Designing Mobile Training Content: Challenges and Open Issues

Mobile training is an evolution of electronic training and is based on mobile learning technology, which is used to design mobile learning courses. Due to the widespread deployment of mobile devices and the need to remain current with developments in mobile technology, it is important to consider the design of appropriate mobile training content to increase learners’ engagement in mobile learning courses. However, studies have emphasized the challenges in this area. Therefore, we conducted a systematic mapping study that offers an overview of the current literature in this domain based on a thorough search of the literature by using a process of selection that involves criteria for inclusion and exclusion, data extraction and synthesis strategies. Of the 194 journal articles identified in the initial search stage, 58 were selected as primary studies; they were published between 2009 and 2019. We applied a classification scheme to answer our research questions. Our study examines the current challenges in the design of mobile training content, identifies the key open issues, determines the trends in publication and emphasizes the most widely researched topics in recent years related to the design of mobile training content. Our study identifies the existing challenges and suggests further work on key open issues. Our study also suggests that, considering the major issues related to pedagogical challenges, the research focus should shift toward the design of attractive, interactive and motivating mobile content that is based on a theoretical framework for mobile training courses, and other technological and managerial challenges that can be addressed should be investigated in order to overcome the existing difficulties in the design of mobile training content and to provide better solutions for the continuity of this research domain.


