阅读说明：本技术 一种大规模mimo系统中基站端导频辅助信道估计方法和系统 (Method and system for estimating pilot frequency auxiliary channel of base station end in large-scale MIMO system ) 是由 彭薇 徐康 谢一梅 江涛 于 2021-07-12 设计创作，主要内容包括：本发明公开了一种大规模MIMO系统中基站端导频辅助信道估计方法和系统,属于大规模MIMO无线通信领域。本发明根据信道数据在频域的强自相关特性,提出了一种大规模MIMO系统中基站端导频辅助信道估计方法,根据一些已知频点的CSI来恢复出所有频点上的CSI。采用的方法是图像处理领域里面的一种利用低分辨率图像重构出高分辨率图像的方法,将CSI看作是图像中的一个像素点,每个时刻上所有频点的CSI为一幅图像,利用其中一些已知频点上的CSI输入卷积残差网络模型来重构出所有频点上的CSI。该方法只需在频域上使用少数的导频信号,通过插值的方法可以恢复出所有频点上的CSI,并且能够保证较高的信道估计精度,有效地提升了系统的频谱利用效率和数据传输速率。(The invention discloses a method and a system for estimating a base station end pilot frequency auxiliary channel in a large-scale MIMO system, belonging to the field of large-scale MIMO wireless communication. The invention provides a method for estimating a pilot auxiliary channel at a base station end in a large-scale MIMO system according to the strong autocorrelation characteristic of channel data in a frequency domain, and the CSI on all frequency points is recovered according to the CSI of some known frequency points. The adopted method is a method for reconstructing a high-resolution image by using a low-resolution image in the field of image processing, the CSI is regarded as a pixel point in the image, the CSI of all frequency points at each moment is an image, and the CSI on all the frequency points is reconstructed by using a CSI input convolution residual error network model on some known frequency points. The method only needs to use a few pilot signals on the frequency domain, can recover the CSI on all the frequency points by an interpolation method, can ensure higher channel estimation precision, and effectively improves the frequency spectrum utilization efficiency and the data transmission rate of the system.)

1. A pilot frequency auxiliary channel estimation method of a base station end in a large-scale MIMO system is characterized in that any antenna A of a mobile end sends OFDM signals to any antenna B of the base station end, the OFDM signals simultaneously comprise OFDM pilot frequency and OFDM data, and after the base station end receives the OFDM signals, the channel state estimation is carried out on a channel between the antennas A and B by adopting the following method:

a preparation stage:

the mobile terminal antenna A sends an OFDM pilot signal s to the base station antenna B at the time t_t(ii) a The base station end receives the pilot signalEstimating the channel state information H of each frequency point of the current channel at the time t_t(ii) a Repeating the transmission n times to obtain a channel state information matrix H ═ H of the current channel₁,H₂,…,H_n]；

For each H_tIs determined from the pilot patternPilot position, reserve H_tCSI at pilot position, at H_tFilling zero at other frequency points except the pilot frequency position to obtainAs a sample, H_tObtaining a training set containing n samples for training a model by taking t as a label of the sample as 1,2, …, n, wherein the pilot pattern is formed by selecting subcarriers with corresponding pilot frequency quantity on frequency points uniformly spaced on an OFDM pilot signal;

constructing a convolution residual error network model;

a training stage: training a convolution residual error network model by adopting a training set, so that the model learns the frequency domain correlation characteristics of the current channel through continuous iterative training;

an application stage: and inputting the received OFDM signal into the trained model, and outputting the channel state information of the current channel.

2. The method of claim 1, wherein the determining is based on received pilot signalsEstimating the channel state information of each frequency point of the current channel at the time t

Wherein the content of the first and second substances,representing pilot signalsOn the ith subcarrier of (1)_t(i) Representing a pilot signal s_tM represents the number of subcarriers.

3. The method of claim 1 or 2, wherein the convolutional residual network model is formed by connecting 9 Conv2D layers in series, wherein the input of the third layer is directly connected with the output of the fifth layer, the input of the sixth layer is directly connected with the output of the eighth layer, and the ReLu activation function is added except for the last layer.

