extfieldsofstudy: Engineering Medicine

text:
A framework for a teaching toolkit in entrepreneurship education

Despite mounting interest in entrepreneurship education (EE), innovative approaches such as multimedia, web-based toolkits including entrepreneurial storytelling have been largely ignored in the EE discipline. Therefore, this conceptual contribution introduces eight propositions as a fruitful basis for assessing a ‘learning-through-real-multimedia-entrepreneurial-narratives’ pedagogical approach. These recommendations prepare the grounds for a future, empirical investigation of this currently under-researched topic, which could be essential for multiple domains including academic, business and society.

rule (e.g., Oosterbeek et al., 2010), a preponderance of studies corroborate that EE has had a positive impact on individuals (e.g., Sternberg and Wennekers, 2005;Fayolle et al., 2006;Acs and Szerb, 2007;Souitaris et al., 2007). However, research done so far stresses disagreements on how to implement EE in practice (Kozlinska, 2011;Albornoz Pardo, 2013;Lourenço et al., 2013). Overall, there is no comprehensive, pedagogical toolkit to educate people in entrepreneurship (Neck and Greene, 2011). In general, EE challenges traditional, educational methods. In addition, for more than a whole decade there has been a serious call for novel and innovative entrepreneurial teaching techniques (Kuratko, 2003;EC, 2008EC, , 2013c as well as for more interactive and awareness-raising learning approaches (Kuratko, 2005;Miller et al., 2012;Neck and Greene, 2011;EC, 2013b). In this regard, Gendron (2004) as well as Lautenschläger and Haase (2011) suggest that there should be increased encouragement of entrepreneurs in class. This represents the point of departure for a multimedia approach to spread entrepreneurial narratives productively and resourceefficiently. Such a multimedia approach should utilise Sarasvathy's (2001) theory of effectuation, which has directed lecturers to consider implementing the entrepreneur's personal resources in the curriculum (Fletcher and Watson, 2007;Gibb, 2011). In line with this theory, EE has become continuously subject to a shift towards a more action-oriented, educational approach. This reallocation is based on a consensus in EE research that entrepreneurship is best imparted by using experimental, pedagogical tools (e.g., Fayolle et al., 2006;Noyes and Deligiannidis, 2013;Mayer et al., 2014;Assudani and Kilbourne, 2015). In light of this situation, this paper draws attention to the following research question: Do multimedia entrepreneurial stories constitute an appropriate teaching tool to change perceptions towards entrepreneurship in a positive way? This qualitative, conceptual paper will first introduce the underlying framework. Within this approach, the propositions will be developed. Finally, a critical discussion will conclude this conceptual paper by taking due account of the limitations and practical and theoretical implications of the framework as well as making recommendations for future research.

Entrepreneurial narratives for multimedia entrepreneurship education
From a pedagogical perspective, entrepreneurial role models need not be used in traditional, pedagogical methods such as teacher-learner interactions. Role models can be presented via multimedia through informal narratives with comparatively high value and impact with respect to low cost for introducing entrepreneurship (Kuratko, 2005). Stories -or narratives or storytelling -have been shown to have a significant impact on individual perceptions. Bruner (1986) claims that individuals classify familiarity and build reality in two basic ways: propositional judgment and narrative judgment. While the propositional method is based on a cause-to-effect analysis, narrative judgment includes information regarding persons, settings, intentions, and behaviour. Through these elements, narratives create a context to achieve high impact on individuals' perceptions (Stewart, 1997). As indicated by Coles (1989), stories touch us and inspire us to take new paths. Moreover, stories tend to make us more open-minded. Therefore, entrepreneurial narratives are anticipated to be an effective educational approach for EE.
In response to a call to utilise the narrative approach (Johansson, 2004;Hamilton, 2006), research on entrepreneurial behaviour and motivational drivers have already experimented with this direction (Pless, 2007;Essers, 2009;Clarke and Holt, 2010;Flottemesch, 2013;Haley and Boje, 2014). Additionally, Kaminski (2003) recommends employing stories to teach leadership. Furthermore, Watson (2001) proposed using narratives including synthesised content from research and personal experiences to educate managers. The narrative approach has been also used to educate individuals in health and safety issues (Smith, 2005). While Friedman and Prusak (2008) illustrate the transdisciplinary, pedagogic value of narratives, which enhances and shapes our understanding of knowledge management, Gabriel and Connell (2010) discuss the pedagogic potential of narratives as a promising vehicle for management learning. Storytelling has also found its way into strategic management to support the introduction of change aspects and education of human resources (Swap et al., 2001;Sumner, 2005). Because the narrative approach has been successfully implemented in other pedagogical disciplines such as management learning and strategic management, the same is expected to hold true for EE.
The model of entrepreneurial potential elaborated by Krueger and Brazeal's (1994) states that the emphasis on entrepreneurship in classrooms and proliferation of entrepreneurial knowledge, confidence building, and self-efficacy foster entrepreneurship as a socially accepted, highly regarded, personally rewarding and attractive career choice. For this reason, including stories of entrepreneurial role models in curricula show great potential as an effective tool to raise awareness of entrepreneurship as an attractive occupational path (Godsey and Sebora, 2009). Additionally, role models are living confirmation of attractive and achievable objectives, which support individuals to define their self-concept (Akerlof and Kranton, 2000) and develop self-efficacy to embark on an entrepreneurial career (Scherer et al., 1989;Krueger and Carsrud, 1993;Lockwood and Kunda, 1997;De Clercq and Arenius, 2006). Thus, entrepreneurial role models afford great potential to enhance aspirations for entrepreneurial activities (Arenius and De Clercq, 2005;Koellinger et al., 2007).
However, while in recent years entrepreneurship scholars have been encouraged to describe entrepreneurship from a life-story perspective, few scholars such as Robertson and Collins (2003), McAdams and Pals (2006), Engstrom (2012) or Flottemesch (2013) have used a selfnarrative approach in EE. Narrative theory is often used for exploring and understanding new concepts. In particular, web-based storytelling has increasingly been used to accommodate several different learning styles (McAdams and Pals, 2006). By interviewing successful entrepreneurs and managers, Hood and Young (1993) suggest that practitioners' recommendations, beliefs and experience should be included in EE curricula. In line with this suggestion and based on Boyles' (2012) work, a conceptual model of expected impact variables as basis for this report's propositions is presented in Figure 1. This study supports the grounds for highlighting that narratives enable potential entrepreneurs to: 1 Become motivated about setting up a company with respect to changing attitudes and perception in a positive way.

2
Become convinced in terms of perceived skills and knowledge to devote resources referring to entrepreneurial opportunities, in compliance with the results of Lurtz and Kreutzer (2014) with respect to feasibility.

3
Make key decisions linked to entrepreneurial intentions with respect to behaviour control. Figure 1 provides the starting point for generating this report's propositions. The variables for studying intended impacts of entrepreneurial narratives using multimedia approaches are outline below.