I. INTRODUCTION
Following rapid advances in technology and the current widespread use of mobile devices, the interest in support for learning has increased. As a result, innovative mobile learning and training initiatives have been developed, such as educational and professional development applications. One of these training initiatives is mobile training (m-training). This is related to self-learning and problem solving, which are executed by trainees based on their training needs via portable devices with the aid and support of trainers [1]. Thus, mobile training can be defined as an activity that is designed to be The associate editor coordinating the review of this manuscript and approving it for publication was Francesco Piccialli. used as an application on mobile devices that targets specific people and includes training materials (content) for them to work on and interact with through their mobile devices in a specific order to reach a predetermined goal based on their actual training needs. The main objective of this type of training is the transfer of knowledge or the improvement of skills based on the needs of remote and mobile participants who move around with their mobile devices and are not tied to a specific location. Mobile training is based on mobile learning technology, which is used to design mobile learning courses, and this technology has developed rapidly in recent years. Mobile training is a new training model that evolved from electronic training (e-training). It is considered to be an extension and development of the traditional training model, as it integrates the main elements of the training process of traditional training with mobile device technology [1]. The advantages of this type of training include its mobility, environmental friendliness, ease of navigation, greater interactivity, greater stimulation in learning, ease of access and wider targeting of learners [2]- [7], as shown in Figure 1. Since employees in this digital era are required to be better trained than ever before, the design of mobile learning courses to train employees via mobile devices has become an important requirement for human resources to gain knowledge and develop skills in this fast, changeable and mobile environment. To remain current with developments in mobile learning technology, there is a need to improve the design and delivery of mobile training courses to train employees in different domains, which indicates an urgent requirement to design appropriate mobile training content for training purposes [6]. As a result, many organizations have begun to provide learning and training services via mobile devices. However, various challenges have appeared in this field, especially in terms of the design of mobile training content. Several studies have indicated that the design of compatible, user-friendly, attractive and interactive content to increase both the engagement of learners with mobile training courses and these courses' accessibility to trainees is different from designing electronic training courses to be used on a mobile device, such as designing a web-based (online) course to be accessed via an internet browser by using a mobile device.
Instructional design is considered to be one of the most important issues for designing mobile training content, and it is linked to learning theories that represent the main role for designing mobile training content as a foundation for a pedagogical theory that enhances the development of mobile learning courses [8]. Moreover, designers should consider the limitations of mobile devices, such as the monitor size, input devices, navigation and challenges in bandwidth during browsing, when designing mobile learning content [6]. Therefore, an effective design for mobile learning materials must be tailored to the characteristics of mobile devices.
For instance, effective designs include a simple and user-friendly interface, easy navigation, content in the simplest possible format, and the use of multimedia in an interactive manner; effective designs also break instructional materials into short segments, offer interactive activities with quizzes and assessments, and provide instant feedback and discussion [8].
Therefore, this section emphasizes several challenges in this field and summarizes the prior research that is closely related to this study.
Certain technological issues need to be considered when designing mobile learning/training content, as found in various studies [9], [10], [6]. These issues are related to the following factors: • The physical characteristics of mobile devices, such as their small screen sizes, substantial weight, low memory and limited battery life.
• Physical conditions, such as issues related to the use of mobile devices in outdoor environments, excessive screen luminance, personal security issues, and radio frequency radiation while using devices.
• Mobile technology infrastructure, such as the network capacity for data transmission and dependability.
• Software development for mobile applications, including challenges such as accessibility, execution within mobile device environments, the development of an effective user interface and conflicts between applications and conditions of use. Some researchers argue that these concerns will be limited in the future. For instance, the authors of [11], [12], and [13] noted how rapidly new mobile devices and merchandise are improving, especially in terms of touchscreens and advanced hardware capabilities; moreover, new mobile technologies are being developed, such as augmented reality, as well as new mobile accessories and applications that include new features. Moreover, the use of mobile technology in learning or training is deployed in many applications from pilot projects to sustained experiments. Thus, all of these capabilities are under development, which means that mobile devices have great potential for rich mobile training experiences. As a result, the technical limitations of mobile devices are considered to be temporary challenges.
Other important challenges are associated with the pedagogical approaches that analyze the need to select appropriate learning strategies and methods that integrate with the technical aspects of mobile device technology to design interactive and attractive training content. This type of challenge has been identified in previous research; for example, the authors of [10], [12], [6], and [14] indicated that there is a lack of a solid theoretical framework that can act as a foundation for the pedagogical theory to design mobile learning. Moreover, there are insufficient or limited instructional design guidelines for mobile content, which hinders the creation of an effective approach to instructional design that can generate high-quality, well-designed course content based on a solid theoretical framework. VOLUME 8, 2020 Some authors have argued that many studies of mobile learning have concentrated more on technologies than on the learning process, e.g., [11], [15], and [16]. Well-designed mobile learning content methodology is one of the most important requirements in expanding the achievements of learners [17]. Therefore, the issue regarding how to design interactive mobile learning and training content arises, as there is no stable theoretical model or framework that can act as a pedagogical theory for the design of mobile learning content that attracts and motivates learners to participate in the learning process via mobile devices [10].
One study [18] suggested the establishment of a theoretical model that uses strategic methods to assist and enable educators not only to design mobile learning content but also to integrate this content into learning as part of the curriculum. In addition, UNESCO presented a series of working papers on mobile learning in 2013 that focused on the year 2030 [19]. The most important recommendations for overcoming these challenges proposed in the UNESCO study were to link learning theories to analyses of mobile learning, enhance and support the training of teachers via mobile training, train these teachers to design mobile learning material and promote mobile learning for all.
Of the review studies described above, most of the earliest research focused on the technical challenges of mobile devices. However, following rapid developments in this field, the topics of research and the trends in designing content for mobile devices have evolved. Content is now specifically designed for mobile devices, is presented differently from previous designs for electronic learning, and is viewed on mobile devices.
In recent years, numerous studies have identified a lack of a pedagogical approach to the design of mobile applications for learning and training. These studies have recommended considering technical features within a pedagogical approach.
To remain current with the revolution in mobile technologies that has produced many new features, such as sensors, GPS, cameras, multimedia options, and new communication tools and applications, recent studies have emerged with updated topics. As a result, many studies have suggested that mobile learning/training designers should consider the capabilities of mobile devices as open issues that they can address by selecting the appropriate mobile technology for the content; they can integrate design principles and pedagogical practices when developing mobile applications to design effective content for mobile training.
The purposes of conducting this study are to investigate the existing challenges in the design of mobile training content (m-learning courses) and to identify the key open issues in this research field. To offer an overview of the relevant studies in this area, we carry out a systematic mapping study (SMS) to answer the following research questions based on a literature review: RQ1 -What are the existing challenges in the design of mobile training content? RQ4 -What is the most widely researched topic in the studies of the design of mobile training content during the last five years?
We create a classification schema from selected journal articles that have been published to clarify and emphasize the challenges and open issues that arise. The outcomes are then discussed to offer clear explanations. Finally, guided by the findings of this study, some recommendations are presented for future studies.

II. BACKGROUND
This section presents the following theoretical foundations: basic concepts and definitions of mobile learning and its relevant concepts, such as ubiquitous and pervasive learning.

