UE and AP grouping and frequency selection method in distributed MIMO system

Abstract

The present invention discloses a method for grouping UE and AP and selecting frequency in a distributed MIMO system, including: constructing a secondary network system, obtaining a connection relationship matrix between a secondary user and a secondary access node according to a large-scale fading coefficient between a secondary access node and a secondary user in the secondary network system; judging whether there is aliasing between the secondary user and the primary user in the time-frequency spectrum, if there is aliasing, deleting the service relationship between the secondary user and the secondary access node serving it in the connection relationship matrix, updating the connection relationship matrix, if there is no aliasing, not updating the connection relationship matrix; constructing a measurement criterion of channel correlation, obtaining the value of the measurement criterion of channel correlation after the secondary access node deletes a secondary user it serves, and selecting the minimum value among the values ​​of all the obtained channel correlation measurement criteria. The present invention can avoid interference with cognitive targets in the primary network.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and in particular relates to a method for grouping UE and AP and selecting frequencies in a distributed MIMO system. Background Art

[0002] As one of the potential key technologies of 6G, the non-cellular massive MIMO system is mainly composed of multiple wireless access points (Access Point, AP) deployed in a dispersed manner and a central processing unit (Central Process Unit, CPU) for unified network management. It breaks the traditional cell architecture and provides services to users in a collaborative manner under the same time-frequency resources. Since the non-cellular massive MIMO system deploys antennas in a dispersed manner within the network, the traditional non-cellular massive MIMO system requires that the wireless access points in the network need to provide services to all users in the network. When the number of users accessing the network gradually increases, the AP will find it difficult to bear the increased wireless signal processing and data forwarding. Therefore, a user-centric non-cellular system is proposed. In such a system, before data processing and transmission, it is necessary to determine the connection relationship between UE (user) and AP to ensure that the system is scalable and minimize performance loss as much as possible.

[0003] In actual communication scenarios, multiple unrelated networks may be deployed in the same geographical location to meet different communication needs. Due to the close spatial distance, if communication is carried out using the same time and frequency resources, different system networks will cause relatively strong interference. Therefore, it is necessary to reasonably allocate time and frequency resources to avoid each system from communicating as much as possible to avoid strong interference and affect communication performance.

[0004] The existing user-centric cellular-free massive MIMO system UE-AP grouping method only focuses on the performance of the network. The most common method is to use information such as location or power, that is, the UE selects a portion of APs with closer locations or larger receiving power to provide services for itself. In addition, their grouping within the network does not consider the interference between users in the network. When a new cellular-free massive MIMO system needs to be deployed in a geographical area where other systems are already communicating, it has to consider the strong interference caused by the new system to the existing system.

[0005] Therefore, there is an urgent need to improve the interference of communications in existing distributed MIMO systems to ensure effective communication. Summary of the invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a method for UE and AP grouping and frequency selection in a distributed MIMO system. The technical problem to be solved by the present invention is achieved by the following technical solution:

[0007] In a first aspect, the present invention provides a method for grouping UE and AP and selecting frequency in a distributed MIMO system, comprising:

[0008] Constructing a secondary network system, and obtaining a connection relationship matrix between the secondary user and the secondary access node according to a large-scale fading coefficient between the secondary access node and the secondary user in the secondary network system;

[0009] Determine whether there is aliasing between the secondary user and the primary user in the time-frequency spectrum. If there is aliasing, delete the service relationship between the secondary user and the secondary access node serving the secondary user in the connection relationship matrix and update the connection relationship matrix. If there is no aliasing, do not update the connection relationship matrix.

[0010] According to the updated connection relationship matrix, a channel correlation measurement criterion is constructed, and the value of the channel correlation measurement criterion after any secondary access node deletes a secondary user it serves is obtained. The minimum value of all the obtained channel correlation measurement criterion values ​​is selected, that is, the frequency with the least interference is used for communication.

