WO2021227482A1 - Secure transmission method in large-scale antenna system - Google Patents

Abstract

The present application relates to a secure transmission method in a large-scale antenna system. As the number of antennas increases, extremely high complexity is introduced into the design of beams, and a practical application is difficult. For a scenario that spatial directions of a legitimate receiver and an eavesdropper with respect to a sender are similar, an obvious interference effect can be generated only by consuming a large amount of transmitting power to generate artificial noise, so that the overall system performance is unsatisfactory. The present application provides a secure transmission method in a large-scale antenna system. The method comprises the following steps: a sender obtains an uplink channel vector of a legitimate receiver by means of channel estimation; the sender extracts an angle feature parameter of a main uplink channel according to the uplink channel vector; and the sender modulates sent data according to the angle feature parameter of the main uplink channel, and performs downlink data transmission within the same coherent time slot. The method ensures that only the legitimate receiver can correctly extract information from the sender, and secure transmission can be ensured in multiple scenarios of the large-scale antenna system.

Description

A safe transmission method in large-scale antenna system Technical field

This application belongs to the field of wireless communication technology, and particularly relates to a secure transmission method in a large-scale antenna system.

Background technique

Massive MIMO (Multiple-Input Multiple-Output) technology is regarded as the most promising technology in the 5G (5th-Generation, fifth-generation mobile communication technology) physical layer. As the number of antennas increases, the spatial resolution of the antenna array increases. Using the characteristics of the channel in the spatial domain, physical layer security has become an effective method to ensure secure transmission in massive MIMO systems.

Due to the narrowing of the user's beam and the increase in the number of transmitting antennas, the massive MIMO system usually adopts low-complexity linear beamforming, so as to have good safe transmission performance. However, if the legitimate receiver and the eavesdropper have almost the same spatial direction relative to the sender, it means that the main channel and the eavesdropping channel are highly correlated. In this case, the design of a secure transmission scheme becomes very necessary.

For MIMO systems, existing solutions mainly include beamforming and artificial noise. Among them, the beamforming solution requires the sender to obtain accurate CSI (Channel State Information) of the receiver and the eavesdropper. The sender designs the transmitting beam so that the nulls are aimed at the eavesdropper, so that the eavesdropper cannot demodulate the correct information under a low signal-to-noise ratio. In a massive MIMO system, as the number of antennas increases, the design of the beam will introduce extremely high complexity, making practical applications very difficult. For the artificial noise scheme, the main step is to introduce noise into the null space of the main channel (the channel between the sender and the legal receiver), which reduces the eavesdropper’s channel quality without affecting the signal-to-noise ratio of the legal receiver, thereby improving the system The safe capacity. For scenarios where the legal receiver and the eavesdropper have similar spatial directions relative to the sender, a large amount of transmission power needs to be consumed to generate artificial noise to produce obvious interference effects, and the overall performance of the system is not ideal.

Summary of the invention

1. Technical problems to be solved

Based on the MIMO system, existing solutions mainly include beamforming and artificial noise. Among them, the beamforming solution requires the sender to obtain accurate CSI (Channel State Information) of the receiver and the eavesdropper. The sender designs the transmitting beam so that the nulls are aimed at the eavesdropper, so that the eavesdropper cannot demodulate the correct information under a low signal-to-noise ratio. In a massive MIMO system, as the number of antennas increases, the design of the beam will introduce extremely high complexity, making practical applications very difficult. For the artificial noise scheme, the main step is to introduce noise into the null space of the main channel (the channel between the sender and the legal receiver), which reduces the eavesdropper’s channel quality without affecting the signal-to-noise ratio of the legal receiver, thereby improving the system The safe capacity. For the scenario where the legal receiver and the eavesdropper are close to the sender in spatial direction, a large amount of transmission power needs to be consumed to generate artificial noise to produce obvious interference effects, and the overall performance of the system is not ideal. This application provides a large-scale antenna system Secure transmission method in.

2. Technical solution

In order to achieve the above objective, this application provides a secure transmission method in a large-scale antenna system. The method includes the following steps:

Step 1): The sender obtains the uplink channel vector of the legal receiver through channel estimation;

Step 2): The sender extracts the angle characteristic parameters of the uplink main channel according to the uplink channel vector;

Step 3): The sender modulates the transmitted data according to the angle characteristic parameters of the uplink main channel, and performs downlink data transmission in the same coherent time slot.

