Experimental study of signal behavior for wireless

Construction Innovation Experimental study of signal behavior for wireless communication in construction Zia Ud Din, Leonhard E. Bernold,

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To cite this document: Zia Ud Din, Leonhard E. Bernold, (2017) "Experimental study of signal behavior for wireless communication in construction", Construction Innovation, Vol. 17 Issue: 4, pp.475-491, https:// doi.org/10.1108/CI-11-2016-0061 Permanent link to this document: https://doi.org/10.1108/CI-11-2016-0061 Downloaded on: 20 November 2017, At: 09:09 (PT) References: this document contains references to 30 other documents. To copy this document: [email protected] The fulltext of this document has been downloaded 13 times since 2017*

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Experimental study of signal behavior for wireless communication in construction Zia Ud Din School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona, USA, and

Leonhard E. Bernold

Signal behavior for wireless communication 475 Received 20 November 2016 Revised 17 March 2017 Accepted 20 April 2017


Department of Civil Works, Federico Santa María Technical University, Valparaiso, Chile

Abstract Purpose – The purpose of this study is to understand the effects of building components of a growing concrete structure and different building materials such as glass and steel on Wi-Fi signals propagation in a construction site. Wireless local area networks are considered effective tools to link the islands-ofcommunication in construction. Still, designing a Wi-Fi network that can grow with a new construction requires that one understands the performance of propagation of electromagnetic signals transmitted at 2.4 GHz. Design/methodology/approach – This paper reviews the theoretical behavior of electromagnetic signals when signal attenuation is caused by various construction materials changing their strengths, directions and possibly leading to total absorption. The authors used a typical building layout to conduct experimental work to measure the effect of common building features and communication technologies on signal strengths. Findings – The measured data not only confirmed the theory-based predictions but also demonstrated the complexity of predicting signal propagation when obstructions inhibit the line-of-sight “travel” of electromagnetic signals. Originality/value – Different to other papers, the experiments were conducted outside a concrete building mimicking the situation where the transmitter is set up at the site office. Keywords Innovation, Signal attenuation, Signal strength, Signals’ behaviour on construction site, Wi-Fi network on construction site, Wireless signals Paper type Research paper

Introduction Observers of the construction industry have witnessed a slow adoption of information technology (Heller and Orthmann, 2014). It is mostly implemented in the main office where it found first uses in administration, estimating, planning, accounting and computer-aideddesign (Peansupap and Walker, 2006). Still, on-site work today relies primarily on paperbased communication (Zekavat et al., 2014). FIATECH (2009) presented a challenging vision that “construction sites will become more “intelligent and integrated” as materials, components, tools, equipment and people become elements of a fully sensed and monitored environment “[. . .] Construction job sites will be wirelessly networked with sensors and communications technologies that enable technology and knowledge-enabled construction workers to perform their jobs quickly and correctly”. Lee & Bernold (2008), Loosemore (2014) and Bernold & AbouRizk (2010) presented models and experimental results with