[0011] Beneficial effects of the present invention:

[0012] The present invention provides a method for UE and AP grouping and frequency selection in a distributed MIMO system. By designing a UE-AP grouping scheme, a UE-AP primary service relationship is established to ensure the criterion of user received signal power. Then, based on the acquired location information of the primary user, the UE-AP service relationship is further adjusted to avoid interference with the cognitive target. Finally, the connection relationship is further optimized based on the principle of minimum correlation, and finally the optimal communication frequency band is selected from the total available bandwidth. In this way, on the one hand, the overall performance of the network itself is guaranteed, and on the other hand, interference with the cognitive target in the primary network is avoided. Finally, the optimal frequency band is selected from the existing frequency band for operation.

[0013] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a flow chart of a method for grouping UE and AP and selecting frequency in a distributed MIMO system provided by an embodiment of the present invention;

[0015] Figure 2 is a schematic diagram of a secondary network system provided by an embodiment of the present invention;

[0016] Figure 3 is a schematic diagram of cognitive target interference avoidance provided by an embodiment of the present invention;

[0017] Figure 4It is a flow chart of an algorithm for optimizing AP-UE service relationship by greedy search provided by an embodiment of the present invention;

[0018] FIG5( a ) is a schematic diagram of a cognitive target and a time-frequency spectrum provided by an embodiment of the present invention;

[0019] FIG5( b ) is another schematic diagram of a cognitive target and a time-frequency spectrum provided by an embodiment of the present invention;

[0020] FIG6( a ) is a schematic diagram of a simulation experiment provided by an embodiment of the present invention;

[0021] FIG6( b ) is another schematic diagram of a simulation experiment provided by an embodiment of the present invention;

[0022] FIG6( c ) is another schematic diagram of a simulation experiment provided by an embodiment of the present invention;

[0023] FIG6( d ) is another schematic diagram of a simulation experiment provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0025] See also Figure 1 , Figure 1 The present invention provides a flow chart of a method for grouping UE and AP and selecting frequency in a distributed MIMO system provided by an embodiment of the present invention. The present invention provides a method for grouping UE and AP and selecting frequency in a distributed MIMO system, including:

[0026] S101, constructing a secondary network system, and obtaining a connection relationship matrix between secondary users and secondary access nodes according to a large-scale fading coefficient between secondary access nodes and secondary users in the secondary network system.

[0027] Specifically, in this embodiment, see Figure 2 , Figure 2 It is a schematic diagram of a secondary network system provided by an embodiment of the present invention. The secondary network system is a non-cellular large-scale MIMO system. A large number of distributed wireless access nodes AP are randomly deployed within the network. All access nodes establish a wired connection with the CPU through a fronthaul link. The access nodes aggregate information to the CPU through the fronthaul link to exchange information. The CPU distributes data through the fronthaul link. All access nodes simultaneously and co-frequency collaboratively provide joint data transmission for users in the network. The secondary network system includes M multi-antenna secondary access nodes and K single-antenna secondary users, and each of the secondary access nodes includes N antennas.

[0028] In order to ensure the signal strength of the terminal, a preliminary screening is performed to create a connection matrix C. An initial connection relationship is determined by the large-scale fading coefficient β between the secondary access node and the secondary user. When the mth secondary access node and the kth secondary user communicate with each other, C mk =1, otherwise C mk =0.

[0029] In this embodiment, in the distributed MIMO system, since the secondary access nodes and the secondary users are widely distributed within the network, for a certain secondary user, some secondary access nodes are far away or blocked by shadows, resulting in the secondary user receiving weak signals from these secondary access nodes. In order to reduce the processing overhead of the secondary access nodes and avoid unnecessary power waste, the serving secondary access node of each secondary user is determined according to large-scale fading to form a preliminary connection relationship matrix.

