Sky wave large-scale MIMO-OFDM channel smooth beam structure precoding transmission method and system

Abstract

The invention discloses a sky wave large-scale MIMO-OFDM (Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing) channel smooth beam structure precoding transmission method and system. A sky wave large-scale MIMO-OFDM base station designs a beam structure precoder of each user in combination with a group of subcarriers on the basis of constraining the equivalent channel smoothness of each user receiving end by using a beam-based channel model, and pre-codes signals sent to each user by using the channel smooth beam structure precoder to realize downlink transmission. The method comprises the following steps of: firstly, pre-coding a data signal and a pilot signal which are sent to each user by a base station on each subcarrier by using a beam domain precoder, and mapping a beam domain pre-coded signal of each user to a complete beam domain sending signal based on a space beam set of each user; and finally, converting the sum of the beam domain sending signals of the users into a spatial domain sending signal. According to the method, the equivalent channel estimation performance can be effectively improved and the signal detection performance can be remarkably improved while the sky wave large-scale MIMO-OFDM downlink precoding transmission performance is ensured.

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 smooth 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 of thousands of kilometers through ionospheric reflection, thereby achieving global network coverage. However, due to many limitations such as limited spectrum resources and complex and changeable ionospheric conditions, traditional point-to-point skywave communication usually has a low data transmission rate, which limits its application. Massive MIMO (Multiple-Input Multiple-Output, MIMO) technology can serve a large number of users simultaneously on the same time-frequency resources by configuring a large number of antennas on the base station side, which can significantly improve spectrum efficiency and power efficiency. The combination of skywave communication and massive MIMO technology, namely skywave massive MIMO communication, can greatly 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 performed separately on each subcarrier. This may make the equivalent channel at the receiving end of each user (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 of 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, and ultimately leads to a decrease in system performance.

[0004] In the Skywave massive MIMO-OFDM system, in order to smooth the equivalent channel and facilitate equivalent channel estimation at the receiving end, the precoders on a group of subcarriers should be jointly designed. How to ensure good sum rate performance while constraining the equivalent channel smoothness at the receiving end is a problem that the Skywave massive MIMO-OFDM system needs to solve. Summary of the invention

[0005] Purpose of the invention: In view of the shortcomings of the prior art, the present invention proposes a skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method and system, which effectively improves the equivalent channel estimation performance and significantly improves the signal detection performance while ensuring the transmission performance.

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

[0007] The skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method comprises:

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

[0009] The base station uses a channel smooth beam structure precoder to precode the signal sent to each user to achieve downlink transmission; including: 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, and finally transforms the sum of each user's beam domain transmission signal 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 equivalent channel smoothness at the receiving end 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 stacked by 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, i.e., 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. The expression of the null space vector of each user is obtained by solving this optimization problem, 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 the 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 whose 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, wherein the base station implements the skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method.

[0018] A computer program product comprises a computer program / instruction, wherein when the computer program / instruction is executed by a processor, the steps of the skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method are implemented.

[0019] Beneficial effect: Compared with the prior art, 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 smooth beam structure precoding transmission system according to an embodiment of the present invention;

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

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

[0023] Figure 4 4 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 bit error rate performance diagram 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 solution provided by the present invention will be described in detail below in conjunction with specific embodiments. It should be understood that the following specific implementation methods 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 smooth 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, and 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 of each user on a group of subcarriers, and introduces channel smoothness constraints to improve the smoothness of the equivalent channel of each user receiving end. The equivalent channel of the receiving end is the cascade of the precoder and the air interface channel. The equivalent channel smoothness of the receiving end 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.

[0028] In this embodiment, 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. In this embodiment, the operations involving the spatial beam matrix or its conjugate transpose multiplication vector can be effectively implemented through chirp-z transformation.

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

[0030] 1. System Configuration

[0031] In this embodiment, a one-wave massive MIMO-OFDM system is considered. The base station is configured with a uniform linear array with M antennas to provide services for U single-antenna users. The user index set is In skywave communication, due to the system's carrier frequency f c The antenna spacing d of the array needs to change with the ionospheric conditions, depending on the highest 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 wireless 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, wherein the spatial beam-based channel model is the spatial beam matrix multiplied by the spatial beam domain channel vector, and 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, wherein 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 space 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 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 spatial-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. The two sets are evenly divided 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 cosines and time delays 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 a frequency beam matrix, and each column of it is called a frequency beam. 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 total power constraints and channel smoothness constraints. 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 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 cascade 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 cause the frequency domain equivalent channel This means that the channel correlation between adjacent subcarriers is weakened, the channel smoothness is deteriorated, and the channel changes faster along the subcarrier. Therefore, this will make equivalent channel estimation difficult and is not conducive to the receiver design of user u. In order to facilitate the equivalent channel estimation of each user and ultimately improve the system performance, it is necessary to jointly design the precoders on a group of subcarriers.