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agent-based wireless communication that would provide the backbone to FIATECH’s vision for construction. In a study, Nuntasunti and Bernold (2006) found that a significant number of the face-to-face meetings could be done through a virtual office reducing the major amount of time spent in traffic to and from the on-site meetings. Communication of critical information does not have to wait for the weekly meeting while the collection, analysis and dissemination of relevant data can be done automatically (Lee and Egbu, 2007). Williams et al. (2007) studied potential benefits through surveys in both the USA and South Korea. The responding practitioners expected major improvements in productivity of their work caused by the speed of communication (e.g. design approvals), the reduction of inefficient use of time while improved service to the customer. While they believed that the cost of installing a wireless network was a major barrier (Nielsen and Koseoglu, 2007), studying its use for a tunneling project asserted that: “[. . .] a wireless network covering the whole construction area represents a minor cost item. The necessary investment for wireless local area network (WLAN) and mobile collaboration is negligible”. Despite the rapid advancement of technologies dedicated to remote sensing, Luo et al. (2011) have highlighted that a major reason why construction companies are not collecting real-time data is the lack of a suitable information and communication technology (ICT) infrastructure. They proposed an ICT infrastructure model that provides site laborers direct access to information relieving them from time spent on material tracking, defect management, etc. A similar model was introduced by Bernold & AbouRizk (2010) which presents “[. . .] an integrated communication system consisting of two Local Area Networks (LANs), one high-speed cable and one wireless Ethernet, connected to the Internet”. They talked about a continuous and mostly automated supply chain of information, which is electronically transmitted where any team member allowed to enter the system has access. Because of a large amount of data being created by sensors or other analog devices, they recommend the use of intelligent software agents that monitor data flows enabled to make a decision within their specialty area. Implementation of the wireless sensor network to monitor the temperature, light and acceleration in and around the buildings was studied by Jang, Healy and Skibniewski (Jang et al., 2008). Over the past half-decade, the building information modeling (BIM) has gained wide acceptance in the construction industry, but its use is mostly limited to designing and planning (Zekavat et al., 2014). To use BIM on the job site to make information readily available to construction workers, a reliable, secure wireless network is a fundamental requirement. Similarly, the use of multimedia-capable devices and the cloud-based services also depends on wireless capacity (Ayyash et al., 2016). Therefore, one of the main challenges for wireless technology on the construction site is the availability of the robust wireless network. For example, recently in a study Wireless-Fidelity (Wi-Fi) performance was criticized for over-utilization and interference from rogue access points (Aps) (Sui et al., 2016). This study calls for designing a wireless network with an appropriate number of secured APs to solve the problems of over-utilization and security. As this short introduction demonstrates, a large number of researchers assessed the relevance and feasibility of the wireless technology and mentioned a robust and reliable Wi-Fi network as a backbone for the wireless infrastructure. But no researcher conducted wireless signal surveys in the built environment to determine the Wi-Fi signal behavior. The goal of the study is to understand the effects of building components of a growing concrete structure and different building materials such as glass and steel on Wi-Fi signals propagation. The study is an effort to provide a guideline for the design of a wireless network where availability of signals is not compromised due to the rugged

construction environment. Following two questions were answered to achieve the goals of the study: Q1. How do Wi-Fi signals behave across the construction site? Q2. How to provide reliable Wi-Fi coverage across the construction site?


To answer Q1, the authors performed tests to measure the effect of different building materials such as concrete, reinforcing steel, steel structure and glass, and other physical obstacles such as trees on wireless signals propagation when they strike with these materials and objects. The authors also measured the effect of elevation on signal strength. To answer Q2, a repeater was used to study its impact of on signals strength especially in the shadow areas where signal strength was insufficient to establish a wireless connection. The following sections will introduce the basics of wireless technology before presenting the results of experiments to test the behavior of radio signals around buildings. Key enabling technologies for wireless communication Over the past 10 years, an ever-increasing number of wireless tools have mushroomed creating not only new ways of communication but also very new markets. Each such application, however, depends on a network be it a W-WAN (wireless wide area network), WLAN (wireless local area network), W-PAN (wireless personal area network) or a W-MAN (wireless metropolitan area network). Novel commercial applications commonly originate with the creation of a new communication standard created in a long process by a volunteer group of the Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA). The first meeting for the IEEE 802, the family of standards dedicated to wireless networks most famously the Wi-Fi, met in 1980. For example, the media access control (MAC) coding rules are defined in the IEEE 802.11standard, which was introduced in 1997. This allows the different devices to identify each other on the hardware level, and “speak the same language” when communicating one-to-one or one-to-many. The 802.11 transmission protocol uses the frame structure to organize packets of data organized into fields that carry, for example, the MAC addresses of the receiver and the transmitter. Built into a network are routers that “push” the data packets across long distances to a wireless access point (WAP) able to read and understand the protocol from the transmitting device and translate it to the Ethernet protocol sending it to the ultimate receiver. A wide variety of antennas, connected to the WAP, receive and emits radio signals. The design of the antenna dictates transmission signal strength, area coverage and reception sensitivity (Engst and Fleishman, 2003). However, larger coverage will diminish signal strengths (Wilton and Charity, 2008). The standard IEEE 802.15.1 covers “Bluetooth” devices and IEEE 802.15.4 defines “ZigBee” network. It is interesting to know that the name ZigBee is related to the zigzagging of bees to find food and their method of communication with other bees in the hive. The communication range of Bluetooth is 10 to 100 m. The data transfer rates are 1 to 2 Mbps. The ZigBee networks data transfer rate is 250 Kbps, and signal transmission range is 10 to 100 m. Both types of network use very low power 10 to 100 mW (Zheng et al., 2009). The IEEE 802.16 family of standards known under the name “WiMAX” (Worldwide Interoperability for Microwave Access) belongs to the 4th generation (4G) of wireless communication standards. It promises to speed up the communication to one Gbit/s by streamlining the protocols of the involved systems. Internationally, WiMAX is finding a home in emerging markets that do not have a decent wired infrastructure (Rao and Radhamani, 2007). While ZigBee can manage communications among thousands of tiny

