CN114785643B - OFDM system channel estimation method based on deep learning - Google Patents

OFDM system channel estimation method based on deep learning Download PDF

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CN114785643B
CN114785643B CN202210409459.1A CN202210409459A CN114785643B CN 114785643 B CN114785643 B CN 114785643B CN 202210409459 A CN202210409459 A CN 202210409459A CN 114785643 B CN114785643 B CN 114785643B
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袁晓军
黄周洋
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University of Electronic Science and Technology of China
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Abstract

The invention belongs to the technical field of information and communication, and relates to an OFDM system channel estimation method based on deep learning. In order to further improve the performance of OFDM system channel estimation, the invention provides an OFDM system channel estimation method based on deep learning, which comprises the following steps: firstly, estimating channel state information of a pilot frequency position from a received signal by using a linear estimation method; secondly, obtaining a channel state information estimated value of the whole OFDM time-frequency resource block by using an interpolation algorithm; finally, the multi-scale parallel expansion convolutional neural network provided by the invention is used for further reducing the noise of the estimated channel state information. The performance test is carried out on the method under the complex TDL mixed scene, and the result shows that the method of the invention exceeds the existing method.

Description

OFDM system channel estimation method based on deep learning
Technical Field
The invention belongs to the technical field of information and communication, and relates to an OFDM system channel estimation method based on deep learning.
Background
Accurate channel estimation and effective channel information feedback are basic conditions for guaranteeing the performance of a wireless communication system. In recent years, deep Learning (DL) or AI has been widely studied in wireless communication systems, both in academia and industry. OFDM is one of the core technologies of the 4G and 5G standards, which can well support multiple access and has stable performance in a frequency selective fading environment. In a wireless communication system, a transmission signal is subject to various fading and multipath propagation. In order to demodulate the transmitted signal, a pilot signal is designed to estimate the channel information. The pilot signal is transmitted with the data signal, precoded using the same data signal, and subjected to similar channel fading. Since the position and sequence of the pilot signals are known to the receiver, the receiver can estimate the channel using the received signals.
Document 1"M.Soltani,V.Pourahmadi,A.Mirzaei and H.Sheikhzadeh," Deep Learning-Based Channel Estimation, "in IEEE Communications Letters, vol.23, no.4, pp.652-655,April 2019,doi:10.1109/lcomm.2019.2898944" considers the time-frequency response of a fast fading communication channel as a two-dimensional image, considers the pilot values as a low resolution image, and uses a super resolution network in cascade with a denoising network to estimate the channel, and names the proposed network as a ChannelNet.
Document 2"l.li, h.chen, h. -h.chang and l.liu," Deep Residual Learning Meets OFDM Channel Estimation, "in IEEE Wireless Communications Letters, vol.9, no.5, pp.615-618,May 2020,doi:10.1109/lwc.2019.2962796," designed a deep neural network for channel estimation based on residual learning, named reenet.
Disclosure of Invention
In order to further improve the performance of OFDM system channel estimation, the invention provides an OFDM system channel estimation method based on deep learning.