A. MOBILE LEARNING (M-LEARNING)
Many studies have defined the concept of mobile learning. Earlier studies of mobile learning were focused on understanding the methods that mobile devices offer to assist learners in obtaining a better education considering the affordances and constraints of m-learning and how it can be addressed and established as a complement to electronic learning or learning in the classroom [20]. Previous definitions of mobile learning have focused on the use of mobile technology such as the definitions stated in [21] and are anchored in learners' ability to learn and gain knowledge.
Due to the research development in this field, the definition of mobile learning has evolved in different ways and directions. Some studies, such as [22], have stated three essential perspectives of m-learning, including learner mobility, learning mobility and the mobility of technology. Other studies have expanded the definition to include other characteristics of mobile learning, such as ubiquity [23] and contextual mobile learning [15]. According to [6], mobile learning is the delivery of learning content and providing support to learners on mobile devices to offer flexibility for learners to learn on the go. services anywhere and at any time to acquire knowledge as a part of daily life [10].
However, another study [25] expressed that u-learning is an emergent model aimed at delivering learning experiences and expanding the learning process into various environments where learners can access content anytime and interact with the content in any way by utilizing the surrounding environment, mobile devices, wireless communication and pervasive and digital components.
Therefore, u-learning is electronic learning based on mobile devices, a pervasive component and wireless communication to gain knowledge anywhere and at any time. It differs from mobile learning in that it concentrates on learners and the learning environment by offering the appropriate information at a suitable place and time through a convenient method for learners.

C. PERVASIVE LEARNING (P-LEARNING)
Pervasive learning as a concept was mentioned in the book entitled ''Flat Army: Creating a Connected and Engaged Organization'' by Pontefract [26]. Based on this concept, the learning process goes beyond gaining knowledge at a specific location and extends to learning everywhere in everyday life. The learning process occurs according to the needs of learners. Dan Pontefract defined the concept of pervasive learning as learning that occurs according to the learner's speed and needs, including formal, informal and social learning types. This indicates that the learning process occurs not only in formal learning environments but also as needed and at the appropriate time. Learning is a part of life with any tool, such as mobile devices.
The idea of pervasive learning has been extracted from the 70-20-10 model, which was established to fulfill actual needs for employee' development. The learning ratios for meeting the specific needs of employee development differ within groups. As stated by Pontefract [26], these numbers represent percentages indicating that we learn 70% from job experiences by completing work tasks or assignments, 20% from other people, such as mentors, managers, or others, and 10% from attending courses, reading books or other resources. Dan devised a model and applied it to the educational aspect. Accordingly, he indicated that pervasive learning occurs in specific proportions (the 3-33 model), with 33% of learning occurring formally, 33%, occurring informally and 33% occurring socially. Therefore, pervasive learning added value to the learning process by allowing learners to learn during their lifetime even if they were not registered at educational institutions. This is because of the availability and ease of accessing resources to acquire knowledge anytime and anywhere via mobile devices or computers when required, which facilitates developing learners' knowledge and skills.
Some research has presented a wider concep; for instance, a study [27] indicated that pervasive learning is related to pervasive education, which is considered to be mobile learning that concentrates on the mobility of learners who use and interact with technolog; pervasive education not only considers the use of mobile devices but also includes the use of fixed devices by learners during their movement. Furthermore, learner mobility can include mobility within a specific space, in various areas of life and at different times. As a result, pervasive education is expected to have a great impact such as merging isolated environments, enhancing learning with interactive content and establishing interactive communication that results in facilitating and evolving the learning process.
A study conducted on a review of the literature related to mobile, ubiquitous, and pervasive learning [24] indicated that each concept has specific features and characteristics that can be distinguished from the other concepts. For instance, m-learning is differentiated by its capability to deliver and access learning content from any location, while u-learning has the capacity to reproduce context-aware scenarios. However, the chance to obtain the related details of the environment and its objects by utilizing embedded appliances at the right time represents p-learning. There are some common characteristics among the concepts of mobile, ubiquitous, and pervasive learning. This integration was stated in the same study, which expressed the union of these three concepts as MUP-learning. MUP-learning is a term that conveys the strength of combining mobile, ubiquitous, and pervasive learning, which is more efficient than these types of learning functioning individually [24]. Therefore, the purpose of MUP-learning, as noted in [24], tends to benefit from the strength of this integration to design interactive mobile learning or training content by applying an appropriate pedagogical design model, establishing efficient evaluations, and applying the new technologies of mobile devices while developing new mobile learning or training apps that target various emerging topics such as augmented reality (AR), artificial intelligence (AI), educational robotics, and metacognition.
Many definitions and terms are available that distinguish, describe, and clarify these concepts; however, because of the rapid development in technology, the concepts have evolved. Therefore, some recent studies have clearly attempted to remain current with developments and build a convergence among these concepts, such as among mobile, ubiquitous, and pervasive learning, that will facilitate establishing the learning process and spreading or extending it from the classroom to an open environment that contains interactive learning content that is easily accessed anytime, anywhere and with various methods.