4. A method according to claim 1 or 2, characterized in that the loss function of the training phase is as follows:

where H (t, f) represents the actual at point f at time tAnd the CSI which is estimated at a frequency point F at the time T is represented, omega represents network parameters which need to be optimized through training, T represents the number of the time, and F represents the number of the frequency points.

5. The method of claim 4, wherein parameters in the network model are adjusted and updated to be optimal by combining gradient descent and an Adam optimization algorithm in the training process, and the optimization is aimed at minimizing the loss function.

6. Method according to claim 1, characterized in that the mobile transmits an OFDM signal s_n+1＝[s_d(1)，s_d(2),s_p(3),…,s_p(m-2),s_d(m-1),s_d(m)]^TThe signal received by the base station is,wherein, subscript d indicates that the position sends a data signal, subscript p indicates that the position sends a pilot signal, and m indicates the number of subcarriers, the application stage includes:

determining a pilot frequency position according to the pilot frequency pattern;

performing channel estimation on the pilot frequency position according to the received signal to obtain CSI at the pilot frequency position, and setting the CSI of the other frequency points except the pilot frequency position to be zero to obtain the CSI

Will be provided withAnd inputting the signal into a trained model for channel estimation, wherein the model output is CSI on all frequency points.

7. A system for base station-side pilot-assisted channel estimation in a massive MIMO system, comprising: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is configured to read executable instructions stored in the computer-readable storage medium, and perform the method for estimating the pilot-assisted channel at the base station in the massive MIMO system as claimed in any one of claims 1 to 6.

Technical Field

The invention belongs to the field of large-scale multiple-input multiple-output (MIMO) wireless communication, and particularly relates to a method and a system for estimating a base station side pilot frequency auxiliary channel in a large-scale MIMO system.

Background

Massive MIMO technology has been extensively studied in recent years as a key technology in 5G. In the large-scale MIMO technology, more antennas, usually hundreds to thousands, are placed at a Base Station (BS) end to serve multiple users simultaneously, so that a larger spatial diversity gain is obtained, and a data transmission rate and reliability of a link are further improved. In order to fully exert the advantages of the massive MIMO system, it is essential to acquire accurate Channel State Information (CSI), and generally, CSI is estimated by a pilot estimation method. However, in the massive MIMO system, due to the increase of the number of antennas and simultaneous users, the consumption of pilot resources for channel estimation is increased, which reduces the pilot resources for data transmission, thereby resulting in a decrease in the performance of the massive MIMO system. Therefore, a channel estimation method with high estimation accuracy and less pilot resource consumption is urgently needed.

Conventional channel estimation methods can be divided into blind channel estimation, semi-blind channel estimation and pilot-assisted channel estimation. Blind channel estimation refers to channel estimation based on the received signal without transmitting a known pilot sequence at the transmitting end. The channel estimation method is also called no-priori knowledge estimation, and utilizes some characteristics inherent in the modulation signal and irrelevant to specific information carrying bits, or adopts a decision feedback method to carry out channel estimation. The blind estimation algorithm has the characteristics of high data transmission rate and high spectrum utilization rate because pilot symbols do not need to be inserted. The blind channel estimation must be able to extract statistical characteristics and estimate the channel when a large amount of data is received, and it also has the disadvantages of high computational complexity and slow algorithm convergence, so the blind channel estimation method is difficult to be applied in practice. The pilot-assisted channel estimation method is applied in practice, and refers to that a pilot symbol and a data symbol are simultaneously transmitted at a transmitting end, because the pilot symbol is known in advance, the CSI at the position of the known pilot symbol can be estimated by some methods, and then the CSI at the position of the data symbol is derived by using the known CSI through a certain method. Although the pilot-assisted channel estimation has the disadvantages of low spectrum utilization efficiency and low data transmission rate, the careful pilot design can improve the channel estimation accuracy and reduce the calculation complexity of the channel estimation, so that the pilot-assisted channel estimation is more applied to practical systems.