Another implementation manner provided by this application is: the uplink channel vector in the step 1) is obtained as follows:

Suppose that the base station has M>>1 antenna and is a uniform linear array, the legal receivers and eavesdroppers are randomly distributed in the coverage of the base station and both are configured with a single antenna, the length of the training sequence is L<T, where T is the channel coherence time length ,

The sender obtains the estimated value of the uplink main channel based on the received signal.

Another implementation manner provided by this application is: in the uplink channel estimation, the signal received by the sender is:

Among them, h _r represents the uplink main channel in the form of an M×1 vector, and s ^H represents the conjugate transpose of the normalized orthogonal training sequence s that satisfies s ^{H s=L,}

Defined as the power normalization coefficient,

Represents the energy of the training sequence. N is an M×L noise matrix, where each element is the same and independently meets the mean value of 0, and the variance is

The complex Gaussian distribution,

Represents the power of noise.

Another implementation manner provided by this application is: the estimated value of the uplink primary channel is:

Among them _, n is an M×1 noise vector, where each element is identical and independently meets a complex Gaussian distribution with a mean value of 0 and a variance of 1.

Another implementation manner provided by this application is: in the step 2), the uplink channel angle characteristic parameter is extracted as follows:

The discrete Fourier transform of the uplink main channel can be expressed as

Where F is an M×M matrix, and the elements in the p-th row and q-th column are

Define q _max as

The subscript of the point with the largest energy in the middle;

Define the spatial rotation matrix Φ(φ) as a diagonal matrix composed of elements {1,e ^jφ ,...,e ^j(M-1)φ }, the discrete Fourier transform of the channel after rotating with φ as the parameter for

The range of φ is

Solve the optimal space rotation angle φ _opt ; define the set

Among them, μ<<M represents the number of DFT points used for channel sparse representation, so

in

Express

The sub-vector of the vector contains the _{elements in the set C spa} as the subscript index

Elements in

Define the spatial characteristic parameters as

Calculate the spatial characteristic parameter a ₁ of the uplink main channel.

Another implementation manner provided by this application is: the downlink data transmission process in step 3) is as follows:

The sender performs the spatial rotation operation of the beam with a parameter of φ ₀ at the sending end, and the signal is received by the legal receiver;

The legal receiver calculates the estimated value of the rotated downlink main channel based on the received signal

according to

_{Obtain the} subscript q max2 of the maximum energy point of the downlink main channel; calculate the sparse estimation value of the downlink main channel after rotation according to the received signal

according to

With reference to the method in step 2), φ _{opt2 of the} downlink primary channel can be obtained;

In summary, the estimated value of the angle characteristic parameter of the downlink main channel obtained by the legal receiver is obtained. According to the interval position of this parameter, the N-bit data sent by the transmitting end can be demodulated according to the mapping relationship.

Another implementation manner provided by this application is: the received signal from the legal receiver is

in,

Is the conjugate transpose of the downlink main channel vector g _r , the normalized training matrix SS ^H is a matrix of τ×L, the parameter τ satisfies μ＜τ＜＜M, and the normalized training matrix as a whole satisfies the trace of ^{SS H not exceeding} τ, other parameter variables are the same as defined above.

Another implementation manner provided by this application is that the parameter φ ₀ depends on the bit data that needs to be sent currently, and the specific determination method is as follows:

For time division duplex and frequency division duplex systems, assuming that the wavelength of the uplink electromagnetic wave is λ ₁ and the wavelength of the downlink electromagnetic wave is λ ₂ , according to the angular reciprocity of the path, the sender can obtain the estimated value of the angular characteristic parameter of the downlink main channel:

Assuming that the DOA range of the uplink and downlink main channel is [θ _r -Δθ _r ,θ _r +Δθ _r ], the corresponding range of the angle characteristic parameter of the downlink main channel is

Assuming that a single transmission needs to send N-bit data, K=2 ^N intervals need to be divided evenly according to the range of the angle characteristic parameters; the parameter φ ₀ satisfies: the downlink main channel is subjected to _{spatial rotation with the parameter φ 0} , and the rotated downlink Estimated value of the angle characteristic parameter of the main channel

It is located exactly in the center of the interval corresponding to the currently sent data.

Another implementation manner provided by this application is: the estimated value of the downlink primary channel is as follows:

in,

Represents the pseudo-inverse matrix of the matrix S ^H.