[0030] Obtaining a connection relationship matrix between secondary users and secondary access nodes, including:

[0031] Define a connection relationship matrix C to represent the service relationship between the secondary access node and the secondary user. The connection relationship matrix C is a 0-1 binary array. The elements in the connection relationship matrix are expressed as:

[0032]

[0033] Among them, C mk is the service relationship between the mth secondary access node and the kth secondary user, if C mk =1, then the mth secondary access node serves the kth secondary user, if C mk =0, the mth secondary access node does not serve the kth secondary user;

[0034] According to the large-scale fading coefficient between the secondary access node and the secondary user in the secondary network system, the service relationship between the mth secondary access node and the kth secondary user is obtained, and its expression is:

[0035]

[0036] Among them, β mk is the large-scale fading coefficient between the mth secondary access node and the kth secondary user, β k is the vector of large-scale fading coefficients between the kth secondary user and all secondary access nodes, Clustering th is the initial grouping threshold (dB);

[0037] A connection relationship matrix is ​​obtained according to the service relationships between all secondary access nodes and all secondary users.

[0038] S102, determining whether there is aliasing between the secondary user and the primary user in the time-frequency spectrum; if there is aliasing, deleting the service relationship between the secondary user and the secondary access node serving it in the connection relationship matrix, and updating the connection relationship matrix; if there is no aliasing, not updating the connection relationship matrix.

[0039] Specifically, in this embodiment, in order to achieve cognitive target interference avoidance, the distance threshold dist is set by obtaining the location information of the primary user. th and angle threshold θ th When the mth secondary access node of the secondary network has a cognitive target k' in the direction of a secondary user k it serves, when the distance between the mth secondary access node and the cognitive target k is less than the threshold distance, and the angle formed by the mth secondary access node and the cognitive target k' and the mth secondary access node and the secondary user k is less than the threshold angle, that is, dist mk' ≤dist th And θ <mk',mk> ≤θ th When , the service relationship between the mth secondary access node and the secondary user k is deleted to prevent the mth secondary access node from forming a beam in the direction of the secondary user k, which may cause strong interference to the cognitive target k'.

[0040] Specifically, since there are already primary users communicating on the existing time-frequency resources, in order to avoid strong interference to the primary users by the secondary communication system, based on the acquired frequency usage of the primary network system and the corresponding location information of the primary users, the interference avoidance distance threshold and beam width threshold are set, and further deletions are made on the basis of the existing connection relationship to avoid strong interference to the primary users.

[0041] See also Figure 3 , Figure 3 This is a schematic diagram of cognitive target interference avoidance provided by an embodiment of the present invention. Since the beam sent by AP1 to the secondary user will cause greater interference to the primary user at frequency f1, that is, the primary user and the secondary user are in the same direction of AP1, and the primary user is close to AP1, the beam signal from AP1 to the secondary user leaks to the primary user. Therefore, it is necessary to change the connection relationship matrix so that AP1 does not serve the secondary user. On frequency f2, although the primary user and the secondary user are in the same direction of AP2, since the primary user is far away from AP2, the beam signal from AP2 to the secondary user may have experienced relatively large attenuation when it reaches the primary user, so it will not cause relatively large interference to the primary user. The cognitive target interference avoidance criteria are as follows:

[0042] dist mk' ≤distth And θ <mk',mk> ≤θ th ;

[0043]

[0044] Among them, dist th is the distance threshold, which means that when the distance between the mth secondary access node and the secondary user k' reaches this value, the received signal power is equivalent to the noise, θ th is the angle threshold, dist mk' is the distance between the mth secondary access node and the kth primary user. The position of the mth secondary access node is known, and the position of the primary user can be obtained based on the collaborative positioning of multiple secondary access nodes. Therefore, the distance between the mth secondary access node and the kth primary user can be obtained. mk' ,θ <mk',mk> is the angle between the mth secondary access node and the kth secondary user and the k'th primary user, that is, the angle between the mth secondary access node and the kth secondary user and the mth secondary access node and the k'th primary user.

[0045] Distance threshold dist th The first condition to be satisfied is:

[0046] P Noise =P t *pathloss(dist th );

[0047] Among them, P Noise is the noise power in the current environment, P t is the transmission power of the mth secondary access node, pathloss(·) is the path loss function, and its value is related to the distance.