Eight impact variables as indicators for an adequate educational vehicle
2.2.1 Impact on entrepreneurial attitudes-The key findings of Shinnar et al. (2009) and earlier results from Carsrud and Olm (1986) state that individuals' views on entrepreneurship often differ. In particular, values and motivations play a crucial role in the intention-behaviour link (Fayolle et al., 2014). The high impact of beliefs about attitude, control, and subjective norms on behaviour mediated by intentions is captured by the Theory of Planned Behavior (TBP) (Ajzen, 1991). According to Ajzen (2011), intention is an individual's eagerness to perform certain behaviour, and attitude refers to a person's evaluation of this intended behaviour. Several researchers have evaluated the impact of EE on individuals' entrepreneurial intentions through attitudes (e.g., Carayannis et al., 2003;Shinnar et al., 2009;Lourenço et al., 2013;Fayolle et al., 2014;Fellnhofer and Kraus, 2015). Several studies discovered that EE can have a positive impact on entrepreneurial attitudes by increasing the perceived attractiveness of entrepreneurship (e.g., Dickson, 2004;Krueger, 2007;Cheung, 2008;Petridou and Glaveli, 2008;Von Graevenitz et al., 2010). Further, researchers have shown that attitudes influence behaviours and behaviours influence attitudes (Brännback et al., 2007;Fretschner and Weber, 2013). This effect of attitude regarding entrepreneurship may vary regionally (Kautonen et al., 2013) or be influenced by culture (Liñán and Chen, 2009). In light of studies highlighting disparities in attitudes among persons who and were and were not participating in EE (Fayolle and Gailly, 2015), we assume that web-based entrepreneurial narratives support individuals in changing their entrepreneurial attitudes. Thus, we predict the following:

Proposition 1
Multimedia entrepreneurial narratives support individuals in changing their attitudes towards entrepreneurship.

Impact on entrepreneurial passion-While
feelings play a role in entrepreneurship (e.g., Baron, 2008;Foo et al., 2009;Cardon et al., 2012), in general entrepreneurship is linked with the exploration and exploitation of entrepreneurial opportunities (De Carolis and Saparito, 2006). Not only are strong, positive emotions fundamental in academic research on passion (e.g., Carver, 2003), but also in organisational psychology (e.g., Shipton et al., 2006;McMahon, 2009) and in the entrepreneurship literature (e.g., Baum and Locke, 2004;Chen et al., 2009;Cardon et al., 2013). Entrepreneurship is often faced with economic or social shifts and disruptive transformation (Metcalfe, 2004;Christensen et al., 2006), which create the need for continuous passion towards seeking entrepreneurial opportunities, developing ground-breaking products or services, and running initial prototypes while addressing the need for necessary resources  (Gundry and Welsch, 2001). Based on these prior findings, we believe that narratives of passionate entrepreneurs can inspire their audience and therefore propose the following:

Proposition 2
Multimedia entrepreneurial narratives support individuals in enhancing their entrepreneurial passion.

Impact on entrepreneurial perceived desirability and feasibility-
According to the social psychology literature, intentions have been demonstrated to be effective predictors of planned behaviours (Krueger et al., 2000). The existing literature suggests two main antecedents of intentions, i.e. perceptions of desirability and feasibility (e.g., Shepherd and Krueger, 2002;Peterman and Kennedy, 2003;Kuehn, 2008;Armstrong, 2011;Wurthmann, 2014). In order to analyse these dependent drivers pre-and post-watching entrepreneurial narratives, this article suggests investigating these key variablesdesirability and feasibility -as well. Many studies such as Ajzen (1991), Shapero (1975) and Shapero and Sokol (1982) discuss the origins of entrepreneurial intentions. In particular, Shapero and Sokol (1982) argue that entrepreneurial intentions originate from feasibility and desirability as well as perceptions and opportunity awareness, which are instrumental in any EE (De Clercq et al., 2013). Nonetheless, whether a generalisation to a narrative EE setting may occur is still an open question (Segal et al., 2005;Lanero et al., 2011). We propose that:

Proposition 3
Multimedia entrepreneurial narratives support individuals in changing their perceived entrepreneurial feasibility and desirability.

Impact on opportunity exploration and exploitation-
The exploration and exploitation of entrepreneurial opportunities act as the cornerstone of successful businesses (Sharma and Salvato, 2011;Gaimon and Bailey, 2013). Information and searching for information are key to recognising and exploiting business opportunities (Shane and Venkataraman, 2000;Fiet, 2007), and individuals can learn how to acknowledge patterns required to recognise these potential opportunities (Baron and Ensley, 2006). According to Patel and Fiet (2011), one can improve such recognition of opportunities. A study published by Heinonen and Poikkijoki (2006) related to opportunity discovery, evaluation, and exploitation provides information on applied teaching techniques in EE. This contribution summarises that the entrepreneurial-directed approach appears to be appropriate for broadening individuals' perspectives and developing the skills for entrepreneurial behaviour. Based on Kirzner's (1973Kirzner's ( , 1979Kirzner's ( , 1997 work regarding cognition theory and efforts by McMullen and Shepherd (2006), Tang et al. (2012) elaborated a reliable and valid model with three diverse elements of awareness including scanning and searching, relationship and connection, and evaluation and judgment. Given that this alertness scale can serve as a central indicator for effective EE, we propose the following:

Proposition 4
Multimedia entrepreneurial narratives support individuals in facilitating their entrepreneurial alertness related to opportunities. in past years (Rauch et al., 2009;Edmond and Wiklund, 2010;Covin and Lumpkin, 2011;Wales et al., 2013). While most academics view the EO construct as a firm-level phenomenon (Covin and Slevin, 1989b;Guth and Ginsberg, 1990;Zahra, 1993;Lumpkin and Dess, 1996;Hisrich, 2001, 2003), it can also be applied to the individual level, although few studies have implemented this approach (e.g., Poon et al., 2006;Joardar and Wu, 2011;Bolton, 2012;Langkamp Bolton and Lane, 2012;Goktan and Gupta, 2013). Based on the effort by Lumpkin and Dess (1996), Langkamp Bolton and Lane (2012) stress the reliability and validity of the used scale with 1,100 individuals. Weaver et al. (2002) applied also the scale of EO. Evidently, individuals are the reasons why enterprises operate entrepreneurially (Joardar and Wu, 2011;Covin and Miller, 2014). Since entrepreneurial role models highlight their EO in their narratives, we assume that entrepreneurs can have an impact on the EO of their audience. In light of the proposed implication, we infer the following:

Proposition 5
Multimedia entrepreneurial narratives support individuals in changing their EO.

