State-of-the-Art Review of Deep Learning Techniques in
Recommendation System
Ishwari Singh Rajput
1
Rakshita Mall
1
Anand Shanker Tewari
1
Arvind Kumar Tiwari
2
1
National Institute of Technology, Patna, Bihar, India
2
Kamla Nehru Institute of Technology, Sultanpur, Uttar Pradesh, India
Keywords: Deep neural network, Recommendation system, Autoencoder etc.
Abstract: On the basis of achievements of deep learning, its use in the field of recommendation system has gained a lot
of attention. This paper provides a review on the various techniques of deep learning which are used in
recommendation system. The ability of deep learning models to analyse the user-item relationship efficiently
is the reason for using it instead of the traditional recommendation models. This paper consists of the
introduction of basic terms and concepts in both recommendation system and deep learning. Then there is a
description of the researches on deep neural network based recommendation system
1 INTRODUCTION
The advancement in the field of information
technology in recent times has made the access of
huge amount of data very easy. There are a vast
variety of products and services whose description is
easily available along with their comments and
reviews. Due to this overload of information (Gantz
et al., 2012), it becomes very difficult for the user to
choose an appropriate product according to his
requirements.
To deal with the above problems,
recommendation systems are used. Recommendation
Systems provide the users with personalized
recommendations. There are many areas where
recommendation system is being used, such as music,
books, movies, shopping etc. Most of the online
vendors have a recommendation engine already
equipped. Recommendations are done on the basis of
user’s previous items choice or the items preferred by
similar user or on the basis of the description of item.
Here the item refers to the product or services which
is to be recommended. Recommendation system is
classified into three parts based on their approach:
(Çano et al., n.d.) Content based, collaborative
filtering and hybrid.
Recently, the use of artificial neural networks has
become very popular in the problems which require
complex computations and huge amount of input
data. Deep learning is a part of ANN architectural
models of deep neural network are efficiently built
and trained. Deep neural networks have its
applications in various fields such as speech
recognition, image processing, object recognition,
image processing, NLP tasks etc. Due to various
advantages of deep learning, researchers have been
encouraged to use its associated techniques in the
field of recommendation system also.
Deep learning is being used successfully in
recommendation system as well as many other fields
in computer science and has shown significant
improvements in the existing models. In 2007, a
collaborative filtering method for movie
recommendation system was given by
(Salakhutdinov et al., 2007) which utilized the
hierarchical model of deep learning.
In 2015, (Sedhain et al., 2015) with the use of auto
encoders, predicted the values which were missing in
the user-item matrix.
The sparsity issue in recommendation system in
collaborative filtering was addressed by (H. Wang et
al., 2015).
Various surveys have been done in the area of
deep neural network based recommendation system.
The state-of-the-art survey for deep recommendation
system has been done by (September & 2016, 2016).
In 2017, a comprehensive review on deep learning
based recommendation system has been done by
(Zhang et al., 2019). The paper proposed the
classifications on the basis of their structure, that is
neural network models and integration models.
182
Rajput, I., Mall, R., Tiwari, A. and Tiwari, A.
State-of-the-Art Review of Deep Learning Techniques in Recommendation System.
DOI: 10.5220/0010564900003161
In Proceedings of the 3rd International Conference on Advanced Computing and Software Engineering (ICACSE 2021), pages 182-189
ISBN: 978-989-758-544-9
Copyright
c
2022 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
This paper is organized in various sections as
follows. First, it consists of the introduction of
recommendation and deep learning techniques.
Section two consists of background and related
terminologies. In Section three, there is a review of
the various approaches of deep neural networks in the
area of recommendation system and its classification.
Section 5 includes the concluding part.
2 BACKGROUND AND
TERMINOLOGIES
A recommendation system is used for information
filtering and outputs a list of specific products in a
personalized manner. For examining how the two
fields that is recommendation system and deep
learning are integrated together, one should know the
basic of both these fields. This section of the paper
includes a brief description about the fundamental
classifications and challenges of both the fields. First,
there is an introduction of types of recommendation
system and then the details about deep learning
methods.
2.1 Recommendation System
It is very important for a user to filter the huge amount
of data available, to find a useful, tailored and
relevant content. The recommendations that are
predicted by the recommendation System helps the
users in taking a decision.
In a traditional recommendation system, the
recommendations can be made in two different
manners, i.e., predicting a particular item or preparing
a ranking list of items for a particular user (Park et al.,
n.d.). The recommendation system is divided into
three broad categories: Content based (Park et al.,
n.d.), Collaborative filtering and hybrid
recommendation system (Park et al., n.d.).