The technical scheme adopted by the invention comprises the following steps:
s1, assume that the system has N t Each OFDM symbol, N f Sub-carriers, M Tx Multiple transmit antennas and M Rx A plurality of receiving antennas; assuming that the Cyclic Prefix (CP) length of the OFDM system is greater than the maximum channel delay, removing the cyclic prefix and performing discrete fourier transform (discrete Fourier transform, DFT), mth Rx The signal on the ith subcarrier of the kth OFDM symbol received by the receiving antenna can be expressed as
Figure BDA0003603554710000021
wherein ,
Figure BDA0003603554710000022
and />
Figure BDA0003603554710000023
Respectively corresponding frequency domain channel and mth Tx A precoding factor of the root antenna; s is(s) k,i For transmitting symbols +.>
Figure BDA0003603554710000024
Is additive white gaussian noise; since both pilot and data signals are transmitted with the same transmit precoder, the above equation can be rewritten as +.>
Figure BDA0003603554710000025
wherein ,
Figure BDA0003603554710000026
is a pre-coded channel. For all OFDM symbols and subcarriers, there are
Figure BDA0003603554710000027
wherein ,
Figure BDA0003603554710000028
s k,i and />
Figure BDA0003603554710000029
Are respectively->
Figure BDA00036035547100000210
S and />
Figure BDA00036035547100000211
(k, i) -th elements of (a); />
Figure BDA00036035547100000212
Representing the product of the corresponding position elements of the matrix;
generally, at total N t In the OFDM symbols, the pilot occupies N p,t <N t And, at total N f Among the number of carriers, the pilot occupies N p,f <N f A plurality of; for the mth Rx A root receiving antenna for receiving pilot signals as
Figure BDA00036035547100000213
At the receiving end, the least square method LS is used for obtaining the known pilot frequency S p From observation of
Figure BDA00036035547100000214
Channel state information +.>
Figure BDA00036035547100000215
I.e.
Figure BDA00036035547100000216
Wherein/represents the division of the matrix's corresponding element positions;
s2, obtaining the channel estimation value of the pilot frequency position of the OFDM time-frequency resource block
Figure BDA00036035547100000217
Then, obtaining the channel state information of the whole OFDM time-frequency resource block by interpolation algorithm>
Figure BDA0003603554710000031
For the kth position of OFDM time-frequency resource block pilot frequency position p1 and kp2 OFDM symbols, using DFT interpolation, i.e
Figure BDA0003603554710000032
Figure BDA0003603554710000033
wherein ,
Figure BDA0003603554710000034
and />
Figure BDA0003603554710000035
Respectively represent the mth Rx On the root receiving antenna, kth p1 All pilot carriers (i) p ) A vector formed by a frequency domain channel and a corresponding delay domain impulse response vector; the same method can obtain +.>
Figure BDA0003603554710000036
and />
Figure BDA0003603554710000037
For each row of OFDM time-frequency resource block, linear interpolation method is adopted, namely
Figure BDA0003603554710000038
So far, the channel estimation value of the whole OFDM time-frequency resource block can be obtained
Figure BDA0003603554710000039
S3, judging whether the multi-scale parallel expansion convolutional neural network (multi-scale parallel dilated convolutional neural network, MPDCNN) has completed training and storing; if the MPDCNN is trained and saved, executing a step S5, otherwise executing a step S4;
s4, performing off-line training on the MPDCNN to reduce noise of the channel estimation value of the whole OFDM time-frequency resource block; the specific structure of the proposed MPDCNN is shown in the accompanying figure 2: the input data firstly passes through a layer of two-dimensional convolution in the MPDCNN, then sequentially passes through a plurality of multi-scale parallel expansion convolution blocks (multi-scale parallel dilated convolutional block, MPDCB) and a layer of two-dimensional convolution, and finally, the output after the two-dimensional convolution is added with the data of the input MPDCNN to be used as the output of the MPDCNN; the input data firstly