Another implementation manner provided by this application is: the sparse estimation value is:

Among them, according to the angle reciprocity, C _spa2 is a

To

The set of integers at the center.

3. Beneficial effects

Compared with the prior art, the beneficial effects of the secure transmission method in a large-scale antenna system provided by this application are:

The secure transmission method in a large-scale antenna system provided in this application is a secure transmission method in a large-scale antenna system based on feature reciprocity in a large-scale antenna system.

The secure transmission method in the large-scale antenna system provided by this application uses the angular reciprocity of the uplink and downlink channels in the massive MIMO system to propose a new feature reciprocity-based secure transmission scheme.

In the secure transmission method in the large-scale antenna system provided by this application, the channel model adopts a typical low-rank channel model, which is based on the average DOA (Direction of Arrivals) and AS (Angle Spread) of the incident path. ) Modeling.

According to the secure transmission method in the large-scale antenna system provided by this application, according to the angular reciprocity of the path, this application proposes a method for extracting the angular feature of the multipath channel, which uses the reciprocity of the single path to realize the feature reciprocity of the multipath. It is easy to ensure that only legitimate receivers can correctly extract the sender’s information, and secure transmission can be ensured in multiple scenarios.

The secure transmission method in the large-scale antenna system provided by this application overcomes the defects of the prior art, uses the physical uplink and downlink angular reciprocity to extract the angular characteristic parameter reciprocity of the channel, and performs the channel performance based on the transmitted data. The spatial rotation at a certain angle ensures that the legitimate receiver can extract the desired data value according to the downlink channel, and secure transmission can be ensured without knowing the eavesdropper's channel.

The secure transmission method in the large-scale antenna system provided by this application is not affected by the eavesdropper's position, channel quality, number of antennas, and other parameters, and is suitable for multi-scene secure transmission under the large-scale antenna system.

The secure transmission method in the large-scale antenna system provided by this application is based on the extraction of the angle characteristic parameters of the uplink and downlink channels. Compared with the traditional beamforming technology, as the number of antennas increases, there is no need to perform complex beam design, and the overall system complexity is not Will increase significantly and is relatively easy to implement.

The secure transmission method in the large-scale antenna system provided by this application is based on the consistency of the arrival angle and the complex gain of each path of the uplink and downlink main channels. As long as the eavesdropper and the legal receiver are separated by several wavelengths, the probability of eavesdropping success is almost zero. This secure transmission premise is easy to implement in the millimeter wave band. Therefore, the sender does not need to know the channel state information of the eavesdropping channel.

The secure transmission method in a large-scale antenna system provided in this application is suitable for transmitting bit-level key parameters within a coherent time. The overall transmission rate is not based on the security capacity theory in the information theory, so arbitrarily increasing the number of antennas on the eavesdropping side will not affect the security performance.

The secure transmission method in the large-scale antenna system provided by this application overcomes the limitations of the existing traditional physical layer secure transmission scheme in the massive MIMO scenario, and has security performance in scenarios where the eavesdropper and the legitimate receiver are particularly close. Unaffected, it provides a new idea for the design of the secure transmission scheme.

Description of the drawings

Fig. 1 is a schematic diagram of a single-antenna single-eavesdropper model of the massive MIMO system of the present application;

Figure 2 is a comparison diagram of the secure transmission performance of the present application under different bit data rates;

Figure 3 is a comparison diagram of the secure transmission performance of the present application under different channel sparsity degrees.

Detailed ways

Hereinafter, specific embodiments of the application will be described in detail with reference to the accompanying drawings. According to these detailed descriptions, those skilled in the art can clearly understand the application and can implement the application. Without violating the principle of the present application, the features in various embodiments can be combined to obtain new implementations, or replace some features in some embodiments to obtain other preferred implementations.

Referring to FIGS. 1 to 3, this application provides reference to FIG. 1. In this application, it is assumed that both the legitimate receiver and the eavesdropper are single-antenna users randomly distributed in the coverage area of the base station (sender). There are few scattering objects around the base station, the incident angle at the base station is extremely narrow, and the correlation between the multipath channel paths is very strong. Assume that the eavesdropper is in a silent eavesdropping state, that is, not actively sending out any signals. The sender, the legal receiver, and the eavesdropper all know the normalized training matrix S, the parameters μ,τ,N,λ ₁ ,λ ₂ , the eavesdropper knows the data demodulation method and will perform the same method according to the eavesdropping channel demodulation.