Impact on perceptions of entrepreneurial self-efficacy-Several
researchers focus on evaluating the impact of EE on individuals' intentions to develop a business on traditional antecedents of intentions such as self-efficacy (e.g., Cox et al., 2002;Maritz and Brown, 2013;Piperopoulos and Dimov, 2015). General self-efficacy refers to an individual's estimation regarding his or her general ability to act successfully (Chen et al., 2001). At its heart, self-efficacy is not necessarily about entrepreneurial skills, but rather, one's judgments of possessed skill capability. However, self-efficacy is still integral to entrepreneurship as many researchers have found that role models only inspire entrepreneurial intentions if they influence self-efficacy (Chen et al., 2001;Kickul et al., 2009). Because entrepreneurial narratives can increase the probability that individuals will consider an entrepreneurial career, such narratives most likely have a positive effect on individuals' self-efficacy. In line with this argumentation, we assume the following:

Proposition 6
Multimedia entrepreneurial narratives support individuals in facilitating their entrepreneurial self-efficacy.

Impact on entrepreneurial intentions-EE
is expected to be associated with entrepreneurial self-efficacy, which may increase entrepreneurial intentions (e.g., Wilson et al., 2007;Liao and Gartner, 2008;Bae et al., 2014). There is a well-established research stream stressing that intentions prepare the grounds for founding a company (Liñán and Chen, 2009;Kautonen et al., 2013). Studies that examine EE's impact on entrepreneurial intentions report positive outcomes (Lorz et al., 2013). As discussed, thus, we presume that the same holds true for a new innovative EE approach, and therefore, we predict:

Proposition 7
Multimedia entrepreneurial narratives support individuals in facilitating their entrepreneurial intentions.
2.2.8 Impact on perceived behavioural control-In order to assess the effect of EE, most studies accept intention as the immediate determiner of behaviour (e.g., Krueger et al., 2000;Fayolle et al., 2006;Liñán and Chen, 2009). In particular, Kautonen et al. (2013)  suggested modified questions and assertions (e.g., "I have applied much effort to activities aimed at starting a business in the last 12 months".) to determine the link between intention and behaviour (Gollwitzer, 1999). In line with this argumentation, the following proposition requires examination when evaluating EE:

Proposition 8
Multimedia entrepreneurial narratives support individuals in facilitating their perceived behavioural control.

Discussion and conclusions
With respect to recommendations by Lorz et al. (2013), essential elements have been discussed in this conceptual paper as a basis for the future measurement of real entrepreneur narratives as an awareness-raising toolkit using multimedia. Based on the methods of other impact studies (e.g., Kolvereid and Moen, 1997;Wilson et al., 2007;Liao and Gartner, 2008;Olomi and Sinyamule, 2009;Kružić and Bulog, 2010), a structured sampling procedure with an adequate sample size for an ex ante/ex post design represents a further essential requirement for evaluating entrepreneurial narratives as an adequate teaching tool (Lorz et al. 2013). In addition, because most EE studies are carried out at higher educational levels (e.g., Vanevenhoven and Liguori, 2013;Volery et al., 2013;Zhang et al., 2014), a focus on currently under-researched target groups such as students of vocational schools (Lorz et al., 2013) will enrich the academic discussion. Only few studies have explored the impact of EE on these groups (Peterman and Kennedy, 2003;Athayde, 2009;Oosterbeek et al., 2010).
As the worldwide dissemination of EE continues (Solomon and Fernald, 1991;Edelman et al., 2008;Joshi and Ganapathi, 2008;Kailer, 2009;Ion and Viorica, 2011;Boyles, 2012) along with growing academic interest in its role and impact (DeJaeghere and Baxter, 2014; Saeed et al., 2014;Sipon and Lope Pihie, 2014;Fayolle and Gailly, 2015), there appears to be little agreement regarding what the content of EE should be as well as how to implement teaching tools in these programs (Honig et al., 2005;Neergaard and Ulhøi, 2007;Edelman et al., 2008). In particular, researchers have paid little attention to the impact of entrepreneurial narratives on individuals (Davidsson, 2006;Pittaway and Cope, 2007;Xavier et al., 2008;de Vries, 2014). In response to this under-researched, phenomenal development and promising topic, this article draws attention to essential impact indicators for a narrative teaching approach within the field of EE privilege. Eight propositions are suggested for a framework to demonstrate that an entrepreneurial narrative approach in EE will produce a positive impact on entrepreneurial attitudes (Proposition 1), entrepreneurial passion (Propositions 2), perceived desirability and feasibility (Propositions 3), alertness to entrepreneurial opportunities (Propositions 4), one's EO (Propositions 5), perceptions of entrepreneurial self-efficacy (Propositions 6), entrepreneurial intention (Propositions 7) and, finally, perceived behavioural control (Propositions 8). Overall, this report stresses that the multimedia entrepreneurial narrative approach affords great potential impact on multiple stakeholders. We strongly suggest that entrepreneurial storytelling could be a useful tool to shape and foster entrepreneurship as an attractive career path. However, this perspective needs to be cast in a formal model with empirical data to ascertain if real entrepreneurial stories support individuals changing their perspectives towards entrepreneurship in a positive way. The proposed framework provides a basis for future research in this discipline.


extfieldsofstudy:
Engineering
Computer Science
Medicine

s2fieldsofstudy:
Business
Computer Science

text:
Tag-to-Tag Interference Suppression Technique Based on Time Division for RFID

Radio-frequency identification (RFID) is a tracking technology that enables immediate automatic object identification and rapid data sharing for a wide variety of modern applications using radio waves for data transmission from a tag to a reader. RFID is already well established in technical areas, and many companies have developed corresponding standards and measurement techniques. In the construction industry, effective monitoring of materials and equipment is an important task, and RFID helps to improve monitoring and controlling capabilities, in addition to enabling automation for construction projects. However, on construction sites, there are many tagged objects and multiple RFID tags that may interfere with each other’s communications. This reduces the reliability and efficiency of the RFID system. In this paper, we propose an anti-collision algorithm for communication between multiple tags and a reader. In order to suppress interference signals from multiple neighboring tags, the proposed algorithm employs the time-division (TD) technique, where tags in the interrogation zone are assigned a specific time slot so that at every instance in time, a reader communicates with tags using the specific time slot. We present representative computer simulation examples to illustrate the performance of the proposed anti-collision technique for multiple RFID tags.

