Skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method and system

Abstract

The present invention discloses a method and system for precoding transmission of a Tianbo massive MIMO-OFDM channel with a smooth beam structure. The Tianbo massive MIMO-OFDM base station utilizes a beam-based channel model to design a beam structure precoder for each user in conjunction with a group of subcarriers on the basis of constraining the smoothness of the equivalent channel at each user's receiving end, and utilizes the channel smooth beam structure precoder to precode the signal sent to each user to achieve downlink transmission. First, the base station utilizes a beam domain precoder on each subcarrier to precode the data signal and pilot signal sent to each user, and then maps the beam domain precoded signal of each user to a complete beam domain transmission signal based on the spatial beam set of each user, and finally transforms the sum of the beam domain transmission signals of each user into a spatial domain transmission signal. The present invention can effectively improve the equivalent channel estimation performance and significantly improve the signal detection performance while ensuring the downlink precoding transmission performance of Tianbo massive MIMO-OFDM.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and in particular relates to a skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method and system. Background Art

[0002] Skywave communication operates in the 3-30MHz frequency band and can achieve beyond-line-of-sight communication over thousands of kilometers through ionospheric reflection, thereby achieving global network coverage. However, traditional point-to-point skywave communication is often subject to low data transmission rates due to numerous limitations, such as limited spectrum resources and complex and changing ionospheric conditions, which restricts its application. Massive MIMO (Multiple-Input Multiple-Output) technology, by configuring a large number of antennas on the base station side, can simultaneously serve a large number of users on the same time-frequency resources, significantly improving spectrum efficiency and power efficiency. The combination of skywave communication and massive MIMO technology, namely skywave massive MIMO communication, can significantly improve system capacity and reliability.

[0003] Downlink precoding transmission plays a very important role in Skywave massive MIMO communication. It can precode the signal sent to each user in the downlink transmission to suppress multi-user interference and improve the sum rate performance. When OFDM is combined with a massive MIMO system, traditional precoding methods such as zero-forcing precoding, minimum mean square error precoding, signal-to-noise ratio precoding, and weighted minimum mean square error precoding are all performed separately on each subcarrier. This may make the equivalent channel at each user's receiving end (i.e., the cascade of the precoder and the air interface channel of each user) not smooth in the frequency domain. In other words, the channel correlation between adjacent subcarriers in the frequency domain equivalent channel is weakened, and the channel changes faster along the subcarrier, which makes the equivalent channel estimation at the receiving end complicated, ultimately leading to a decline in system performance.

[0004] In Skywave massive MIMO-OFDM systems, precoders for a group of subcarriers should be jointly designed to smooth the equivalent channel and facilitate equivalent channel estimation at the receiver. How to ensure good sum rate performance while constraining the equivalent channel smoothness at the receiver is a challenge that Skywave massive MIMO-OFDM systems must address. Summary of the Invention

[0005] Purpose of the invention: To address the deficiencies of the existing technology, the present invention proposes a skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method and system, which effectively improves the equivalent channel estimation performance and significantly improves the signal detection performance while ensuring transmission performance.

[0006] Technical solution: In order to achieve the above-mentioned purpose, the present invention provides the following technical solution:

[0007] A skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method includes:

[0008] The base station uses a beam-based channel model to design a beam structure precoder for each user by combining a set of subcarriers while constraining the smoothness of the equivalent channel at each user's receiving end.

[0009] The base station uses a channel smoothed beam structure precoder to precode the signal sent to each user to achieve downlink transmission; this includes: the base station uses a beam domain precoder on each subcarrier to precode the data signal and pilot signal sent to each user, and then maps each user's beam domain precoded signal to a complete beam domain transmission signal based on each user's spatial beam set. Finally, the sum of each user's beam domain transmission signal is transformed into a spatial domain transmission signal.

[0010] Furthermore, the receiving end equivalent channel is a cascade of a precoder and an air interface channel.

