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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>GADGN: A dual graph convolutional architecture for trafic flow prediction</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Yanan Song</string-name>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Siwei Wei</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Donghua Liu</string-name>
          <email>liudonghua@wti.ac.cn</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="editor">
          <string-name>PCWrEooUrckResehdoinpgs ISSNc1e6u1r-3w-0s0.o7r3g</string-name>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>CCCC Second Highway Consultants Co. Ltd.</institution>
          ,
          <addr-line>Wuhan, 430056</addr-line>
          ,
          <country country="CN">China</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Information Center, China Waterborne Transport Research Institute</institution>
          ,
          <addr-line>Beijing, 100080</addr-line>
          ,
          <country country="CN">China</country>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>School of Computer Science, Hubei University of Technology</institution>
          ,
          <addr-line>Wuhan, 430070</addr-line>
          ,
          <country country="CN">China</country>
        </aff>
      </contrib-group>
      <fpage>28</fpage>
      <lpage>41</lpage>
      <abstract>
        <p>With the rapid growth of urban trafic, the importance of trafic flow prediction has increased. However, existing models still have significant limitations in capturing complex spatio-temporal dependencies: 1) The data collected by sensors is often incomplete, which can seriously afect the accuracy and reliability of trafic flow prediction. However, most existing models for processing missing values focus only on temporal dependencies, ignoring the complexity of spatio-temporal correlations in trafic data; 2) Relying only on predefined graph structures based on a priori knowledge or adaptive graph structures generated by node embedding often makes it dificult to accurately reflect the complexity and temporal variability of trafic flow data. However, most current models usually adopt only a single graph structure, fail to fully exploit the complementary advantages of multiple graph structures, and thus have certain limitations in capturing spatio-temporal features. To this end, this paper proposes a generative adversarial dual graph network for trafic flow prediction, viz, GADGN. In this paper, we use the GAN-TCT (Generative Adversarial Network-Transformer-CNN Fusion Module) module to achieve efective missing value filling. The generator of GAN-TCT combines a long short-term memory network (LSTM), a convolutional neural network (CNN) and a transformer to efectively extract the time-series features and spatial correlations in the input data, thus enhancing the realism of the generated data. In addition, this paper designs the dual graph convolutional fusion coding mechanism (DSGCE), which simultaneously considers the information contained in the node features and the adjacency matrix, generates an adaptive graph structure using node embeddings, constructs a predefined graph structure based on a priori knowledge through the adjacency matrix, and extracts the features contained in the two graph structures by convolution of the two graphs, and then extracts a more comprehensive graph structure from the two graph structures through the graph features learned by the self-encoder. It then extracts a more comprehensive graph structure from the two graph structures using the graph features learned by the self-encoder, and then extracts a more comprehensive spatio-temporal information from the two graph structures using the graph features learned by the self-encoder, and then combines this information to generate a new feature representation containing more potential patterns and dependencies, and ifnally captures the temporal features through a gating mechanism to obtain the final prediction results. Extensive experiments on real road datasets PeMSD4 and PeMSD8 have shown that the GADGN model reduces the MAE by about 3.5%, the RMSE by about 1.5% and the MAPE by about 1.8% compared to the existing state-of-the-art method (SOTA). These results fully demonstrate the superiority and efectiveness of the GADGN model in trafic lfow prediction tasks.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;generative adversarial networks</kwd>
        <kwd>dual graph convolutional fusion coding mechanism</kwd>
        <kwd>missing value filling</kwd>
        <kwd>gating mechanism</kwd>
        <kwd>trafic flow prediction</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>With the acceleration of urbanisation, the transport system is facing serious problems such as congestion
and uneven public transport services. Trafic flow prediction has become a key issue in urban trafic
management. Most of the traditional trafic flow prediction methods rely on cloud-based data centres
for computation, but with the rapid development of sensor networks and the application of edge
computing, more and more real-time data needs to be processed on edge devices. Edge computing,
as a distributed computing architecture, can efectively reduce latency and improve data processing
eficiency by distributing computing tasks to edge nodes close to the data source. In this context, this
paper proposes a trafic flow prediction model (GADGN) based on generative adversarial network and
dual graph convolutional architecture. The model combines the GAN-TCT module with the dual-graph
convolutional fusion mechanism and is optimised for eficient data gap filling and trafic flow prediction
in an edge computing environment.</p>
      <p>Missing values in the data not only weaken the validity of the data, but also directly afect the
reliability of model training, leading to inaccurate prediction results. Traditional filling methods, such
as mean-filling, linear interpolation or forward filling, although simple and easy to use, often fail to
capture the true distribution of the data. In addition, the relationships between nodes in urban trafic
networks change dynamically, and trafic flows vary significantly depending on time, location and
environment. Traditional models that rely only on static modelling are dificult to adapt to complex
changes in trafic flows. Static neighbourhood matrices reflect the fixed connectivity of nodes in a
transport network, and understanding these relationships is critical to capturing the underlying trafic
patterns. Relying solely on dynamic modelling can result in models that do not fully capture changing
trafic flow patterns. Therefore, the construction of efective trafic flow prediction models must focus
on both static and dynamic spatio-temporal features to better adapt to complex trafic flow changes.</p>
      <p>To solve these problems, this paper proposes a method that combines the GAN-TCT module with the
dual graph convolutional fusion mechanism (GADGN) to improve the accuracy and robustness of trafic
lfow prediction. In the missing value filling session, we design the GAN-TCT module to eficiently
generate pseudo-data that is highly similar to the real trafic flow to fill the missing parts of the data.
