Can Be Applied for Bluetooth Low Energy to i-construction? : Characterization of BLE on Excavator

The Government of Japan has declared that construction sites must implement i-construction, in which information communication technology (ICT) is used to improve construction site productivity and throughput. Examples of iconstruction include remote and automatic control systems for construction machinery, for which wireless communication is generally required; however, the wireless characteristics that must be considered in i-construction remain unclear. To clarify these characteristics, Bluetooth Low Energy (BLE) beacon module is implemented for construction machines, and we investigate its communication characteristics. We focussed on two evaluations: characterisation of the BLE unit module and its feasibility in i-construction. Experimental results were used to characterise BLE communication in a residential area and construction site. In the latter, BLE beacon module communication was possible between a sender located in an excavator attachment and a receiver in its cabin.


Introduction
An earlier version of this paper was presented at the 12th International Workshop on Autonomous Self-Organizing Networks (ASON 2019) [1]. Please read the additional evaluations reported in the body of this paper.
How can the efficiency of labourers at a construction site be increased? To solve this issue, Japan is promoting work style reforms. In 2015, the Ministry of Land, Infrastructure, Transport, and Tourism of Japan declared that construction sites must drastically introduce information and communications technology (ICT) to improve productivity and throughput throughout the construction site [2]. This approach is called iconstruction. In i-construction, many ICT technologies are applied to the operation of a construction site. Remote [3] or automatic [4] control of construction machinery such as excavators, cranes, and other vehicles has been widely discussed. Automatic measurement of construction sites using drones is also another popular approach. Other i-construction applications include preventive maintenance [5] for construction machinery. In prevention maintenance, data gathered by various sensors installed on the construction machines are sent to servers via cellular or satellite networks. Each communication solution in i-construction is based on an Internet of Things (IoT) system. In IoT systems, networks comprise numerous data-gathering IoT devices (such as sensor devices), with each device sending data through wireless communication. Therefore, specific requirements specific to IT devices such as desktop PCs and servers must be met in order to implement these kinds of networks. For example, because IoT devices can be directly accessed by anyone at any time, additional security solutions must be Strength Indicator (RSSI) and communication availability using real machines was analysed.

Organisation
The rest of this paper is organised as follows. Section 2 describes the works related to this study. The experimental evaluation of BLE using a beacon module is explained and the results are discussed in Section 3. Section 4 describes the characteristics evaluation of BLE communication in construction excavators. Finally, Section 5 concludes this paper and explains future work.

Related works
This section summarises the works related to this study.
In the area of Bluetooth communication, there have been many fundamental studies. For example, previous work [7] provides an overview of the communication and architecture of the Bluetooth framework. In addition, some protocols for operating Bluetooth are also summarised in this study. More recently, BLE has been proposed and evaluated for inter-vehicular communication [8]. From this work, BLE can be used to form the infrastructure of Vehicle-to-everything (V2X) systems. BLE can be used to construct not only a singlehop network (in which the sender and receiver communicate directly) but also multi-hop networks (in which the sender and receiver communicate via multiple terminals). The performance of Bluetooth-based multihop networks was evaluated in [9]. From the findings of this study, Bluetooth-using terminals were found to be able to communicate over long distances via relay terminals. The studies discussed above demonstrated that Bluetooth is a very useful communication protocol. In addition, the impact of Bluetooth communication broadcast has also been investigated [10]. This study shows that careful network design is necessary. Some studies have evaluated the Bluetooth communication characteristics considering an IEEE 802.11 and a ZigBee coexistence environment [11,12]. However, previous studies do not focus the communication characteristics considering construction machines such as excavators. This study evaluates it using real machines. The results of this paper are useful to design the application for construction machines using BLE.
Applications of Bluetooth communications have been described in many papers. For example, [13] describes a position prediction method using BLE. In [14], a system for multiple device control was implemented using Bluetooth.
Actuator control using Bluetooth communication is evaluated in [15] and a manufacturing network application using Bluetooth is discussed in [16]. Although much research effort has focussed on the topic of Bluetooth, the literature on the detailed evaluation of Bluetooth communication characteristics is still insufficient. In particular, there have been fewer experimental evaluations of the use of Bluetooth in construction machinery than on the use of Wi-Fi or WiMAX [17]. Thus, the evaluation results described in this paper can be utilised as implementation guidelines in i-construction. In the next section, experimental evaluations of BLE communication in two specific environments are presented. The characterisation was conducted using a BLE communication module installed in different excavators. Note that this paper is not Can Be Applied for Bluetooth Low Energy to i-construction? : Characterization of BLE on Excavator 3 proposing the application using BLE considering construction machines. In future, we will propose some applications based on this paper's results.