sequentially passes through a layer of two-dimensional convolution and three parallel expansion convolution blocks (ilated convolutional block, DCB) with different expansion rates in the MPDCB; then, the outputs of the 3 DCBs are spliced according to the dimension of the tensor channel and then sent to a channel attention module (channel attention, CA), and the outputs after CA are subjected to one-layer two-dimensional convolution; finally, adding the output after two-dimensional convolution with the data input to the MPDCB to be used as the output of the MPDCB; the input data can pass through a plurality of residual blocks (ResBlock) in the DCB, and finally, the output of the last ResBlock is added with the input of the DCB to be used as the output of the DCB; the input data sequentially passes through a two-dimensional convolution layer, a parameterized rectifying linear unit (parametric rectifed linear unit, PRelu) and a two-dimensional convolution layer in the ResBlock; finally, adding the output of the second two-dimensional convolution layer and the input of ResBlock to be used as the output of ResBlock; the input data sequentially pass through two-dimensional convolution, PRelu, two-dimensional convolution, global average pooling, 1X1 convolution, PRelu, 1X1 convolution and Sigmoid function in the CA module; finally, multiplying the output of the Sigmoid function and the input of the CA module in the channel dimension according to broadcast multiplication to be used as the output of the CA module;
define the MPDCNN noise reducer used as
Figure BDA0003603554710000041
Where θ is a network parameter. Handle->
Figure BDA0003603554710000042
After being sent into the noise reducer, the noise-reduced output can be obtained:
Figure BDA0003603554710000043
the loss function used in the offline training phase is
Figure BDA0003603554710000044
S5, loading the saved MPDCNN to reduce the noise of the channel estimation value of the whole OFDM time-frequency resource block, namely
Figure BDA0003603554710000045
The invention has the beneficial effects that: the method has the advantages that the structural characteristics of the OFDM system channels are utilized by using the deep neural network to reduce noise of the time-frequency domain resource block channels with larger noise, and the method has outstanding performance on the problem of estimating the OFDM system channels.
Drawings
Fig. 1 is a diagram illustrating pilot configuration in a 5G OFDM system used;
FIG. 2 is a flow chart of the proposed channel estimation method;
FIG. 3 is a block diagram of the proposed denoising network;
fig. 4 is a simulation curve of an OFDM system using different channel estimation methods.
Detailed Description
The following describes in further detail the embodiments of the present invention with reference to the drawings and examples.
A specific implementation method of the present invention based on the above method is given below, and parameters of the specific method are set as follows:
fig. 1 is a typical pilot resource allocation in a current 5G system. In the frequency domain, there are 12 subcarriers and 14 OFDM symbols in one Resource Block (RB). The pilot occupies 6 subcarriers and 2 OFDM symbols in one RB and one slot. In the simulation, 8 RBs shown in FIG. 1 are used in the frequency domain, and 14 OFDM symbols are used in the time domain, i.e., N t =14,N f =96,N p,t =2,N p,f =48. Number of transmitting antennas M Tx Number of receiving antennas m=32 Rx =2. The modulation scheme is QPSK.
The link-level simulator follows a 3GPP Tapped Delay Line (TDL) model, which has been calibrated. The carrier frequency is 3.5GHz, and the subcarrier space is 15KHz. A new hybrid training data construction method is presented to solve the generalization problem in DL. For training data, the channel model is a mixed model of TDL-A, TDL-B, TDL-C, TDL-D and TDL-E, wherein one sample randomly selects one channel model from the 5 models; the delay spread is randomly selected from 0 seconds to 300 ns; randomly generating the speed between 0km/h and 50 km/h; the signal-to-noise ratio of each sample is randomly chosen from 0dB to 20 dB.
