Buenos, how do you make sure PI in this case by having signals mixed with powers? Besides, shouldn't we control the impedance for fanout also during high speed condition?Another thing is in most of my designs I don't have power planes, I use mixed signal/power layers and lots of GND planes. If you have lots of gigabit signals, then I would recommend this approach to you. I have power planes on 14+ layer PCBs, between the out-most GND plane and the outer (top or bottom) layer. Also, the top/bottom layers don't have any or many impedance controlled tracks, only fanout. Usually short fanout tracks are not impedance controlled anyway.
Marce, I have stackup constraint that does not allow me to have true stripline, between ground and mixed power planes, how should I route my critical signals after fanout?Or you can have specific layers for high speed signals that are true stripline (between two grounds) and run other non-critical slower signals on other layers, where the signal coupling isn't as critical.