Introduction
Recent advancements in digital electronics, signal processing, and wireless communications have given rise to low-cost and low-power automatic wireless sensor systems, which allow non-contact and non-line-of-sight reading of data using electromagnetic signals. Radio-frequency identification (RFID) is an important building block for the Internet of Things (IoT), which helps to connect the status of a physical object to the Internet by capturing meaningful data in order to trace and share information [1,2]. It has become a key technology that spans a wide range of applications such as asset management, supply chains, and construction. An RFID system consists of a reader (interrogator), a tag (transponder), and middleware software that helps with data processing from the tag to the reader using RF signals [3,4]. Tags are classified as passive, semi-passive, and active depending on their functionality. Since a passive tag is not equipped with a power source, it requires energy from the reader in order to send back the re-modulated signal containing its ID information [4,5]. RFID communication based on backscatter tag signals depends on various factors including the power of the signal, the radar cross section (RCS) of the tag, and propagation loss.
For example, the RFID tags used in the construction industry for materials management suffers a lot from interference signals originating from neighboring tags, which ultimately results in the tag-to-tag collision problem. In order to minimize data collisions caused by neighboring tags, in this paper, we propose an anti-collision algorithm based on the time-division (TD) technique. In the TD technique, each tag's response is differentiated by a specific time interval that corresponds to that particular tag for the transmission of data to the reader within the same reader-coverage area. Therefore, a data frame is divided into several timeslots, each of which is assigned to individual tags, and the reader can read all of the tags in chronological order [6]. Furthermore, the data frame considered in this paper includes the preamble frame, which helps with the channel estimation, signal-to-noise ratio (SNR) estimation, synchronization, etc. When transmitting data from a tag to the reader, the tag sends the preamble data before transmitting its main ID information to broadcast the destination of the tag information. After the reader receives the preamble data, it records TD slots for the data transmission corresponding to tags. In RFID applications, the TD method is required to group tags using the same time slot such that they only transmit within a specific time interval. In this paper, we compare the performance of the proposed TD method to the conventional method without the consideration of interference suppression and anti-collision technology and the CDMA technique based on the gold-code sequence, which spreads the tag backscatter signals to separate the signals of each individual tag. To verify the performance of our system, we perform our simulation in various reader interrogation zones. The rest of the paper is organized as follows. Section 2 describes the RFID collision problems, and discusses and analyzes some previous works related to the collision problem. In Section 3, we present details of the received signal model employed in this study. In Section 4, we introduce the proposed TD solution for the anti-collision problem for a single reader, while in Section 5, we present a performance analysis of the proposed method in terms of bit error rate (BER) and frame error rate (FER). Finally, in Section 6, we conclude the paper.

RFID Collision Problem
In this section, we discuss types of RFID collisions and related previous works.

Types of RFID Collision
The occurrence of interference due to tag signals is a common issue in RFID systems. Simultaneous transmission from multiple tags leads to collision as the readers and the tags normally use the same channel. Collisions are classified as reader-to-reader, reader-to-tag, and tag-to-tag collisions [7,8].

Reader-Reader Collision
A reader-to-reader collision occurs when neighboring readers simultaneously interrogate a specific tag in the same frequency band. Furthermore, it may occur when the tag is within the readable range of one reader and the interference range of the other reader; the signal from the latter reader may interfere with the return signal from the tag. Reader-to-reader collision is discussed in more detail in Colorwave [9][10][11][12].

Reader-to-Tag Collision
A reader-to-tag collision (tag interference) occurs when a tag receives multiple concurrent queries from different readers. This problem can be solved by assigning different channels to nearby readers [13].

Tag-to-Tag Collision
A tag-to-tag collision occurs when multiple signals simultaneously arrive at the reader, preventing the reliable detection of all tags within its interrogation zone. This scenario is illustrated in Figure 1, where tags (T1, T2, T3 and T4) simultaneously send signals to a single reader, preventing the reader from correctly recognizing a particular tag. Without an anti-collision protocol, the data from these tags would not only collide at the reader, hence degrading their identification, but will also lead to wasted bandwidth and energy [14].

Related Works on Collision Problem
RFID is an accurate object identification system that is used commercially to detect and manage information pertaining to various items. However, the large-scale acceptance of RFID for industrial applications is lower than expected, and this is primarily due to several issues including environmental factors, multipath fading [15], as well as the tag-collision problem. The tag-collision problem generally occurs due to a lack of proper co-ordination between the reader and tags. The backscattering response from the tag to the reader can collide with a neighboring tag, which ultimately obscures the reader from properly reading tag IDs.
In this paper, we focus on the problem of a single reader in an environment with multiple tags. Further, we focus on the practical application of RFID technology to the construction industry. Typically, on a construction site, there are a large number of tagged objects. Thus, it is extremely important to simultaneously improve the object identification performance and minimize operating costs. A variety of multiple access technologies have been used in recently developed RFID systems such as space-division multiple access (SDMA), frequency-domain multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA), which is known as spread-spectrum [16][17][18].
SDMA reuses certain resources (channel capacity) in spatial slots. SDMA protocols separate the channel using electronically controlled directional antennas or multiple readers to identify tags. However, drawbacks of this approach include the high implementation cost, the need to design a complicated antenna system, and its limitation to only a few specialized applications. On the other hand, FDMA uses different frequency ranges for data transmission to and from the transponders, which result in a high cost to implement the reader. This requires a complex receiver at the reader end because a dedicated receiver must be provided for each reception channel. Although CDMA exhibits good performance for multiple tags, it requires that their data be multiplied by a pseudo-random noise (PN) sequence before transmission. On the contrary, the TDMA anti-collision algorithm allocates several timeslots in each frame to solve the collision problem. The TDMA anti-collision technique is the most popular RFID reader-to-reader anti-collision scheme, and it is used in the Colorwave algorithm [19,20], where each reader chooses a random time slot to transmit data. The TDMA anti-collision method chronologically divides the total available channel capacity among participants. Its procedures are classified as synchronous reader-driven and asynchronous tag-driven. In the reader-driven procedure, at a particular time interval, an individual tag is first selected from a group of tags using specific algorithms, and then a data communication link is made between the selected tag and reader. On the other hand, with the tag-driven procedure, the tag announces itself to the reader by transmitting its IDs in the presence of a reader. The former and