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sensors at close range, WiMAX creates efficient communication channels over distances of 25-30 miles. Visible light communication, which uses wireless optical in the wavelength interval of 380-780 nm, can provide seamless communication coverage indoor over long distances. It is hypothesized that the light fidelity (Li-Fi) technology (Deicke et al., 2012) will eventually support a data transfer rate of over 1 Gbps (Bao et al., 2015). Construction is one industry that is poised to benefit from these developments.


Behavior of 2.4 GHz Wi-Fi radio waves In 1947, the International Telecommunication Union (ITU) designated 12 bands for electronic communication. While both the 2.4 and the 5 GHz bands could be used for unlicensed usage, it was the lower frequency that became the commercial favorite and is now supports Wi-Fi, Bluetooth, phones and even baby monitors. The result is overcrowding of the band that can lead to interferences slowing down communication. The 5 GHz band is still relatively less crowded, but the consequence of the higher frequency is a smaller coverage and, most important for construction, a lower amount of signal penetration through solid objects. For these two reasons, the 2.4 GHz bandwidth is favored for data transfer and coverage in construction (Cisco Systems, 2008a). Recently, more Wi-Fi enabled applications, such as surveillance cameras, laptops and personal digital assistants are available, which use the 802.11x standard to link up to each other through an AP or connect to the internet. The 4G technology deployed by cellular companies is limited to mobile phones only. Therefore, Wi-Fi technology is more appropriate on the construction site able to transmit the data at 54 Mbps, sufficient to handle an average of 50 wireless cameras at a time (Bernold, 2006; Lee and Bernold, 2008). The IEEE 802.11 wireless standard specifies WLAN computer communication at 2.4 GHz, which translates into a wavelength of 125 mm. These electromagnetic waves, traveling at the speed of light, are transmitted and received via special antennas that are tuned to the same frequency band. The performance of such wireless systems depends on several factors related to the deterioration of the wave energy between the transmitter and the receiver referred as signal attenuation. Before installing a Wi-Fi network on a construction site, with ever-changing spatial parameters, one should be able to predict the signal strength at the receiver. On the other hand, data security concerns have to be considered adding to the complexity of the network design. Figure 1 presents a simplified construction site for a multi-story office building with an underground garage, one elevator shaft, a staircase and reinforced concrete slabs built on the site. As indicated, it is planned to install a Wi-Fi antenna on the roof of the on-site office building. What are the signal-related issues need to be considered when planning a seamless wireless network that reaches the entire building from beginning to end?

Figure 1. The changing spatial conditions of a growing construction project


Before discussing the result of field experiments targeted on measuring signal quality around obstacles, the paper will provide a quick overview of the principles related to signal, “weakening”. Principles of signal propagation While a transmitter emits the electromagnetic signals, and they propagate with the speed-oflight through the atmosphere more-or-less linearly in all directions. However, very seldom will a signal be able to follow a line-of-sight (LOS) to hit the antenna of a receiver. An LOS path also called a direct path, is a straight-line path that connects the transmitter and the receiver. With the absence of the LOS path, the transmitted signal could only reach the receiver through reflected, diffracted or scattered paths (Chen, 1999). Of course, as the signal spreads out like a circular wave, it will lose strength referred to as attenuation the rate of which can be calculated when traveling through the air. As soon as a wave hits an obstacle in its way, many different effects are possible including being very absorbed. Let us review the two most important issues. Free space signal loss. The signal loss is logarithmic which means that after a “cave-in” of amplitude close to the transmitter it is able to travel very far with minimal energy reduction. The unit used to define the energy drop of a particular antenna is decibel (dB). Davies et al. (2008) showed that following simplified model provides a reasonable approximation for 802.11 networks using 2.4 GHz: X PLðdBÞ ¼ PL0 þ 10 n logðD=d0 Þ þ (1) LOi Distance cumulative where: PLðdBÞ ¼ Total loss in dB along a path d PLoðdBÞ ¼ Loss at the distance do ðmÞ D ¼ Distance ðmÞ n ¼ Path loss exponet: Open space ¼ 2 Large inside space ¼ 3:3 PLðdo ¼ 1mÞ ¼ 41 dB ðfor 2:4 GHzÞ