Content based recommendation system: In
content-based recommendation, the items which
are similar in content is searched. The profile of
user is established on the basis of items on which
the user is interested in. According to the profile
generated, the recommendation system searches
the database for the appropriate items using the
descriptive attributes of the item. If we use this
recommendation system (Lops et al., 2011) for
an item which is newly added, then content based
recommendation system works very efficiently.
The problem with new inserted item is that it may
not have any rating, but still the algorithm works
since it uses descriptive information for
recommendations. The limitation of this method
is that it cannot recommend diverse range of
products since the algorithm does not take the
information from similar users.
Collaborative filtering recommendation systems:
This recommendation system assumes that users’
who have previously preferred same items,
would have same choice in future also. In this
system, the recommendations are done on the
basis of similar users’ pattern rather than
descriptive features of items. A correlation
among the users is determined, depending upon
the choice of similar users, the items are
recommended.
There are two methods which the CF algorithm
follows: memory based algorithm and model
based algorithms.
In memory based algorithm the complete user-
item matrix is taken into consideration for
identifying similarity. After finding out the
nearest neighbour, on the basis of neighbours
past rating, the recommendations are provided.
In model based algorithms, an offline model is
built with the use of machine learning algorithms
and data mining methods. These models include
clustering models, Decision models, Bayesian
model and singular value decomposition model.
Hybrid Recommendation System: Hybrid
recommendation system is a combination of
content based and collaborative filtering model,
it incorporates the benefits of both the methods
and tries to eliminate the limitations of the above
models (Tran et al., 2000). There are many
hybrid systems proposed, some of them are as
following:
Cascade: The output of one approach is later on
used by other approach
Switching: Recommendation output is given on
the basis of current situation and either of the one
approach is used.
Weighted: the combination of the scores of
various approaches is used for recommendation
2.2 Deep Learning
Deep learning is based on learning many layers of
representations with the help of artificial neural
networks, and is a part of machine learning. Deep
learning has its applications in various fields such as
Computer vision, recognition of speech, natural
language processing etc. The important factors which
increase the importance of deep neural network as the
State-of-the-Art Review of Deep Learning Techniques in Recommendation System
183
state-of-the-art machine learning methods are as
following:
Big data: As the amount of data increases, better
representations are learnt by the deep learning model.
Computational power: The complex computations of
the deep learning model is done using the GPU.
In this section, there is a description of various deep
learning models which are used in recommendation
systems.
2.2.1 Autoencoder
It is a type of a feed forward network in which some
representations from the encoded input are found by
the training, so that the input can be restored back
from these representations. An autoencoder consists
of three layers, which are the input layer, the hidden
layer and the output layer. There are equivalent
numbers of neurons in the input layer and the output
layer. The representations are obtained from the
hidden layer, and with the help of these
representations, the input layer is reconstructed at the
output layer (Deng et al., n.d.).
In the learning process, two mappings are used,
with the help of encoder and decoder. Encoder is used
for mapping the data form input to hidden layer while
decoder is used to map the data from hidden to output
layer (Strub et al., n.d.).
2.2.2 Recurrent Neural Network
The use of sequential information is done in RNN,
which is a class of artificial neural network. In RNN,
the sequence of values, i.e., x
(0)
, x
(1)
,..., x
(t)
is
processed. On each element of the sequence, the same
task is performed and the output is based on previous
computations. RNN (Wu et al., n.d.) uses an internal
memory to hold the values of previous computations,
so that it may be used later. Some major problems that
occur in RNN is exploding gradient or vanishing
gradient. To remove this problem, Long short term
memory (LSTM) and gated recurrent unit is used. It
is used when the processing is to be done for
predicting events which has comparatively longer
interval and delays. LSTM has a processor to
distinguish the useful information from the
information which are not useful. This processor is
known as cell. There are three gates in LSTM namely
input gate, output gate and forget gate. The
information that does not complies with the
algorithm’s certification, is forgotten with the help of
forgot gate. RNN are useful while dealing with
temporal dynamics of interactions and when the
user’s behavior has a certain sequence of pattern.
2.2.3 Convolution Neural Network
It is a neural network of fed forward type. In CNN
(Wu et al., n.d.), the use of convolution operations is
done rather than the usual matrix multiplication in
one or more layers. There are many applications in
which CNNs are applied such as object recognition,
self-driving cars, audio processing etc. while
transforming the input to output, the CNN uses three
major components that are convolution layer, pooling
layer and fully connected layers. These three are
stacked together to form a CNN. The layers of CNN
are used for the following operations.
Convolution: It is a core operation and is used for
extracting the features from the input. It is done by
applying convolution filters involving some
mathematical operation.