[0048] Distance threshold dist th The second condition that is satisfied is:

[0049]

[0050]

[0051] Among them, factor is the conversion factor, which is the ratio of the overlapping area of ​​the secondary network and the primary network on the time-frequency spectrum to the total area of ​​the secondary network on the time-frequency spectrum. overlap is the overlapping area of ​​the primary network system and the secondary network system in the time-frequency spectrum, S secondary is the area occupied by the secondary network system on the time-frequency spectrum, P Noise is the noise power in the current environment, P tis the transmit power of the mth secondary access node, and pathloss() is the path loss function.

[0052] In the above embodiment, the distance threshold dist th When the second condition is met, the secondary network and the primary network may partially overlap in the time-frequency spectrum. In this case, it can be considered that the secondary network only interferes with the primary user in part of the time and frequency. Therefore, the above formula can be converted to appropriately relax the distance threshold dist th .

[0053] Through signal measurement and processing, the communication start and end time and start and end frequency of the primary user have been obtained, so the area of ​​the time-frequency spectrum can be obtained by multiplying the corresponding time by the frequency.

[0054] Of course, since the attenuation of signal power with communication distance is not linear, the actual distance th A recommended threshold range can be selected based on simulation experiments.

[0055] The angle threshold θ th The expression is:

[0056]

[0057]

[0058]

[0059] Among them, the above formula is the estimation formula for the beam width of any linearly distributed array antenna, which is the signal wavelength, d is the average antenna spacing, N is the number of antennas, (rad) is the unit of radians, v2 is the quadratic polynomial approximation of the power pattern, and v4 is the quartic polynomial approximation of the power pattern.

[0060] The UE-AP service relationship matrix of the secondary network is updated through the interference avoidance distance threshold and angle threshold to avoid interference to the primary user.

[0061] S103. Construct a channel correlation criterion based on the updated connection relationship matrix, obtain the value of the channel correlation criterion after any secondary access node deletes a secondary user it serves, and select the minimum value from all the obtained channel correlation criterion values, that is, communicate at the frequency with the least interference.

[0062] Specifically, in this embodiment, the minimum correlation grouping between terminals in the network is used to further optimize the UE-AP service relationship through a greedy search method in order to reduce interference between user terminals in the secondary network and make the channels between users as orthogonal as possible.

[0063] By defining a channel correlation criterion Calculate the number of secondary users a secondary access node serves after deleting the secondary user. The value is large or small, and finally selected from all candidate results In the smallest case, after all the secondary access nodes are traversed, the above greedy search operation is executed repeatedly until the result converges.

[0064] Specifically, in order to reduce the interference between user terminals in the secondary network, the channels between users are made as orthogonal as possible. A frequency selection scheme based on minimum correlation of favorable propagation is proposed.

[0065] Favorable propagation is a characteristic that the more base station antennas a MIMO system has, the more orthogonal the channels between users tend to be. The minimum correlation frequency selection scheme for favorable propagation is constructed, and its expression is:

[0066]

[0067] Among them, g k is the channel vector of the kth secondary user, g j is the channel vector of the jth secondary user, j is the jth secondary user, (·) H is the conjugate transpose operation, M is the total number of antennas in the secondary network system;

[0068] The secondary user channel is split into large-scale fading and small-scale fading, and the minimum correlation frequency selection scheme that is beneficial to propagation is simplified to:

[0069]

[0070] Among them, β mk is the large-scale fading coefficient between the mth secondary access node and the kth secondary user, β mj is the large-scale fading coefficient between the mth secondary access node and the jth secondary user, h mk is the small-scale fading coefficient between the mth secondary access node and the kth secondary user, h mj is the small-scale fading coefficient between the mth secondary access node and the jth secondary user;

[0071] According to the channel vector g of the kth secondary user k , define the cumulative distribution function P θ , whose expression is:

[0072]

[0073] Among them, P is the sign of the probability of event occurrence, and θ is the preset favorable propagation threshold;

[0074] According to Chebyshev's inequality, we can obtain Pθ A lower bound of is expressed as:

[0075]

[0076] in, An indicator for measuring the correlation of signals between two secondary users;

[0077] For spatially correlated channels, when there are many antennas, using large-scale fading to measure the channel correlation between users actually shows that the farther two users are apart in space, the smaller their channel correlation is. From another perspective, it also measures the interference between secondary users k and j.

