Positivity of Time-frequency Distribution Functions

This paper deals with the question how various lInaturalll conditions posed on time-frequency distribution functions prevent them to be non-negative everywhere for all signals. The attention is mainly restricted to distribution functions that involve the signal bilinearly. This paper summarizes and extends results recently obtained; the emphasis is on a coherent presentation of these results, rather than on rigorous proofs of them (which can be found in or obtained from existing literature). Distribution functions of the Wigner-Rihaczek type are considered in some more detail.