[0011] Furthermore, the receiving end equivalent channel smoothness is the speed at which the equivalent channel changes along the subcarrier; the better the channel smoothness, the slower the equivalent channel changes along the subcarrier; the worse the channel smoothness, the faster the equivalent channel changes along the subcarrier.

[0012] Furthermore, the beam structure precoder means that the spatial domain precoder of each user on each subcarrier has a beam structure, that is, the product of the spatial beam matrix and the spatial beam domain precoder.

[0013] Furthermore, the specific process of the channel smooth beam structure precoder design is to first make the vector formed by stacking the spatial domain precoders on a group of subcarriers of each user have a structure of a space-frequency beam matrix multiplied by a space-frequency beam domain vector, and express the precoder design as an optimization problem with the space-frequency beam domain vector as the optimization variable, maximizing the system performance as the optimization goal, and including total power constraints and channel smoothness constraints; by solving this optimization problem, the expression of the space-frequency beam domain vector of each user is obtained, and then the beam domain precoder on each subcarrier of each user can be obtained by using the frequency beam matrix.

[0014] Furthermore, the channel smoothness constraint is to limit the number of non-zero elements of the equivalent channel of each user receiving end in the delay domain.

[0015] Furthermore, a corresponding null space matrix is ​​constructed according to the smoothness constraint of each user channel, and the optimization variable, that is, the space-frequency beam domain vector, is made to have a structure of a null space matrix multiplied by a null space vector. At this time, the optimization problem is converted into an optimization problem with the null space vector as the optimization variable, maximizing system performance as the optimization goal, and including a total power constraint. By solving this optimization problem, the expression of the null space vector of each user is obtained, and then the expression of the space-frequency beam domain vector of each user is obtained using the null space matrix.

[0016] Furthermore, the spatial beam set of each user is a set of spatial beams corresponding to non-zero elements of the spatial beam domain channel in the spatial beam-based channel model of each user, or a selected set including the spatial beam set; when adding a new spatial beam to the spatial beam set, the selected spatial beam belongs to a user for whom a non-zero spatial beam has been selected into the existing spatial beam set.

[0017] The invention discloses a skywave massive MIMO-OFDM channel smooth beam structure precoding transmission system, comprising a base station and a group of user terminals. The base station implements the skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method.

[0018] A computer program product includes a computer program / instruction, which, when executed by a processor, implements the steps of the skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method.

[0019] Beneficial effects: Compared with the existing technology, the present invention jointly designs the precoders on a group of subcarriers and introduces channel smoothness constraints to improve the smoothness of the equivalent channel of each user receiving end. The channel smooth beam structure precoder formed can significantly and effectively improve the equivalent channel estimation performance and significantly improve the signal detection performance while ensuring the downlink transmission performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a block diagram of a skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission system according to an embodiment of the present invention;

[0021] Figure 2 This is a schematic diagram of the channel smoothed beam structure precoding transmission process according to an embodiment of the present invention;

[0022] Figure 3 1 is a graph showing the sum rate performance of a channel smoothed beam structure precoder under different equivalent channel delay spread constraints in an embodiment of the present invention;

[0023] Figure 4 1 is a normalized mean square error performance diagram of a channel smoothed beam structure precoder under different equivalent channel delay spread constraints in an embodiment of the present invention;

[0024] Figure 5 1 is a diagram showing the bit error rate performance of a channel smoothed beam structure precoder under different comb pilot configurations when an estimated equivalent channel is used in an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The technical solutions provided by the present invention will be described in detail below with reference to specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0026] like Figure 1 As shown, an embodiment of the present invention discloses a skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method, wherein each base station uses a channel smooth beam structure precoder to precode the signal sent to each user to achieve downlink transmission; specifically, the base station uses a beam-based channel model to design a beam structure precoder for each user in conjunction with a group of subcarriers on the basis of constraining the equivalent channel smoothness of each user's receiving end; wherein the channel smooth beam structure precoder is mainly composed of a beam domain precoder for each user on each subcarrier, a beam mapping module for each user, and a beam transformation module. Figure 2 A flow chart of the channel smoothed beam structure precoding transmission method is given, which mainly includes the base station using a beam domain precoder on each subcarrier to precode the data signal and pilot signal sent to each user, then mapping each user's beam domain precoded signal to a complete beam domain transmission signal based on each user's spatial beam set, and finally transforming the sum of each user's beam domain transmission signal into a spatial domain transmission signal.