Evaluation of Bluetooth Low Energy in Various Situations
This section shows the experimental evaluation using a Bluetooth Low Energy beacon module. Specifically, we focused on two environments: a public road and a construction site. Note that, because this section reports on a basic version of the study described in the following section, we describe a simple experimental set up. Moreover, we used black box test for testing approach.

Experimental Settings for Unit Test
The sender was a beacon module using BLE 4.2 with a communication distance (Tx power) of 100 m. The receiver was an iPad mini. Two points were evaluated: • The relationship among the distance between the sender and the receiver, the RSSI, and the communication establishment time (Tc) • The relationship among the distance between the sender and the receiver, the RSSI, and communication success percentage (Ps) Tc is the time difference between the start of beacon transmission and the completion of reception. Ps is the percentage of beacon signals received for the trials at each distance. As this study represented the first step in assessing the applicability of BLE for i-construction, we used a simple evaluation item. In the future, we will carry out evaluations for BLE using more detailed and complex      characteristics. The evaluations were carried out at a public road ( Fig. 1) in Chofu, Tokyo and at a construction site ( Fig. 2) in Sagamihara, Kanagawa. Heavy vehicular traffic was observed in the public road and some excavators were working at the construction site, indicating that the network environments of both areas changed dynamically. However, the trends were nearly the same at the two sites. The beacon module was fixed and the iPad (Fig. 3) was mobile. The RSSI, Tc, and Ps were measured over a distance from 50 to 150 m. At each distance, 10 trials were conducted and the average was recorded as the result. The payload size was 70 bytes. The BLE communication mode was set as connection. The total measurement time was 60 s. The evaluation system's algorithm is summarized in the Appendix A (see Algorithm 1 and Algorithm 2).

Experimental results and discussion in each situation
First, we show the results obtained at the public road. Fig.  4 shows the relationship among communication distance, RSSI, and Tc. Additionally, Fig. 5 plots the relationship among communication distance, RSSI, and Ps. As shown in Fig. 4, Tc increased when the distance increased. Furthermore, RSSI decreased when the distance increased. As shown in Fig. 5, as the distance increased, Ps showed a decreasing trend. These results indicate that a decrease in radio field intensity affects the communication establishment time and hence the success rate of the communication. The maximum difference of Tc was 25 s. In Fig. 5, the percentage appears to decrease significantly