In order to solve the problems of serious pilot resource consumption and low estimation precision when the traditional pilot frequency auxiliary channel estimation method is applied to a large-scale MIMO system, the invention provides a pilot frequency auxiliary channel estimation method based on frequency domain correlation.

Disclosure of Invention

Aiming at the defects and improvement requirements of the prior art, the invention provides a method and a system for estimating a pilot frequency auxiliary channel at a base station end in a large-scale MIMO system, and aims to solve the technical problems of serious pilot frequency resource consumption, low channel estimation precision and the like in the prior art.

To achieve the above object, according to a first aspect of the present invention, there is provided a method for estimating a pilot-assisted channel at a base station in a massive MIMO system, where any antenna a at a mobile terminal sends an OFDM (Orthogonal Frequency Division Multiplexing) signal to any antenna B at the base station, the OFDM signal includes both OFDM pilot and OFDM data, and after receiving the OFDM signal, the base station performs channel state estimation on a channel between the antennas a and B by using the following method:

a preparation stage:

the mobile terminal antenna A sends an OFDM pilot signal s to the base station antenna B at the time t_t(ii) a The base station end receives the pilot signalEstimating the channel state information H of each frequency point of the current channel at the time t_t(ii) a Repeating the transmission n times to obtain a channel state information matrix H ═ H of the current channel₁，H₂，…，H_n]；

For each H_tDetermining pilot positions based on the pilot pattern, reserving H_tCSI at pilot position, at H_tFilling zero at other frequency points except the pilot frequency position to obtainAs a sample, H_tObtaining a training set containing n samples for training a model by taking t as a label of the sample as 1,2, …, n, wherein the pilot pattern is formed by selecting subcarriers with corresponding pilot frequency quantity on frequency points uniformly spaced on an OFDM pilot signal;

constructing a convolution residual error network model;

a training stage: training a convolution residual error network model by adopting a training set, so that the model learns the frequency domain correlation characteristics of the current channel through continuous iterative training;

an application stage: and inputting the received OFDM signal into the trained model, and outputting the channel state information of the current channel.

Preferably, the method further comprises receiving a pilot signal based on the received pilot signalEstimating the channel state information of each frequency point of the current channel at the time t

Wherein the content of the first and second substances,representing pilot signalsOn the ith subcarrier of (1)_t(i) Representing a pilot signal s_tM represents the number of subcarriers.

Preferably, the convolution residual error network model is formed by connecting 9 Conv2D layers in series, the input of the third layer is directly connected with the output of the fifth layer, the input of the sixth layer is directly connected with the output of the eighth layer, and ReLu activation functions are added to the rest layers except the last layer.

Has the advantages that: the optimal network structure can extract the characteristics of the local image characteristics to be applied to the channel, automatically learns the frequency domain correlation characteristics of the channel through model training, and carries out channel estimation by utilizing the learned frequency domain correlation of the channel. Two direct connection structures are inserted into the preferable network structure, and the direct connection structure is introduced to solve the problem of network degradation, so that the performance of the model is improved.

Preferably, the loss function of the training phase is as follows:

where H (t, f) represents the actual CSI at frequency point f at time t,and the CSI which is estimated at a frequency point F at the time T is represented, omega represents network parameters which need to be optimized through training, T represents the number of the time, and F represents the number of the frequency points.

Preferably, parameters in the network model are adjusted and updated through gradient descent combined with an Adam optimization algorithm in the training process to achieve the optimal parameters, and the optimization target is to minimize the loss function.