[0027] In this embodiment, the base station jointly designs the precoders for each user on a group of subcarriers and introduces channel smoothness constraints to improve the smoothness of the equivalent channel at each user's receiver. The equivalent channel at the receiver is the concatenation of the precoder and the air interface channel. The equivalent channel smoothness at the receiver measures the speed at which the equivalent channel varies along the subcarrier. Better channel smoothness indicates slower variations along the subcarrier, while worse channel smoothness indicates faster variations along the subcarrier.

[0028] In this embodiment, the beam structure precoder refers to the spatial domain precoder for each user on each subcarrier having a beam structure, that is, the product of the spatial beam matrix and the spatial beam domain precoder. In this embodiment, all operations involving the spatial beam matrix or its conjugate transpose multiplied by a vector can be effectively implemented using a Chirp-Z transform.

[0029] The method of the present invention is primarily applicable to skywave massive MIMO-OFDM systems equipped with a large-scale antenna array at the base station side to simultaneously serve multiple users. The specific implementation of precoding involved in the present invention is described in detail below, using a specific communication system example. It should be noted that the method of the present invention is applicable not only to the specific system model exemplified below, but also to system models with other configurations.

[0030] 1. System Configuration

[0031] In this embodiment, a large-scale MIMO-OFDM system with one day wave is considered. The base station is configured with a uniform linear array with M antennas, providing services to U single-antenna users. The user index set is In skywave communication, since the carrier frequency f c The antenna spacing d of the array needs to change with the ionospheric conditions, and the maximum operating frequency f of the system o To set, that is, d = λ o / 2, where λ o =c / f o , c is the speed of light.

[0032] In OFDM modulation, the number of carriers is N c , the length of the cyclic prefix is ​​N g , the subcarrier spacing is Δ f , the sampling interval is T s =1 / N c Δf. Where N v subcarriers are used to transmit data, and their index set is The duration of each OFDM symbol with CP is T sym =(N c +N g )T s .

[0033] Skywave massive MIMO-OFDM system operates in time division multiplexing (TDD) mode. Each radio frame contains N F time slots, each of which contains N S OFDM symbols, so the total number of OFDM symbols in a frame is N = N F N S In each time slot, the nth p OFDM symbols are used to transmit pilot sequences for channel estimation, and the remaining OFDM symbols are used to transmit uplink and downlink data.

[0034] 2. Beam-based Channel Model

[0035] The beam-based channel model includes a spatial beam-based channel model and a space-frequency beam-based channel model. The spatial beam-based channel model is the spatial beam matrix multiplied by the spatial beam domain channel vector, while the space-frequency beam-based channel model is the space-frequency beam matrix multiplied by the space-frequency beam domain channel vector. The spatial beam matrix is ​​a matrix composed of the spatial domain direction vectors corresponding to a selected set of direction cosine sampling grid points, and each spatial domain direction vector is called a spatial beam. The space-frequency beam matrix is ​​a matrix composed of the space-frequency domain direction vectors corresponding to a selected set of direction cosine and time delay sampling grid points, and each space-frequency domain direction vector is called a space-frequency beam. Each space-frequency beam is composed of the spatial beam corresponding to the direction cosine sampling grid points and the frequency beam corresponding to the time delay sampling grid points.