Related Works on Collision Problem
RFID is an accurate object identification system that is used commercially to detect and manage information pertaining to various items. However, the large-scale acceptance of RFID for industrial applications is lower than expected, and this is primarily due to several issues including environmental factors, multipath fading [15], as well as the tag-collision problem. The tag-collision problem generally occurs due to a lack of proper co-ordination between the reader and tags. The backscattering response from the tag to the reader can collide with a neighboring tag, which ultimately obscures the reader from properly reading tag IDs.
In this paper, we focus on the problem of a single reader in an environment with multiple tags. Further, we focus on the practical application of RFID technology to the construction industry. Typically, on a construction site, there are a large number of tagged objects. Thus, it is extremely important to simultaneously improve the object identification performance and minimize operating costs. A variety of multiple access technologies have been used in recently developed RFID systems such as space-division multiple access (SDMA), frequency-domain multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA), which is known as spread-spectrum [16][17][18].
SDMA reuses certain resources (channel capacity) in spatial slots. SDMA protocols separate the channel using electronically controlled directional antennas or multiple readers to identify tags. However, drawbacks of this approach include the high implementation cost, the need to design a complicated antenna system, and its limitation to only a few specialized applications. On the other hand, FDMA uses different frequency ranges for data transmission to and from the transponders, which result in a high cost to implement the reader. This requires a complex receiver at the reader end because a dedicated receiver must be provided for each reception channel. Although CDMA exhibits good performance for multiple tags, it requires that their data be multiplied by a pseudo-random noise (PN) sequence before transmission. On the contrary, the TDMA anti-collision algorithm allocates several timeslots in each frame to solve the collision problem. The TDMA anti-collision technique is the most popular RFID reader-to-reader anti-collision scheme, and it is used in the Colorwave algorithm [19,20], where each reader chooses a random time slot to transmit data. The TDMA anti-collision method chronologically divides the total available channel capacity among participants. Its procedures are classified as synchronous reader-driven and asynchronous tag-driven. In the reader-driven procedure, at a particular time interval, an individual tag is first selected from a group of tags using specific algorithms, and then a data communication link is made between the selected tag and reader. On the other hand, with the tag-driven procedure, the tag announces itself to the reader by transmitting its IDs in the presence of a reader. The former and latter procedures are also called reader-talk-first (RTF) and tag-talk-first (TTF), respectively. TTF is relatively slow and inflexible compared to RTF [16,21,22]. For anti-collision algorithms in the RFID system, Abramson's Logic of Hiring Access (ALOHA) protocol and binary tree algorithms, which are based on deterministic and probabilistic approaches, are widely used. Most of the standards employed for ultra-high frequency (UHF) RFID systems refer to binary tree searches for anti-collision algorithms as deterministic schemes because each roof-to-leaf path denotes a unique tag ID, and it is possible to simultaneously retrieve all IDs after all branches are completely searched. On the other hand, the ALOHA anti-collision algorithm is usually referred to as a probabilistic scheme, where each tag ID will have a probability of successful retrieval [23].
Tree-based anti-collision algorithms use a virtual tree to read all tag data within the read range of the reader by iteratively querying a subset of tags at different levels, and tag IDs are distributed based on their prefixes. The root is the set of tags to be identified, intermediate nodes represent the group of colliding tags, and the leaves represent individual tag responses [24]. They are further classified as tree splitting (TS), query tree (QT), binary search (BS), and bitwise arbitration (BTA) [17]. In the TS protocol, multiple responding tags are grouped using a random number generator. On the other hand, the QT protocol stores the tree information at the reader. The reader broadcasts a query with a bit-string prefix, which is then matched with the tag ID's prefix, after which the tag with the matched prefix responds to the query with its ID to the reader. This process continues until only one tag responds [25]. This overcomes the problem of generating a random number using TS, and decreases the cost and computational complexity. In BS, the communication starts by the reader transmitting the serial number of tags. The serial number is then compared against their ID. In this protocol, communication is possible if tags with an ID equal to or lower than the serial number respond to the reader. Unlike the above protocols, BTA operates by requesting tags to respond bit-by-bit from the most significant bits to the least significant bit of their ID [17].
The ALOHA protocol is a probabilistic approach, where a signal responds within a random time slot in a frame to identify tags. Of the different ALOHA-based protocols, the frame-slotted ALOHA (FSA) and dynamic frame-slotted ALOHA (DFSA) are most commonly used in RFID systems because they can reduce the collision probability between tags. In FSA, the reader determines the frame size and the actual duration of the slot, and tags use random slots from among the entire set of slots (the total number of slots and the frame size are fixed). Therefore, in a reader interrogation zone, for a small number of tags, there is the problem of wastage because of the empty slots, and for a large number of tags, there is a collision problem. This drawback of FSA is partially solved by the DFSA. In DFSA, the reader will add the number of slots in one frame when there a large number of slots have collided. Similarly, the reader will decrease the number of slots in one frame size when there are many empty slots [26]. In DFSA, the size of the next frame is adjusted after estimating the number of remaining tags at the end of each frame without performing any operation for the ideal slots and collided time slots within the current frame. Therefore, the request time by the reader during these time slots are wasted. Consequently, if we try to reduce the request time, the probability of tag collisions may be increased [27].

Received Signal Model
The received signal, r(k), at a sample index k can be modeled as which can be summarized as where l represents the lth tag, L is the total number of tags transmitting the signal to the reader, x l (k) is the transmitted signal of the lth tag, and n(k) is the additive white Gaussian noise (AWGN) with independent and identically distributed components with zero mean and variance σ 2 . Using Friis' free-space equation, the relationship between the signal strength and distance, and between the received power at the antenna of the tag, P tag , and the transmitted power from the reader, P r is given by where G r and G tag are the transmitter and receiver antenna gains, respectively, λ is the wavelength (m), and d is the distance between the transmitter and the receiver antennas [28,29]. Equation (3) can be expressed in decibel form as which describes the amount of power available to the RFID tag for operation. Note that in this paper, [dB] refers to the decibel scale of the specific value. Since the power of the radio-frequency signal decreases when it propagates into space, there is a difference between the power delivered from the transmitting antenna and the power obtained at the receiving antenna, and this is known as the path loss [30,31]. In Equation (4), 10log 10 4πd λ 2 is the free-space path loss factor, and is defined as P loss [dB]. The decreasing trend of the signal in terms of the log-distance path loss is mathematically modeled as [32] P loss (dB) = P loss (r 0 ) + 10nlog d r 0 where r 0 is a reference distance, P loss (r 0 ) is the free-space path loss at a distance r 0 , n is a value that depends on the surroundings and building type (a more hostile environment results in a higher value of n), and the path loss; accordingly, it is higher for the same distance when compared to an environment with a lower value of n [33][34][35].

Anti-Collision for RFID Based on TD
In this section, we propose an efficient and simple anti-collision RFID system that is specifically for smart green construction technology. The goal of this construction technique is to reduce the construction cost and to enhance the speed of construction by reusing and recycling the materials used in a previous building. In order to improve the traditional construction technology, the smart green construction generally employs a construction management system that is based on a sensing technique such as RFID. In this case, RFID provides the vital roles for managing a variety of construction materials, monitoring the status of new or reused materials, and classifying the generation of each construction materials. On the construction site, the tracking and storing of materials' information using RFID remain challenging because a large number of materials are used in narrow spaces. In this case, we need to employ a low-cost RFID system utilizing a simple protocol, unlike the ALOHA system or gold-code based system, and it should have a good anti-collision performance. In addition, the RFID tag does not need to adaptively allocate time slots in a data frame, as is the case with ALOHA, because construction materials may not move for a relatively long time.
In this paper, we propose a simple and efficient RFID system that is based on the TD method. The proposed RFID tag transmits data using the specific time slot, and this time slot is fixed. Before tags are attached to construction materials, the tags are grouped according to their time slots (note that the number of groups may be determined according to the size of the building or the amount of construction materials). When we attach tags with specific time slots to the construction materials, they should be managed to avoid that tags that are in the same group are located in a neighbor. For example, if the tag of the first group is attached to a specific construction material, tags belonging to other groups should be attached to the neighboring materials. This process of managing the locations of RFID tags is simple because construction materials are generally on the same site for relatively long periods of time until the building is demolished and the materials reused in another building. Although we focus on an application of the proposed RFID system on the construction site in this paper, it might be applied to other areas, where objects are placed in the same site for relatively long periods, such as the storage, library, and industrial sites with various devices installed.
The proposed anti-collision RFID algorithm focuses on the case involving a single handheld reader and multiple tags. In addition to efficiently reducing the probability of collisions due to other tags by assigning tags to previously allocated time slots to transmit data, it also employs a simple RFID protocol for one-way communication. This simple RFID protocol may reduce the implement cost, because the RFID tag transmits data using the fixed specific time slot, without the complicated mathematical operation process, unlike the anti-collision RFID system based on the gold-code or ALOHA. Note that the gold-code anti-collision RFID tag requires a significant amount of numerical operations including the multiplication and addition, and the ALOHA technique requires an adaptive resource process for allocating time slots within a data frame.