III. RESEARCH METHOD A. SYSTEMATIC MAPPING PROCESS
We conducted an SMS based on the guidelines developed by Petersen et al. [28] and Kitchenham et al. [29]. This approach was selected because it offers a broader way to systematically analyze the selected studies to answer the research questions and gather proof as guidelines for further studies.

1) PLANNING OF THE SYSTEMATIC MAPPING STUDY
This SMS offers an overview of the current literature and has the following goals [30]: to structure the type of study and outcomes by classifying the?eld; identify the frequency of publications in the scheme based on these categorie; specify the coverage of a specific domain; gather outcomes to answer specific research questions; and present a visual summary by mapping the results.
The process of an SMS contains the following five phases, each of which has a specific outcome, as shown in Figure 2  Phase 4: Produce a classification scheme as the layout of the SMS, which is generally created based not only on elements extracted from the keywords of the abstracts of the essential and current studies but also on the researcher' knowledge of the selected domain of study.
Phase 5: Extract data from the selected studies and map them by using a classification scheme to specify the trends in the research field.

B. RESEARCH OBJECTIVES
This study aims to achieve the following research objectives To gain a better understanding of these challenges, we reviewed the recent literature and considered it to be a primary source to answer our research questions. Initially, to perform a general review of the results of various searches within academic databases, an SMS is conducted by exploring the current literature, concentrating on a classification scheme and a thematic analysis of studies. The following steps clarify the systematic search of the literature and the selection procedures based on the criteria for inclusion and exclusion, the classification scheme, and the extraction of data for the mapping process.

D. LITERATURE SEARCH
Our study began in January 2020, and based on the rapid advancements in mobile devices and recent emerging technologies and to remain current with the updates in research studies, we did not target older studies; to review more studies, we considered studies conducted during the previous ten years, i.e., between 2009 and 2019. The literature search process started with the Web of Science (WoS) database, and we then queried the other electronic databases of Scopus, the IEEE Xplore Digital Library, the Education Resources Information Center (ERIC), and Science Direct. The aim of using various types of academic database search engines was to obtain an overview of the current status of the research in the selected domain to give us a better understanding of the research problem. We selected databases that were most closely related to the field of research, that were reputable and that represented the largest available scientific online resources. A secondary method of searching called a snowballing technique was also used, which involved looking up the references of references and bibliographies by using key articles or books related to the research topic [31].

E. SEARCH TERMS
To find relevant articles, the search was started in January 2020 using the WoS database followed by searching the other databases with several keywords (i.e., design, instructional design, mobile training courses, mobile-based training courses, mobile learning courses, mobile professional learning courses, course content, and learning materials). To narrow the search process, a Boolean We prepared for the filtering process by defining criteria for the inclusion and exclusion of articles. This allowed us to eliminate and select relevant studies via a selection process. In addition, we exported studies to the EndNote reference management software to gather the results and complete the filtering process.  Table 1 summarizes the inclusion and exclusion criteria applied in the process of selecting papers. Table 2 shows the results of the database searches, the implementation of the inclusion and exclusion criteria in the filtering process, and the snowballing technique.

H. EXTRACT THE KEYWORDS OF ABSTRACTS (CLASSIFICATION SCHEME)
This step involves the process of designing the classification scheme, as shown in Figure 3.
1) Reviewers read the abstracts and look for keywords and concepts related to the research objectives by using synonyms.
2) If the abstracts do not contain sufficient information, the researcher reads the introduction, conclusions, and other sections if necessary. VOLUME 8, 2020 3) When keywords are found, a classification scheme can be established, and articles can be sorted into categories.

