Which is best as Primary Cell in case of CA L21 (10M) or L18 (5M)?

If we have

  • L2100: 10 MHZ
  • L1800: 5 MHZ

Which will be better to be primary cell in case of CA L21 or L18?

It seems the wider bw will be better due to signalling part, but what is the roughly decrease % of the throughput in case of being p cell is L18?

2100 PCell.
Carrier scheduling strategy is present then only PDCCH is PCell and PUCCH is only transmitted in PCell bandwidth is less so 5 MHz not ideal as priority will be less.

If CA actived all the time and BO is splitted amongs the both CCs, then ideally there should not major change in throughput, even when any one is Pcell.
But yes signallinh point of view, more precisely PUCCH capacity point of view, higher BW pcell should be preffered.