Positivity of time-frequency distribution functions

This paper deals with the question how various 'natural' conditions posed on time-frequency distribution functions prevent them to be nonnegative everywhere for all signals. The attention is restricted mainly 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