CN111683033A

CN111683033A - Encryption and transmission method based on constellation rotation in TR _ OFDM system

Info

Publication number: CN111683033A
Application number: CN202010458656.3A
Authority: CN
Inventors: 陈善学; 冯叶青; 李方伟
Original assignee: Chongqing University of Post and Telecommunications
Current assignee: Chongqing University of Post and Telecommunications
Priority date: 2020-05-27
Filing date: 2020-05-27
Publication date: 2020-09-18
Anticipated expiration: 2040-05-27
Also published as: CN111683033B

Abstract

The invention relates to the technical field of wireless communication, in particular to an encryption and transmission method based on constellation rotation in a TR _ OFDM system; the encryption method comprises the steps that a sending end and a receiving end respectively quantize an estimated channel and judge the channel into a quaternion as a secret key; the sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal to obtain three-dimensional constellation points, and the quaternion is utilized to carry out rotary encryption on the modulated QPSK signal; modulating in an OFDM mode; the sending end carries out time reversal on the estimated channel state information to obtain a time reversal mirror; after parallel-serial conversion, the modulated OFDM symbols are transmitted by a channel through a time reversal mirror, and for the transmission method, a receiving end receives the encrypted constellation points; the quaternion generated by the channel is used for corresponding decryption, and the channels of the eavesdropper and the transmitting end have difference, so that the corresponding quaternion cannot be obtained for decryption, and the safety of the TR _ OFDM system is ensured.

Description

Encryption and transmission method based on constellation rotation in TR _ OFDM system

Technical Field

The invention relates to the technical field of wireless communication, in particular to an encryption and transmission method based on constellation rotation in a TR _ OFDM (Time reverse _ orthogonal frequency Division Multiplexing) system.

Background

With the rapid development of wireless communication technology, people's work and life become more and more convenient, and mobile terminals such as mobile phones and notebook computers are gradually changing people's life style. Especially, with the advent of technologies such as mobile payment, internet of things, automatic driving, etc., it is becoming more critical to ensure the security of communication. The traditional encryption scheme is mainly to encrypt data through a secret key encryption system, and by utilizing the high complexity of an encryption algorithm, if equipment of an eavesdropper cannot be attacked and cracked for a long time, the problems are not solved along with the continuous development of computer technology, especially the occurrence of a quantum computer, and great challenges are provided for the security of wireless communication.

The existing common security encryption mechanism is relatively independent from a physical layer during design, the security of a communication system is guaranteed by a security technical means based on traditional cryptography on the upper layer of a wireless communication system, and the characteristics of the physical layer are not fully utilized. In recent years, the physical layer security technology based on the information theory security principle draws wide attention, a new idea is provided for wireless communication security research, compared with the traditional encryption mechanism, the strong security strategy can be proved by theory, the wireless communication security strategy is designed from the perspective of the physical layer, and the strong security strategy is used as the improvement and supplement of the traditional encryption algorithm.

Time Reversal (TR) was first proposed by m.fink in 1992. Signals processed through the time reversal technique have temporal focusing and spatial focusing, whether in a homogeneous medium environment or a heterogeneous medium environment, where temporal focusing means that the signal energy of each path will be focused at the same time. Spatial focusing means that electromagnetic energy is focused at a target point in space, and the electromagnetic energy is very low or even negligible at other positions of the target point. The so-called time reversal, i.e. the inversion of the received signal in the time domain, is equivalent to a phase conjugation in the frequency domain. In a communication system, a receiving end first transmits a short time-domain probe pulse, which undergoes reflection, scattering, diffraction in a channel, and is finally received by a transmitting end. The transmitting end performs a time reversal operation on the received signal. Under a complex multipath environment, after the signals processed by time reversal are retransmitted, the space-time focusing property of the time reversal technology is presented at a target point, and the space-time focusing property is the most obvious property of the time reversal under the multipath complex condition. Meanwhile, it should be noted that the space-time focusing characteristic is adaptive to the environment, and the more complex the environment is, the more obvious the space-time focusing effect is. The focusing effect is applied to the communication field, the performance of a communication system can be improved, and the safe transmission of information is ensured.

Orthogonal Frequency Division Multiplexing (OFDM) techniques decompose a wideband channel into many parallel narrow sub-channels, with the bandwidth of each sub-channel being less than the coherence bandwidth of the channel, so that the fading experienced by each channel is approximately flat fading. Therefore, intersymbol interference caused by time dispersion of a wireless channel can be effectively inhibited, and the complexity of equalization in a receiver is reduced.

