Research On Physical Layer Watermarking Schemes For MSK Signals

In recent years,the physical layer security technology has attracted a lot of attention of researchers,and attention has been paid to the physical layer watermarking of information hiding.Secret transmission technology has always been a hot topic,but the carrier signals in most of the research are multimedia digital signals,namely digital watermarking technology.This article focuses on the radio frequency signals and studies the secret information transmission technology in the radio frequency signals,which is called the physical layer watermarking or the radio frequency watermarking.MSK modulation signals have been widely adopted in military and civil communication fields,because of the advantages of continuous phase,constant envelope,high spectral efficiency,small bandwidth and strong anti-interference ability.Therefore,it is of great significance to study the watermark transmission algorithm based on the minimum shift keying(MSK)signal.This paper mainly studies the transmission schemes of watermark signals under the background of MSK signals,embeds the watermark signals to MSK modulation signals at the transmitters,extracts the watermark signals and demodulates the original MSK signals at the receivers.In this paper,a watermark embedding model is proposed.By using the principle of watermark superposition,a multiplicative model can be used to realize the phase superposition of the MSK carrier signals and the watermark signals.Two kinds of algorithms of the watermark generation,embedding and extraction for MSK signals are proposed.In order to ensure that the watermark extraction and carrier demodulation procedures do not affect each other,we design appropriate watermark extraction algorithms to make the watermark extraction and demodulation procedures orthogonal,and derive the watermark signals frequency that satisfies the orthogonal conditions mentioned above.Then it also makes a theoretical and quantitative analysis of imperceptibility and reliability which are two indicators of system performance.Imperceptibility is measured by the bit-error-rate(BER)of the carrier signals,and reliability is measured by the BER of the watermark bits.Therefore,the bit-error-rate(BER)formulas over addition white Gaussian noise are deduced.The theoretical analysis and simulation results show that watermark signals make little effect on the BER performance of the original MSK receivers,when embedded to the MSK signals with the orthogonal frequencies.The simulation results also prove that imperceptibility and reliability are contradictory.The imperceptibility decreases with the increase of the watermark strength,while the reliability is contrary.The imperceptibility and reliability are inversely proportional to the signal-to-noise ratio of the system,and the reliability increases with the increase of the watermark embedding ratio.In addition,qualitative analysis about the security of watermarking system is made by studying the power density of the signals.It comes to the conclusion that the security of watermarking system is guaranteed by the secret spread spectrum sequence and the frequency of the watermark signals.Finally,a complete watermarking communication system based on MSK signals is established on the hardware platform NI USRP 2920 and the software platform NI LabVIEW.The key parameters of affecting the system performances are tested,including the watermark strength,symbol rate ratio and the frequencies of watermarking signals.And the practice is consistent with the expectation.To summarize,in practical applications,we should consider the impact of system performance on the basis of the actual scenario requirements,reasonably configure related parameters,and optimize system performance.