The generator extracts time-series features related to missing values using a Long Short-Term Memory
(LSTM) network, a Convolutional Neural Network (CNN) captures local spatial dependencies, and global
context modelling is performed by the Transformer module. The structure not only integrates local and
global features, but also efectively enhances the spatio-temporal consistency of the data through the
feature fusion layer. In addition, this paper designs a dual-image convolutional fusion coding mechanism,
which adopts a dual-image convolutional structure to fully capture the multi-scale features of both static
and dynamic images, thus enhancing the feature extraction capability of the model. Specifically, static
graph convolution uses a predefined graph structure to extract fixed neighbourhood features, while
dynamic graph convolution dynamically generates time-varying neighbourhood matrices through node
embedding, efectively characterising dynamic spatio-temporal dependencies. However, since static and
dynamic graph convolution may only reflect local relationships when fused, and may not adequately
model the deeper features in the data, we consider introducing an adaptive feature reconstruction
module (self-encoder), which is capable of extracting complex spatio-temporal features through a hidden
space representation, and helps to better represent the global and non-linear properties of the data. This
mechanism efectively combines the a priori information of the fixed topology with the flexibility of the
time-varying topology, enabling the model to better capture the long- and short-term dependencies
and multi-scale features in the data when dealing with complex spatio-temporal data (e.g. trafic flow
prediction), thus improving the overall prediction performance.).</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related work</title>
      <sec id="sec-2-1">
        <title>2.1. Trafic flow forecasts</title>
        <p>
          In the field of trafic flow prediction, researchers have developed a variety of deep learning models
to improve prediction accuracy and capture complex spatio-temporal dependencies, each proposing
innovative solutions to specific problems. For example, Graph WaveNet [
          <xref ref-type="bibr" rid="ref1">1</xref>
          ] significantly improves
shortterm prediction accuracy by combining the Graph Convolutional Network (GCN) and the Temporal
Convolutional Network (TCN) to efectively capture local spatial dependencies and time-series patterns
of trafic data. However, the model has limited ability to model dynamically changing spatio-temporal
dependencies. AGCRN [
          <xref ref-type="bibr" rid="ref2">2</xref>
          ] improves the robustness of the model by introducing adaptive graph
convolution and dynamically adjusting the relationships between nodes to adapt to diferent trafic conditions,
but its reliance on dynamic graph structure may neglect the stabilising efect of fixed relationships.
ASTGCN [
          <xref ref-type="bibr" rid="ref3">3</xref>
          ] combines graph convolution with a long- and short-term memory network (LSTM) to model
the spatio-temporal dynamic properties in trafic data, which shows a good ability to capture temporal
dependencies, but still falls short in processing complex spatial features. In addition, STHSGCN [
          <xref ref-type="bibr" rid="ref4">4</xref>
          ]
and STFGCN [
          <xref ref-type="bibr" rid="ref5">5</xref>
          ] further improve the modelling ability of trafic flow data through hierarchical design
and fusion of multiple spatial features, respectively. STHSGCN emphasises on capturing multi-level
spatio-temporal dependencies, while STFGCN focuses on fusion of multi-dimensional spatial features to
improve prediction accuracy. However, these models still have shortcomings in dealing with long-term
dependencies and dynamic changes, especially in capturing and balancing fixed and time-varying
relationships. The efective combination of fixed relationships, which contribute to robustness, and
time-varying relationships, which better reflect real-time dynamics, remains a challenge.