Non linearity: An additional operation is used for
introducing non linearity after every convolution
operation and usually ReLU is used.
Pooling: This operation is done for reducing the
dimensionality of the feature maps and decreases the
processing time.
Figure 1: Convolutional Neural Network Model
2.2.4 Restricted Boltzmann Machine
It consists of two layers of neural network namely a
visible and a hidden layer. The complex computations
and learning in RBM (Wu et al., n.d.) is based on
inherent intrinsic expression of data. The word
“restricted’ is used for intra-layer communication as
it is not present in both hidden layer and visible layer.
Due to this restriction, the learning efficiency
increases. There is a full connection between the
nodes of different layers which are stacked together
and there is no connection between the nodes of same
layer. Since RBM uses a simple forward encoding
operation, so it is very fast when compared to other
models such as autoencoder.
ICACSE 2021 - International Conference on Advanced Computing and Software Engineering
184
3 DEEP NEURAL NETWORK
BASED RECOMMENDATION
SYSTEM
DL techniques are widely used in various real world
applications such as sentiment analysis, speech
recognition image classification, text classification
etc. Various researchers also include deep neural
network based techniques in the field of
recommendation system to improve its performance
as compared to traditional recommendations systems.
This section describes the various categories
recommendation systems which are based on deep
learning. The categorization is based on the types of
recommendation systems used which is as follows:
Collaborative filtering recommendation system
based on deep neural network.
Content-based recommendation system based on
deep neural network.
Hybrid recommendation system based on deep
neural network.
Social network-based recommendation system
based on deep neural network.
Context aware recommendation system based on
deep neural network.
Integration model and neural network model are
the two categories of deep neural network based
recommendation system. Integration model is further
divided into two categories on the basis of whether it
combines any traditional recommendation system
model with deep neural network technique or depends
solely on deep learning method.
Neural network model is also divided into two
categories on the basis of deep neural network based
technique used: models which uses single deep neural
network based technique and deep
neural network
based composite model. In deep neural network based
composite model, different deep neural network
techniques are used to build a hybrid system having
more capability. The framework of recommendation
system
based on deep neural network is presented in
Figure 2.
Figure 2. Deep neural network based Recommendation
system
3.1 Collaborative Filtering
Recommendation Systems based on
Deep Neural Networks
Collaborative filtering (CF) is one of the commonly
implemented techniques in recommendation systems
in order to tackle various real-life issues. The state of
the art CF-based methods uses the rating matrix for
recommending the items. But this approach faces the
problem of data sparseness and cold start problem.
The sparsity of the user-item matrix, the learned
features is not effective which reduces the
performance of recommendation system. Various
researchers propose deep neural network based
collaborative filtering techniques to enhance its
effectiveness in recommendation.
3.1.1 Collaborative Filtering Method based
on Generative Adversarial Network
Generative Adversarial Network is a neural network
which is generative and having discriminator and
generator functions. These both functions are
simultaneously trained in competition with one
another in architecture of minimax game. The first
model to implement GAN in the field of Information
Retrieval is (IRGAN) (J. Wang et al., 2017) which
stands for Information retrieval generative
adversarial network. The state of the art GAN model
has two modules a discriminator and a generator. The
generative retrieval module predicts appropriate
documents with given query, whereas discriminative
retrieval module predicts relevancy given with a pair
of query and document.
The IRGAN model combines above two
Information Retrieval models in order to play a
minimax game with them: the generative retrieval
model produces (or selects) relevant documents that
are relevant documents like ground truth, while the
discriminating retrieval model separates the relevant
documents from those generated by the generative
retrieval model.
3.1.2 Recurrent Neural Network based
Collaborative Filtering Method
In order to deal with the information in sequential
form, recurrent neural network (RNN) proves to be a
very effective network. Concepts of loops are used in
place of feedforward network to remember
sequences. Variants of RNN viz. Long Short-Term
Memory (LSTM) and Gated Recurrent Unit (GRU)
network are used to deal with the problems of long
term dependencies and vanishing gradient problem.
State-of-the-Art Review of Deep Learning Techniques in Recommendation System
185
In collaborative filtering method based on RNN, the
impact of user historical sequence is modelled on the
current behavior of user, recommendation is
performed and user’s behavior is predicted [19].
Figure 7 shows the framework of collaborative
filtering method based on RNN (Wu et al., n.d.). Let
the input set is {I
1
, I
2
. . . I
t
}, and output is O
t
= σ (f
(Wꞏh
t−1
+ VꞏI
t
)ꞏV), σ represents a softmax function, f
represents the activation function, which specifies the
selection probability of any item at time t. h
t
represents the hidden state vector.