[0036] For the system model exemplified above, in downlink transmission, the received signal on the kth subcarrier on the nth OFDM symbol at user u is

[0037]

[0038] in(·) H represents the conjugate transpose, is the transmitted signal of user u, z u,n,k is distributed as Additive Gaussian white noise, is the downlink spatial domain channel of user u on the kth subcarrier on the nth OFDM symbol, that is,

[0039]

[0040] in

[0041]

[0042] is the direction vector in the spatial domain. In (2), P u is the number of paths between the base station and user u, is the complex-valued gain of the p-th path of user u, where β u,p is the gain, is the initial phase uniformly distributed on [0,2π). ν u,p It is the Doppler frequency caused by the ionosphere and user mobility. are the direction cosines, where and are the azimuth arrival angle and the elevation arrival angle respectively. τ =d / c,τ u,p is the propagation delay of the pth path between user u and the base station. By stacking the spatial domain channels on a set of subcarriers, the downlink space-frequency domain channel can be obtained as

[0043]

[0044] in

[0045]

[0046] is the frequency domain direction vector.

[0047] Note that the path parameter Ω u,p and τ u,p The values ​​of are restricted to the set [-1,1) and p0,τ max ), where τ max =N g T s is the maximum delay spread. Divide the two sets evenly into multiple non-overlapping subsets, that is,

[0048]

[0049] where N τ =N v N g / N c , and The number of partition subsets for direction cosine and time delay are and N de =F de N τ , where F an and F de It is called the refinement factor. Definition and Where × represents the Cartesian product of two sets, the spatial domain channel in (2) and the spatial frequency domain channel in (4) can be approximated as

[0050]

[0051] in is the spatial beam domain channel,

[0052]

[0053] is the space-frequency beam domain channel,

[0054]

[0055] and is the spatial beam matrix, whose columns are called spatial beams, and is the frequency beam matrix, and its columns are called frequency beams. Therefore, (6) and (7) are called beam-based channel models. (7) can also be expressed as

[0056]

[0057] in is the space-frequency beam matrix, which is composed of the space beam matrix and the frequency beam matrix. T represents transpose, represents the Kronecker product.

[0058] 3. Channel Smoothed Beam Structure Precoding

[0059] The specific process of channel smooth beam structure precoder design is as follows: first, let the vector formed by stacking the spatial domain precoders on a group of subcarriers of each user have the structure of a space-frequency beam matrix multiplied by a space-frequency beam domain vector, and express the precoder design as an optimization problem with the space-frequency beam domain vector as the optimization variable and the maximization of the designed system performance as the optimization goal, and including the total power constraint and the channel smoothness constraint. By solving this optimization problem, the expression of the space-frequency beam domain vector of each user can be obtained, and then the beam domain precoder on each subcarrier of each user can be obtained by using the frequency beam matrix. For the system model exemplified above, consider the downlink transmission on the nth OFDM symbol. For the sake of simplicity, the index n in the subscript is omitted below. According to (1), the received signal on the kth subcarrier at user u can be rewritten as

[0060]

[0061] in is the spatial domain precoder of user u on the kth subcarrier, x u,k is the data symbol with zero mean and unit variance of user u on the kth subcarrier. Then the rate of user u on the kth subcarrier is

[0062]

[0063] At this time, the traditional subcarrier-by-subcarrier precoder design is described as an example of the weighted sum rate maximization problem under the total power constraint, which can be expressed as

[0064]

[0065] where w u is the priority weighting factor of user u, P k represents the power budget on the kth subcarrier.

[0066] By stacking the received signals on a group of subcarriers of user u, we can get

[0067]

[0068] in

[0069]

[0070] in(·) T represents the transpose, and

[0071]

[0072] is the equivalent channel between the signal sent to user u and the received signal of user u (i.e., the concatenation of the precoder and the air interface channel). It should be noted that the traditional precoder design in (12) is performed separately on each subcarrier, which may make the frequency domain equivalent channel This means that the channel correlation between adjacent subcarriers is weakened, the channel smoothness deteriorates, and the channel varies more rapidly along the subcarrier. This makes equivalent channel estimation difficult and is not conducive to receiver design for user u. To facilitate equivalent channel estimation for each user and ultimately improve system performance, it is necessary to jointly design the precoders on a group of subcarriers.