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Internet of Things 07 2020 -10 2020 | Volume 6 | Issue 23 | e3 (40%) when the distance reaches 100 m. The reason for this is the interference of other radio waves. Fig. 6 shows the utilisation of the 2.4-GHz band at a distance of 100 m (by Wi-Fi analyser [18]). It is seen from the results that many radio wave generators were present. In this experimental location, there were many residences near the area that probably have Wi-Fi access points, a factor that appears to have adversely affected the results.
Second, we show the results obtained at the construction site. Fig. 7 describes the relationship among the communication distance, the RSSI, and Tc; Fig. 8 shows the relationship among the communication distance, the RSSI, and Ps. In this evaluation, results were measured at distances of 50, 80, and 100 m. In Figs. 7 and 8, the dotted black circles indicate the results of communication outside prospect, in which the excavator serves as an obstruction. As shown in Fig. 7, the difference between the in-and out-of-prospect Tc is 7 s, indicating that the communication establishment time is strongly affected by the obstruction. The reason for this is the decrease in RSSI introduced by the obstruction. It is difficult to implement stable communication at this RSSI (approximately -90 dBm). As shown in Fig. 8, the outside prospect communication results differ significantly from the in-prospect results; in terms of Ps, the results differ by 60%. From these results, we can conclude that obstructions must be considered when implementing a BLE network. Fig. 9 shows the utilisation of the 2.4-GHz band at a distance of 80 m. It is seen that, although the utilisation trend is the same as in the public road (Fig. 6), the number of radio wave generators is reduced. The reason for this is that the experimental area was surrounded by an industrial park; as a result, there were few Wi-Fi access points and the distances between the radio wave generator and beacon module or iPad were    smaller than in the public road case. Finally, we evaluated the effect of the payload size of the Tc and Ps. Fig. 10 shows the relationship among payload size, Tc, and Ps at a distance of 50 m in construction site. The results show the communication in the prospect case. As shown in the figure, the difference in Tc is approximately 1 s and Ps is 100% at each payload size. Therefore, in this evaluation the payload size did not have a significant impact on the BLE communication performance.

Evaluation of Bluetooth Low Energy using Excavators
This section describes an experimental evaluation using a BLE beacon module installed in several excavators. We used three different excavator classes. Note that, the testing approach is same as Section 3. Same as Section 3, furthermore, the evaluation system's algorithm is summarized in the Appendix A (see Algorithm1 and Algorithm 3).

Experimental Settings for Unit Test
In the evaluations, a beacon module using BLE 4.2 was used as a sender (communication distances: 10 and 30 m) and an in-vehicle computer housing an Intel Atom E3950 (Apollo Lake) processor made by Interface [19] was used as a receiver. We evaluated the relationships among distance between sender and receiver, RSSI, and communication establishment time (Tc). Note that the evaluations described in this section focused only on Tc because the communication success percentage was either 0 or 100%. When Ps was 0%, the result of Tc was NG. In the evaluation, we used an application program to receive  the signal produced by a beacon module. Unlike the system used in Section 3.1, the application of the receiver took approximately 10 s to display results; therefore, when a signal was successfully received, the Tc used as the evaluation time was considered to be 10 s. That means that the results were the same when the connection was established in the first attempt (10 s). The total measurement time was 60 s.
In the evaluations, we attached the beacon module to several excavators. Specifically, we used an SK135-5 [20], an SK470- 10 [21], and an SK210DLC-10 [22], which are examples of 13-, 45-, and 20-ton class excavator, respectively. The latter is used for metal recycling. In the evaluation, the module was attached to both the SK135-5 and SK470-10 via a nibbler used for building demolition. For the SK210DLC-10, the sender module was positioned on the excavator's magnetic attachment (Fig. 11); in all three cases, the receiver's antenna was placed in the cabin (Fig. 12). The utilisation of the 2.4-GHz band in the evaluation area was measured by a Wi-Fi analyser and is shown in Fig. 13. As shown in the figure, the signal strengths of other wireless communications sources were not large. The sender was placed in a bracket (Fig. 14) with an opening for transmission of radio waves. The hole was set near the BLE antenna. If the module were mounted upside down, the antenna would be further from the radiation hole; therefore, we evaluated the two installation cases. We changed the posture of the excavator (Fig. 15) several times during each evaluation. The different postures were (1) Parking, (2) Highest, (3) Furthest, and (4) Dismantling work. In each posture, the distance between the sender and receiver changed, and we measured the RSSI and Tc for each posture over 10 trials and averaged the results. The payload size was 70 bytes and the BLE communication mode was set to connection. Fig. 16 shows the results when the beacon module's communication distance was set to 10 m. As shown in the figure, the receiver was able to receive the beacon signal in the Parking, Furthest, and Dismantling work postures. Therefore, the Tc of each case was 10 s. Moreover, RSSI was approximately -65 dBm. In the Highest posture, however, the beacon signal was too weak and could not be received. The RSSI was approximately -95 dBm. From these results, when the excavator was in the Highest posture, communication could not be established at a beacon module communication distance of 10 m,. Fig. 17 shows the results at a communication distance of 30 m. As shown in the figure, unlike the previous   9 however, communication could be established between the sender and receiver. Thus, we can conclude that BLE communication can be applied in the use of 13-ton class excavators.