In training the MPDCNN network, the training set and the test set contained 189000 samples (9000 samples per signal-to-noise ratio) and 21000 samples (1000 samples per signal-to-noise ratio), respectively. In MPDCNN, 4 different MPDCBs were used, each containing 4 different resblocks. Note that the first layer convolution of MPDCNN is 48 different 3×3×2 filters, the last layer convolution of MPDCNN is two different 3×3×48 filters, and the 1×1 convolution in CA is 48 different 1×1×48 filters. All other convolution layers of MPDCNN are 48 different 3×3×48 filters. An Adam optimizer was used and the learning rate was set to 1e -4
According to the above parameter settings, the specific steps of the simulation are as follows:
s1, assume that the system has N t Each OFDM symbol, N f Sub-carriers, M Tx Multiple transmit antennas and M Rx A plurality of receiving antennas; assuming that the Cyclic Prefix (CP) length of the OFDM system is greater than the maximum channel delay, removing the cyclic prefix and performing discrete fourier transform (discrete Fourier transform, DFT), mth Rx The signal on the ith subcarrier of the kth OFDM symbol received by the receiving antenna can be expressed as
Figure BDA0003603554710000051
wherein ,
Figure BDA0003603554710000052
and />
Figure BDA0003603554710000053
Respectively corresponding frequency domain channel and mth Tx A precoding factor of the root antenna; s is(s) k,i For transmitting symbols +.>
Figure BDA0003603554710000054
Is additive white gaussian noise; since both pilot and data signals are transmitted with the same transmit precoder, the above equation can be rewritten as
Figure BDA0003603554710000061
wherein ,
Figure BDA0003603554710000062
is a pre-coded channel. For all OFDM symbols and subcarriers, there are
Figure BDA0003603554710000063
wherein ,
Figure BDA0003603554710000064
s k,i and />
Figure BDA0003603554710000065
Are respectively->
Figure BDA0003603554710000066
S and />
Figure BDA0003603554710000067
(k, i) -th elements of (a); />
Figure BDA0003603554710000068
Representing the product of the corresponding position elements of the matrix;
generally, at total N t In the OFDM symbols, the pilot occupies N p,t <N t And, at total N f Among the number of carriers, the pilot occupies N p,f <N f A plurality of; for the mth Rx A root receiving antenna for receiving pilot signals as
Figure BDA0003603554710000069
At the receiving end, the least square method LS is used for obtaining the known pilot frequency S P From observation of
Figure BDA00036035547100000610
Channel state information +.>
Figure BDA00036035547100000611
I.e. < ->
Figure BDA00036035547100000612
Wherein/represents the division of the matrix's corresponding element positions;
s2, obtaining the channel estimation value of the pilot frequency position of the OFDM time-frequency resource block
Figure BDA00036035547100000613
Then, obtaining the channel state information of the whole OFDM time-frequency resource block by interpolation algorithm>
Figure BDA00036035547100000614
For the kth position of OFDM time-frequency resource block pilot frequency position p1 and kp2 OFDM symbols, using DFT interpolation, i.e
Figure BDA00036035547100000615
Figure BDA00036035547100000616
wherein ,
Figure BDA0003603554710000071
and />
Figure BDA0003603554710000072
Respectively represent the mth Rx On the root receiving antenna, kth p1 All pilot carriers (i) p ) A vector formed by a frequency domain channel and a corresponding delay domain impulse response vector; the same method can obtain +.>
Figure BDA0003603554710000073
and />
Figure BDA0003603554710000074
For each row of OFDM time-frequency resource block, linear interpolation method is adopted, namely
Figure BDA0003603554710000075
So far, the channel estimation value of the whole OFDM time-frequency resource block can be obtained
Figure BDA0003603554710000076
S3, judging whether the multi-scale parallel expansion convolutional neural network (multi-scale parallel dilated convolutional neural network, MPDCNN) has completed training and storing; if the MPDCNN is trained and saved, executing a step S5, otherwise executing a step S4;