[0073] Similar to the frequency beam matrix U, define Then the frequency domain equivalent channel It can be expressed as

[0074]

[0075] in It is called the delay domain equivalent channel. The equivalent channel smoothness actually depends on the delay domain equivalent channel Specifically, the greater the delay spread of the time domain equivalent channel, the lower the correlation between the subcarriers of the frequency domain equivalent channel, and the faster the change of the equivalent channel along the subcarrier, which reduces the channel smoothness and is not conducive to the downlink equivalent channel estimation of each user. Therefore, it is necessary to limit To maintain the smoothness of the equivalent channel and facilitate the estimation of the downlink equivalent channel for each user. e express Limited delay spread, i.e. Only the first N e elements can be non-zero, and the rest are zero, that is,

[0076]

[0077] in Represents a vector Nth e arrive The element (16) is called the channel smoothness constraint. Then the precoding design on a set of subcarriers is formulated as the following problem

[0078]

[0079] Where P = Σk∈K P k is the total power budget for a group of subcarriers.

[0080] Let a set of subcarrier stacked precoders p u Has the following structure

[0081]

[0082] in Right now Before List, is the number of frequency beams used, and Then the delay domain equivalent channel can be expressed as

[0083]

[0084] in and

[0085]

[0086] yes A truncated version of And for the matrix represents a block circulant matrix. In this case, The delay spread can be limited to This depends on the number of frequency beams used when When , the structure in (18) can naturally satisfy the channel smoothness constraint in (16). When , the channel smoothness constraint in (16) can be transformed into

[0087]

[0088] in is a selection matrix. Therefore, we can flexibly select To adjust And bigger It provides more optimization freedom, but also has higher complexity.

[0089] According to (18), the spatial domain precoding of user u on the kth subcarrier is

[0090]

[0091] in Right now The k-k0th row of . Then the rate in (11) becomes

[0092]

[0093] And the problem in (17) is transformed into

[0094]

[0095] In skywave massive MIMO communications, due to the small angular spread and limited number of propagation paths, the spatial beam domain channel usually exhibits sparsity, which means that most elements of the spatial beam domain channel are approximately zero. This channel sparsity can be exploited to reduce the complexity of precoder design. The ordered index set of non-zero elements of the spatial beam domain channel of user u is defined as and The order in the index set is the order of increasing index. When , the optimal solution of the problem in (24) can be expressed as

[0096]

[0097] in is the beam selection matrix, i.e.

[0098]

[0099] and

[0100]

[0101] in

[0102]

[0103]

[0104] and

[0105] As the number of users increases, the condition that the spatial beam domain channels of any two users do not overlap may not be well met. In this case, it is necessary to consider using more spatial beams to improve the sum rate performance of the precoder, that is, the spatial beam set of each user, in addition to selecting the set of spatial beams corresponding to the non-zero elements of the spatial beam domain channel of each user, You can further add more spatial beams to the set, that is, a set of spatial beams When M→∞, the given spatial beam index set At this time, the user Add a new spatial beam, i.e. and Available

[0106]

[0107] The conditions for the equality sign to hold are

[0108]

[0109] in (31) indicates that when adding a new spatial beam to the spatial beam set, the selected spatial beam should belong to a user that has a non-zero spatial beam selected to the existing spatial beam set. The above provides a criterion for selecting additional spatial beams in the precoder design. A spatial beam that satisfies condition (31) should not be selected because it will not improve the performance of the precoder, while a spatial beam that satisfies condition (31) should not be selected because it will not improve the performance of the precoder. In other words, additional spatial beams are preferably selected from users with overlapping spatial beam domain channels, as this can lead to inter-user interference, which should be considered when designing the precoder.