RFID Signal Power Model Considering Interference Signals
In RFID systems, once the tag is powered up by the transmitted power from the reader, the selected tag starts to respond to the reader. Since the modulated signal scattered from the RFID tag suffers from noise and interference signals that originate from other tags, the received signal at the RFID reader, which is given by Equations (1) and (2), consists of the modulated backscattered signal of interest, noise, and interference signals [36,37]. At the reader, the amount of power available to the receiver antenna for it to operate is given by where P R−receiver [dB] is the power received by the receiver antenna of the reader, G R−receiver is the gain of the receiver antenna at the reader (which is equal to the transmitter antenna gain at the reader), and σ is the RCS of the RF tag, which is important for the system-level efficiency and reliability [35,36]. The RFID tag's cross-section area can be written as [38,39] where τ is the impedance-matching coefficient between the tag chip and its antenna. Substituting the value of σ antenna into Equation (6), the modulated power from the RF tag to the reader becomes: Equation (8) can be expressed in a general form as Equations (8) and (9) show that the total energy consumption of passive RFID systems depends on the power transmitted from the reader to the tag. Since those equations show that the effective distance r between the tag and the reader can be adjusted by maintaining the power supplied at the reader antenna, the total backscattering power of the passive RFID tag and interrogation range depends on the transmission power level of the reader. In our computer simulations, we used the above-mentioned RFID signal power models.

RFID Anti-Collision Scheme Based on TD
In order to suppress the interference signals from surrounding multiple RFID tags, we propose an anti-collision method based on the TD technique that allocates the tag information on the specific time slot. During reader-to-tag communication, the reader transmits a carrier signal with transmit power P r to a set of arbitrary multiple tags within their interrogation zone. If the power received by the reader is greater than the minimum power received from the tag, which is known as the reader sensitivity, data transmission may be possible from the tag to the reader. All of the tags in the coverage area of the reader then become activated and transmit their identification information to the reader with a power of P R−receiver whenever the energy received by the reader is higher than an activation threshold. If the measured P R−receiver value is higher than the activation threshold, the transmitting signal is effective for transmitting its payload. On the contrary, if the measured value is lower than the activation threshold, the transmitting signal does not effectively transmit its payload and the data transmission may fail. In the proposed RFID system, while the particular tag using the specific time slot transmits data to the reader, other tags using other time slots do not transmit any data, but maintain their sequence order. Since tags transmit data according to their turn, in the reader, interference signals from other tags are efficiently suppressed and the communication reliability is enhanced compared to conventional RFID system which has the low performance of reading capability for multiple tags, because they do not have the interference suppression or anti-collision technique. Figure 2 shows an example of the transmitted signals at the tags and the received signal at the reader based on binary phase shift keying (BPSK), which uses a constant carrier signal amplitude with an independent data modulation technique and helps to provide stable power transfer in wireless communication [40][41][42]. In the figure, to explain the proposed system, we assume that each tag employs a unique time slot. Although in a real system, some tags can be allocated into the same time slots, interference signals from other tags are effectively suppressed compared to conventional RFID systems. We discuss the interference suppression performance for the proposed RFID system based on TD by performing computer simulations.

RFID Anti-Collision Scheme Based on TD
In order to suppress the interference signals from surrounding multiple RFID tags, we propose an anti-collision method based on the TD technique that allocates the tag information on the specific time slot. During reader-to-tag communication, the reader transmits a carrier signal with transmit power to a set of arbitrary multiple tags within their interrogation zone. If the power received by the reader is greater than the minimum power received from the tag, which is known as the reader sensitivity, data transmission may be possible from the tag to the reader. All of the tags in the coverage area of the reader then become activated and transmit their identification information to the reader with a power of − whenever the energy received by the reader is higher than an activation threshold. If the measured − value is higher than the activation threshold, the transmitting signal is effective for transmitting its payload. On the contrary, if the measured value is lower than the activation threshold, the transmitting signal does not effectively transmit its payload and the data transmission may fail. In the proposed RFID system, while the particular tag using the specific time slot transmits data to the reader, other tags using other time slots do not transmit any data, but maintain their sequence order. Since tags transmit data according to their turn, in the reader, interference signals from other tags are efficiently suppressed and the communication reliability is enhanced compared to conventional RFID system which has the low performance of reading capability for multiple tags, because they do not have the interference suppression or anti-collision technique. Figure 2 shows an example of the transmitted signals at the tags and the received signal at the reader based on binary phase shift keying (BPSK), which uses a constant carrier signal amplitude with an independent data modulation technique and helps to provide stable power transfer in wireless communication [40][41][42]. In the figure, to explain the proposed system, we assume that each tag employs a unique time slot. Although in a real system, some tags can be allocated into the same time slots, interference signals from other tags are effectively suppressed compared to conventional RFID systems. We discuss the interference suppression performance for the proposed RFID system based on TD by performing computer simulations.

Data Architecture for RFID Anti-Collision Based on TD
In this subsection, we propose an efficient data architecture that is based on TD to prevent RFID collisions. Here, we allocate specific time slots in the RFID data frame, and a fixed number of data bits for the preamble data and the payload. Within a particular time interval, the reader recognizes data bits from a particular tag, which consists of preamble data followed by the payload. In order to prevent collisions between tags in the same reader interrogation zone, a specific time interval is reserved for the specific tags.
The functions of the preamble and payload are summarized as follows:

Preamble
In a noisy and interference environment that may prevent us from accurately detecting the desired signal, the preamble helps to prevent performance degradation of the signal detection [43]. Thus, preamble data are used for the channel estimation, SNR estimation, synchronization, etc. At the receiver of the reader, the efficient channel and SNR estimations should be obtained to enable the accurate detection of the transmitted data from the RFID tags and the estimation of the number of tags in the interrogation zone, respectively. In addition, perfect synchronization is required for the proposed RFID approach because the tag transmits data using a specific time slot. In this paper, we assume that the channel estimation, SNR estimation, and synchronization are performed at the receiver of the reader.
Assuming that there are four tags that each employs unique time slots, Figure 3 shows the data architecture for the proposed anti-collision RFID system. In this system, preamble data in the lth tag are transmitted using the lth time slots in the figure before transmitting the payload.