In view of the above system model, this application provides a secure transmission method in a large-scale antenna system. The method includes the following steps:

Step 1): The sender (base station) obtains the uplink channel vector of the legal receiver through channel estimation;

Step 2): The sender (base station) extracts the angle characteristic parameters of the uplink main channel according to the uplink channel vector;

Step 3): The sender (base station) modulates the transmitted data according to the angle characteristic parameters of the uplink main channel, and performs downlink data transmission in the same coherent time slot.

Further, the uplink channel vector in the step 1) is obtained as follows:

Suppose that the base station has M>>1 antenna and is a uniform linear array, the legal receivers and eavesdroppers are randomly distributed in the coverage of the base station and both are configured with a single antenna, the length of the training sequence is L<T, where T is the channel coherence time length ,

The sender obtains the estimated value of the uplink main channel based on the received signal.

Further, in the uplink channel estimation, the signal received by the sender is:

Among them, h _r represents the uplink main channel in the form of an M×1 vector, and s ^H represents the conjugate transpose of the normalized orthogonal training sequence s that satisfies s ^{H s=L,}

Defined as the power normalization coefficient,

Represents the energy of the training sequence. N is an M×L noise matrix, where each element is the same and independently meets the mean value of 0, and the variance is

The complex Gaussian distribution,

Represents the power of noise.

Further, the estimated value of the uplink primary channel is:

Among them, n is an M×1 noise vector, where each element is identical and independently satisfies a complex Gaussian distribution with a mean value of 0 and a variance of 1.

Further, in the step 2), the uplink channel angle characteristic parameters are extracted as follows:

The discrete Fourier transform of the uplink main channel can be expressed as

Where F is an M×M matrix, and the elements in the p-th row and q-th column are

When the number of base station antennas tends to infinity, the energy of each path is concentrated at a point in the DFT domain, and the position of this point reflects the angular characteristics of the path; when the number of base station antennas is limited, due to the limitation of spatial resolution, the energy Leakage occurs, according to the characteristic of decreasing energy leakage edge; define q _max as

The subscript of the point with the largest energy in the middle; q _max roughly reflects the distribution characteristics of the incident angle of the multipath channel. In order to more accurately extract the angle characteristics of the channel, a spatial rotation operation needs to be introduced.

Define the spatial rotation matrix Φ(φ) as a diagonal matrix composed of elements {1,e ^jφ ,...,e ^j(M-1)φ }, the discrete Fourier transform of the channel after rotating with φ as the parameter for

The range of φ is

Solve the optimal space rotation angle φ _opt ; define the set

Among them, μ<<M represents the number of DFT points used for channel sparse representation, so

in

Express

The sub-vector of the vector contains the _{elements in the set C spa} as the subscript index

Therefore, it is only necessary to take several values within the value range of the parameter φ and compare the corresponding channel energy to determine the maximum channel energy and the corresponding optimal spatial rotation angle φ _opt .

Define the spatial characteristic parameters as

Calculate the spatial characteristic parameter a ₁ of the uplink main channel.

Further, the downlink data transmission process in the step 3) is as follows:

The sender performs the spatial rotation operation of the beam with a parameter of φ ₀ at the sending end, and the signal is received by the legal receiver;

The legal receiver calculates the estimated value of the rotated downlink main channel based on the received signal

according to

_{Obtain the} subscript q max2 of the maximum energy point of the downlink main channel; calculate the sparse estimation value of the downlink main channel after rotation according to the received signal

according to

With reference to the method in step 2), φ _{opt2 of the} downlink primary channel can be obtained;

In summary, the estimated value of the angle characteristic parameter of the downlink main channel obtained by the legal receiver is obtained. According to the interval position of this parameter, the N-bit data sent by the transmitting end can be demodulated according to the mapping relationship.

Further, the received signal of the legal receiver is

in,

Is the conjugate transpose of the downlink main channel vector g _r , the normalized training matrix SS ^H is a matrix of τ×L, the parameter τ satisfies μ＜τ＜＜M, and the normalized training matrix as a whole satisfies the trace of ^{SS H not exceeding} τ, other parameter variables are the same as defined above.