[0110] Although M is finite in practice, the above discussion shows that it is possible to design a precoder for a group of subcarriers in the spatial beam domain instead of designing it in the spatial domain. Due to the significant sparsity of the spatial beam domain channel in skywave massive MIMO communication, designing the precoder in the spatial beam domain can significantly reduce the complexity. Define a new set of spatial beam indices in Inspired by (18) and (25), the vector of the spatial domain precoders on a set of subcarriers is constrained to have a beam structure of a space-frequency beam matrix multiplied by a space-frequency beam domain vector

[0111]

[0112] in The spatial beam index set is The beam selection matrix is

[0113] (33) Then the rate in (11) becomes

[0114]

[0115] in So we can optimize q instead u To maximize the sum rate, that is

[0116]

[0117] in After the above process, the original precoder design problem is expressed as an optimization problem with the space-frequency beam domain vector as the optimization variable, the rate maximization as the optimization goal, and the total power constraint and channel smoothness constraint. By solving this optimization problem, the expression of the space-frequency beam domain vector of each user can be obtained, and then the beam domain precoder on each subcarrier of each user can be obtained by using the frequency beam matrix. By solving the problem (35), we can get q u Then, according to (32), the spatial domain precoder on each subcarrier is given by

[0118]

[0119] in It is called the spatial beam domain precoder of the kth subcarrier. Due to the sparseness of the spatial beam domain channel in skywave massive MIMO communication, The dimension is usually larger than p u,k The dimension of is much smaller. Since (35) only involves the beam domain channel and the spatial domain precoder p u,k With the beam structure shown in (36), the spatial beam matrix V u and spatial beam-domain precoder Multiplication takes into account the channel smoothness constraint, so this precoder design is called a channel smoothing beam structure precoder.

[0120] According to the channel smoothness constraint (i.e., the second constraint) in (35), q u Should be The null space of , whose dimension is In this case q u can be rewritten as

[0121]

[0122] in is The null space matrix of the basis of the null space can be obtained by singular value decomposition, and By combining (32) and (37), p u It can be further rewritten as

[0123]

[0124] This naturally satisfies the channel smoothness constraint in (35). Then problem (35) can be further transformed into the following problem

[0125]

[0126] in

[0127]

[0128] Next, we solve the problem in (39). First, consider the following unconstrained problem

[0129]

[0130] And the optimal solution of (41) The optimal solution with (39) The relationship between

[0131]

[0132] Therefore, we can solve the unconstrained optimization problem in (41) and then perform the scaling operation. In addition, the problem in (41) can be equivalently transformed into the following weighted minimum mean square error problem

[0133]

[0134] in

[0135]

[0136] Then, the block coordinate descent method is applied to solve the problem in (43). Specifically, in each iteration, a variable block is optimized while other variables are fixed. The subproblem of each variable block can be solved by the first-order optimal condition while ensuring convergence. Finally, the channel smoothing beam structure precoding design is summarized as follows:

[0137] Step 1: Input Iteration number T

[0138] Step 2: Initialize to meet of

[0139] Step 3: Updated u,k :

[0140]

[0141] Step 4: Update α u,k :

[0142]

[0143] Step 5: renew

[0144]

[0145] Step 6: t=t+1, then determine whether t is greater than T. If so, continue; if not, return to step 3.

[0146] Step 8: Scaling To meet the total power constraint. Then the spatial domain precoder of each subcarrier is

[0147]

[0148] in(·) * After the conjugate precoder is designed, it needs to be applied to each user data and generate a transmission signal. According to (36), the transmission signal on each subcarrier is

[0149]

[0150] And V can be regarded as the chirp-z transformation matrix and rewritten as

[0151]

[0152] in F N×G Indicated by F N The matrix consists of the first G (G≤N) columns of N. (S) is greater than or equal to M+N an An integer of -1. So the matrix V and the vector The multiplication of consists only of the product of a diagonal matrix or a discrete Fourier transform matrix and a vector, which can be implemented efficiently.

