Military Communications and Information Technology: A Trusted ...

Chapter 4: Information Assurance & Cyber Defence
501
the coding of a single bit will require 1024 signal samples which, given the sampling
frequency of f s = 44 100 Hz, allows for creating a hidden transmission channel with
the bitrate of 43 bps. In particular the data bit rate of the proposed algorithm:
where:
f s – sampling frequency,
N – number of samples.
f s
bit rate [ bps]
(2)
N
B. Watermark extractor – principle of operation
The schematic of the watermark extractor is provided in Fig. 5. The extraction
mechanism of binary signature belongs to the class of algorithms using the socalled
blind decoding method, i.e. the receiver side does not require the host signal.
The watermarked signal first encounters the angle phase drift scanner, which corrects
the angle phase drift for the pilot samples (cf. red-marked lines in fig. 3). The angle
phase drift is caused by differing stability of clocks tacking the sampling circuits
in the transmitter and the receiver of the watermark (e.g. in the case of a watermark
signal transmission in a VHF circuit). As previously mentioned, the watermark
embedder has 6 pilot spectral lines with the assigned angle phase value of 0[rad/s].
The same angle phase value is expected on the receiver side. The angle phase scanner
allows for reproducing the drift value of the angle phase by analyzing the values
of the virtual line module, which is the sum of pilot line modules. In the proposed
algorithm the individual increment of the angle phase Δφ 1 is Δφ 1 = 0.000767 [rad/s].
Figure 4. Pattern selection method for differential coding