CN108055105B - A kind of radio frequency watermark insertion and extracting method towards CPM signal - Google Patents

Abstract

The invention discloses a kind of radio frequency watermark insertion and extracting method towards CPM signal, belong to radio frequency watermark field, it include: using CPM baseband signal as carrier signal, ratio is embedded according to watermark, and watermark signal is superimposed in the phase of carrier signal, baseband signal is obtained, CPM radiofrequency signal will be formed on modulates baseband signals to carrier frequency；The carrier frequency of CPM radiofrequency signal is reduced to zero intermediate frequency, baseband signal is obtained, baseband signal is demodulated, after obtaining carrier symbol, carrier signal phase is reconstructed, phase unwrapping is carried out to baseband signal and obtains signal phase, remove reconstruct carrier signal phase from signal phase, obtains watermark signal；Check up polarity is carried out to watermark signal, demodulates the bit information that watermark signal includes.The present invention will not destroy the phase continuity of signal when watermark bit switches over, and ensure that system performance is constant.

Description

A method of embedding and extracting radio frequency watermark for CPM signal

technical field

The invention belongs to the field of radio frequency watermarks, and more specifically relates to a method for embedding and extracting radio frequency watermarks oriented to CPM signals.

Background technique

Information hiding technology represented by digital watermarking technology has been greatly developed, and with the enhancement of computer performance, it is necessary to develop new information hiding technology. As a new information hiding method, radio frequency watermarking technology increases the diversity of information hiding methods. Traditional digital watermarking technology mainly uses multimedia signals as information carriers, and cannot be applied to lower-level physical signals; radio frequency watermarking introduces watermarking technology into radio frequency communication signals.

The CPM signal is a continuous phase modulation signal. This modulation method is characterized by continuous phase and constant envelope, so the spectrum utilization rate is high, and the linearity requirement of the power amplifier is low, so it is widely used. There is an existing watermark embedding method based on continuous phase frequency shift keying (CPFSK) signal. The CPFSK modulation method is a special case of the CPM modulation method, and its frequency pulse adopts a rectangular wave form. The method of modifying the CPFSK signal modulation index is used to embed the watermark. When the watermark bit is switched, the phase continuity of the CPFSK signal will be destroyed and the system performance will be degraded.

It can be seen that there is a technical problem in the prior art that the continuity of the signal phase is destroyed after the watermark is embedded, resulting in a decrease in system performance.

Contents of the invention

Aiming at the above defects or improvement needs of the prior art, the present invention provides a CPM signal-oriented radio frequency watermark embedding and extraction method, thereby solving the existing problem in the prior art that the continuity of the signal phase is destroyed after the watermark is embedded, resulting in a decrease in system performance. technical problem.

In order to achieve the above object, the invention provides a kind of radio frequency watermark embedding and extracting method for CPM signal, comprising:

(1) Taking the CPM baseband signal as the carrier signal, superimposing the watermark signal on the phase of the carrier signal according to the watermark embedding ratio to obtain the baseband signal, and modulating the baseband signal to the carrier frequency to form a CPM radio frequency signal;

(2) The carrier frequency of the CPM radio frequency signal is reduced to zero intermediate frequency to obtain the baseband signal, and the baseband signal is demodulated. After obtaining the carrier symbol, the phase of the carrier signal is reconstructed, and the baseband signal is phase unwrapped to obtain the signal phase. From the signal The phase of the reconstructed carrier signal is removed from the phase to obtain the watermark signal; the polarity of the watermark signal is detected, and the bit information contained in the watermark signal is demodulated.

Further, the carrier signal is expressed as:

x(t)＝e ^jφ(t)

Among them, x(t) is the carrier signal, j is the imaginary unit, and φ(t) represents the phase of the carrier signal.

Further, the phase of the carrier signal is:

Wherein, a _k = ± 1, ± 3, ..., ± M-1, a _k is the carrier symbol of the M system, T is the symbol period, h is the modulation index of the CPM, and q (t) is the Phase pulse, n is the total number of symbol periods, k is a positive integer greater than or equal to 0 and less than or equal to n-1.

Further, the watermark signal is:

w(t)=ρd sin(2πft)

Among them, w(t) is the watermark signal at time t, d=±1 represents the embedded watermark bit; ρ is the strength of the watermark signal; f is the frequency of the embedded sine wave.

Further, the baseband signal is:

Among them, s _w (t) is the baseband signal, j is the imaginary number unit, and φ _w (t) is the phase of the CPM signal after embedding the watermark.

Further, the watermark signal is a sine wave controlled by watermark symbols, that is, the initial phase of the sine wave is determined by the watermark bits, the amplitude of the sine wave is determined by the watermark strength, and the frequency of the sine wave is an integer multiple of the symbol rate of the carrier signal.

Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

(1) The present invention uses the CPM baseband signal as the carrier signal, superimposes the watermark signal on the phase of the carrier signal according to the watermark embedding ratio to obtain the baseband signal, modulates the baseband signal to the carrier frequency to form a CPM radio frequency signal, and then demodulates the watermark signal The contained bit information, the present invention does not destroy the phase continuity of the signal when the watermark bit is switched, and ensures that the system performance remains unchanged.

(2) Embedding watermark information in the normal CPM modulated signal, the common CPM receiver can normally receive the CPM signal, but cannot detect the watermark signal; the present invention can not only receive the original CPM signal, but also detect the embedded watermark signal, so as to realize the specific Covert transmission of information. The present invention can embed and extract the watermark in any CPM signal, not limited to CPFSK signal, so the application range is wider.

Description of drawings

Fig. 1 is a schematic diagram of embedding a watermark in a CPM modulated signal provided by an embodiment of the present invention;

Fig. 2 is a schematic diagram of the phase comparison of the CPM signal before and after embedding the radio frequency watermark provided by the embodiment of the present invention;

FIG. 3 is a structural diagram of a watermark transmitting system provided by an embodiment of the present invention;

FIG. 4 is a structural diagram of a watermark extraction system provided by an embodiment of the present invention;

Fig. 5 is a schematic diagram of a watermark signal extracted from a phase signal provided by an embodiment of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not constitute a conflict with each other.

The CPM signal RF watermark embedding and extraction method provided by the present invention includes two parts: watermark embedding and watermark extraction, which will be described respectively below.

Watermark embedding consists of the following steps:

(1.1) Generate carrier signal: In CPM modulation, the carrier signal is expressed as

x(t)＝e ^jφ(t)

Among them, j is the imaginary unit, φ(t) represents the phase of the carrier signal, which is defined as follows

Wherein a _k = ± 1, ± 3, ..., ± M-1 is the carrier symbol of the M system, T is the symbol period, h is the modulation index of CPM, n is the total number of the symbol period, and k is greater than or equal to 0 is a positive integer less than or equal to n-1, q(t) is the phase pulse at time t, and its definition is as follows

Among them, g(t) is the frequency pulse, which can be selected in various forms, the most common is the rectangular pulse, which is defined as follows:

where L is the correlation length. The watermark scheme proposed by the invention can be applied to any modulation index, frequency pulse and correlation length. For the sake of simplicity, in this example, the carrier symbol is binary, the symbol period T=0.001s, the modulation index h=1/2, the frequency pulse g(t) is a rectangular pulse, and the correlation length L=1.

(1.2) The watermark embedding ratio is W, as shown in Figure 1, a watermark of 1 bit is embedded in W carrier symbols; W=10 in this embodiment; it can be adjusted by adjusting the watermark embedding ratio in other specific applications Watermark reliability; in order to enhance the reliability of the watermark, the W value can be increased.

The watermark signal is defined as follows:

w(t)=ρd sin(2πft)

Among them, d=±1 represents the embedded watermark bit; ρ is the watermark signal strength; is the frequency of the embedded sine wave, due to the carrier symbol rate The sine wave frequency can also be written as f=mR, that is, the sine wave frequency is an integer multiple of the carrier symbol rate. It can be seen from the definition of the watermark signal that when the watermark bit d=1, the initial phase of the sine wave is 0; when the watermark bit d=-1, the initial phase of the sine wave is π. And because the frequency of the sine wave is an integer multiple of the carrier symbol rate, when the watermark symbol changes, the sine wave just returns to the original position, so the continuity of the watermark signal can be guaranteed. Since the phase of the CPM signal itself is continuous, the superposition of the watermark signal on the phase of the CPM signal is still continuous, so the embedding of the watermark in this scheme can still ensure the continuous phase of the signal. In this embodiment, for the sake of simplicity, take m=1, namely The watermark strength ρ=0.05, in other specific applications, the concealment of the watermark can be adjusted by adjusting the value of ρ; in order to enhance the concealment of the watermark, the value of ρ can be reduced.

The baseband signal embedded with watermark can be expressed as

Among them, j is the imaginary number unit, φ _w (t) is the phase of the CPM signal after embedding the watermark.

The signal phase comparison before and after watermark embedding is shown in Fig. 2, in which the phase waveforms of 20 carrier symbols are drawn, in which 2 watermark bits are embedded. It can be seen from the figure that there is an obvious change in the phase, because in this example, when ρ=0.05, the watermark embedding strength is relatively large; in other specific applications, the value of ρ can be reduced to 0.001, and after the value of ρ is reduced The phase distortion caused by the embedded watermark is so small that it is hardly noticeable.