(2)

Figure 2 presents two figures depicting the propagation and attenuation of signals from a directional antenna, which concentrates the radio energy into a smaller area, creating the main beam, thus increasing the apparent signal strength, referred to as antenna gain. Figure 2(a) shows a polar plot of the radiation from a directional 2.4 GHz antenna transmitting over a distance of 100 meters at which point the energy of the main beam has reached 0 dB. The existence of side and the back lobes point to the fact that not all energy can be focused into the main direction resulting in some energy being wasted. The beamwidth of an antenna indicates not only its gain but also the area that will be covered meaning, within which a receiver can get a signal. As mentioned earlier, obstructions to the LOS require the signal to “expend” extra energy to penetrate, if at all. Attenuation properties of common building materials. Similar to a tollgate, obstructions to a radio signal ask for a token in the form of an amount of energy that has to be spent to get through. The size of the token is proportional to the dielectric constant of the material. Table I presents the loss of energy of a radio wave passing through some common building materials (Bugaj, 2014; Davies et al., 2008; Jang and Healy, 2010; Ogunjemilua et al., 2009).


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Figure 2. Energy loss models of a directional antenna operating at 2.4 GHz

Material

Table I. Absorption values for different building materials

Hollow wooden door Hollow plasterboard/drywall Brick/block wall, marble cladding Steel rollup door Double pane coated glass window Reinforced concrete wall Glass, tinted Reinforced concrete wall

Thickness

Loss (dB)

4.5 cm 15 cm 7.5 cm 4 cm 2.5 cm 25 cm N/A 70 cm

4 9 5-8 11 13 16 21 25

According to function (1), the absorption losses of a radio signal having to pass through a certain material has to be added to the losses from traveling through free space. However, most often a signal wave hits an obstacle not at a perpendicular angle causing it to divert from its path. Signal reflection, diffraction, refraction and scattering. Reflection or bouncing from an obstacle surface is the most common propagation behavior of radio signals. However, on construction sites with many odd-shaped temporary structures, equipment and half-finished


components diffraction and scattering are equally important. Figure 3 depicts four important propagation features that redirect, hamper or even “capture” Wi-Fi signals. The layout presents a birds-eye view of the building site, laid out in Figure 1, made of reinforced concrete slab, elevator shaft, round columns and a staircase. A transmitter has been mounted on the roof of the office trailer, and a receiver is positioned to the right of the elevator shaft. Six signal paths intend to demonstrate effects of various building features on the radio wave. The first example shows when a ray strikes a surface made of a material that is denser than air, the ray reflects (R) at an angle that mirrors the incident angle on the normal to the reflecting surface. Consequently, round columns disperse or scatter (S) signals in almost any directions as illustrated by path five at the corner column. In fact, the scattering on a round object is always accompanied by signal diffraction (D), as is shown by Path 2, where one wave is hitting a small obstacle, such as small protruding stones on a rough concrete surface, creates a secondary circular wave with the same frequency but less energy. Still, this “bending effect” results in that the signal is reaching the “shadow” area behind a large obstacle. Two examples are occurring at the first column met by Path 2 and the corner of the elevator shaft met by Path 4. The last phenomenon is signal refraction (F) when a wave passes from one space into another with a different density. This situation is displayed by path three, as it reaches the concrete wall of the elevator. While one part of the signal is reflected, another is penetrating the concrete thus losing speed and energy. Therefore, it is changing its angle toward the normal to the surface until it reaches the exit surface, concrete-to-air, where the angle is changed back and the speed of light is picked up again. However, Table I shows that the radio signal lost at least 16 dB when entering the shaft with the crane tower. The example assumes that the 16-dB loss resulting from passing through the second wall, 32 dB in total will make the signal strength too small to be picked up resulting in the shadow area. However, the signal will be reflected by the shaft walls and even the steel elements of the tower crane. The effect of this situation is that most of the electromagnetic signals being bouncing back and forth continuously losing energy, and a small amount of signals will “escape” through the open door with still some small energy left.