[0078] After obtaining an indicator that can measure the channel correlation between users Afterwards, this can be used as a criterion for greedy search to further optimize the UE-AP service relationship.

[0079] Finally, combined with the UE-AP grouping matrix formed previously, the correlation index of the signal between the two secondary users is used as the greedy search criterion, and the updated connection relationship matrix is ​​optimized to obtain the channel correlation measurement criterion, which is expressed as:

[0080]

[0081] Among them, C mj is the service relationship between the mth secondary access node and the jth secondary user. For the greedy search algorithm flow, please refer to Figure 4 , Figure 4 This is an algorithm flow chart for optimizing AP-UE service relationship by greedy search provided by an embodiment of the present invention. First, the grouping matrix C and the large-scale fading coefficient of the secondary network system are loaded, the correlation of the channels under the current grouping relationship is calculated, and the connection relationship of each secondary access node is changed in turn until convergence. The updated connection relationship matrix is ​​output to obtain the final connection relationship matrix.

[0082] After the UE-AP service relationship optimization is completed, according to the communication requirements of the secondary network, its communication bandwidth only occupies a part of the total available bandwidth. Therefore, the total available bandwidth in the current environment is divided into several sub-bands to form an available frequency band set Ω f , which contains multiple available frequency starting point information. Finally, from the available frequency band set Ω f Select a frequency band with the smallest channel correlation for communication to ensure the minimum interference between users in the network, that is, f working =min(f corr ), where f working Represents the final selected communication frequency, min(f corr) represents taking out the frequency with the smallest correlation among all candidate frequencies.

[0083] In summary, the present invention provides a method for UE and AP grouping and frequency selection in a distributed MIMO system. By designing a UE-AP grouping scheme, the UE-AP primary service relationship is established to ensure the criterion of user received signal power. Then, based on the acquired location information of the primary user, the UE-AP service relationship is further adjusted to avoid interference with the cognitive target. Finally, the connection relationship is further optimized based on the principle of minimum correlation, and finally the optimal communication frequency band is selected from the total available bandwidth. In this way, on the one hand, the overall performance of the network itself is guaranteed, and on the other hand, interference with the cognitive target in the primary network is avoided, and finally the optimal frequency band is selected from the existing frequency band for operation.

[0084] In an optional embodiment of the present invention, the beneficial effects of the UE and AP grouping and frequency selection method in the distributed MIMO system proposed by the present invention are verified by the following simulation experiments, specifically:

[0085] 1. Simulation conditions

[0086] The hardware platform of the simulation experiment of the present invention is: Intel(R) Core(TM) i5-12500H CPU, the main frequency of the processor is 4.5GHz, and the memory is 16GB.

[0087] The simulation was implemented using MATLAB R2021b software, with parameters set to 30 secondary access nodes in the secondary network, each secondary access node having 8 antennas, and 8 single-antenna users. The primary network dot diagram and the corresponding time-frequency spectrum are shown in Figures 5(a) and 5(b). Figure 5(a) is a schematic diagram of the cognitive target and time-frequency spectrum provided by an embodiment of the present invention, and Figure 5(b) is another schematic diagram of the cognitive target and time-frequency spectrum provided by an embodiment of the present invention. The radius of the primary network and the secondary network are both 500m.

