What are the conditions you typically set to trigger IRAT handover?

RSCP and Ec/Io are used to trigger IRAT handover:

1.       RSCP ≤ -100dBm.

2.       Ec/Io ≤ -16dBm.

Find more document on IRAT
WCDMA IRAT Handover
Details on IRAT Parameter Optimization Document