        </p>
        <p>To address the above issues, this paper proposes a Dual Graph Convolutional Fusion Coding (DSGCE)
mechanism that comprehensively captures both fixed relationships and dynamically changing features in
trafic data by combining an adaptive graph structure and a predefined adjacency matrix. The mechanism
enhances the adaptability to dynamic changes in spatio-temporal dependence while maintaining the
model’s sensitivity to long-term stable features, providing a more comprehensive and eficient solution
to the complex task of trafic flow prediction.</p>
      </sec>
      <sec id="sec-2-2">
        <title>2.2. Missing value processing</title>
        <p>
          In the field of missing value processing, traditional methods such as mean-filling and k-nearest neighbour
algorithms [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ] are widely used due to their ease of implementation, but such methods fail to capture
the dynamic features in time series and are particularly inadequate for dealing with long-term missing
values. Recent studies have shown that generative models have significant advantages in missing
value filling. For example, generative models proposed by Dong et al. [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ] and Yoon et al. [
          <xref ref-type="bibr" rid="ref8">8</xref>
          ] perform
well in long-term interpolation tasks. However, these models typically treat the filling of missing
values separately from the subsequent prediction, which can lead to information loss and a decrease in
prediction accuracy. To overcome this limitation, researchers have attempted to use joint modelling
approaches. For example, GRU-D [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ] achieves the fusion of filling and prediction by directly estimating
the missing values within the GRU, while LSTM-i [
          <xref ref-type="bibr" rid="ref10">10</xref>
          ] improves the modelling capability in the time
dimension by using the LSTM state to infer the current missing values. However, these methods mainly
focus on feature mining in the time dimension and fail to fully account for the complex spatio-temporal
features and dynamic relationships between road segments in trafic data.
        </p>
        <p>To address the above shortcomings, this paper proposes a Generative Adversarial
NetworkTransformer-CNN Fusion Module (GAN-TCT), which aims to comprehensively model the temporal
variability characteristics and spatial dependence of trafic flow data. This multi-model fusion design
efectively integrates local and global features, and achieves high robustness and accuracy in dynamic
and complex trafic flow data.The GAN-TCT module not only provides an innovative method to fill in
missing values, but also lays a solid foundation for improving the performance of trafic flow prediction.</p>
        <p>
          The application of edge computing to trafic flow prediction is also gaining attention. In recent
years, many studies have attempted to combine edge computing techniques with trafic flow prediction,
aiming at real-time data processing and prediction using edge computing devices. For example, Yu et al.
[
          <xref ref-type="bibr" rid="ref11">11</xref>
          ] proposed a short-term trafic flow prediction method based on spatio-temporal correlation using
edge computing, which achieves fast trafic flow prediction and anomaly detection by deploying a deep
learning model on the edge device, reducing the data transmission delay and improving the prediction
accuracy. The GADGN model in this study precisely considers the application requirements of the
edge computing environment, and by designing an eficient computing module adapted to the edge
devices, it enables the model to better cope with the processing requirements of large spatio-temporal
data in trafic flow prediction. In particular, the GADGN model proposed in this paper can operate
eficiently in the edge computing environment. The GAN-TCT module of the model is able to process
trafic flow data in real time on edge devices and generate high quality missing value filled data. The
dual graph convolutional fusion coding mechanism is then able to extract spatio-temporal features
through static and dynamic graph convolution and combine with a self-encoder for multi-scale feature
learning. This design makes the model not only applicable to traditional cloud computing platforms,
but also able to perform real-time trafic prediction and analysis on edge devices, improving real-time
trafic management and decision support capabilities.
        </p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>3. Modifications</title>
      <p>The architecture of our proposed GADGN model is shown in figure 1, and consists of two main modules:
the missing value filling module and the trafic prediction module. In the missing value filling module,
we design the GAN-TCT module, which is diferent from the traditional GAN.The GAN-TCT module
combines a Convolutional Neural Network (CNN) to extract local spatial features, a Long Short-Term
Memory Network (LSTM) to capture time-series dependencies, and a Transformer module to model
global spatio-temporal contextual information. Based on this structure, the Generative Adversarial
Network (GAN) generates missing value filling results that are very close to the real data through
adversarial training, thus achieving an important breakthrough in filling quality and spatio-temporal
consistency, ensuring that the generator can simultaneously account for spatio-temporal dependencies
and generate more realistic filling data. The filled data is divided into q time steps, and the data from
each time step is used to generate a predefined knowledge-based adjacency matrix and a dynamic
adjacency matrix based on node embeddings (without prior knowledge). These two adjacency matrices
are then fed into two graph convolution modules to extract the spatial dependencies in the dynamic
graph structure and the predefined knowledge-based graph structure, respectively. By combining the
features from these two graph convolutions and feeding them into the self-encoder for feature learning,
we are able to extract richer spatial feature information and complete comprehensive spatial feature
extraction. Finally, by replacing the traditional update gate and reset gate with the extracted spatial
features, we compute the gate control parameters to better capture the temporal dependencies in the
data and obtain the final prediction results. The design not only improves the model’s ability to model
complex spatio-temporal dependencies, but also significantly improves the accuracy of the prediction.</p>
      <sec id="sec-3-1">
        <title>3.1. Missing value processing module</title>