The TR _ OFDM system can reduce the intersymbol interference and the intercarrier interference, shorten the length of the cyclic prefix and improve the frequency spectrum efficiency. Stable, high-speed, green communication can be achieved in a complex multipath environment, but if an eavesdropper appears in the TR _ OFDM system, the security of the system cannot be guaranteed.

Disclosure of Invention

Based on the problems in the prior art, the invention aims at the problem that the information cannot be safely transmitted if an eavesdropper exists in a TR _ OFDM system, and the problem to be solved by the invention is the physical layer safety problem of the TR _ OFDM system.

The invention solves the technical problem by adopting the scheme of the encryption and transmission method based on constellation rotation in the TR _ OFDM system.

In a first aspect of the present invention, the present invention provides an encryption method based on constellation rotation in a TR _ OFDM system, the encryption method is based on the TR _ OFDM system and adopts constellation encryption, and the encryption method includes:

the sending end and the receiving end respectively quantize the estimated channel, judge the channel as a quaternion, and use the quaternion as a secret key for encryption and decryption of both parties;

the sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal; carrying out constellation rotation encryption on the mapped sending signal by using the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

the sending end carries out time reversal on the estimated channel state information to obtain a time reversal mirror;

and after parallel-serial conversion, the modulated OFDM symbols pass through a time reversal mirror and are transmitted through a channel, and a receiving end receives the encrypted constellation points.

Further, the channel estimation process includes using a rayleigh channel, performing channel estimation between the transmitting end and the receiving end, and transmitting a probe signal between the transmitting end and the receiving end to perform channel estimation on the probe signal.

Furthermore, the judgment mode includes that the receiving end adopts the minimum distance to demodulate, namely the receiving end decrypts the signal after the fast Fourier transform and then compares the signal with the original mapping point, and judges the point closest to the original mapping point as the corresponding quaternary signal.

Further, before the parallel-serial conversion, the transmitting end performs inverse fast fourier transform on the transmitted signal after constellation rotation, and adds a cyclic prefix.

In a second aspect of the present invention, the present invention further provides a safe transmission method based on constellation rotation in a TR _ OFDM system, where the safe transmission method includes:

before quantizing the estimated signal, the transmitting end and the receiving end carry out channel estimation mutually, and the eavesdropping end carries out passive detection on the detection channel;

the sending end and the receiving end respectively quantize the estimated channel and judge the channel into a quaternion which is used as a secret key for encryption and decryption of both parties;

the sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal; mapping into a three-dimensional constellation point, carrying out constellation rotation encryption on the mapped transmission signal at the angle of the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

the modulated OFDM symbols are subjected to parallel-serial conversion, pass through a time reversal mirror and are transmitted through a channel;

the receiving end and the eavesdropping end carry out serial-parallel conversion on the received OFDM symbols to obtain encrypted constellation points;

the receiving end decrypts the encrypted constellation point by using the secret key, and the eavesdropping end cannot decrypt the constellation point.

Further, performing inverse fast Fourier transform and cyclic prefix addition on the constellation-rotated transmission signal; and carrying out corresponding fast Fourier transform and cyclic prefix removal on the transmitted signal before constellation reverse rotation.

Further, the decrypting, by the receiving end, the encrypted constellation point by using the secret key includes performing fast fourier transform on the encrypted constellation point, and performing constellation inverse rotation by using the secret key; after the constellation inverse mapping result is converted in parallel and serial, the receiving end resolves the initial sending signal.

The invention has the beneficial effects that:

the invention generates a constellation encrypted secret key by using a channel for nodes Alice and Bob of two parties of legal communication, a sending end Alice encrypts a constellation point corresponding to a modulated signal by using the secret key, and a legal receiving end Bob decrypts the generated constellation point by using the secret key. The eavesdropping node Eve does not know the encrypted secret key, so that the encrypted constellation points cannot be decrypted, and the safety of the system is ensured.