Preferably, the OFDM signal s transmitted by the mobile terminal_n+1＝[s_d(1)，s_d(2)，s_p(3)，…，s_p(m-2)，s_d(m-1)，s_d(m)]^TThe signal received by the base station is,wherein, subscript d indicates that the position sends a data signal, subscript p indicates that the position sends a pilot signal, and m indicates the number of subcarriers, the application stage includes:

determining a pilot frequency position according to the pilot frequency pattern;

according to the receptionPerforming channel estimation on the pilot frequency position by the signal to obtain CSI at the pilot frequency position, and setting the CSI of the other frequency points except the pilot frequency position to be zero to obtain the CSI

Will be provided withAnd inputting the signal into a trained model for channel estimation, wherein the model output is CSI on all frequency points.

To achieve the above object, according to a second aspect of the present invention, there is provided a system for base station-side pilot-assisted channel estimation in a massive MIMO system, comprising: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is configured to read executable instructions stored in the computer-readable storage medium, and execute the method for estimating a pilot-assisted channel at a base station in a massive MIMO system according to the first aspect.

Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:

the invention provides a method for estimating a pilot auxiliary channel at a base station end in a large-scale MIMO system according to the strong autocorrelation characteristic of channel data in a frequency domain, and the CSI on all frequency points is recovered according to the CSI of some known frequency points. The adopted method is a method for reconstructing a high-resolution image by using a low-resolution image in the field of image processing, the CSI is regarded as a pixel point in the image, the CSI of all frequency points at each moment is an image, and the CSI on all the frequency points is reconstructed by using a CSI input convolution residual error network model on some known frequency points. The method can recover the CSI on all the frequency points by using only a few pilot signals on the frequency domain through an interpolation method of uniform arrangement, and can ensure higher channel estimation precision. Compared with the existing channel estimation method based on interpolation, on the premise of achieving the same channel estimation precision, the number of the pilot frequencies used by the method is obviously less than that of the existing method, namely, the spectrum utilization efficiency and the data transmission rate of the system are effectively improved.

Drawings

Fig. 1 is a flowchart of a method for estimating a pilot auxiliary channel at a base station in a massive MIMO system according to the present invention;

fig. 2 is a network structure of a deep convolution residual error network according to an embodiment of the present invention;

fig. 3 is a schematic diagram of a channel estimation process according to an embodiment of the present invention;

fig. 4 is a comparison of Normalized Mean Squared Error (NMSE) between the estimated value and the true value under different pilot numbers according to some prior art pilot-assisted channel estimation methods based on interpolation according to an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in fig. 1, the method for estimating a pilot-assisted channel at a base station in a massive MIMO system provided by the present invention specifically includes the following steps:

(1) obtaining a channel state information matrix

(1-1) the Mobile terminal transmits an OFDM pilot signal s to the base station terminal_t＝[s_t(1)，s_t(2)，…，s_t(m)]^TAnd one OFDM pilot symbol comprises pilot signals on m subcarriers.

(1-2) the signal receiving terminal receives the pilot signalEstimating the channel state information of each subcarrier of the channel at the time t as

H_tContaining frequency domain channel state information on N subcarriers.

(1-3) repeating the two steps for n times to obtain the channel state information matrix

Wherein H_tRepresenting the channel state information of all frequency points at the moment t.

(2) Setting a pilot pattern

(2-1) setting the number of pilots, in the embodiment of the present invention, four different pilot numbers are set for comparing the estimation effects of different pilot numbers, which are respectively: 80. 160, 240, and 320;

and (2-2) selecting subcarriers corresponding to the number of the pilot frequencies to be used as the pilot frequencies according to a certain rule (uniform interval) on the OFDM symbols to obtain pilot frequency patterns.

(3) Constructing training set and verification set according to channel state information matrix and pilot frequency pattern

(3-1) H for each time instant_tThe following treatments were carried out: filling H in pilot frequency position according to the pilot frequency pattern set in the step (2)_tAnd filling zero in other frequency points except the pilot frequency position to obtain the CSI corresponding to the frequency pointsTaking the reference signal as the input of a sample, namely, at the moment t, except the pilot frequency position, CSI of other frequency points is unknown;

(3-2) reacting H_t＝[H_t(1)，H_t(2)，…，H_t(m)]^TForming n samples for training the model with the sample input constructed in the step (3-1) as the labels of the samples;

(3-3) carrying out the following steps on the training samples obtained in the step (3-2) according to the ratio of 5: the scale of 1 is divided into a training set and a validation set, wherein the training set is used for training the model, and the validation set is used for cross validation in the model training process.