        <p>Missing value handling is a key issue in spatio-temporal data analysis. This study proposes a new
GAN-TCT module for missing value filling in trafic flow data. The module combines the generator and
discriminator of Generative Adversarial Networks (GAN) while introducing a spatio-temporal feature
extraction mechanism. The generator part consists of LSTM, CNN and Transformer architectures, which
can efectively capture the temporal dependence and spatial relationship of the data. The discriminator
part adopts MLP (Multi-Layer Perceptron), which discriminates the diference between the generated
data and the real data through multiple fully connected layers to ensure that the generated data has high
authenticity and consistency. The generator and discriminator optimise each other through adversarial
training, improving the quality of the filling results. The model not only fills in missing values, but
also captures more accurate spatio-temporal dependencies when dealing with complex spatio-temporal
data, thus generating more realistic and accurate missing value filled data.</p>
        <p>Specifically, the generator first captures long-term dependencies in the time series using LSTM,
and then extracts local spatial features using CNN. The CNN-processed features are then fed into the
Transformer encoder, which models the global spatio-temporal context information through the
selfattention mechanism, further enhancing the ability to model complex spatio-temporal dependencies. To
obtain global features, the generator also uses a global pooling layer that fuses the global representation
of spatial features with the spatio-temporal features output by the Transformer to produce more realistic
data filled with missing values. The generator is formulated as follows:
 =   ()
(1)</p>
        <p>The LSTM process the output lstm, where x is the input data and lstm is subjected to 1D convolution
to extract local spatial features:
o = Re  (  ())
(2)</p>
        <p>= ( , , dim = − 1)</p>
        <p>=  −  ()
where  represents generated data. And the structure of the discriminator is based on several
fully connected layers and its task is to distinguish between real and generated data. Its goal is to</p>
        <p>The features output from the convolutional layer are mapped to the input dimensions of the
Transformer by a Fully Connected Layer (FC):</p>
        <p>=  ( )
The transformer input is processed by the encoder to extract global spatio-temporal features:
  =   ()</p>
        <p>To further enhance the features, the generator extracts global features from the output of the
convolutional layer (by average pooling) and passes them to a fully connected layer:
 =  −  (( , dim = 1))</p>
        <p>Finally, the generator splices the features that come out of the transformer with the global features
and fuses them together through a fully connected layer:
(3)
(4)
(5)
(6)
(7)
maximise the discrimination between real data  and generated data . The loss function of the
discriminator is defined as:
 = −  [log ()] −  [log(1 − ()]
where (∙ ) is the discriminator output.</p>
        <p>In addition, we introduce the Kullback-Leibler Dispersion (KLD) as a measure of the diference
between the distribution of the generated data and that of the real data:
(‖) = ∑︁ () log

()
()
where  and  represent the probability distribution of real and generated data respectively.</p>
        <p>The total loss function of the generator is:
 = −  ⌊︀ log ( )︀⌋ +  ∙  (︀  ‖
︀)
where the first term is the loss of the generator to fool the discriminator, the second term is the
KLD between the distribution of the generated data and the distribution of the real data, and  is the
hyperparameter that controls the trade-of between the two.</p>
        <p>During the training process, the generator and the discriminator improve each other through
adversarial training, where the generator is continuously optimised to generate more realistic complementary
missing value data, while the discriminator gradually improves its ability to discriminate between
generated and real data. In addition, we design an adaptive data generation mechanism that enables
GAN-TCT to dynamically adjust the generation process according to the spatio-temporal dependence
of the data, in order to better capture patterns in complex trafic flow data and produce high-quality
complementary results. This adaptive mechanism significantly reduces the filling error and improves
the overall performance of the trafic flow prediction model.</p>
      </sec>
      <sec id="sec-3-2">
        <title>3.2. Flow forecasting module</title>
        <p>The trafic prediction module is the core part of this paper, and the accuracy of trafic prediction is
crucial for trafic management. In order to improve the prediction accuracy, this paper proposes a dual
graph convolutional fusion (DSGCE) coding mechanism, which not only efectively captures the spatial
dependence in the data, but also dynamically adjusts the gating parameters by the spatial features of
the self-encoder output to capture the temporal dependence to achieve more accurate trafic prediction.
Specifically, this paper uses graph convolution based on the Laplace operator to capture the time-varying
relationships in the dynamic neighbourhood matrix that reflect the instantaneous changes in trafic flow.
Meanwhile, Chebyshev polynomial-based graph convolution is used to process the predefined adjacency
matrix to model the stable relationships between nodes, thus capturing the long-term dependence of
trafic flow. On this basis, this paper extracts and learns the spatial features using a self-encoder to
obtain a feature representation with comprehensive spatial information. Based on the spatial features
output from the self-encoder, the model dynamically adjusts the gating parameters to better capture the
time dependence in the trafic flow data. By combining the spatial features obtained by the dual-image
convolutional fusion coding mechanism with the temporal features adjusted by the gating mechanism,
the model is able to more accurately predict the changes in the spatio-temporal characteristics of the
trafic flow.</p>
        <p>We divide the filled data combined with temporal information into q time segments, and the input
data x of each time segment is the combination of the input data  of the time step and the output
extraction, which maps the learned dynamic features to the feature matrix :
features H− 1 of the previous time step, and pass x to the Multilayer Perceptron (MLP) layer for feature
 =   (x)</p>
        <p>Finally, the resulting dynamic graph is fed into the Laplace graph convolution for adaptive graph
feature extraction:</p>
        <p>D− 21 AD− 21 = D− 21 (︁ Re LU
︁( GGA )︁
i
where D is the diagonal matrix of time step i , whose diagonal elements are the degrees of each node.