s4, performing off-line training on the MPDCNN to reduce noise of the channel estimation value of the whole OFDM time-frequency resource block; the specific structure of the proposed MPDCNN is shown in the accompanying figure 2: the input data firstly passes through a layer of two-dimensional convolution in the MPDCNN, then sequentially passes through a plurality of multi-scale parallel expansion convolution blocks (multi-scale parallel dilated convolutional block, MPDCB) and a layer of two-dimensional convolution, and finally, the output after the two-dimensional convolution is added with the data of the input MPDCNN to be used as the output of the MPDCNN; the input data firstly sequentially passes through a layer of two-dimensional convolution and three parallel expansion convolution blocks (ilated convolutional block, DCB) with different expansion rates in the MPDCB; then, the outputs of the 3 DCBs are spliced according to the dimension of the tensor channel and then sent to a channel attention module (channel attention, CA), and the outputs after CA are subjected to one-layer two-dimensional convolution; finally, adding the output after two-dimensional convolution with the data input to the MPDCB to be used as the output of the MPDCB; the input data can pass through a plurality of residual blocks (ResBlock) in the DCB, and finally, the output of the last ResBlock is added with the input of the DCB to be used as the output of the DCB; the input data sequentially passes through a two-dimensional convolution layer, a parameterized rectifying linear unit (parametric rectifed linear unit, PRelu) and a two-dimensional convolution layer in the ResBlock; finally, adding the output of the second two-dimensional convolution layer and the input of ResBlock to be used as the output of ResBlock; the input data sequentially pass through two-dimensional convolution, PRelu, two-dimensional convolution, global average pooling, 1X1 convolution, PRelu, 1X1 convolution and Sigmoid function in the CA module; finally, multiplying the output of the Sigmoid function and the input of the CA module in the channel dimension according to broadcast multiplication to be used as the output of the CA module;
define the MPDCNN noise reducer used as
Figure BDA0003603554710000077
Where θ is a network parameter. Handle->
Figure BDA0003603554710000078
After being sent into the noise reducer, the noise-reduced output can be obtained:
Figure BDA0003603554710000081
the loss function used in the offline training phase is
Figure BDA0003603554710000082
S5, loading the saved MPDCNN to reduce the noise of the channel estimation value of the whole OFDM time-frequency resource block, namely
Figure BDA0003603554710000083
Fig. 4 is a simulation curve of different channel estimation methods. Wherein LS corresponds to the channel estimation result obtained after executing the first two steps of the method of the invention; reenet corresponds to channel estimation and results obtained using the method of literature 2; the channel net corresponds to the channel estimation result obtained by using the method in document 1; MPDCNN corresponds to the performance results of the proposed method. From the figure, the performance of the method provided by the invention is far superior to that of the other three methods, and the effectiveness of the method is proved.

Claims (2)

1. The OFDM system channel estimation method based on deep learning is characterized by comprising the following steps:
s1, defining system N t Each OFDM symbol, N f Sub-carriers, M Tx Multiple transmit antennas and M Rx A plurality of receiving antennas; the Cyclic Prefix (CP) length of the OFDM system is greater than the maximum channel delay, and after the cyclic prefix is removed and Discrete Fourier Transform (DFT) is performed, the mth Rx The kth OFDM symbol received by the receiving antenna, the signal on the ith subcarrier is expressed as:
Figure FDA0004094012090000011
wherein ,
Figure FDA0004094012090000012
and />
Figure FDA0004094012090000013
Respectively corresponding frequency domain channel and mth Tx A precoding factor of the root antenna; s is(s) k,i For transmitting symbols +.>
Figure FDA0004094012090000014
Is additive white gaussian noise; since both pilot and data signals are transmitted with the same transmit precoder, the above equation is rewritten as:
Figure FDA0004094012090000015
wherein ,
Figure FDA0004094012090000016
is a pre-coded channel; for all OFDM symbols and subcarriers, there are:
Figure FDA0004094012090000017
wherein ,
Figure FDA0004094012090000018
s k,i and />
Figure FDA0004094012090000019
Are respectively->
Figure FDA00040940120900000110
S and />
Figure FDA00040940120900000111
(k, i) -th element of (c); />
Figure FDA00040940120900000112
Representing the product of the corresponding position elements of the matrix;
at total N t In the OFDM symbols, the pilot occupies N p,t <N t And, at total N f Among the number of carriers, the pilot occupies N p,f <N f A plurality of; for the mth Rx The root receiving antenna receives the pilot signals:
Figure FDA00040940120900000113
wherein ,
Figure FDA00040940120900000114
S p and />
Figure FDA00040940120900000115
Are respectively->
Figure FDA00040940120900000116
S and />
Figure FDA00040940120900000117
A sub-matrix formed by corresponding pilot frequency position elements;
at the receiving end, the least square method LS is used for obtaining the known pilot frequency S p From observation of
Figure FDA00040940120900000118
Channel state information +.>
Figure FDA00040940120900000119
I.e.