Figure 3. Propagation of radio signals in 2-D across a hypothetical construction site

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Plate 1. Cut-and-cover construction

Providing reliable access and security Figure 3 includes two problems that have not been addressed yet. First, some of the signals reach outside the construction environment offering “foreign” receivers to interact with the transmitter. Of course, access can be password protected, but we know passwords can be broken. Clearly, the most effective protection is to limit signal strength to the site itself. The theoretical signal patterns of a directional antenna depicted in Figure 2 hint at three unique features that could be used to restrict the radio signals: maximum energy of the transmitter, beam width and direction of the main beam. The second problem is related to the staircase into the basement of the building, similar to the situation of a cut-and-cover railroad construction depicted in Plate 1. The challenge is to provide network access from an area that is totally cut-off from any signal reaching it either it being outside the beam width, such as the operator of the climbing crane, or the underground space. Of course, the simplest action would be to install another AP inside the tunnel or the garage. However, this would only alleviate the problem for some time, as the large support walls, with heavy rebar, will soon cut those signals off as well. One flexible solution to such problems is signal repeaters designed to receive a signal at a low and transmit it out at a desired higher strength. Wi-Fi repeaters only need electric power and receive and send at 2.4 GHz. Of course, the maximum power and the antenna configuration will dictate the area that can be covered by one repeater. On the other hand, such devices can be daisy-chained together by setting one within the area of coverage of the second. This section presented some of the theoretical issues related to an electromagnetic wave that propagates from the antenna of a Wi-Fi AP. As was indicated, walls, doors, fixed or mobile equipment, scaffolds and other obstacles cause varying degrees of attenuation. The remainder of the paper will present the results of a study to prove the hypothesis that the effect of building components of a growing concrete structure can be predicted both qualitatively and quantitatively.


Signal survey concepts The ultimate goal of a wireless signal survey is to determine the optimal number of APs and their placement that provides adequate signal coverage throughout a construction site (Zvanovec, 2003), so the network will not suffer from coverage holes, resulting in areas of poor network performance. With most implementations, “adequate coverage” means support of a minimum data rate or throughput. A wireless site survey also detects the presence of radio frequency (RF) interference coming from other sources that could degrade the performance of the WLAN (Cisco Systems, 2008b). The need and complexity of a wireless signal measurement will vary depending on the facility. For example, a small construction site may not require a site survey. This scenario will probably get by with a single Wi-Fi AP (or router) located anywhere in the construction site and still maintain adequate coverage. A larger construction site, such as an office complex, airport, hospital or warehouse, generally requires an extensive wireless site survey. Without a survey, users will probably end up with inadequate coverage and suffer from low performance in some areas. Design of field experiments To simulate the construction job site, five construction scenarios were created: flat site free of obstacles; vertical concrete walls; steel structure; glass façade; and higher floor addition. The objective of each experiment was to measure the effect of: interference, reflection, diffraction and refraction. The understanding of the fundamental principles of signal propagation will be instrumental in creating a construction network that will provide ubiquitous access to Wi-Fi during the entire construction phase. A directional antenna connected to an AP served as a transmitter and a laptop loaded with signal strength measuring software was used to measure signal strength. The observed data were subsequently used to create contour maps of signals. For each experiment, a Wi-Fi transmitter was mounted on a tripod so that its center was set at 2.10 meters above the ground level as shown in Figure 4(a). Table II presents an overview of the selected field experiments. The purpose of experiment T-1 was to measure the Wi-Fi signal variation in an open space while away from the antenna in the horizontal plane. The experiment established the baseline to compare to other readings coming from the test performed in five different scenarios. For this reason, a Wi-Fi antenna mounted on a tripod was positioned at the edge of a 90.0-meters long and 50.0-meters wide paved empty parking lot with the only obstruction of two light posts. The signal strength was measured along the horizontal centerline (x-direction) at the height of 1.3 meters from the ground level. The signal strength readings were also taken at a perpendicular (y-direction) to the horizontal centerline. Experiment T-2 was designed to understand the effect of scattering and diffraction caused by the edge of a thick concrete wall creating a dead spot behind. Figure 4 shows the set-up of the hardware positioned next to the Civil Engineering building, facing the corner of two rectangular concrete walls. The collected data should show the effects of reflection, scattering and diffraction as sketched in Figure 3. However, due to a thickness of the two concrete walls, no refraction effect was noticed. The goal of experiment T-3 was to map the changes in signal strength after installing the repeater at a distance of 11.0 meters from the


