signal of the neighboring cell is significantly better than that of the serving cell.
*Oneigh, freq: indicates the frequency-specific offset for the inter-frequency neighboring cell.
*Oserv, freq: indicates the frequency-specific offset for the inter-frequency serving cell.
Time to Trigger: this timer helps to avoid irregular measurement and handover.

* Note: Oneigh, freq & Oserv, freq are used when the Event A3 is configured for Inter-Frequency Handover. In Intra-Frequency handover case these are not used.

** Note: Tuned Mneigh & Tuned Mserv are combination of signals and other related parameters which was described above formula.

*** 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 not triggered.

Conclusion:

Unlike Inter-Frequency and Inter-RAT handover, UE may keep measuring Intra-Frequency neighboring cells but only sends measurement reports to eNodeB when neighboring cell signal becomes better than serving cell signal. Based on measurement reports, eNodeB makes decision and sends handover command to UE for handing over to the neighbour cell. This procedure is called Event A3 for Intra-Frequency handover.