[0088] 2. Simulation content and result analysis

[0089] Simulation 1. Under the above simulation conditions, the performance of the d-ZF average user SE and edge user SE under different thresholds in step S101 is compared. As can be seen from Figure 6(a), Figure 6(a) is a schematic diagram of a simulation experiment provided by an embodiment of the present invention. Both the average user SE and the edge user SE increase first and then decrease. The reason why the SE in the first half increases with the increase of the grouping threshold is that d-ZF can eliminate the inter-user interference of the secondary access node serving the secondary users. Therefore, as the number of secondary users connected to the secondary access node increases, the inter-user interference becomes smaller and smaller until its magnitude is lower than the noise. The reason why SE in the second half decreases as the grouping threshold increases is that ZF precoding forces zero interference between users at the expense of the expected signal power. The secondary access node has only 8 antennas. When the number of users served by the secondary access node gradually increases until it equals 8, a large number of degrees of freedom are used to eliminate interference between users, causing the expected signal power to decrease. At this time, although the noise has been completely eliminated, there is still fixed noise in the system. Therefore, on the whole, the system in the first half is interference-limited, and the system in the second half is noise-limited, reflecting the characteristic that ZF precoding will amplify noise.

[0090] Simulation 2: Although the formula for determining the distance threshold has been given in step S102, due to the nonlinear relationship between distance and signal, a reasonable interference avoidance distance threshold value is determined through simulation.

[0091] FIG6(b) takes the logarithm of the interference power, and FIG6(b) is another schematic diagram of the simulation experiment provided by the embodiment of the present invention, wherein the solid line is the interference power curve, and the dotted line is the fixed noise power curve. It can be seen that the distance corresponding to the intersection of the two curves is approximately 1000m, that is, when the communication distance is 1000m, the interference signal power will decay to the noise level, so if according to the formula in step S102, 1000m should be selected as the interference avoidance distance threshold, but according to the channel characteristics, the signal power does not decay linearly with the distance, so the simulation compares the variation trend of the linear value of the interference signal power with the distance.

[0092] As can be seen from Figure 6(c), Figure 6(c) is another schematic diagram of the simulation experiment provided by an embodiment of the present invention. When the distance reaches about 600m, the interference signal power is almost equal to the noise power. Therefore, the interference avoidance distance threshold can be narrowed to 600m. While not causing strong interference to primary users, a high connection rate of the secondary network is maintained, thereby ensuring the normal operation of the secondary network.

[0093] Simulation 3, finally compared the performance of random frequency selection and the performance of frequency selection with minimum correlation after optimizing the connection relationship by the minimum correlation criterion. As can be seen from Figure 6(d), Figure 6(d) is another schematic diagram of the simulation experiment provided by the embodiment of the present invention. After the interference avoidance is completed, the UE-AP grouping is further optimized based on the minimum correlation criterion, and the performance of the frequency band with the minimum correlation is finally selected, which is better than the random frequency selection performance as a whole.

[0094] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the term "include", "comprise" or any other variant is intended to cover non-exclusive inclusion, so that the article or device including a series of elements includes not only those elements, but also other elements that are not explicitly listed. In the absence of more restrictions, the elements defined by the sentence "including one..." do not exclude the existence of other identical elements in the article or device including the elements. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The orientation or position relationship indicated by "up", "down", "left", "right", etc. is based on the orientation or position relationship shown in the accompanying drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention.

[0095] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification.

