I want to understand, as most rocket uses LAN cable and switches to communicate between stages. Is it possible to replace with WiFi? Although it's will be complex system but one idea to make each stages wireless. How can I improve reliability? With combination of wireless and wireless can we communicate within its structure. How to improve coupling between antennas?
The biggest issue between stages is that wifi doesn't penetrate metal well. Engine bells and plumbing are generally metal, even if tanks and exterior airframe skin aren't. Since cables are highly reliable, it's worth having the weight to avoid dropouts when flight conditions add enough interference on top of the signal blockage to cut off your guidance at just the wrong time.
Too, signal wires needn't be heavy gauge; therefore, they can be fairly light (though the conduit they run in usually can't be lightened as much, it has to stand up to stresses of one kind or another).