  Wireless communications are significantly impacted by the propagation
environment, particularly in doubly selective channels with variations in both
time and frequency domains. Orthogonal Time Frequency Space (OTFS) modulation
has emerged as a promising solution; however, its high equalization complexity,
if performed in the delay-Doppler domain, limits its universal application.
This article explores domain-adaptive system design, dynamically selecting
best-fit domains for modulation, pilot placement, and equalization based on
channel conditions, to enhance performance across diverse environments. We
examine domain classifications and connections, signal designs, and
equalization techniques with domain adaptivity, and finally highlight future
research opportunities.