Further, the parameter φ ₀ depends on the bit data that needs to be sent currently, and the specific determination method is as follows:

For TDD (Time Division Duplex, Time Division Duplex)/FDD (Frequency Division Duplex, Frequency Division Duplex) systems, assuming that the uplink electromagnetic wave wavelength is λ ₁ and the downlink electromagnetic wave wavelength is λ ₂ , according to the angular reciprocity of the path, the sender The estimated value of the angle characteristic parameter of the downlink main channel can be obtained

Assuming that the DOA range of the uplink and downlink main channel is [θ _r -Δθ _r ,θ _r +Δθ _r ], the corresponding range of the angle characteristic parameter of the downlink main channel is

Assuming that a single transmission needs to send N-bit data, K=2 ^N intervals need to be divided evenly according to the range of the angle characteristic parameters; the parameter φ ₀ satisfies: According to the interval corresponding to the current transmission data, the downlink main channel is set to the parameter φ ₀ The space rotation, the estimated value of the angle characteristic parameter of the downlink main channel after rotation

It is located exactly in the center of the interval corresponding to the currently sent data.

Further, the estimated value of the main downlink channel is as follows:

in,

Represents the pseudo-inverse matrix of the matrix S ^H.

Further, the sparse estimation value is:

Among them, according to the angle reciprocity, C _spa2 is a

To

The set of integers at the center.

Figures 2 and 3 show the bit error rate performance of eavesdroppers and legitimate receivers under the secure transmission method in this large-scale antenna system. In the simulation, it is assumed that the system is a TDD system, that is, λ ₁ =λ ₂ , the number of base station antennas is 128, the average arrival angle of the legal receiver and the eavesdropper is 30°, the unilateral angle extension is 2°, and the system parameter μ= 12. L=128, the signal-to-noise ratio of the simulation independent variable is defined as

It can be seen that with the change of the signal-to-noise ratio and system parameters, the bit error rate of the eavesdropper has always been maintained above 0.45, that is, basically no useful information can be deciphered from the data demodulation.

Figure 2 compares the variation of the bit error rate of the legal receiver and the eavesdropper with the signal-to-noise ratio under different bit transmission rates. Set τ=128 in the simulation. Since the angular characteristic parameter range of the channel is a fixed value, as the bit transmission rate increases, the single interval becomes narrower, the interval center distance decreases, and the bit error rate rises under the same signal-to-noise ratio. Therefore, in practical applications, data transmission is required. Fastness and reliability are compromised. For eavesdroppers, the angular characteristic parameters of the eavesdropping channel and the main channel basically satisfy independent and identical distributions, so the bit error rate of the demodulated bit data is always close to 0.5.

Figure 3 compares the variation of the bit error rate of the legal receiver and the eavesdropper with the signal-to-noise ratio under different channel sparsity levels. Set N=2 in the simulation. Channel sparse processing is an effective means to reduce the impact of noise in the massive MIMO scenario. In order to further improve the system performance, in the process of extracting the channel angle characteristic parameters of the downlink data transmission, the channel is sparsely processed to improve the feature reciprocity Accuracy of extraction. Since the security performance of the system has nothing to do with the quality of the eavesdropper's channel, the security transmission performance will not be affected.

Although the application is described above with reference to specific embodiments, those skilled in the art should understand that within the principles and scope disclosed in the application, many modifications can be made to the configuration and details disclosed in the application. The protection scope of this application is determined by the appended claims, and the claims are intended to cover all modifications included in the literal meaning or scope of equivalent technical features in the claims.

Claims (10)

1. A secure transmission method in a large-scale antenna system, characterized in that: the method includes the following steps:

   Step 1): The sender obtains the uplink channel vector of the legal receiver through channel estimation;

   Step 2): The sender extracts the angle characteristic parameters of the uplink main channel according to the uplink channel vector;

   Step 3): The sender modulates the transmitted data according to the angle characteristic parameters of the uplink main channel, and performs downlink data transmission in the same coherent time slot.
2. The secure transmission method in a large-scale antenna system according to claim 1, wherein the uplink channel vector in said step 1) is obtained as follows:

   Suppose that the base station has M>>1 antenna and is a uniform linear array, the legal receivers and eavesdroppers are randomly distributed in the coverage of the base station and both are configured with a single antenna, the length of the training sequence is L<T, where T is the channel coherence time length ,

   The sender obtains the estimated value of the uplink main channel based on the received signal.
3. The secure transmission method in a large-scale antenna system according to claim 2, wherein in the uplink channel estimation, the signal received by the sender is:

   

   Among them, h _r represents the uplink main channel in the form of an M×1 vector, and s ^H represents the conjugate transpose of the normalized orthogonal training sequence s that satisfies s ^{H s=L,}