[0073] Similar to the frequency beam matrix U, we 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 larger the delay spread of the delay 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 The delay spread of N is used 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 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, To adjust And a bigger It can provide more optimization freedom, but the complexity will also be higher.

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

[0090]

[0091] in Right now 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 the 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 ascending order of the index. When , the optimal solution of the problem in (24) can be expressed as

[0096]

[0097] in is the beam selection matrix, that is

[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→∞, given a set of spatial beam indices At this time, the user Add a new spatial beam, namely and Available

[0106]

[0107] The condition for the equality to hold is

[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 for which a non-zero spatial beam has been 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, the additional spatial beams are preferably selected from users with overlapping spatial beam domain channels, since this will cause 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. Since the spatial beam domain channel in skywave massive MIMO communication is significantly sparse, 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 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 is the spatial beam index set The beam selection matrix is

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

[0114]

[0115] in So we can optimize q 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 target, 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 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 It has a beam structure as shown in (36), that is, the spatial beam matrix V u With spatial beam-domain precoder Multiplying, while taking into account the channel smoothness constraint, so this kind of precoder design is called a channel smoothing beam structure precoder.

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

[0121]

[0122] in Is The null space matrix of the null space basis of can be obtained by singular value decomposition, and By combining (32) and (37), p u 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] The optimal solution of (41) The optimal solution is (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 alpha 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 for 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 viewed as a 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 the diagonal matrix or the discrete Fourier transform matrix and the vector, which can be implemented efficiently.

[0153] 4. Equivalent Channel Estimation

[0154] From (13), it can be seen that in order to perform signal detection for each user in downlink transmission, it is necessary to obtain the equivalent channel (i.e., the cascade 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 as 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 estimation of user u can be obtained by using the least squares estimation, that is,

[0161]

[0162] The equivalent channel on the data subcarrier can be obtained by interpolating the channel estimation results on the pilot subcarrier. Since the delay spread of the equivalent channel is not constrained, the equivalent channel is not smooth and changes rapidly along the subcarrier, which means that the equivalent channel on the data subcarrier obtained 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 configuration 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, the 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 ), 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 designed by subcarrier, 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 equivalent channel in the delay domain 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 method 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. It can be seen from the figure that when using an estimated equivalent channel, the bit error rate performance of the proposed BSP has a great advantage over WMMSE due to the improvement in the equivalent channel estimation performance. When perfect channel information is used, the bit error rate performance of BSP can still be close to WMMSE. In addition, the performance of WMMSE when using estimated channel information for signal detection is very different from that when using perfect channel information, while the performance of BSP using estimated channel information for signal detection can be close to that when using perfect channel information.

[0179] The embodiment of the present invention also 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] The embodiment of the present invention also 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] The matters not described in detail in the present invention are all known technologies to those skilled in the art.

[0182] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method, characterized in that: include: The base station uses the beam-based channel model to design the beam structure precoder for each user by combining a group of subcarriers on the basis of constraining the smoothness of the equivalent channel at each user's receiving end. The base station uses a channel smooth beam structure precoder to precode the signal sent to each user to achieve downlink transmission; including: 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, and finally transforms the sum of each user's beam domain transmission signal into a spatial domain transmission signal.

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 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.

5. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 1, characterized in that: The channel smooth beam structure precoder design is specifically designed as follows: first, a vector formed by stacking the space domain precoders on a group of subcarriers of each user has a structure of a space-frequency beam matrix multiplied by a space-frequency beam domain vector, and the precoder design is expressed as an optimization problem with the space-frequency beam domain vector as an optimization variable, system performance maximization as an 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.

6. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 5, characterized in that: 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.

7. The skywave massive MIMO-OFDM channel smoothed beam structure precoding transmission method according to claim 5, characterized in that: According to the smoothness constraint of each user channel, the 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, the maximization of system performance as the optimization goal, and a total power constraint. The expression of the null space vector of each user is obtained by solving this optimization problem, and then the expression of the space-frequency beam domain vector of each user is obtained using the null space matrix.

8. 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 whose non-zero spatial beam has been selected into the existing spatial beam set.

9. A Skywave massive MIMO-OFDM channel smooth beam structure precoding transmission system, comprising a base station and a group of user terminals, characterized in that: The base station implements the skywave massive MIMO-OFDM channel smooth beam structure precoding transmission method according to any one of claims 1-8.

10. 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 8 are implemented.

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