Data Architecture for RFID Anti-Collision Based on TD
In this subsection, we propose an efficient data architecture that is based on TD to prevent RFID collisions. Here, we allocate specific time slots in the RFID data frame, and a fixed number of data bits for the preamble data and the payload. Within a particular time interval, the reader recognizes data bits from a particular tag, which consists of preamble data followed by the payload. In order to prevent collisions between tags in the same reader interrogation zone, a specific time interval is reserved for the specific tags.
The functions of the preamble and payload are summarized as follows:

Preamble
In a noisy and interference environment that may prevent us from accurately detecting the desired signal, the preamble helps to prevent performance degradation of the signal detection [43]. Thus, preamble data are used for the channel estimation, SNR estimation, synchronization, etc. At the receiver of the reader, the efficient channel and SNR estimations should be obtained to enable the accurate detection of the transmitted data from the RFID tags and the estimation of the number of tags in the interrogation zone, respectively. In addition, perfect synchronization is required for the proposed RFID approach because the tag transmits data using a specific time slot. In this paper, we assume that the channel estimation, SNR estimation, and synchronization are performed at the receiver of the reader.
Assuming that there are four tags that each employs unique time slots, Figure 3 shows the data architecture for the proposed anti-collision RFID system. In this system, preamble data in the lth tag are transmitted using the lth time slots in the figure before transmitting the payload.

Payload
In this RFID system, when the reader sends the wake-up signal to the tags (for simplicity, we did not include the wake-up signal in the data architecture), tags sequentially transmit the preamble and payload using the specific time slot shown in Figure 3. The payload of this RFID

Payload
In this RFID system, when the reader sends the wake-up signal to the tags (for simplicity, we did not include the wake-up signal in the data architecture), tags sequentially transmit the preamble and payload using the specific time slot shown in Figure 3. The payload of this RFID system includes tag information such as the identification (ID) number, electronic product code (EPC) number, and other important information [44,45].
Using the proposed data architecture, when a specific tag transmits the data to a reader, no other tags transmit any data, as shown in Figure 3. Therefore, there is no interference signal when simultaneously communicating between multiple tags and a reader. While the TD-based anti-collision architecture for RFID in Figure 3 considers four tags and a reader, it may be extended for more tags. As mentioned previously, in the case where some tags can be allocated into the same time slots in a real system, interference from other tags are effectively suppressed, compared to the case with a conventional RFID system without the anti-collision technique.

Computer Simulations
In this section, we present the computer simulation results obtained to illustrate the performance of the proposed RFID system compared to that of the conventional system without the anti-collision technique and anti-collision RFID systems that are based on the gold-code sequence [46,47]. For the simulation, we considered three data frame sizes, 96 bits, which is the minimum length required for the International Standards Organization (ISO) standard [48], 144 bits, which is the recommended frame length in the ISO standard, and 512 bits, which is the maximum frame length used in typical passive RFID systems in the ISO standard. In addition, we assume that the length of the preamble is 16 bits, which is the length generally used in RFID systems [28,48]. For the simulation involving the gold-code RFID system, we assume that the length of the gold code is seven.
These simulations focus on determining the suppression performance for interference signals from circumjacent tags within the corresponding reader interrogation zone. As mentioned in the previous section, for this simulation, we assume that the channel estimation, SNR estimation, and synchronization are perfectly performed at the receiver of the reader.

Four Tags with Unique Time Slots
In this subsection, to illustrate the interference suppression performance of the proposed system, we present simulation results based on four tags with unique time slots in a reader interrogation zone. For this simulation, we considered four RFID tags located at different distances within the reader interrogation zone. Moreover, we employed passive RFID tags (T1, T2, T3 and T4) with a frequency of 900 MHz and a fixed transmitted power for a distance of 2 m with a minimum loss in each bit. After we present the results for only the four tags considered without any other tags, we present other results for the same four tags as well as many other circumjacent tags within the reader interrogation zone. Figure 4 shows the variation of the BER with different SNR values. There are twelve curves because the four tags are at different distances (1.8-2.4 m), for three different systems, namely the proposed system, a conventional system, and an anti-collision RFID system that is based on the gold-code sequence. In Figure 4, we observe that the BER curves for the proposed system are much lower than those of the conventional system, and are slightly lower than those of the gold-code system, which means that the performance of the proposed system is better than that of both the conventional and gold-code systems. We also observe that the conventional system does not work properly when there exist interference signals from other tags, and when the distance between a tag and the reader is not close enough. Although the gold-code system has an anti-collision performance that is similar to that of the proposed system, it requires a complicated protocol, which results in an increased cost. If we increase the length of the gold code, the anti-collision performance is improved, but the computational complexity is higher, which results in a long tag-response time.  Figure 5 illustrates the case where the conventional system works properly in an interference environment. The distances between tags and the reader are 0.6, 1, 2 and 2.5 m, respectively. Moreover, we observe that only the tag at a distance of 0.6 m works properly, but other tags do not work because their BER is too high for all SNRs; this is due to the interference signals from other tags. On the other hand, the proposed system works properly for all tags in a similar case. Next, to illustrate the interference suppression performance of the proposed RFID system compared to the conventional system, we present the simulated results for the variation in the FER as the SNR is varied in cases involving various frame lengths. Figures 6-8 show FER curves for data frame lengths of 96, 114 and 512 bits, respectively, employing four tags with distances of 1.8, 2, 2.2  Figure 5 illustrates the case where the conventional system works properly in an interference environment. The distances between tags and the reader are 0.6, 1, 2 and 2.5 m, respectively. Moreover, we observe that only the tag at a distance of 0.6 m works properly, but other tags do not work because their BER is too high for all SNRs; this is due to the interference signals from other tags. On the other hand, the proposed system works properly for all tags in a similar case.   Figure 5 illustrates the case where the conventional system works properly in an interference environment. The distances between tags and the reader are 0.6, 1, 2 and 2.5 m, respectively. Moreover, we observe that only the tag at a distance of 0.6 m works properly, but other tags do not work because their BER is too high for all SNRs; this is due to the interference signals from other tags. On the other hand, the proposed system works properly for all tags in a similar case. Next, to illustrate the interference suppression performance of the proposed RFID system compared to the conventional system, we present the simulated results for the variation in the FER as the SNR is varied in cases involving various frame lengths. Figures 6-8 show FER curves for data frame lengths of 96, 114 and 512 bits, respectively, employing four tags with distances of 1.8, 2, 2.2 Next, to illustrate the interference suppression performance of the proposed RFID system compared to the conventional system, we present the simulated results for the variation in the FER as the SNR is varied in cases involving various frame lengths. Figures 6-8 show FER curves for data frame lengths of 96, 114 and 512 bits, respectively, employing four tags with distances of 1.8, 2, 2.2 and 2.4 m. In addition, Figures 9-11 show FER curves for the same frame lengths as in previous figures, but they employ four tags with distances of 1.5, 1.8, 2.1, and 2.5 m. In these figures, it appears that there is a single FER curve for the conventional system because the FER values for all tags are close to 0.5, which means that the reader cannot distinguish the tag identification information from the received data due to the presence of interference signals. On the other hand, in all cases, the reader that employs the proposed system correctly distinguishes the identification information for each tag. Note that in all figures, the FER for the proposed RFID system increases as the length of the data frame is increased.
Sensors 2017, 17, 78 11 of 18 and 2.4 m. In addition, Figures 9-11 show FER curves for the same frame lengths as in previous figures, but they employ four tags with distances of 1.5, 1.8, 2.1, and 2.5 m. In these figures, it appears that there is a single FER curve for the conventional system because the FER values for all tags are close to 0.5, which means that the reader cannot distinguish the tag identification information from the received data due to the presence of interference signals. On the other hand, in all cases, the reader that employs the proposed system correctly distinguishes the identification information for each tag. Note that in all figures, the FER for the proposed RFID system increases as the length of the data frame is increased.    Figures 9-11 show FER curves for the same frame lengths as in previous figures, but they employ four tags with distances of 1.5, 1.8, 2.1, and 2.5 m. In these figures, it appears that there is a single FER curve for the conventional system because the FER values for all tags are close to 0.5, which means that the reader cannot distinguish the tag identification information from the received data due to the presence of interference signals. On the other hand, in all cases, the reader that employs the proposed system correctly distinguishes the identification information for each tag. Note that in all figures, the FER for the proposed RFID system increases as the length of the data frame is increased.