IV. RESULTS
This section presents the outcomes of our study that offer answers to the research questions. The first research question is: RQ1 -What are the existing challenges in the design of mobile training content? To enable a better understanding, these challenges have been classified into three categories as follows. (a) Technological challenges are associated with technological issues such as problems that relate to application development, challenges in mobile networks that relate to network availability, difficulties in optimization when designing interactive content since the learning process occurs in spatiotemporal contexts and issues with the physical specifications of mobile devices and operating systems. (b) Pedagogical challenges include issues related to learning theories, strategies and instructional design guidelines. (c) Managerial challenges involve administrative problems, such as a lack of training in the skills necessary to design mobile content and poor project management skills, for instance, miscommunication among project team members and design team members that hinder the delivery of good content on time. From the results of this classification, we developed nine subcategories to examine these challenges in more detail, as shown in Figure 4. Table 3 shows the number of challenges in each category, as extracted from the selected studies. Seventeen studies emphasized the existence of technical challenges, and 8 of these 17 studies were related to application development challenges, which represents the major technical issue. Twenty-four studies indicated challenges that related to a lack of implementation and development of effective methods for the design of the mobile training/learning content of 50 studies classified under pedagogical challenges. Consequently, these challenges have been identified as the main issue in this category. Regarding the managerial challenges, only 2 of the 3 studies in this category underlined the existence of challenges related to poor project management skills in mobile learning projects. The third study emphasized a lack of training skills in designing professional mobile training content. This indicates that based on the classification of these studies, the major challenges in the design of mobile training/learning content are pedagogical challenges, and most of the challenges relate to a lack of implementation and development of effective methods for designing mobile training/learning content, such as the use of pedagogical models, frameworks or learning strategies. The category with the fewest challenges is managerial challenges.
The   A total of 58 articles were selected that were published in various academic journals. The chart in Figure 5 shows the numbers of journal articles on the design of mobile training content published during the last 10 years. It can be observed that the highest number of publications occurred in 2019; the lowest occurred in 2010. This result reflects the scarcity of publications in the field at the earlier stages of the mobile device technology revolution.
Our study found that these 58 studies were published in 48 academic journals. We grouped the results of the 48 journals into three categories, which represent journals that had published one, two and three articles.
There are 2 journals categorized with three articles, 6 journals are classified with two articles and 40 journals published only one article. Table 5   on this research topic. This result clearly reflects the trends in the publication of journal articles related to the research topic.
The fourth research question is RQ4 -What is the most widely researched topic in the studies of the design of mobile training content during the last five years? To identify the most prominent topic studied in recent years, we categorized the most common topics in each year between 2014 and 2019 based on the selected studies. For instance, in 2014, there were two studies on the topic of mobile game applications and another two on the usage of MAR technology as a learning tool in the design of mobile learning content. However, in 2015, there were no prominent topics. In 2016, most attention was given to the use of mobile devices for language learning and the design of mobile applications for motivation in learning and for the enhancement of the education process. Three studies in 2017 mostly concentrated on the frameworks for designing and developing mobile learning via mobile technologies. The topics studied in 2018 were varied and did not focus on a specific area.
In 2019, most studies focused on the use of MAR technology as a learning tool in the design of mobile learning content. Table 6 summarizes the most widely researched topics in the studies each year between 2014 and 2019.
We can see from Table 6 that the predominant topic in related studies during the last five years is the use of MAR in the design of mobile learning content. Figure 6 shows a bubble plot of the main topics of the selected studies by year.
It is clearly shown in the bubble plot that during the last five years the predominant topic in related studies is the use of MAR technology as a learning tool in the design of mobile learning content. This was expressed in 2 studies [S12] and