(1.3) Modulate and transmit the baseband signal embedded with the watermark; the modulated signal expression is:

Where f _c is the carrier frequency, in this embodiment f _c =92 MHz; the above-mentioned entire watermark embedding process is shown in FIG. 3 .

The flowchart of watermark extraction in this embodiment is shown in FIG. 4 . The demodulation block diagram includes carrier symbol demodulation and watermark bit demodulation, wherein the carrier symbol demodulation module is not expanded, and researchers in the field can obtain a complete solution of CPM demodulation according to open documents. This scheme only reconstructs the phase signal according to the demodulation result of the carrier symbol, thereby assisting the demodulation of the watermark signal.

In this embodiment, the watermark demodulation method includes the following steps:

(2.1) The carrier frequency of the received CPM signal is reduced to zero intermediate frequency by a mixer to obtain a baseband signal s _w (t).

(2.2) According to the existing CPM signal demodulation scheme, the baseband signal s _w (t) is demodulated to obtain the carrier symbol a _k , so that the phase of the reconstructed CPM signal is as follows

(2.3) Perform phase unwrapping on the baseband signal s _w (t) to obtain φ _w (t), and subtract the reconstructed phase φ(t) from the phase φ _w (t) to obtain the watermark signal

_w (t)＝φw(t)-φ(t)

It can be seen from the above steps that no matter what parameter the CPM signal adopts, it can be restored by reconstruction, which has no effect on the extraction step of the watermark signal, so this scheme can be applied to any CPM modulation format. In this embodiment, the watermark signal extracted from the signal phase shown in FIG. 2 is shown in FIG. 5 .

(2.4) Correlate the watermark signal w(t) with the sine wave to get

where f = 1000Hz, the same frequency as the sine wave used in the previous embedding scheme. Finally, polarity detection is performed to demodulate the watermark bits

This scheme first demodulates the carrier symbol from the received signal, then reconstructs the phase of the carrier signal through the demodulation result, then subtracts the reconstructed phase from the phase of the received signal to obtain the embedded watermark signal, and finally performs correlation calculation and polarity analysis on the watermark signal. Sex detection, demodulation of watermark bits.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (6)

1. a kind of radio frequency watermark insertion and extracting method towards CPM signal characterized by comprising

(1) using CPM baseband signal as carrier signal, according to watermark insertion than being superimposed watermark letter in the phase of carrier signal Number, baseband signal is obtained, CPM radiofrequency signal will be formed on modulates baseband signals to carrier frequency；

(2) carrier frequency of CPM radiofrequency signal is reduced to zero intermediate frequency, obtains baseband signal, baseband signal is demodulated, is carried After body symbol, carrier signal phase is reconstructed, phase unwrapping is carried out to baseband signal and obtains signal phase, is removed from signal phase Carrier signal phase is reconstructed, watermark signal is obtained；Related operation and Check up polarity are carried out to watermark signal, demodulate watermark signal The bit information for including.

2. a kind of radio frequency watermark insertion and extracting method towards CPM signal as described in claim 1, which is characterized in that institute State carrier signal expression are as follows:

X (t)=e^jφ(t)

Wherein, x (t) is carrier signal, and j is imaginary unit, and what φ (t) was represented is the phase of carrier signal.

3. a kind of radio frequency watermark insertion and extracting method towards CPM signal as claimed in claim 2, which is characterized in that institute State the phase of carrier signal are as follows:

Wherein, a_k=± 1, ± 3 ..., ± M-1, a_kIt is the carrier symbol of M system, T is code-element period, and h is that the modulation of CPM refers to Number, q (t) are the phase impulses of t moment, and n is the sum of code-element period, and k is the positive integer for being less than or equal to n-1 more than or equal to 0.

4. a kind of radio frequency watermark insertion and extracting method towards CPM signal as claimed in claim 1 or 2, which is characterized in that The watermark signal are as follows:

W (t)=ρ dsin (2 π ft)

Wherein, w (t) is the watermark signal of t moment, and d=± 1 indicates the watermark bit of insertion；ρ is watermark signal strength；F is embedding The sine wave freuqency entered.

5. a kind of radio frequency watermark insertion and extracting method towards CPM signal as claimed in claim 4, which is characterized in that institute State baseband signal are as follows:

Wherein, s_wIt (t) is baseband signal, j is imaginary unit, φ_wIt (t) is to be embedded in the later CPM signal phase of watermark, φ (t) What is represented is the phase of carrier signal.

6. a kind of radio frequency watermark insertion and extracting method towards CPM signal as claimed in claim 1 or 2, which is characterized in that The watermark signal is the sine wave controlled by watermark bit, i.e. the initial phase of sine wave is determined by watermark bit, sinusoidal The amplitude of wave is determined that the frequency of sine wave is the integral multiple of carrier signal chip rate by watermark signal strength.