The dynamic graph convolution operation is performed by constructing a normalised Laplace matrix to
generate the output feature V:</p>
        <p>V =
︂(</p>
        <p>I + D− 21 AD− 12 )︂</p>
        <p>X + 
represents the number of nodes of the matrix, D is the trafic feature captured by the sensor, and
where I is the unit matrix, X ∈ R×  is the input feature for dynamic graph convolution, N
 and
term  =  ∙ , which is obtained by combining the above two formulas:
b denote the weight and bias, respectively. And in this paper, the weight and bias matrices are also
done an update operation through the node matrix to get the weight tensor W =  ∙  and the bias
V =
︂(</p>
        <p>I + D− 21 (︁ Re LU  )︁
︁(</p>
        <p>D− 21 )︂</p>
        <p>XW + 
 combined with temporal information:</p>
        <p>Similarly, our predefined-based adjacency matrix  is generated from the original adjacency matrix</p>
        <p>The obtained dynamic feature matrix  is combined with the node information matrix containing
time information to generate the dynamic graph G for that time period. The first filled node embedding
matrix  and the time information are subjected to an element-by-element product operation to
obtain the node information matrix  with periodicity:
where   denotes the daily embedding and   denotes the weekly embedding. Secondly, the
correthe dynamic feature matrix  and the periodic node matrix  for that time period:
sponding dynamic graph G is obtained by a further element-by-element multiplication operation of
 =  ⊗   ⊗</p>
        <p>G = tanh (︀  ⊗  )︀</p>
        <p>The generated pre-defined neighbourhood based matrix  is fed into the Chebyshev polynomial
based graph convolution for feature extraction operation and finally outputs the pre-defined graph
feature E:</p>
        <p>=  ⊗   ⊗  
 = 2− 1 − − 2,  ≥ 2</p>
        <p>E = ∑︁
− 1
=0
X
input features of the Chebyshev graph convolution.
where 0 and 1 are the initial polynomials, usually taken as 0 = I and 1 = , i.e. the unit
matrix I and the adjacency matrix  of the predefined graph. In this paper,  = 2 and X are the</p>
        <p>Next, the two graph features are combined and fed to the autocoder to further learn the spatial
features and obtain a more comprehensive feature representation R. Specifically, the coder compresses
the input features into the potential space Z via a two-layer linear transformation (weight matrices
1 and 2) and an activation function, and the decoder starts from the potential space Z and
reduces the feature matrices by using the two-layer linear transformation (weight matrices 1 and
2) and activation functions to increase the similarity of the reconstructed features to the original
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
input, reduces the feature matrix, and finally adds a residual join term
of the reconstructed features to the original input:
Rcorv to increase the similarity
where  denotes sigmoid activation, W , W , W ,  ,  ,  denote learnable parameters, [∙ ] denotes
splicing operation, R denotes reset and update gates,  is an intermediate temporary variable, and H
denotes the spatio-temporal characteristics of the output of the gating mechanism at the i -th time step.</p>
        <p>The spatio-temporal dependent information captured at each time step is output through the gating
mechanism. We take the gated feature output of the q -th time step as the final result, i.e., H , and
further process it through the convolutional layer to produce the final prediction result yˆ:
yˆ = (H)</p>
      </sec>
      <sec id="sec-3-3">
        <title>3.3. Loss function</title>
        <p>The loss function plays a crucial role in the training process and is used to evaluate the performance of
the model during the process. By minimising the loss function, the model can appropriately reduce
the diference between the predicted values and the true labels, thus improving the accuracy of the
prediction. We use L1 loss as the loss function to train the whole model. Where K is the historical time
step, T is the future time step and  is the absolute value of the diference between the true value
Y+ and the predicted value Yˆ +:
 = 1 ∑=︁1 ⃒⃒⃒ Yˆ + − Y+⃒⃒⃒</p>
        <p>=   +</p>
        <p>Finally, we use the spatial features of the self-encoder output to compute gating parameters to
dynamically extract temporal features from the data. The specific gating mechanism can be expressed
as follows:</p>
        <p>Rconv = V + E</p>
        <p>Z =  (2 · ( (1 · Rconv)))
R =  (2 · ( (1 · Z))) + Rconv
 =  ([, H− 1] RW + )
 =  ([, H− 1] RW + )
 = tanh ([, H− 1 ⊙ ] RW + )</p>
        <p>H =  ⊙ H− 1 + (1 − ) ⊙</p>
        <p>Finally, we combine the total loss function  of the generator with the loss values  trained on
the whole model to obtain the total loss:
where  is a weighting parameter to balance the contribution of the two loss components.</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>4. Experimental setup</title>
      <sec id="sec-4-1">
        <title>4.1. Datasets</title>
        <p>To evaluate the performance of the GADGN proposed in this paper, we use two publicly available real
trafic flow datasets from the PeMS data: the PeMSD4 and PeMSD8. The PeMS data are collected in real
time from more than 40,000 detectors in California, USA.</p>
        <p>Specific dataset information is provided below:
• PeMSD4: This dataset consists of 307 detectors collecting data at 5-minute intervals for a total of
59 days of trafic flow data. The data was collected between January and February 2018.