Figure FDA0004094012090000021
Wherein/represents the division of the matrix's corresponding element positions;
s2, obtaining the channel estimation value of the pilot frequency position of the OFDM time-frequency resource block
Figure FDA0004094012090000022
Then, the channel state information of the whole OFDM time-frequency resource block is obtained by interpolation algorithm>
Figure FDA0004094012090000023
For the kth position of OFDM time-frequency resource block pilot frequency position p1 and kp2 The OFDM symbols adopt a DFT interpolation method, namely:
Figure FDA0004094012090000024
Figure FDA0004094012090000025
wherein ,
Figure FDA0004094012090000026
and />
Figure FDA0004094012090000027
Respectively represent the mth Rx On the root receiving antenna, kth p1 All pilot carriers (i) p ) A vector formed by a frequency domain channel and a corresponding delay domain impulse response vector; the same method can obtain +.>
Figure FDA0004094012090000028
and />
Figure FDA0004094012090000029
For each row of the OFDM time-frequency resource block, a linear interpolation method is adopted, namely:
Figure FDA00040940120900000210
so far, the channel estimation value of the whole OFDM time-frequency resource block is obtained
Figure FDA00040940120900000211
S3, judging whether the multi-scale parallel expansion convolutional neural network (MPDCNN) has completed training and storing; if the MPDCNN is trained and saved, executing a step S5, otherwise executing a step S4;
s4, performing off-line training on the MPDCNN to reduce noise of the channel estimation value of the whole OFDM time-frequency resource block; define the MPDCNN noise reducer used as
Figure FDA00040940120900000212
Wherein θ is a network parameter, handle->
Figure FDA00040940120900000213
After being sent into the noise reducer, the noise-reduced output is obtained:
Figure FDA00040940120900000214
the loss function used in the offline training phase is:
Figure FDA00040940120900000215
s5, loading the stored MPDCNN to reduce noise of the channel estimation value of the whole OFDM time-frequency resource block, namely:
Figure FDA0004094012090000031
2. the method for estimating the channel of the OFDM system based on deep learning as claimed in claim 1, wherein in the step S4, the training process of MPDCNN is:
the method comprises the steps that input data firstly pass through a layer of two-dimensional convolution in an MPDCNN, then pass through a plurality of multi-scale parallel expansion convolution blocks (MPDCB) and a layer of two-dimensional convolution in sequence, and finally, the output after the two-dimensional convolution is added with the data of the input MPDCNN to be used as the output of the MPDCNN;
the input data firstly sequentially passes through a layer of two-dimensional convolution and three parallel expansion convolution blocks (DCBs) with different expansion rates in the MPDCB; then, the outputs of the 3 DCBs are spliced according to the dimension of the tensor channel and then sent to a channel attention module (CA), and the outputs after CA are subjected to one-layer two-dimensional convolution; finally, adding the output after two-dimensional convolution with the data input to the MPDCB to be used as the output of the MPDCB;
the input data can pass through a plurality of residual blocks (ResBlock) in the DCB, and finally, the output of the last ResBlock is added with the input of the DCB to be used as the output of the DCB;
the input data sequentially passes through a two-dimensional convolution layer, a parameterized rectifying linear unit (PRelu) and a two-dimensional convolution layer in the Resblock; finally, adding the output of the second two-dimensional convolution layer and the input of ResBlock to be used as the output of ResBlock;
the input data sequentially pass through two-dimensional convolution, PRelu, two-dimensional convolution, global average pooling, 1X1 convolution, PRelu, 1X1 convolution and Sigmoid function in the CA module; and finally, multiplying the output of the Sigmoid function and the input of the CA module in the channel dimension according to broadcast multiplication to obtain the output of the CA module.
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