Drawings

FIG. 1 is a diagram of a TR-OFDM based system according to the present invention;

FIG. 2 is a flowchart of an encryption method based on constellation rotation in a TR _ OFDM system according to the present invention;

FIG. 3 is a flowchart of a safe transmission method based on constellation rotation in a TR _ OFDM system according to the present invention;

fig. 4 is a block diagram of a preferred method for transmitting a signal in a TR _ OFDM system based on constellation rotation.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Fig. 1 is a scene diagram of a TR-OFDM-based system according to the present invention, as shown in fig. 1, it is assumed that there are three nodes, a sending node Alice, a receiving node Bob, and an eavesdropping node Eve (passive eavesdropping); alice and Bob are legal communication nodes and respectively have 1 transmitting antenna and 1 receiving antenna. Eve is a eavesdropping node (passive eavesdropping) with 1 receiving antenna and a distance between Alice and Bob greater than half a wavelength.

In an actual scenario, since a channel has time-varying characteristics (fading characteristics of the channel change at any time due to the influence of position variations such as reflectors in both communication parties and a wireless communication environment, and channels are different at 2 or more time points with an interval exceeding a channel coherence time length), reciprocity (channel fading characteristics experienced by two received signals are the same when users at both ends of the same communication link exchange information within the channel coherence time) and diversity (in a rich multipath scattering environment, when information from the same signal sender is received, and when two signal receivers are separated by more than half a wavelength, channels experienced by the two received signals are uncorrelated).

Based on the above scenario, as shown in fig. 2, the present invention provides an encryption method based on constellation rotation in a TR _ OFDM system, including:

101. the sending end and the receiving end respectively quantize the estimated channel and judge the channel into a quaternion which is used as a secret key for encryption and decryption of both parties;

since the transmitting end and the receiving end transmit the sounding channel to each other, it can be assumed that channel estimation is ideal, i.e., it is assumed that the results of quantizing the same channel by both are identical.

The channel estimation is used to generate a key for constellation encryption and to generate a subsequent time-reversal mirror.

Alice and Bob respectively send detection signals, and the channel impulse response between the sender Alice and the receiver Bob is represented as:

suppose the channel impulse response between Alice and Eve is h_ij ^e[k]：

It can be seen that the channel impulse response is the same for the transmitting end at the receiving end as well as the eavesdropping end.

Alice and Bob are simultaneously paired with h^r[k]And quantizing the data to obtain a quaternion q which is used as a key for encrypting and decrypting the data by two parties.

For the judgment mode, the embodiment of the invention adopts a minimum distance judgment mode, namely, a receiving end adopts the minimum distance to demodulate, namely, the receiving end decrypts the signal after fast Fourier transform and then compares the signal with the original mapping point, and judges the point closest to the original mapping point as the corresponding quaternary signal.

The decision can be made using the mapping result table as shown in table 1:

TABLE 1 mapping results Table

	1	2	3	4	5	6	7	8	9	10
											1	1.4142	2.0004	1.4134	1.4119	1.4180	1.4122	1.4205	2.0021	1.4114	1.9997
2	2.0000	1.4154	2.0006	1.9988	0.0050	2.0000	0.0090	1.4235	2.0024	1.4212
											3	1.4143	0.0014	1.4158	1.4148	1.4108	1.4161	1.4078	0.0112	1.4204	0.0101
4	4.0029	1.4135	0.0017	0.0023	2.0003	0.0027	1.9999	1.4078	0.0068	1.4068

Assuming that mapping table mapping is [1+1i, -1+1i, -1-1i, 1-1i ] (1/sqrt (2)), in the mapping table, a quaternary signal 0 corresponds to 1+1i, 1 corresponds to-1 +1i, 2 corresponds to-1-1 i, and 3 corresponds to 1-1 i; the first row is the distance from the received signal to the first quaternary signal, the second row is the next to the fourth row, and so on; the corresponding quadtrain number is taken from the minimum distance row, for example, the first row of minimum values is 1.4142e-16, which means that the first received symbol is closest to the 1 st mapping point, so the received signal is determined to be the quaternary signal 0.

The quaternion employed in the present invention will be described in detail below:

q＝[w,x,y,z]＝w+xi+yj+zk

wherein [ w, x, y, z ] are respectively expressed as a set of quaternions; the four numbers can be obtained in a random mode; wherein i, j, k are imaginary numbers and satisfy:

i²＝j²＝k²＝-1；

ij＝-ji＝k；

jk＝-kj＝i

ki＝-ik＝j

for the conjugate of a quaternion, it is expressed as:

for the modulus length of a quaternion, the calculation formula is expressed as:

the inverse of this set of quaternion can be expressed as:

and satisfy, qq^-1＝1；

102. The sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal; mapped to three-dimensional constellation points s (n).