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Figure 4. Hardware layout for T-2 and T-3

Main feature Wave behavior

Table II. Experimental Wi-Fi signal behavior test matrix

Propagation Reflection Scattering Diffraction Refraction

Open space (T-1)

Dead spot (T-2)

Repea-ter (T-3)

Steel frame (T-4)

Glass facade (T-5)

x x x x

x x x x

x x

x

x

Eleva-tion (T-6) x

x

transmitter as shown in Figure 4(b). The function of a repeater is to receive the Wi-Fi signals from a source, such as a transmitter, amplify their strength and emit them again. Again, the signal values were measured. Figure 4(a) presents a basic transmitter set up which consists of a directional Wi-Fi transmitter and a mobile receiver used for the experiments. Figure 4(b) shows an arrangement consisting of an antenna facing the corner of a solid concrete wall and a repeater installed beyond the corner of the building. Test T-4 was performed to study reflection and scattering of Wi-Fi signals resulting from a large steel frame structure. The transmitter was placed outside while signal data was collected inside the building. Experiment T-5 was designed to understand the effect of a large glass façade. Here again, the transmitter was installed on a tripod outside, and signals were measured in before and after passing through the glass façade. The last experiment T-5 simulated the changing distance and height of a Wi-Fi receiver that moves with a crew relative to a fixed transmitter. It was designed to quantify the expected deterioration of signal strength as a receiver was moved from the first- to the second-, third-, fourth- and fifth-floor levels. The collected signal strength data were entered as percentage values into the SigmaPlot software to produce the contour maps. The following section will present and analyze the measured readings for the different experiments.


Discussion of results Experiment T-1 Figure 5 displays the signal maps from the open parking lot. Clearly observable are the curved signal distribution that dropped to 74 dBm (20 per cent) at 90.0 meters not sufficient for establishing reliable communication. As modeled in Figure 2, the 2.4 GHz beam of a directional antenna produces higher strength along the centerline. The scattering and reflections caused by the metallic light poles are clearly visible. A separate plot was created along the cross-section A-A presented in Figure 6. Section A-A was positioned at 30 meters from the antenna. As expected, the main energy is concentrated in the core section with some small variations along the centerline. The antenna design might cause this small, surprising dip of 2.5 dBm (3 per cent).


Figure 5. Contour of the signal strength for experiment T-1

Figure 6. Signal contour for cross section A-A

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Experiment T-2 Figure 7 displays the result of the three wave behaviors to be studied with experiment T-2. Of course, one observes not only a striking difference in signal strength between the left and the right side of the centerline but also the changes compared to the contour in Figure 5. Reflections of the electromagnetic waves on the concrete wall, certainly containing vertical and horizontal rebar, caused dispersion and boosting of the signal on the right. The scattering enhanced this superimposing effect at the edge of the wall creating an area with 42 dBm (60 per cent) signal strength at 8.0 meters from the antenna. Similarly, diffraction and scattering at the corner “helped” the signal strength in the “dead spot” of Point A. Both effects created a bending effect reminiscent of an eddy in flowing water. The overlays allow the signals to reach the region behind the wall. The small tree on the right side and at a distance of 16.0 meters have mixed consequences. The foliage seems to reflect some signals while, at the same time, diminishing its energy. From the T-1 experiment, we learned that at a distance of 16.0 meters, close to the trees in the back, the signal should still be at a minimum 58 dBm (40 per cent). Experiment T-3 The drastic effect of the signal repeater at 11.0 meters distance is shown in Figure 8. The signal strength at the farthest corner increased from 74 dBm (20 per cent) to 50 dBm 50 per cent. It is interesting to notice how the signals coming from the antenna and the repeater overlap. On the other hand, its bi-directional nature creates two halves with drastically different contour shapes. At 16.0 meters distance, around the small trees, signals were improved from 74 dBm (20 per cent) to 34 dBm (70 per cent). The most important observations are related to the changes occurred behind the wall corner indicated by Point A. It is apparent that the repeater elevated the signal strength from 74 dBm (20 per cent) to