Figure 3. Collaborative filtering model based on RNN.
3.1.3 Collaborative Filtering Method based
on Autoencoder
The first ever developed autoencoder-based
collaborative recommendation model is Autoencoder
based Collaborative filtering (Sedhain et al., 2015). It
decomposes the vectors by integer ratings. The model
proposed by (Sedhain et al., 2015) takes user or item
based ratings as inputs in rating matrix R. The output
is produced by the process of encoding and decoding
by optimizing the parameters of model and reducing
the reconstruction error. Consider an example, if the
range of integers [1-5] represents the rating score,
then each r
ui
can be divided into five vectors.
Figure 4. Collaborative filtering method based on
Autoencoder
Above figure represents the 1 to 5 rating scale in
which blue boxes represents the user rated item. The
cost function which is to be reduced is taken as Mean
Square Error. The rating prediction in this approach
is found by making the summary of each of the five
vectors, which are scaled by rating K. Pretraining of
parameters and local optimum avoidance is
performed by RBM. Stacking multiple autoencoder
collectively shows the slight improvement in
accuracy. This method based on autoencoder suffers
from the problem of dealing with non-integer ratings
and sparseness of input data due to decomposition of
partial observed vectors.
Collaborative Denoising Auto-Encoder (Strub et
al., n.d.) is primarily used for prediction rankings.
User feedback is taken as input to the CDAE. If the
user enjoys a movie, the input value is 1 otherwise it
is 0. It shows the vector preference to display the
user's interest in some item. Gaussian noise corrupts
the CDAE input.
3.1.4 Collaborative Filtering Method based
on Restricted Boltzmann Machine
Restricted Boltzmann Machine (RBM) is a two-layer
neural network capable to deal with typical learning
based problems. The efficiency of learning is
improved by removing the connections between same
layers. This recommendation method is proposed by
(Salakhutdinov et al., 2007). Further a conditional
RBM model is proposed to consider information in
form of feedback. The visible layer of RBM can take
only binary values so only one hot vector can be used
to represent the rating score. The architecture of RBM
based model is represented as shown in Figure 5.
Figure 5. Collaborative filtering model based on RBM.
There are equal hidden layers in each RBM and
have softmax units which are visible for movie
ratings given by user. There is one training case in
ICACSE 2021 - International Conference on Advanced Computing and Software Engineering
186
each RBM unit with combined weights and biases.
Hidden units hold the binary states which are
different for separate users. Let there are N movies
rated by user and there are n visible units. Suppose M
represents X × N size matrix where 𝑟

= 1 if movie i
is rated as x by user u, otherwise 𝑟

= 0. So,
𝑝
𝑟

1
|
 



 



(1)
𝑝ℎ
1M σ b
∑∑
𝑟



w

) (2)
where σ (x) represents a logistic function, 𝑟

:
interaction, 𝑏
:bias of rating x for movie i and b
j
:
hidden unit bias.
3.2 Content-based Recommendation
Systems based on Deep Neural
Networks
Content-based recommendation systems recommend
on the basis of descriptive attributes of items and
users’ profiles such as texts, pictures and videos
(Meteren et al., n.d.).
The use of deep neural network in content-based
recommendation systems is to capture the non-linear
relationships between user and item (Meteren et al.,
n.d.). It also captures the intra-relationship between
data, from large available data sources.
3.3 Hybrid Recommendation System
based on Deep Neural Networks
The state-of-the-art CF-based approach uses the
ranking matrix to suggest products. But this method
suffers from a data sparsity and a cold start crisis. Due
to the scarce existence of the user-item matrix, the
features learned are not accurate, which decreases the
efficiency of the recommendation framework. Hybrid
recommendation model based on deep neural
networks incorporates a content-based
recommendation model with collective
recommendation-based filtering models, which
combines the mechanism of feature learning and
recommendation into a single model. (Meteren et al.,
n.d.) suggested a layered, self-encoder-based hybrid
paradigm that learns the latent space factors of users
and items and simultaneously performs mutual
filtering from the ranking matrix.
An autoencoder is a variant of neural network,
having encoder and decoder as two components. The
encoder converts the input into its hidden
representation, while the decoder converts the hidden
representation back to the restored input form. The
parameters corresponding to the autoencoder are
trained to minimize the error due to the
reconstruction, which is measured by the loss
function. Denoising autoencoder (DAE) tries to
reconstruct the input from a corrupted version for
improved representation from the input. More
variants of autoencoder have been developed for
better outcomes. The hybrid recommendation model
based on the stacked denoising autoencoder uses both
the rating matrix and the side information and
integrates both the SDAE (Meteren et al., n.d.) and
the matrix factorisation. Matrix factorization is a
widely used model with improved accuracy, and
SDAE is a powerful model for extracting high-level
features from inputs. The integration of the above two
model will become a powerful model for more
benefits.
