Coming back to the older statement where TDD cell in PCC cannot have FDD cell as SCC.
What is the procedure happening when an user is being served by a TDD cell without CA and then say the traffic buffer requirement states the user needs CA, will there be an inter frequency handover to FDD cell and then TDD scc addition will happen ?
Yes there is feature called inter frequency HO for CA.
If UE not support TDD TDD CA or there is no another TDD carrier, then such handover can happen to FDD based on PDCP buffer.
Hi Does LTE new release Support TDD_FDD CA?
I`d say the behavior is purely dependent on the RAN network features which are enabled. It may perform a PCELL swap (handover or redirection from TDD to FDD then configure CA) or may even do nothing, keep the UE in TDD.
Imagine you have TDD with 100MHz and FDD 10MHz, the DL CA FDD+TDD will produce almost no gain, to be honest, it may even reduce the aggregated throughput depending on the vendor implementation of SRS, CSI-RS, etc…