yes, the G18 level might be too low to setup a call in the end.. you’ll have to fine tune this parameter. maybe increase a little on the G18 and reduce a little on G9… it really depends on your G18 cells density.
Depending on priority, the HO can push/keep a call within the G18 layer. But it also depends on various parameters (which types of HO are activated, the preferred band, …). It looks like you know a good deal about dualband networks, so I believe you set up your network correctly. Next week, I’ll try to have a look through some documents and check whether there is anything else we could use.
Are you having RACH or SDCCH congestion ? Or a critical KPI ? Maybe, if everything is good, you just have to fine tune your parameters. An interesting indicator is the amount of traffic (erlang) carried by G18 cells and G9 cells, and the traffic load. If it is balanced, then I think it is as good as it can get.