Let the corresponding three-dimensional constellation points of QPSK signal s (n) be:

four points, which can just form a regular tetrahedron.

Carrying out constellation rotation encryption on the mapped transmission signal s (n) by using the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

will map the three-dimensional constellation points s: (n) constellation rotation, and the rotation-encrypted signal s '(n) is represented by s' (n) ═ qs (n) q^-1

s'(n)＝qs(n)q^-1The meaning of is: rotating the three-dimensional constellation point s (n) by an angle theta around the axis l;

wherein:

θ＝2arccos(w)；

then, performing inverse discrete Fourier transform to generate OFDM symbols:

103. the sending end carries out time reversal on the estimated channel state information to obtain a time reversal mirror TRM (TimeReversal mirror);

the time-reversal mirror is represented as

104. And after parallel-serial conversion, the modulated OFDM symbols pass through a time reversal mirror and are transmitted through a channel, and a receiving end receives the encrypted constellation points.

Adding the generated OFDM symbols into a cyclic prefix by Alice, and performing parallel-serial conversion on the OFDM symbols and then passing through a time reversal mirror; at this time, the signal received by the receiving end is represented as:

the above process may be equivalent to that an equivalent channel exists between Alice and Bob, which is an autocorrelation function, and a peak exists when K is L-1; the equivalent channel is represented as:

based on the above embodiment, as shown in fig. 3, the present invention further provides a safe transmission method based on constellation rotation in a TR _ OFDM system, where the safe transmission method includes:

201. the transmitting end and the receiving end mutually transmit detection signals to perform channel estimation on the detection signals, and the eavesdropping end performs passive detection on the detection channels before the estimated signals are quantized;

alice and Bob send sounding signals to each other, assuming that the channel state information estimated by Alice and Bob is ideal.

202. The sending end Alice and the receiving end Bob respectively sum and pair h^r[k]And quantizing the data to obtain a quaternion q which is used as a key for encrypting and decrypting the data by two parties.

203. The sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal; carrying out constellation rotation encryption on the mapped sending signal by using the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

in this embodiment, the transmission signal is a binary signal, and after the binary signal is subjected to serial-to-parallel conversion, constellation mapping is performed; mapping into three-dimensional coordinate points, and performing constellation rotation encryption on the mapped signal s '(n) by using the generated key, wherein s' (n) is represented as qs (n) q^-1. Then IFFT is carried out, cyclic prefix is added, and parallel-serial conversion is carried out.

Wherein S' (n) is a rotation-encrypted signal; x (k) represents the modulated OFDM symbol, expressed as:

204. the sending end carries out time reversal on the estimated channel state information to obtain a time reversal mirror;

in this embodiment, the sending end Alice estimates the channel state information H_r(k) And performing time reversal to obtain a time reversal mirror. The OFDM symbols x (k) pass through a time-reversal mirror and are transmitted over a channel.

205. The modulated OFDM symbols are subjected to parallel-serial conversion, pass through a time reversal mirror and are transmitted through a channel;

206. the receiving end and the eavesdropping end carry out serial-parallel conversion on the received OFDM symbols to obtain encrypted constellation points;

for the receiving end, refer to the above embodiment, and for the eavesdropping end:

the signal received by the eavesdropping end Eve is as follows:

wherein, the above process can be equivalent to that an equivalent channel exists between Alice and Eve, which is a cross-correlation function and is far less than h_rq[k]. The equivalent channel is represented as:

207. the receiving end decrypts the encrypted constellation point by using the secret key, and the eavesdropping end cannot decrypt the constellation point.

The receiving end performs inverse fast Fourier transform on the received signal, and the symbol s' (n) ═ qs (n) q after parallel-to-serial conversion^-1Perform constellation inverse rotation, expressed as: s (n) ═ q^-1(qs(n)q^-1)q。

The eavesdropping end performs inverse fast Fourier transform on the received signal, and the symbol s (n) q after parallel-to-serial conversion^-1(qs(n)q^-1) q, because Eve cannot acquire the channel state information between Alice and Bob, and cannot generate a corresponding quaternion, it cannot perform constellation inverse rotation on the received signal. Therefore, the safety of communication between Alice and Eve is ensured.