[0096] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A method for grouping UE and AP and selecting frequency in a distributed MIMO system, characterized in that: include: Constructing a secondary network system, and acquiring a connection relationship matrix between the secondary user and the secondary access node according to a large-scale fading coefficient between the secondary access node and the secondary user in the secondary network system; The secondary network system includes M multi-antenna secondary access nodes and K single-antenna secondary users, each of the secondary access nodes includes N antennas; The acquiring of the connection relationship matrix between the secondary user and the secondary access node comprises: A connection relationship matrix C is defined to represent the service relationship between the secondary access node and the secondary user. The elements in the connection relationship matrix are represented as follows: Among them, C mk is the service relationship between the mth secondary access node and the kth secondary user, if C mk =1, then the mth secondary access node serves the kth secondary user, if C mk =0, the mth secondary access node does not serve the kth secondary user; According to the large-scale fading coefficient between the secondary access node and the secondary user in the secondary network system, the service relationship between the mth secondary access node and the kth secondary user is obtained, and its expression is: Among them, β mk is the large-scale fading coefficient between the mth secondary access node and the kth secondary user, β k is the vector of large-scale fading coefficients between the kth secondary user and all secondary access nodes, Clustering th is the initial grouping threshold; Acquire the connection relationship matrix according to the service relationships between all the secondary access nodes and all the secondary users; Determine whether there is aliasing between the secondary user and the primary user on the time-frequency spectrum, if there is aliasing, delete the service relationship between the secondary user and the secondary access node serving the secondary user in the connection relationship matrix, and update the connection relationship matrix; if there is no aliasing, do not update the connection relationship matrix; The expression for judging whether there is aliasing between the secondary user and the primary user in the time-frequency spectrum is: dist mk' ≤dist th And θ <mk',mk> ≤θ th ; Among them, dist th is the distance threshold, θ th is the angle threshold, dist mk' is the distance between the mth secondary access node and the k'th primary user, θ <mk',mk> is the angle between the mth secondary access node and the kth secondary user and the k'th primary user; According to the updated connection relationship matrix, a channel correlation measurement criterion is constructed, and the value of the channel correlation measurement criterion after any secondary access node deletes a secondary user it serves is obtained, and the minimum value is selected from all the obtained channel correlation measurement criterion values, that is, the frequency with the least interference is used for communication; The process of constructing the channel correlation measurement criterion includes: According to the characteristics that the more days the secondary network system base station has, the more orthogonal the secondary user channels tend to be, a frequency selection scheme with minimum correlation that is beneficial to propagation is constructed, and its expression is: Among them, g k is the channel vector of the kth secondary user, g j is the channel vector of the jth secondary user, j is the jth secondary user, (·) H is the conjugate transpose operation, M is the total number of antennas in the secondary network system; The secondary user channel is split into large-scale fading and small-scale fading, and the correlation-minimizing frequency selection scheme that is beneficial to propagation is simplified to: Among them, β mk is the large-scale fading coefficient between the mth secondary access node and the kth secondary user, β mj is the large-scale fading coefficient between the mth secondary access node and the jth secondary user, h mk is the small-scale fading coefficient between the mth secondary access node and the kth secondary user, h mj is the small-scale fading coefficient between the mth secondary access node and the jth secondary user; According to the channel vector g of the kth secondary user k , define the cumulative distribution function P θ , whose expression is: Among them, P is the sign of the probability of event occurrence, and θ is the preset favorable propagation threshold; According to Chebyshev's inequality, we can obtain P θ A lower bound of is expressed as: in, An indicator for measuring the correlation of signals between two secondary users; The correlation index of the signal between the two secondary users is used as a greedy search criterion to optimize the updated connection relationship matrix to obtain a measurement criterion for channel correlation, which is expressed as: Among them, C mj is the service relationship between the mth secondary access node and the jth secondary user.

2. The method for grouping UE and AP and selecting frequency in a distributed MIMO system according to claim 1, characterized in that: The distance threshold dist th The first condition to be satisfied is: P Noise =P t *pathloss(dist th ); Among them, P Noise is the noise power in the current environment, P t is the transmit power of the mth secondary access node, and pathloss(·) is the path loss function.

3. The method for grouping UE and AP and selecting frequency in a distributed MIMO system according to claim 1, characterized in that: The distance threshold dist th The second condition that is satisfied is: Among them, factor is the conversion factor, S overlap is the overlapping area of ​​the primary network system and the secondary network system in the time-frequency spectrum, S secondary is the area occupied by the secondary network system on the time-frequency spectrum, P Noise is the noise power in the current environment, P t is the transmit power of the mth secondary access node, and pathloss(·) is the path loss function.

4. The method for grouping UE and AP and selecting frequency in a distributed MIMO system according to claim 1, characterized in that: The angle threshold θ th The expression is: Where λ is the signal wavelength, d is the average antenna spacing, N is the number of antennas, and θ th The unit is radians, v2 is the quadratic polynomial approximation of the power pattern, and v4 is the quartic polynomial approximation of the power pattern.

5. The method for grouping UE and AP and selecting frequency in a distributed MIMO system according to claim 1, characterized in that: The minimum value of the measurement criterion of the channel correlation is calculated, and the frequency with the least interference is selected for communication.

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