GNU Radio Watermarking
This page discusses some of the work done by CSPL relating to physical layer watermarking using GNU Radio.
Use of Watermarking
Physical Layer Watermarking can be used as an added on authentication mechanism to digital communications. By modifying the transmitted signal in some known way a low power signal is added on top of the message signal. This method preserves the original communication which allows watermark enabled clients to communicate with non-watermark aware clients.
We have so far focused on phase-dithered watermarking. This rotates a symbol after standard modulation by a small amount
Dr. Scheets has been the primary investigator. Bruce Lebold worked on DBPSK watermarking as a MS student. Nathan is following up on this work looking at coherent PSK modulations (BPSK, QPSK, and QAM) with phase-dithered watermarking.
DBPSK with Standard GR Blocks
Bruce used standard GNU Radio blocks to implement watermarking on differential BPSK. His thesis can be found through the OSU Library . This used standard GNU Radio blocks to map D8PSK symbols to a rotated constellation and ignored one of the received bits.
Nathan derived theoretical bit error rates for a few coherent modulations (BPSK, QPSK, QAM) that have been watermarked. To test this a new constellation class is derived from the standard gr-digital constellation class. This class adds a watermark angle to generate dynamic constellations (the watermarked points can rotate while the receiver is running). To do over the air testing we added the GNU Radio SNR estimator to generic_mod_demod. Then a custom file sink was created to store SNR readings and received bits. Analysis is currently done off-line; but, there is interest in showing line rates. An accidental outcome of this may be an OTA bit error rate testing application for GNU Radio.
The constellation looks roughly similar to a standard constellation, but points are split in to two more points that are clustered together.
The affect on an unaware receiver is that the constellation looks like it has a small frequency offset.
The transmitter uses a modified PSK transmitter with an additional argument: watermark angle. The GNU Radio block diagram looks like the attached image.
We have (so far unpublished) analytic and simulated results for the bit error rates of the message and watermark that agree. We are working on live over the air testing with GNU Radio. Once the OTA tests are done everything will be published through Nathan's MS thesis. An incidental outcome will be a BER testing application.