Figure 7. Signal contours for T-2

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Figure 8. Signal strength contours for T-3

a strong 50 dBm (50 per cent), making the communication between the transmitter and Point A feasible. The histograms in Figure 9 compare the signal strength values measured for T-2, T-3 and the baseline T-1 experiments at different distances. One can clearly recognize the drops from T-1 to T-2 as well as the increases due to the repeater. Most interesting is the large drop in strength from the open space T-1 to the T-2 experiment. For example, at 8.0 meters from the centerline, T-1 shows a value of 54 dBm (45 per cent), while T-2 only 72 dBm (22 per cent). Because of the reflections from the concrete, one would expect values above 54 dBm (45 per cent) for T-2. However, the

Figure 9. Comparison of T-2, T-3 and baseline at 6 meters from antenna

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concrete wall is not smooth and made of a uniform material. While some electromagnetic waves are reflected by the semi-smooth concrete surface and others, refract through the cover only to reflect from the round surfaces of the rebar. Thus, the wall not only “swallows” the refracted signals but also reflects waves from surfaces at different distances (e.g. rebar vs concrete surface) thus causing additional losses. The decaying effect of the wall is further highlighted by the increasing differences starting with the only 1.6 dBm (2 per cent) at the centerline to approximately 38 dBm (70 per cent) at 8.0 meters distance from it.


Experiment T-4 Figure 10 displays histograms representing the signal strength at different distances along the centerline. Apparent is the sharp drop at a distance of 14.0 meters from the antenna. As discussed in literature review, signals scatter and reflect after striking to the metal which results in making the signal either become stronger or lead to losses resulting in the distorted propagation of the signals. Figures 10 illustrates the result of the experiments conducted in the steel framed building. Experiment T-5 In this experiment, changes in elevation or distance of the receiver from the antenna showed large influence on RF signal strength. Figure 11 depicts signal strength attenuation for four different receiver heights. It is interesting to notice that the strength patterns are following similar patterns. At 3.0 meters horizontal distance from the antenna, the first floor still falls into the area covered by the main beam (Figure 2) and thus shows the highest signal strength of approximately 50 dBm (50 per cent). On the second floor at 8.0 m, the value has fallen to 66 dBm (30 per cent). Considering the doughnut-cone-shaped energy losses of a directional antenna presented in Figure 2, the overall configuration of the four profiles

Figure 10. Signal propagation in steel framed structure (T-4)

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Figure 11. Degradation of signals with height and distance (T-5)

follows expected patterns. It is important to remember that a 74 dBm (20 per cent) strength is too weak to offer a reliable Wi-Fi communication. Summary and conclusions The availability of up-to-date and accurate data where needed has been identified as key to improving productivity in construction. However, the large and ever-changing construction site has resulted in many “islands-of-communication” void of real-time connection to the internet. Advancements in wireless technology have the potential to open new paths to overcome the obstacles hampering the reliable coverage of the site with electromagnetic signals. Still, the key to an efficient network is up-front planning applying basic principles relevant to an ever-changing construction site. The paper presents the results of a study to understand the behavior of Wi-Fi (Wireless-Fidelity) signals when emitting from a semi-directional antenna. After a theoretical review of the signal attenuation, scattering, refraction and reflection results from field tests were presented. The experiments were performed in the built environment, which did not include the electric noise created by equipment and the effect of temporary structures. Thus, the authors recommend that extended experiments on an active construction site should be conducted. The results from the current study can help in understanding the general trends in attenuation of Wi-Fi signals due to the presence of obstacles. But the attenuation values may change if field tests are conducted on an active construction site. Therefore, signal strength studies on an active construction site could provide better understanding of signal behavior. Usually, a construction site has a very rugged environment due to the presence of heavy machinery, interfering noise and elevation changes which affect the propagation of signals. The additional experiments will help in determining the requirement of number repeaters or their placement on the site. The baseline experiment confirmed the theoretical energy loss model of a directional antenna. Similarly, the effect of the discontinued concrete wall and steel trusses was measured. While the result of other studies, showing the drastic attenuation of signals passing through thick concrete walls, the measurements affirmed the scattering and refraction effect at the corner edge of an outside concrete wall. As expected, the installation

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