OTFS is a modulation scheme that multiplexes QAM information symbols in a new signal representation called the delay-Doppler representation
The delay-Doppler representation generalizes time and frequency representations
2.1.2 Three fundamental properties
Invariance means that the coupling pattern is the same for all QAM symbols
Separability (sometimes referred to as hardening) means that all the diversity paths are separated from one another and each QAM symbol experiences all the diversity paths of the channel.???
orthogonality means that the coupling is localized, which implies that the QAM symbols remain roughly orthogonal to one another at the receiver
2.2 The Delay-Doppler Signal Reprsentation
The mathematical expression of this complementarity is captured by the Heisenberg uncertainty principle which states that a signal cannot be simultaneously localized to any desired degree in time and in frequency
As it turns out, there exists signals which behave as if they are simultaneously localized to any desired degree both in time and in frequency, a property which renders them optimal both for delay-Doppler Radar multi-target detection and for wireless communication