V. DISCUSSION
The main objectives of this SMS were to investigate the current challenges and identify the key open issues in the design of mobile training content (m-learning course content). We followed the guidelines provided by Petersen et al. [28] to conduct an SMS, in conjunction with the approach used by Kitchenham et al. [29] to ensure a better understanding of the outcomes of this study.  Based on 58 studies selected from five academic databases, we identified several challenges and extracted various key open issues for conducting future studies related to the research topic of this study.
To answer the first research question, the challenges were classified into the following three categories: technological challenges, pedagogical challenges and managerial challenges. To allow us to examine these challenges in more detail, they were grouped into subcategories, as shown in Figure 4. The outcomes shown in Table 3 indicate that 17 studies emphasized the existence of technical challenges in the design of mobile training content. The subcategory of technical challenges includes four types of challenges related to application development, wireless networks, physical specifications and operating systems. Application development issues include the development of mobile application interfaces, software integration, the development of mobile educational games and the authoring of learning tools. Challenges in a mobile network are related to facing limited network resources while the learning process is executed across a networked environment, such as the design of video content as learning material to be broadcast via mobile device networks for a large number of learners and the design of interactive content including multimedia, social communication and GPS. As a result, these subjects become a challenge for determining the best practices to design and deliver interactive and communicative content. Consequently, these challenges must consider the infrastructure characteristics of various types of mobile device networks to ensure the delivery of appropriate and good quality mobile learning content to trainees at different locations with different network configurations. The third subcategory is related to the physical specifications of mobile devices, such as their small screen sizes, substantial weight, low memory and limitations on battery life. The fourth type relates to an operating environment, such as following standards for the design content to be compatible for different environments.
Because the outcome has identified that the major technical issue related to application development, this indicates a lack of programming skills to support the latest mobile technology. Conversely, the lack of challenges related to physical specifications emphasizes the point discussed in the literature that physical issues such as hardware capabilities will become temporary challenges because of rapid developments in mobile device technology. Therefore, more attention is required to improve the skills related to application development.
Of these studies, 50 identified the existence of pedagogical challenges in the design of mobile training content, while only three mentioned the existence of managerial challenges, as shown in Table 3. Pedagogical challenges are therefore considered to be significant in the design of mobile training/learning content. However, managerial challenges represented a minority and were related to poor project management in mobile learning projects [6] and [17] as well as the need for training design team staff to perform their tasks professionally and to follow the development of emerging mobile technology [6].
Pedagogical challenges can be divided into the following three subcategories: (a) the need for identifying and using instructional design guidelines to design mobile training content; (b) the adaptation and appropriation of mobile learning technology for learning in a way that is compatible with suitable learning theories, objectives and actual training/learning needs; and (c) the lack of the implementation and development of effective methods to design mobile training/learning  VOLUME 8, 2020 content, such as the use of pedagogical models, frameworks or learning strategies.
Nine [12], [6], [46]- [52] of the 50 studies indicated that there were challenges that related to a need for identifying and using instructional design guidelines to design mobile training content. Seventeen studies [6], [44], [53]- [67] mentioned challenges related to adapting and appropriating mobile learning technology to learn in a compatible way with suitable learning theories, objectives and actual training/learning needs. The remaining twenty-four studies [1], [6], [8], [10], [12], [16], [37], [38], [45], [15], [68]- [81] confirmed the existence of challenges related to a lack of implementation and development of effective methods for designing mobile training/learning content, such as the use of pedagogical models, frameworks or learning strategies. These represented the majority of the 50 studies that mentioned pedagogical challenges, and this is therefore considered to be the most significant pedagogical challenge in the design of mobile training content.
This result supports what has been found in the literature in terms of pedagogical challenges, for example, in [10], [12], [6], and [14]. Other studies have also presented recommendations; for instance, [18] suggested establishing a theoretical model that uses strategic methods. In addition, UNESCO conducted research in 2013 [19] that focused on the year 2030 and that recommended linking learning theories to analyses of mobile learning to enhance and support the professional development of teachers via mobile training, to train teachers in the design of material, and to promote mobile learning for all. A concerted effort is therefore needed to overcome these challenges.
Although few of these studies emphasized managerial challenges, which include a lack of training in the skills necessary to design mobile content and poor project management skills in mobile learning projects, we believe that staff training must be a high priority and that a lack of staff training is critical, especially in remaining current with the rapid development of emerging mobile technology. In addition, managing the process of designing mobile training content is also significant, and a critical factor is the cooperation and communication among all project team members with the design team following proper procedures, such as establishing standards and guidelines and dividing the work into phases and the product into small segments, to make the best use of the available resources [17]. However, this issue remains an important aspect of the on-time delivery of a professional design of mobile training content of high quality. Consequently, the outcome of poor project management skills in mobile learning projects indicates that proper project management techniques must be implemented [6]; therefore, future studies should examine the importance of this challenge.
