  In this paper, if the time-modulated array (TMA)-enabled directional
modulation (DM) communication system can be cracked is investigated and the
answer is YES! We first demonstrate that the scrambling data received at the
eavesdropper can be defied by using grid search to successfully find the only
and actual mixing matrix generated by TMA. Then, we propose introducing symbol
ambiguity to TMA to defend the defying of grid search, and design two
principles for the TMA mixing matrix, i.e., rank deficiency and non-uniqueness
of the ON-OFF switching pattern, that can be used to construct the symbol
ambiguity. Also, we present a feasible mechanism to implement these two
principles. Our proposed principles and mechanism not only shed light on how to
design a more secure TMA DM system theoretically in the future, but also have
been validated to be effective by bit error rate measurements.