• PeMSD8: This trafic dataset consists of 170 detectors collecting data at 5-minute intervals for a
total of 62 days of data, collected between July and August 2016.</p>
        <p>Each collection of both datasets contains features in three dimensions: volume, average speed and
average occupancy.</p>
      </sec>
      <sec id="sec-4-2">
        <title>4.2. Experimental setup</title>
        <p>We ran our experiments on a Win10 PC with an NVIDIA GeForce RTX 2070 graphics card. Specifically,
our data are divided in a ratio of 6:2:2, where 60% of the data set is used as the training set, 20% of the data
set is used as the validation set, and the remaining 20% is the test set. Regarding the hyperparameters
of the GADGN model, we set the hidden size to 64, the kth-order Chebyshev polynomial expansion
to 2, the embedding dimension D of PEMSD4 to 10, and the embedding dimension D of PEMSD8 to
5. The model was trained using the Adam optimiser and the training time was set to 100. Due to the
combination of several modules such as biplot convolution, generative adversarial and self-encoder,
the model has a high memory requirement, the batch size of the PEMSD4 dataset was set to 8 and the
learning rate to 0.00075 for the PEMSD4 dataset, and the batch size was set to 24 and the learning rate
to 0.0015 for the PEMSD8 dataset. The MAE was used as the loss function for training the model. Three
evaluation metrics were used to assess performance: (1) mean absolute error (MAE), (2) root mean
square error (RMSE) and (3) mean absolute percentage error (MAPE).
(1) Mean absolute error:
(2) Root mean square error:
(3) Mean absolute percentage error:
  =</p>
        <p>=1
1 ∑︁ ⃒ ⃒
 ⃒⃒ Y+ − Yˆ +⃒⃒
⎯
  = ⎷⎸⎸ 1 ∑︁ (︁ Y+ − Yˆ +︁) 2</p>
        <p>=1
   =
100% ∑i=︁1 ⃒⃒⃒⃒⃒ Ŷ︀+Y −+Y + ⃒⃒⃒⃒
⃒
(30)
(31)
(32)</p>
      </sec>
      <sec id="sec-4-3">
        <title>4.3. Baseline</title>
        <p>
          We compare the proposed GADGN with the following 10 representative trafic forecasting methods:
• ARIMA [
          <xref ref-type="bibr" rid="ref12">12</xref>
          ]: combines three techniques, namely autoregressive (AR), diferential (I) and moving
average (MA), which can efectively capture trends and cyclical changes in data by modelling
time series for future data forecasting.
• VAR [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ]: captures the dynamic interactions between individual variables by using the lagged
values of multiple time series as explanatory variables. It is a multivariate linear model widely
used in time series analysis and forecasting, and is particularly suitable for dealing with the
interdependence between multiple time series variables.
• STGCN [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ]: it is a model that combines Graph Convolutional Networks (GCNs) and
Convolutional Neural Networks (CNNs), specifically designed to deal with complex spatial and temporal
dependencies in spatio-temporal data.
• ASTGCN [
          <xref ref-type="bibr" rid="ref3">3</xref>
          ]: combines the ideas of Graph Convolutional Networks (GCNs) and attention
mechanisms to capture spatial dependence through GCNs, enhance time-dependent modelling through
attention mechanisms, and adaptively select important features in spatio-temporal dimensions to
improve modelling and prediction of complex spatio-temporal data.
• STSGCN [
          <xref ref-type="bibr" rid="ref15">15</xref>
          ]: it is a deep learning approach to modelling spatio-temporal data that aims to capture
the complex correlations and dynamic evolution between spatio-temporal data by processing
spatial and temporal dimensions simultaneously. The model models the relationships between
data in the spatio-temporal dimension by combining graph convolution and temporal convolution,
and by processing spatial and temporal features synchronously.
• Graph WaveNet [
          <xref ref-type="bibr" rid="ref1">1</xref>
          ]: combines the ideas of graph neural networks and WaveNet models. Eficient
modelling of complex spatio-temporal data is achieved by efectively fusing spatial and temporal
information, using graph neural networks to capture spatial dependencies in the data and WaveNet
to capture temporal dependencies.
• STFGNN [
          <xref ref-type="bibr" rid="ref16">16</xref>
          ]: it is a deep learning model based on Graph Neural Network (GNN) architecture
that aims to efectively capture and fuse spatial and temporal features in spatio-temporal data.