To identify the key open issues, we selected the latest studies and classified the results into a list based on the proposed future studies every year. Table 4 presents a list that contains 23 key open issues extracted from studies published between 2014 and 2019. We found that there were various modern topics that remain current with the developments in the research topic field. For instance, in 2014, studies proposed that further research should be conducted regarding adapting suitable electroencephalography (EEG) devices, integrating software frameworks with the available games related to mobile learning such as in [S39], and utilizing mobile sensor technology in mobile learning for workers as indicated in [S54]. In 2015, researchers in [S6] suggested concentrating on eye-tracking and brain wave monitoring technologies to analyze the attention of students when viewing the course content. Four topics were identified in 2016. One study [S2] recommended concentrating on the learner and the use of multimedia strategies combined with interface design and learning theories. However, the same study [S2] also proposed the use of sensors within the human body to establish communication with outside devices and the increased usage of biometrics. A third study [S10] suggested supporting mobile learning/training content in the health sciences by developing mobile applications, and a fourth study [S40] proposed the use of more appealing techniques such as location-based AR applications, animations and 3D models.
New topics were proposed in 2017 and 2018. These topics included the use of a conceptual framework as a guideline for creating and designing course content for mobile training as mentioned in [S42] and the evaluation of mobile authoring tools in [S1] and [S8], which suggested the use of QR codes in designing content for workplace environments (MAR technology). In 2018, one study [S28] proposed developing the Visual Environment for Designing Interactive Learning Scenarios (VEDILS) framework, a second study [S22] suggested developing mobile learning tools that consider cultural background, and a third study [S17] proposed embracing techniques such as artificial reality to build virtual worlds.
However, the largest number of key open issues was identified in 2019. Three studies [S5], [S31], and [S38] focused on the integration of MAR technology into teaching, training and designing interactive activities at different educational levels. One study [S45] recommended the use of frameworks for the design of motivational mobile training content for the vocational training domain and identified another issue related to utilizing artificial intelligence techniques. Additionally, another study [S37] proposed advanced object recognition libraries to identify real objects and including social network features into the authoring tools. One study [S14] suggested incorporating appraisal methods into the designed content to evaluate the learner's performance. Some studies supported the improvement or updating of usability guidelines for the design of good mobile content as in [S52] by promoting device compatibility, increasing the ease of navigation and the readability of content, and optimizing the format of the design of mobile training content as indicated in [56].
As previously mentioned, some studies proposed more than one issue in one year. For  These issues are therefore important and should be considered in the design of mobile content to be delivered via mobile devices; these key open issues support the recommendations of the outcome of our study to overcome the existing challenges in designing mobile training content.
We found that a few journal articles were published each year in this area during the previous ten years as illustrated in Figure 5. Only two articles were published in 2009, and then, the number of articles declined to only one article in 2010, which represented the lowest number of publications between 2009 and 2019. The number of research papers related to the design of mobile learning/training content rose from one in 2010 to nine in 2019. Although the number of publications declined during the initial stages when there was limited development in mobile technology, we observed in this study that research activities began to grow during the last five years and reached a peak in 2019. This can be explained because of extensive developments in mobile technology and the widespread use of mobile devices in recent years. We found that 48 journals published a total of 58 articles. The journals were categorized into three groups based on whether they published one, two or three of the studies. The results showed that two journals published three articles each, and six journals published two articles each. The remaining 40 journals published only one article. We identify eight prestigious journals in which these studies were published. These journals are considered to be the top in the field, and they published 18 of the articles on the design of mobile training content, as shown in Table 5 The results showed that although 48 journals published articles related to the research topic during a 10-year period, the total number of journal articles was relatively low. However, the top eight journals are clearly reputable, and they recognize the importance of articles on the design of mobile training content. Due to this low level of research attention, there is a need for more investigation in and a focus on this area. Future studies should therefore investigate how to stimulate the number of publications by considering the integration of the capabilities of mobile devices as an emerging technology into the design of interactive mobile training content.
We also identified the major topics addressed in the selected papers between 2014 and 2019, as shown in Table 6. Based on this, we categorized the most common topics each year; during the five-year period, there were 18 in total, as presented in Figure 6. Finally, we specified the most popular topic of the most commonly addressed topics during the five-year period.
The results showed that the topics addressed varied over the years. For example, the topics studied in 2015 and 2018 were different, and the studies did not focus on a specific area.  Figure 6, which presents a bubble plot of the topics covered in related studies by year of publication. This could be interpreted as a result of the new revolution of mobile technology. However, our results reveal that more research on implementing and developing effective methods for the design of mobile training/learning content, such as the use of pedagogical models, frameworks and learning strategies, is required to overcome the most significant challenges in the design of mobile training content.