3.4 Social Network-based
Recommendation System using
Deep Neural Networks
Conventional recommendation models never
consider social connections among the users. But we
always take verbal recommendations from our friends
in reality. These verbal recommendations are termed
as social recommendation which occurs daily
(Meteren et al., n.d.). Hence, for improved
recommendation systems and for more personalized
recommendations, social network must be employed
among users. Every user will interact with various
types of social relationships.
The quality of recommendation system is very
crucial task which can be achieved by implementing
the effect of social relationship among the users.
Items with location attributes and sequential pattern
of user behaviour in spatial and temporal frame are
used to form spatiotemporal pattern which is used to
improve recommendation accuracy. Recently, a very
few recommendation techniques have been proposed
which is based on the users’ trust relations improve
conventional recommendation systems. These trust
based recommendation models proves to be an
effective move in the field of recommendation system
models.
In current scenario an integration of deep learning
and social network based recommendation system
provides a platform for various research solutions.
The limitations which are inherent to the social
recommendation must be addressed in the future
research.
.
State-of-the-Art Review of Deep Learning Techniques in Recommendation System
187
3.5 Context-aware Recommendation
Systems based on Deep Neural
Networks
A context-aware recommendation system, integrates
context based information into a recommendation
model. This integration is effectively performed by
the deep learning techniques in different conditions of
recommending item. Deep neural network based
methods are used to extract the latent space
presentation from the context based information.
Deep learning-based model can be integrated into
diverse data to reduce data sparsity problem.
Sequential nature of data plays a significant part in
implementing user behaviours. Recently, recurrent
neural networks (RNNs) are commonly used in a
variety of sequential simulation activities. However,
for real-world implementations, these approaches
have trouble modeling contextual knowledge, which
has been shown to be very essential for behavioural
modelling.
Currently this method based on deep neural
networks focused towards situation information. A
novel approach is proposed called context-aware
recurrent neural networks. It uses two types of
matrices: input and transition matrices. They both are
specific to the context and adaptive in nature. Input
matrices are used to extract various situations such as
time, place, weather condition where actually the user
behaves.
3.6 Comparisons
As deep learning plays a significant role in most of
the fields as it has the capability of dealing with large
and complex problems with improved results. Deep
learning technology also contributes in the field of
recommendation system for improved customer
satisfaction. Deep learning technology overcomes the
shortcomings of traditional models to get high quality
recommendations (Liang et al., 2018).
All the above discussed methods of
recommendation systems use deep neural networks
and hence also achieve the quality in recommending
items to the users (Liang et al., 2018). Different
recommendation models use different deep learning
methods to obtain improve results.
The summary of various deep neural network
based recommendation systems are given below:
Table 1: Comparison
Application
of Deep
Learning in
Findin
g
s
Conten
t
-
b
ased
recommendation
s
y
stems
Extract non-linear use
r
-
item relationship and intricate
relationship with data itself.
Collaborative
filtering
recommendation
systems
Takes the use
r
-item
ranking matrix as input and
uses a deep neural network
based model to learn the latent
space presentation that
corresponds to users or
objects. Use the loss function
to construct a deep neural
network-based model
optimization function. On the
basis of the latent space
presentation,
recommendations are made.
Hybrid
recommendation
systems
Integrate the individual or
object learning process and the
suggestion process into a
single architecture.
Social network-
based
recommendation
systems
Focuses on social
relationship between users,
and extracts the effect of
location of user, movement
patterns and other various
factors.
Context-aware
recommendation
systems
Deep neural network
based methods combines the
context information into the
recommendation model and
obtain the latent space
presentation of the context
based information. Also
reduce data sparsit
y
in model.
4 CONCLUSIONS
The massive volume of data generates the necessity
of applications and technology to efficiently process
and information analysis for providing the maximum
benefit to the target users. Recommendation system
based on deep neural networks proves to be optimal
solution for above mentioned challenges. Deep neural
network based recommendation systems can easily
learn the features of items along with users from large
volume of information, to build a system for effective
recommendation to users. This article compares the
conventional recommendation models with the deep
neural network based recommendation techniques
which extracts the features of users along with items
ICACSE 2021 - International Conference on Advanced Computing and Software Engineering
188
to increase the accuracy of recommending items to
users. It also integrates data from multiple sources to
extract the preferences of users.
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