  Time-modulated arrays (TMA) transmitting orthogonal frequency division
multiplexing (OFDM) waveforms achieve physical layer security by allowing the
signal to reach the legitimate destination undistorted, while making the signal
appear scrambled in all other directions. In this paper, we examine how secure
the TMA OFDM system is, and show that it is possible for the eavesdropper to
defy the scrambling. In particular, we show that, based on the scrambled
signal, the eavesdropper can formulate a blind source separation problem and
recover data symbols and TMA parameters via independent component analysis
(ICA) techniques. We show how the scaling and permutation ambiguities arising
in ICA can be resolved by exploiting the Toeplitz structure of the
corresponding mixing matrix, and knowledge of data constellation, OFDM
specifics, and the rules for choosing TMA parameters. We also introduce a novel
TMA implementation to defend the scrambling against the eavesdropper.