[0153] 4. Equivalent Channel Estimation

[0154] From (13), we can see that in order to perform signal detection for each user in downlink transmission, it is necessary to obtain the equivalent channel (i.e., the concatenation of the channel and the precoder), not just the channel. Therefore, in order to perform equivalent channel estimation, the pilot should also be precoded using the same precoder as the data in downlink transmission. When estimating the equivalent channel at the receiving end, the delay domain equivalent channel is first estimated, and then the frequency domain equivalent channel estimation result is obtained using the frequency beam matrix.

[0155] Consider the grooming pilot and make Denote the index set of subcarriers used to transmit pilot signals. is the transmitted pilot signal of user u, definition According to (13), the pilot signal received by user u can be expressed as

[0156]

[0157] and The phase-shifted pilot is selected as the pilot sequence, and its expression is

[0158]

[0159] in represents the Hadamard product, is the phase shift factor, and x c is a sequence of elements with unit modulus shared by all users.

[0160] First, we discuss the equivalent channel estimation when the precoder is designed separately on each subcarrier as shown in (12). A famous example is the weighted minimum mean square error precoder. In this case, the channel smoothness constraint is not considered, and the frequency domain equivalent channel estimate of user u can be obtained by least square estimation, that is,

[0161]

[0162] The equivalent channel on the data subcarrier can be obtained by interpolating the channel estimation results on the pilot subcarriers. Since the delay spread of the equivalent channel is not constrained, the equivalent channel is not smooth and varies rapidly along the subcarrier, which means that the equivalent channel on the data subcarrier may not be accurate.

[0163] Next, we discuss the equivalent channel estimation when the transmitter uses a channel smoothing beam structure precoder. Note that in this case, the channel smoothness constraint is taken into account and the equivalent channel delay spread of user u is limited to N e ,Right now It can be expressed as

[0164]

[0165] Using the pilot in (48) and (50), the received signal in (47) is rewritten as

[0166]

[0167] where X c =diag{x c}, In this case, the least squares estimate can be used in (51) to obtain The estimate of

[0168]

[0169] but The estimate is

[0170]

[0171] And including pilot and data subcarriers The estimate is

[0172]

[0173] V. Implementation Effect

[0174] In order to enable those skilled in the art to better understand the solution of the present invention, performance results of the channel smoothed beam structure precoding method in this embodiment under specific configurations are given below.

[0175] Considering the skywave massive MIMO-OFDM communication system, the system parameters are configured as follows: number of antennas M = 256, carrier frequency f c =16MHz, subcarrier spacing Δ f =250Hz, number of subcarriers N c =512, number of effective subcarriers N v =384, cyclic prefix length N g =128, base station antenna spacing d = 9m, refinement factor F an =F de = 2. The noise power of all users is set to be the same, and the priority weighting factor of all users is is set to 1. The power budget on each subcarrier is also set to be the same, that is, For the convenience of description, the channel smoothed beam structure precoding method is abbreviated as BSP, and the weighted minimum mean square error precoding method used for comparison is abbreviated as WMMSE.

[0176] Figure 3 Different equivalent channel delay spread constraints (i.e., N e ) under BSP and rate performance, where N de =96. As can be seen from the figure, as N e With the increase of , the performance gap can be narrowed, and the sum rate performance of BSP can be further approached to WMMSE.

[0177] Figure 4 The normalized mean square error performance of BSP under different equivalent channel delay spread constraints in the embodiment is given, where N de =96, N comb =2. As can be seen from the figure, the equivalent channel estimation performance when using BSP is much better than that of WMMSE using subcarrier-by-subcarrier design, especially when the transmit power is high. This is because the lower N e This means that the equivalent channel is smoother and changes more slowly along the subcarrier. Correspondingly, the number of non-zero elements of the delay domain equivalent channel is smaller, which is beneficial to the equivalent channel estimation.