(4) The method comprises the steps of constructing a convolution residual error network model, wherein the network structure of the model is shown in figure 2 and is formed by 9 Conv2D layers, ReLu activation functions are added to the rest except the last layer, and two direct connection structures are inserted into the network structure and are introduced to solve the problem of network degradation, so that the performance of the model is improved. The convolution residual error network adopts a two-dimensional convolution neural network, so that H needs to be converted_tConverted into a two-dimensional matrix form. The convolution residual network model utilizes the characteristic that a convolution neural network can extract local features of an image, and automatically extracts frequency domain correlation features of a channel through model training for channel estimation.

(5) And (4) applying the training set and the verification set constructed in the step (3) to the deep learning model constructed in the step (4) for model training, wherein the loss function adopted by the model training is as follows:

where H (t, f) represents the actual CSI at frequency point f at time t,and the estimated CSI at the frequency point f at the time t, and omega are network parameters needing to be optimized through training. In the training process, parameters in the network model are adjusted and updated through gradient descent combined with an Adam optimization algorithm to achieve the optimal state, and the optimization aim is to minimize a loss function.

(6) The trained model is used for channel estimation, and the channel estimation process is shown in fig. 3 and mainly includes two stages: an interpolation stage and an estimation stage, the specific steps are as follows:

(1) interpolation stage

(1-1) the base station side sends OFDM signals, pilot signals are sent at the pilot frequency positions of each OFDM symbol, the rest data symbols needing to be transmitted are sent, the pilot frequency positions are determined by the pilot frequency patterns in the step (2), and the OFDM signals are supposed to be sent

s_n+1＝[s_d(1)，s_d(2)，s_p(3)…，s_p(m-2)，s_d(m-1)s_d(m)]^T

Wherein s is_dTransmitting a data signal, s, representative of the location_pIndicating that the location is transmitting a pilot signal;

(1-2) receiving end receives signal

Because the pilot signal sent by the pilot position is known in advance, the channel estimation can be carried out on the pilot position according to the received signal to obtain the CSI at the pilot position, and the CSI of the other frequency points except the pilot position is set to be zero to obtain the CSI

(2) Estimation phase

Subjecting the product obtained in step (1)And inputting the CSI into a trained model for channel estimation, outputting the model as CSI on all frequency points, and estimating the CSI on all the frequency points through the CSI of partial frequency points in the process.

Fig. 4 is a comparison of normalized mean square error between the estimated value and the true value under different pilot numbers according to the embodiment of the present invention and some existing pilot assisted channel estimation methods based on interpolation, where the comparison methods include a pilot assisted channel estimation method based on linear interpolation, a pilot assisted channel estimation method based on gaussian interpolation, and a pilot assisted channel estimation method based on cubic interpolation. It can be seen from the figure that the NMSE estimated by each method decreases gradually as the number of pilots increases. The increased number of pilots means that the more information is available for channel estimation, and the lower the NMSE of the channel estimation is. The lowest curve is an NMSE change curve of the method provided by the invention, and the NMSE generated by estimation is obviously smaller than that of the other three methods, which can show that the estimation accuracy of the pilot frequency auxiliary channel estimation method based on the convolution residual error network model provided by the invention is obviously better than that of the other methods. Meanwhile, it can be seen from the figure that the estimated NMSE of the method provided by the present invention is lower when the number of pilots is 80 than that of the other three methods when the number of pilots is 320, in other words, the number of pilots used by the method provided by the present invention is significantly less than that of the other three methods under the condition of achieving the same estimation effect. Therefore, the pilot frequency auxiliary channel estimation method provided by the invention can save pilot frequency resources to a great extent, thereby improving the utilization rate of frequency spectrum.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.