Military Communications and Information Technology: A Trusted ...

500 Military Communications and Information Technology...
j 2
i t i j ft
x t Ae Ae
i
i
(1)
i i i
where:
A i – amplitude of i – harmonic,
f i – frequency of i – harmonic,
φ i – starting phase of i – harmonic.
In order to ensure the imperceptibility of the OFDM signal in the background
of the audio signal, the amplitude value of each harmonic is determined by the corresponding
LT min level.
Fig. 3 presents the amplitude and phase spectrum of the OFDM signal.
The values of the 0 rad/s angle phase are assigned to spectral levels with indexes
of (1,4,7,8,11,14). Amplitudes of lines in fig. 3 are marked in red.
Figure 3. Amplitude and phase spectrum of the OFDM signal
Of 14 harmonics presented in fig. 3, 6 harmonics (marked in red) are utilized
on the receiver side to determine the correction values of the angle phase drift.
The necessity to determine the correction of the angle phase drift arises from
the differing stability of clocks tacking the sampling circuits in the transmitter
and the receiver. The remaining 8 harmonics (marked in blue) are utilized on
the receiver side to reproduce the time synchronization. The harmonics used for
determining the correction of the angle phase drift are assigned the angle phase
value of 0 [rad/s], whereas the lines responsible for time synchronization are assigned
the angle phase values of π/2 and –π/2.
Furthermore, in order to improve the efficiency of watermark signal extraction,
the embedder utilizes the differential coding mechanism [9]. The principle
of the mechanism is that the coding of the same bit takes place in two adjacent frames
(subframes) of the host signal, but using opposing patterns. The manner of pattern
selection is provided in fig. 4. Assuming that the subframes contain 512 samples each,