To keep the UE (user equipment) latched to the best serving cell (with best radio conditions) during RRC connected mode, it is important to switch the call/session from one cell to another in an efficient manner (without much delay or interruption) during mobility.
During handover from one cell to another within same carrier frequency, different events occur based on signal strength (RSRP) or signal quality (RSRQ) –
- Event A1: Serving becomes better than threshold
- Event A2: Serving becomes worse than threshold
- Event A3: Neighbor becomes offset better than serving
- Event A4: Neighbor cell becomes better than a threshold value. Event A4 is mainly used to configure UE to perform measurements on inter‐frequency carriers for offloading purposes.
- Event A5: Serving becomes worse than threshold1 and neighbor becomes better than threshold2
Following A3 related parameters need to be optimized to enable efficient handover procedures:
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A3 Offset - Neighbor becomes offset better than serving. Handover margin for better cell HO. Used in measurement event type A3 where the event is triggered when the neighbor cell becomes better than the serving cell by the value of the A3 offset. Normally, its value is 3dB.
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A3 Hysteresis - Related Hysteresis of Handover Margin for Better Cell Handover of Intra-Frequency Parameter is used within the entry and leave condition of the A3 triggered reporting condition. Normally, its value is 0 but to avoid ping-pong its value can be increased.
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Time to trigger - Time for which the specific criteria for the measurement event A3 must be met to trigger a measurement report. Normally, its value is 320ms.
Note – Above parameter settings can be optimized from one scenario to another changing with mobility & other factors.
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