  When wireless communication signals impinge on a moving human they are
affected by micro-Doppler. A passive receiver of the resulting signals can
calculate the spectrogram that produces different signatures depending on the
human activity. This constitutes a significant privacy breach when the human is
unaware of it. This paper presents a methodology for preventing this when we
want to do so by injecting into the transmitted signal frequency variations
that obfuscate the micro-Doppler signature. We assume a system that uses
orthogonal frequency division multiplexing (OFDM) and a passive receiver that
estimates the spectrogram based on the instantaneous channel state information
(CSI). We analyze the impact of our approach on the received signal and we
propose two strategies that do not affect the demodulation of the digital
communication signal at the intended receiver. To evaluate the performance of
our approach we use an IEEE 802.11-based OFDM system and realistic human signal
reflection models.