It is understood that, in the embodiments of the present invention, for the parameter interpretation of the formula expression of signals, channels, etc., reference may be made to the prior art, for example, related papers and patents related to the transmission method of time reversal, for example, refer to patents such as CN 107911191A; the focus of the present invention is not here, and the core is to use quaternion as the key of the constellation encryption process, therefore, the present invention does not explain these parameters more specifically.

Fig. 4 shows a preferred safe transmission method based on constellation rotation in TR _ OFDM system, under this method, the whole transmission process is a mirror image process, including an encryption process and a decryption process, the corresponding encryption process needs to perform serial-parallel exchange before constellation mapping, then constellation rotation is performed, IFFT and cyclic prefix are used for modulation processing, then parallel-serial conversion is performed, and the result of parallel-serial conversion is transmitted to the receiving end through a multipath channel by a time reversal mirror; at this time, the receiving end carries out a decryption process including serial-to-parallel conversion; a demodulation process of removing cyclic prefix and FFT; and after demodulation is finished, constellation reverse rotation is carried out, and a signal subjected to constellation reverse mapping is output through parallel-serial conversion.

The node in the embodiment of the present invention may be understood as an abstract machine that responds to an external specific trigger condition and performs state conversion according to a certain rule, and may be a device that can be installed with application software and can be networked, such as a mobile phone, a tablet computer, a palm computer, a personal PC computer, a server, and the like; it is also understood that the physical machines capable of data communication may be modems, repeaters, routers, gateways, and so on.

In the description of the present invention, it is to be understood that the terms "coaxial", "bottom", "one end", "top", "middle", "other end", "upper", "one side", "top", "inner", "outer", "front", "center", "both ends", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplicity of description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention.

In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "disposed," "connected," "fixed," "rotated," and the like are to be construed broadly, e.g., as meaning fixedly connected, detachably connected, or integrally formed; can be mechanically or electrically connected; the terms may be directly connected or indirectly connected through an intermediate, and may be communication between two elements or interaction relationship between two elements, unless otherwise specifically limited, and the specific meaning of the terms in the present invention will be understood by those skilled in the art according to specific situations.

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. An encryption method based on constellation rotation in a TR _ OFDM system, the method comprising:

the sending end carries out constellation mapping after carrying out serial-parallel conversion on the sent quaternary signal; mapping into a three-dimensional coordinate point, performing constellation rotation encryption on the mapped transmission signal by using the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

2. The encryption method based on constellation rotation in TR _ OFDM system as claimed in claim 1, wherein the estimating of the channel comprises using a rayleigh channel, and the transmitting end and the receiving end transmit a sounding signal to each other to perform channel estimation on the sounding signal.

3. The encryption method based on constellation rotation in TR _ OFDM system according to claim 1, wherein the decision manner includes that the receiving end performs demodulation with minimum distance, that is, the receiving end decrypts the signal after fast fourier transform and then compares it with the original mapping point, and decides the closest point of the original mapping point as its corresponding quaternary signal.

4. The encryption method based on constellation rotation in the TR _ OFDM system according to claim 1, wherein before the parallel-to-serial conversion, the transmitting end performs inverse fast fourier transform on the transmission signal after the constellation rotation and adds a cyclic prefix.

5. A safe transmission method based on constellation rotation in a TR _ OFDM system is characterized in that the safe transmission method comprises the following steps:

the transmitting end and the receiving end mutually transmit detection signals to perform channel estimation on the detection signals, and the eavesdropping end performs passive detection on the detection channels;

the sending end carries out constellation mapping after carrying out serial-parallel conversion on the sending signal; carrying out constellation rotation encryption on the mapped sending signal by using the angle of the secret key, and modulating by adopting an OFDM (orthogonal frequency division multiplexing) mode;

modulating the received OFDM symbols by a receiving end and an eavesdropping end, and then carrying out serial-to-parallel conversion to obtain encrypted constellation points;

6. The safe transmission method based on constellation rotation in TR _ OFDM system according to claim 5, wherein said transmitted signal after constellation rotation is inverse fast Fourier transformed and added with cyclic prefix; and carrying out corresponding fast Fourier transform and cyclic prefix removal on the transmitted signal before constellation reverse rotation.

7. The method according to claim 5, wherein the decrypting the encrypted constellation points by the receiving end using the key comprises performing fast fourier transform on the encrypted constellation points, performing constellation inverse rotation using the key, and performing constellation inverse mapping on the encrypted signals using the key; after the constellation inverse mapping result is converted in parallel and serial, the receiving end resolves the initial sending signal.