Results for SK-210DLC-10
This subsection shows the results obtained for the SK210DLC-10, a 20-ton class excavator. Excavators of this class can be equipped with a magnetic attachment for gathering metal and scrap, making them useful in metal recycling factories. In these analyses, therefore, we evaluated the BLE communication characteristics with the magnetic field effect taken into consideration under three magnet operational patterns (Fig. 21): • Magnet ON continuously; • Magnet OFF continuously; • ON/OFF switching (changing every 3 s).
In the evaluations, we focused on the results for Tc and RSSI according to under different magnet operational patterns. Fig. 22 shows the posture of the SK210DLC-10, which in this case remained constant for all tests. In the evaluations, the sender was fixed to the magnet attachment and the receiver was located in the cabin. Note that the trend of utilisation for the 2.4-GHz band in the evaluation field was the same as that shown in Fig. 13. Fig. 23 plots the relationship among RSSI, Tc, and the magnet operational pattern. As shown in the figure, the operational did not have a significant effect on the RSSI. Thus, communication could be established normally. Note that communication could be carried out continuously for periods of 60 s. From these results, we can conclude that BLE communication can be applied to a wide range of construction machinery, thus verifying the applicability of BLE to i-construction.

Limitations and constraints
This study includes some limitations and constraints as follows: • Evaluation system In this study, we used simple evaluation system. Communication module is only sending/receiving the BLE signal, and we do not implement any applications that considering communication QoS/QoE, and control of construction machines. Therefore, the functions of the system are simple, only sending the signal by BLE beacon and showing the result by tablet PC (monitor). Our evaluation showed the results with the above assumptions.

• Evaluation metrics
In the evaluation, we showed the characteristics of BLE considering construction machines using RSSI, the communication establishment time, and communication success percentage. This study is the first step for wireless communication characteristics considering construction machines. Therefore, we use simple metric. In future, we will evaluate the communication characteristics using other metric. After implementing applications such as iconstruction systems, especially, we will evaluate the usability of it using the metrics proposed by [23]. Also, we will show the analysis of i-construction systems using Use Cases Tool such as UML diagram.

Conclusion
In this study, BLE communication was characterised and demonstrated using several real pieces of construction machinery. Specifically, we focused on the applicability of BLE for i-construction. In the evaluation, a beacon module that can communicate using BLE 4.2 was used. From the experimental evaluations, we confirmed the following results: • We demonstrated the characteristics of communication in both a residential area and at a construction site. The results of this study confirmed the applicability of BLE for i-construction. We recommend BLE for iconstruction applications because of the payload size that can be communicated and the communication connectivity that does not so affect the size of the construction machines and the type of attachments for construction machines.
Future work will include more detailed evaluations carried out in environments with multiple senders to obtain a better understanding of loss rate and communication delay. We will also propose specific applications/systems of i-construction using BLE. After proposing i-construction systems, we will evaluate the usability of it. Moreover, we will analyze the systems using Use Cases tools. In Section 3, we focused the relationship between the connection and the distance between the sender and the receiver. Thus, we do not set the decision interval. On the other hand, in Section 4, we focused the maximum time to reach the signal. In other words, the motivation of the evaluation of Section 4 was to check that the signal (data) is received within the prescribed time. Therefore, we changed the receiver's algorithm in each evaluation.
Can Be Applied for Bluetooth Low Energy to i-construction? : Characterization of BLE on Excavator