   

   Defined as the power normalization coefficient,

   

   Represents the energy of the training sequence. N is an M×L noise matrix, where each element is the same and independently meets the mean value of 0, and the variance is

   

   The complex Gaussian distribution,

   

   Represents the power of noise.
4. The secure transmission method in a large-scale antenna system according to claim 2, wherein the estimated value of the uplink main channel is:

   

   Among them, n is an M×1 noise vector, where each element is identical and independently satisfies a complex Gaussian distribution with a mean value of 0 and a variance of 1.
5. The secure transmission method in a large-scale antenna system according to claim 1, characterized in that: in the step 2), the angle characteristic parameters of the uplink channel are extracted as follows:

   The discrete Fourier transform of the uplink main channel can be expressed as

   

   Where F is an M×M matrix, and the elements in the p-th row and q-th column are

   

   Define q _max as

   

   The subscript of the point with the largest energy in the middle;

   Define the spatial rotation matrix Φ(φ) as a diagonal matrix composed of elements {1,e ^jφ ,...,e ^j(M-1)φ }, the discrete Fourier transform of the channel after rotating with φ as the parameter for

   

   The range of φ is

   

   Solve the optimal space rotation angle φ _opt ; define the set

   

   Among them, μ<<M represents the number of DFT points used for channel sparse representation, so

   

   in

   

   Express

   

   The sub-vector of the vector contains the set

   

   The element in the index as a subscript

   

   Elements in

   Define the spatial characteristic parameters as

   

   Calculate the spatial characteristic parameter a ₁ of the uplink main channel.
6. The method for secure transmission in a large-scale antenna system according to claim 1, wherein the downlink data transmission process in step 3) is as follows:

   The sender performs the spatial rotation operation of the beam with a parameter of φ ₀ at the sending end, and the signal is received by the legal receiver;

   The legal receiver calculates the estimated value of the rotated downlink main channel based on the received signal

   

   according to

   

   _{Obtain the} subscript q max2 of the maximum energy point of the downlink main channel; calculate the sparse estimation value of the downlink main channel after rotation according to the received signal

   

   according to

   

   With reference to the method in step 2), φ _{opt2 of the} downlink primary channel can be obtained;

   In summary, the estimated value of the angle characteristic parameter of the downlink main channel obtained by the legal receiver is obtained. According to the interval position of this parameter, the N-bit data sent by the transmitting end can be demodulated according to the mapping relationship.
7. The method of secure transmission in a large-scale antenna system according to claim 6, wherein the received signal from the legal receiver is

   

   in,

   

   Is the conjugate transpose of the downlink main channel vector g _r , the normalized training matrix SS ^H is a matrix of τ×L, the parameter τ satisfies μ＜τ＜＜M, and the normalized training matrix as a whole satisfies the trace of ^{SS H not exceeding} τ.
8. The secure transmission method in a large-scale antenna system according to claim 6, wherein the parameter φ ₀ depends on the bit data that needs to be sent currently, and the specific determination method is as follows:

   For time division duplex and frequency division duplex systems, assuming that the wavelength of the uplink electromagnetic wave is λ ₁ and the wavelength of the downlink electromagnetic wave is λ ₂ , according to the angular reciprocity of the path, the sender can obtain the estimated value of the angular characteristic parameter of the downlink main channel

   

   Assuming that the DOA range of the uplink and downlink main channel is [θ _r -Δθ _r ,θ _r +Δθ _r ], the corresponding range of the angle characteristic parameter of the downlink main channel is

   

   Assuming that a single transmission needs to send N-bit data, K=2 ^N intervals need to be divided evenly according to the range of the angle characteristic parameters; the parameter φ ₀ satisfies: the downlink main channel is subjected to _{spatial rotation with the parameter φ 0} , and the rotated downlink Estimated value of the angle characteristic parameter of the main channel

   

   It is located exactly in the center of the interval corresponding to the currently sent data.
9. The method for secure transmission in a large-scale antenna system according to claim 6, wherein the estimated value of the downlink main channel is as follows:

   

   in,

   

   Represents the pseudo-inverse matrix of the matrix S ^H.
10. 8. The secure transmission method in a large-scale antenna system according to claim 6, wherein the sparse estimation value is:

    

    Among them, according to the angle reciprocity,

    

    Is one that contains

    

    To

    

    The set of integers at the center.

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