This model is able to integrate spatial and temporal dependencies in graph-structured data by
introducing a spatio-temporal fusion mechanism.
• AGCRN [
          <xref ref-type="bibr" rid="ref2">2</xref>
          ]: its main innovation is to adaptively learn the spatial relationships between nodes in
a trafic network by introducing an attention mechanism, combining the advantages of graph
convolution and recurrent neural networks, which significantly improves the model’s performance
and prediction accuracy in spatio-temporal data modelling.
• STHSGCN [
          <xref ref-type="bibr" rid="ref4">4</xref>
          ]: it is a novel spatio-temporal graph convolutional network model designed to
address the shortcomings of traditional spatio-temporal graph neural networks in modelling
spatial and temporal heterogeneity. The model is able to capture heterogeneity and causality in
spatio-temporal data more accurately by designing a separate extended causal spatio-temporal
synchronisation graph convolutional network and combining it with a causal spatio-temporal
synchronisation graph mechanism.
• STFGCN [
          <xref ref-type="bibr" rid="ref5">5</xref>
          ]: combines the advantages of graph convolution and time series learning to propose
a new fusion mechanism capable of handling complex trafic flow data. Special emphasis is
placed on learning the spatial dependence and temporal correlation in trafic sequences, while
the model’s ability to model spatio-temporal data is further enhanced by the introduction of a
continuous time correlation learning module and a transformer-based global time correlation
learning module.
        </p>
        <p>MAE</p>
        <p>A comparison of the results of our model (GADGN) with the other 10 benchmark models on the
PeMSD4 and PeMSD8 datasets is shown in table 1. All models are experimented on the same dataset</p>
        <p>MAPE
24.18%
17.24%
14.70%
14.18%
13.90%
17.29%
16.77%
12.97%
12.89%
12.36%
12.14%</p>
        <p>MAE
31.09
19.19
19.00
18.25
17.13
18.28
16.94
15.95
15.50
15.23
14.27</p>
        <sec id="sec-4-3-1">
          <title>PEMSD8</title>
          <p>RMSE
44.32
29.81
28.70
28.06
26.80
30.05
26.25
25.22
24.51
24.35
23.67</p>
          <p>MAPE
22.73%
13.10%
11.32%
11.64%
10.96%
12.15%
10.60%
10.09%
9.95%
9.83%
9.38%
and their performance is evaluated over 12 time steps. It can be seen that GADGN shows excellent
performance in all three metrics, MAE, RMSE and MAPE, outperforming all the benchmark models.
• Comparison with traditional methods: Traditional time series models such as ARIMA and VAR
have significant advantages in modelling temporal correlation. However, they are limited in that
they do not account for spatial heterogeneity in the transport network, making it dificult to
deal with spatial dependencies in complex transport systems. This leads to poor performance
in modelling non-linear features, especially on the PEMS04 and PEMS08 datasets, which have a
large number of nodes and high data complexity, and significantly poor prediction performance.
In contrast, our model GADGN is able to comprehensively capture spatio-temporal features
by introducing a spatio-temporal dependency-based missing value filling method and a
dualplot convolutional fusion coding mechanism, while retaining the ability to model temporal
dependencies, which significantly improves the prediction accuracy and stability.
• Comparison with fixed neighbourhood matrix: Models such as STGCN, which are based on a
ifxed adjacency matrix, are able to capture spatial dependencies in trafic flows using a predefined
spatial structure. However, the limitation of such models lies in the static nature of the adjacency
matrix, which is unable to adapt to the dynamically changing characteristics of the trafic network
and thus lacks flexibility in dealing with complex spatio-temporal relationships, limiting their
predictive performance. Our GADGN model innovatively combines an adaptive adjacency matrix
and a self-encoder, which can not only flexibly capture the static spatial relationships, but also
dynamically adjust the spatial dependencies according to diferent time steps, comprehensively
expressing the global and non-linear characteristics of the data, thus improving the sensitivity to
trafic flow changes and the prediction accuracy.
• Compared to Adaptive Neighborhood Matrix: Models such as Graph WaveNet and AGCRN are
able to dynamically adjust the adjacency matrix by learning the spatial correlations in the data,
thus outperforming models based on fixed adjacency matrices in capturing dynamic changes in
the trafic network. However, these models rely too heavily on adaptive adjacency matrices, often
ignoring the a priori knowledge already present in the trafic network and making it dificult to
fully exploit fixed spatial relationships. Our GADGN model innovatively combines the adaptive
adjacency matrix with a predefined graph structure and uses the Dual Graph Convolutional
Fusion Coding (DSGCE) mechanism to achieve eficient fusion of static and dynamic spatial
relationships. Through this mechanism, the model not only flexibly adjusts the adjacency matrix
to adapt to dynamic changes, but also fully exploits the a priori knowledge to comprehensively
capture the multi-scale spatio-temporal features in the trafic flow, which further improves the
prediction performance.</p>
          <p>To more intuitively demonstrate the performance diference between GADGN and other models, we
visualise the above tabular data as shown in figure 2.</p>
        </sec>
      </sec>
      <sec id="sec-4-4">
        <title>4.4. Ablation experiments</title>
        <p>To evaluate the performance of our proposed GADGN model, we conducted a series of ablation
experiments using the PEMS04 and PEMS08 datasets. By removing diferent components of the model,
we analysed the contribution of each module to the overall performance of the model with the following
experimental design:
• w/o Trans: Removal of the Transformer module from the GAN-TCT framework to assess its
impact on the accuracy of missing value filling.