[0178] Figure 5 The bit error rate performance of BSP and WMMSE under different comb pilot configurations when using the estimated equivalent channel in the embodiment is given, where N de =96, N e =96. The modulation scheme is Quadrature Phase Shift Keying (QPSK), and a zero-forcing detector is used. The average bit error rate refers to the average bit error rate of all users. As can be seen from the figure, when using an estimated equivalent channel, the bit error rate performance of the proposed BSP is significantly superior to that of WMMSE due to the improved equivalent channel estimation performance. When using perfect channel information, the bit error rate performance of BSP can still approach that of WMMSE. In addition, the WMMSE performance when using estimated channel information for signal detection is significantly different from that when using perfect channel information, while the performance of BSP using estimated channel information for signal detection is close to that when using perfect channel information.

[0179] An embodiment of the present invention further discloses a skywave massive MIMO-OFDM channel smooth beam structure precoding transmission system, comprising a base station and a group of user terminals, wherein the base station implements the skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method.

[0180] An embodiment of the present invention further discloses a computer program product, including a computer program / instruction, which, when executed by a processor, implements the steps of the skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method.

[0181] Anything not described in detail in the present invention is well known to those skilled in the art.

[0182] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method, characterized in that: include: The base station uses a beam-based channel model to design a beam-structured precoder for each user, based on a set of subcarriers and constraining the smoothness of the equivalent channel at each user's receiving end. The beam-structured precoder means that the spatial domain precoder for each user on each subcarrier has a beam structure, that is, the product of the spatial beam matrix and the spatial beam domain precoder. The base station uses a channel smoothed beam structure precoder to precode the signal sent to each user for downlink transmission. This includes: the base station uses a beam domain precoder on each subcarrier to precode the data signal and pilot signal sent to each user, then maps each user's beam domain precoded signal to a complete beam domain transmit signal based on each user's spatial beam set, and finally transforms the sum of each user's beam domain transmit signals into a spatial domain transmit signal. The channel smoothing beam structure precoder design process is as follows: first, the vector formed by stacking the spatial domain precoders on a group of subcarriers for each user has a structure of a space-frequency beam matrix multiplied by a space-frequency beam domain vector. The precoder design is then formulated as an optimization problem with the space-frequency beam domain vector as the optimization variable and system performance maximization as the optimization goal, including total power constraints and channel smoothness constraints. By solving this optimization problem, we can obtain the expression of each user's space-frequency beam domain vector, and then use the frequency beam matrix to obtain the beam domain precoder on each subcarrier of each user. The channel smoothness constraint is to limit the number of non-zero elements of the equivalent channel of each user receiving end in the delay domain; according to the channel smoothness constraint of each user, a corresponding null space matrix is ​​constructed, and the optimization variable, that is, the space-frequency beam domain vector is made to have a structure of a null space matrix multiplied by a null space vector. At this time, the optimization problem is converted into an optimization problem with the null space vector as the optimization variable, maximizing system performance as the optimization goal, and including a total power constraint; by solving this optimization problem, the expression of the null space vector of each user is obtained, and then the expression of the space-frequency beam domain vector of each user is obtained using the null space matrix.

2. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 1, characterized in that: The receiving end equivalent channel is a cascade of a precoder and an air interface channel.

3. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 1, characterized in that: The receiving end equivalent channel smoothness is the speed at which the equivalent channel changes along the subcarrier; the better the channel smoothness, the slower the equivalent channel changes along the subcarrier; the worse the channel smoothness, the faster the equivalent channel changes along the subcarrier.

4. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 1, characterized in that: The spatial beam set of each user is a set of spatial beams corresponding to non-zero elements of the spatial beam domain channel in the spatial beam-based channel model of each user, or a selected set including the spatial beam set; when adding a new spatial beam to the spatial beam set, the selected spatial beam belongs to a user for whom a non-zero spatial beam has been selected into the existing spatial beam set.

5. A Skywave massive MIMO-OFDM channel smooth beam structure precoding transmission system includes a base station and a group of user terminals, characterized in that: The base station implements the skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to any one of claims 1-4.

6. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instructions are executed by a processor, the steps of the skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to any one of claims 1 to 4 are implemented.

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