VI. LIMITATIONS
Various issues need to be considered when generalizing the outcomes of this SMS. Our study is limited in terms of the criteria selected and the research method followed. We used five online academic databases and considered only journal articles published in an electronic format between 2009 and 2019. The outcomes may change if a search of articles from a different period and from different databases is conducted. In addition, certain papers were excluded since we did not consider conference papers from this period, and there may be some papers that were not published online.
However, we selected the most appropriate search strings, utilized the search engines of specialized academic databases, adopted a snowballing technique and followed a systematic classification process to gain an overview of the current status of research in this field, which results in a better understanding of the research problem. Consequently, this study clarifies the current challenges and identifies open issues to thus contribute to future research and the development of this field.

VII. CONCLUSION
This SMS provided a review of the existing knowledge based on studies related to the design of mobile training content and it identified future challenges and key open issues.
We conducted a systematic search of the literature by using a method of selection that applied criteria for inclusion/exclusion and data extraction and synthesis strategies. We followed a classification process to answer our research questions. This study examined the current challenges, specified the key open issues in the design of mobile training content, and identified publication venues, trends in journal publications, and the topic of greatest interest in the published studies. Interesting issues and trends related to the research in this area were also emphasized.
The outcomes of our study showed that pedagogical challenges are considered to be the major challenges in the design of mobile training/learning content. We observed relatively few published research articles in 48 journals. It can be argued that researchers' attention has been mostly on technology. The number of publications has slightly increased recently, and this result shows the need to expand the number of publications. However, most of the articles did not focus specifically and deeply on the design of attractive, interactive and motivating mobile content based on a theoretical framework for mobile training courses. Therefore, there is a need for more investigation and refocusing in future studies, which should examine the possibility of developing this issue.
Based on our knowledge and a literature search of this important and constantly evolving field, we did not find comprehensive SMSs of the design of content specifically for mobile training courses. Therefore, this systematic mapping review can offer a better understanding of the research area, and the findings offer solutions for future studies on existing challenges and contribute to the development of this domain. SURAYA BINTI HAMID received the bachelor's degree in information technology (industrial computing) and the master's degree in information technology (computer science) from The National University of Malaysia, in 1998 and 2000, respectively, and the Ph.D. degree from the Department of Computing and Information Systems, The University of Melbourne, Australia, in 2013. She has supervised many master's and Ph.D. students with the University of Malaya, Malaysia. She has supervises students' activities as a Post Graduate Club Advisor and the Head of the Think Tank Talent Recruitment for undergraduate. She is currently an Associate Professor with the Department of Information Systems, Faculty of Computer Science and Information Technology, University of Malaya. She has published several articles in respectable ISI and Scopus indexed journals. She has presented at various international conferences. She has involved with the Association of Information Systems (MyAIS), (AIS), AKRAB UM, and the participation at various community projects. Her research was widely acknowledged. Her research interests include open data, learning analytics, information, computer and communication technology (research method in information systems, qualitative research, mix method, thematic coding, and content analysis), emergent information technology (information seeking, online behavior and its impact, activity theory, and social technology appropriation), web technology (online social networking in higher education), information services and e-learning, social media, e-government, and IS for sustainability (green IS). Her research on online social networks in higher education was recognized by the Australian Government. She received the Australian Education International (AEI) Bursary Award at ISANA, in 2011, and the Google (Australia) Travel Prize, in 2012.
ABDULLAH BIN GANI (Senior Member, IEEE) received the Diploma degree in computer science from ITM, the B.Phil. and M.Sc. degrees in information management from the University of Hull, U.K., the Ph.D. degree in computer science from The University of Sheffield, U.K., and the Teaching Certificate from the Kinta Teaching College, Ipoh. He has a vast teaching experience in a number of educational institutions locally and abroadschools, such as the Malay Women Teaching College, Ministry of Education, the Rotterham College of Technology and Art, Rotterham, U.K., and the University of Sheffield. He is currently a Professor with the Dean of the Faculty of Computing and Informatics, University Malaysia Sabah. He is an Honorary Professor with the Department of Computer System and Technology, Faculty of Computer Science and Information Technology, University of Malaya, Malaysia. He is also working on mobile cloud computing, big data, and the IoT. He has published more than 150 academic articles in respectable journals internationally with top ten % ranking. He received a very good number of citation in Web of Science and Scopus databases. His interest in research kicked off, in 1983, when he was chosen to attend the three-Month Scientific Research Course with RECSAM, Ministry of Education, Malaysia. His current research interests include self-organized systems, machine learning, reinforcement learning, and wireless related networks. He was elected as a Fellow of the Academy of Sciences Malaysia (FASc) for Engineering and Computer Science Discipline.