• w/o CNN: Removal of Convolutional Neural Networks (CNNs) from the GAN-TCT module to
evaluate the importance of CNNs in capturing spatially dependent features during missing value
iflling.
• w/o PG: Removing the neighbourhood matrix based on predefined knowledge and using only the
dynamic neighbourhood matrix generated based on node embedding to assess the importance of
a priori knowledge in the construction of the graph structure.
• w/o AE: The self-encoder module is removed and the output features of the biplot convolution are
passed directly to the following module without going through the learning and fusion process
of the self-encoder.</p>
        <p>The results of the experiments are summarised in table 2. As can be seen from the table, the results
of the ablation experiments are inferior to the GADGN model for both the PEMSD4 and PEMSD8
datasets. In particular, the results of the w/o Trans experiments show that the removal of Transformer
significantly reduces the model’s ability to capture global spatio-temporal dependent features, which
further validates the important role of Transformer in the missing value filling task for global feature
extraction. The w/o CNN experiments show that the removal of Convolutional Neural Network (CNN)
significantly reduces the model’s ability to capture global spatio-temporal dependent features, which
fully illustrates the key role of CNN in learning spatial dependencies in the GAN-TCT module. The w/o
PG experiments show that using only adaptive adjacency matrices significantly weakens the model’s
ability to capture complex spatio-temporal dependencies, leading to a decrease in prediction accuracy,
highlighting the importance of predefined knowledge-based graph structures in modelling complex
spatial dependencies. Finally, the w/o AE experimental results show that the removal of the self-encoder
mechanism significantly degrades the model performance, further confirming the indispensability of
the self-encoder in fusing and reconstructing graph convolutional features, which efectively enhances
the model’s ability to capture and express spatial dependency features. In summary, the design and
implementation of the individual components contribute significantly to the overall performance of the
GADGN model, further demonstrating its efectiveness in spatio-temporal data prediction tasks.</p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>5. Conclusion</title>
      <p>The GADGN model proposed in this paper significantly improves the missing value filling and prediction
ability of spatio-temporal data by introducing the GAN-TCT module and the dual graph convolutional
fusion encoding mechanism (DSGCE). The missing value filling module adopts the Generative
Adversarial Network (GAN), which combines the Long Short Term Memory Network (LSTM), Convolutional
Neural Network (CNN) and Transformer to accurately capture spatio-temporal dependencies and
produce filling results that are closer to the real data. The feature extraction module efectively integrates
static and dynamic adjacency matrices through the synergistic action of the dual-image convolutional
fusion coding mechanism and the gating mechanism, which not only strengthens the learning ability of
complex spatial dependencies, but also dynamically captures temporal dependencies, thus improving</p>
      <sec id="sec-5-1">
        <title>Method (PEMSD4)</title>
        <p>GADGN
w/o Trans
w/o CNN
w/o PG
w/o AE</p>
      </sec>
      <sec id="sec-5-2">
        <title>Method (PEMSD8)</title>
        <p>GADGN
w/o Trans
w/o CNN
w/o PG
w/o AE</p>
      </sec>
      <sec id="sec-5-3">
        <title>RMSE</title>
        <p>the prediction performance of the model.</p>
        <p>Despite the excellent results achieved by the GADGN model in centralised computing architectures,
its application in edge computing environments still faces challenges in terms of computational
resources and real-time performance, especially in terms of inference speed and computational eficiency,
which need to be further optimised. In addition, although the dual-image convolutional fusion coding
mechanism can efectively capture spatio-temporal dependencies, the model may have dificulty dealing
with anomalies caused by extreme events such as unexpected accidents or severe weather in certain
complex trafic networks, which may afect the accuracy of the prediction. Future research will focus on
improving the computational eficiency and inference speed of the model on edge devices, and consider
the introduction of external influences to promote widespread adaptation and extension of the model in
practical applications.</p>
        <p>Acknowledgments: This work was supported in part by the National Natural Science Foundation of China under Grants
62206116 and Chunhui Plan Collaborative Research Project, Ministry of Education, China (HZKY20220350)&amp; Key R&amp;D Plan
of Hubei Provincial Department of Science and Technology 2023BCB041.</p>
        <p>Declaration on Generative AI: The authors have not employed any generative AI tools.</p>
        <p>MAE</p>
      </sec>
    </sec>
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