Many Tags with Random Time Slots
Finally, in order to illustrate the interference-suppression performance of the proposed RFID system compared to the conventional system and an anti-collision RFID system based on the

Many Tags with Random Time Slots
Finally, in order to illustrate the interference-suppression performance of the proposed RFID system compared to the conventional system and an anti-collision RFID system based on the

Many Tags with Random Time Slots
Finally, in order to illustrate the interference-suppression performance of the proposed RFID system compared to the conventional system and an anti-collision RFID system based on the gold-code sequence, we present simulation results based on the use of a large number of tags with random time slots in a reader interrogation zone. Figure 12 shows BER curves (at a distance of 0.8 m) for different SNR values with the proposed RFID system, the conventional system without the anti-collision technique in the presence of 250 interference tags (30 tags at distances ranging from 1.5 m to 4 m, 80 tags at distances ranging from 4 m to 7 m, and 140 tags at distances ranging from 7 m to 10 m) in an interrogation zone. In Figure 12, we observe that the proposed RFID system has a much better performance than the conventional system, because the BER curve of the proposed system is much lower than that of the conventional system. from 4 m to 7 m, and 150 tags at distances ranging from 7 m to 10 m) in an interrogation zone. In Figures 13 and 14, we compare the performance of the proposed RFID system to the conventional system and an anti-collision RFID system based on the gold-code sequence. From both figures, we observe that the proposed RFID system works properly in the interference environment for all cases and it has better performance than that of the gold-code anti-collision RFID system, while the conventional system without the anti-collision technique has high BERs for all SNRs in all cases. The SNR analyses for results of Figures 13 and 14 are shown in Tables 1 and 2, respectively. For these analyses, we consider the data reliability rates at 90%, 95%, 97%, and 99% at different distances and we indicate the minimum SNRs for achieving the considered data reliability rates in Table 1 and 2 for Figures 13 and 14, respectively, for the proposed RFID systems, the gold-code anti-collision RFID system, and the conventional system. From both Tables, we observe that the required minimum SNRs for achieving all reliability rates for the proposed RFID system are lower than them of the gold-code anti-collision RFID system in both cases, while the conventional system does not make the considered data reliability rates for all SNRs in both cases.  In Figure 13, we consider four tags of interest, which have different distances (0.5, 0.6, 0.7 and 0.8 m) and unique time slots in the presence of 510 interference tags (60 tags at distances ranging from 1.5 m to 4 m, 150 tags at distances ranging from 4 m to 7 m, and 300 tags at distances ranging from 7 m to 10 m) in an interrogation zone. Similarly, in Figure 14, we consider four tags of interest, which have different distances (0.5, 0.7 and 0.8 m) and unique time slots, in the presence of 270 interference tags (30 tags at distances ranging from 1.5 m to 4 m, 90 tags at distances ranging from 4 m to 7 m, and 150 tags at distances ranging from 7 m to 10 m) in an interrogation zone. In Figures 13 and 14, we compare the performance of the proposed RFID system to the conventional system and an anti-collision RFID system based on the gold-code sequence. From both figures, we observe that the proposed RFID system works properly in the interference environment for all cases and it has better performance than that of the gold-code anti-collision RFID system, while the conventional system without the anti-collision technique has high BERs for all SNRs in all cases. The SNR analyses for results of Figures 13 and 14 are shown in Tables 1 and 2, respectively. For these analyses, we consider the data reliability rates at 90%, 95%, 97%, and 99% at different distances and we indicate the minimum SNRs for achieving the considered data reliability rates in Tables 1 and 2 for Figures 13 and 14, respectively, for the proposed RFID systems, the gold-code anti-collision RFID system, and the conventional system. From both Tables, we observe that the required minimum SNRs for achieving all reliability rates for the proposed RFID system are lower than them of the gold-code anti-collision RFID system in both cases, while the conventional system does not make the considered data reliability rates for all SNRs in both cases.    Figure 13. BER curves of the proposed system, conventional system without anti-collision technique, and anti-collision system based on the gold-code sequence, for four tags of interest with different distances in the presence of 510 interference tags.    Figure 14. BER curves of the proposed system, conventional system without anti-collision technique, and anti-collision system based on gold-code systems, for four tags of interest with different distances in the presence of 270 interference tags.

Conclusions
In order to minimize the number of tag-to-tag collisions for RFID systems, we propose an anti-collision technique and data structure that employ the TD technique. In the proposed system, since only specific tags transmit the data to the reader in a specific time slot, interference signals from other tags are effectively suppressed. Therefore, when we simultaneously employ multiple tags and a single reader on an industrial site such as a construction site, the proposed RFID architecture improves the data reliability and performance, enabling the identification of information for specific objects or materials. We illustrated the interference suppression performance of the proposed RFID system by performing computer simulations, and compared the results with a conventional system without the anti-collision technique and an anti-collision RFID system that is based on the gold-code sequence. In the future, we will explore in greater detail some practical applications of the proposed RFID system.


Meeting the Challenge of Risk-Sensitive and Resilient Urban Development in sub-Saharan Africa: Directions for Future Research and Practice

: At the heart of the papers in this Special Issue is the call for research and practice to move to understand and act on the direct and indirect interlinkages between urban development and risk accumulation processes; a broader conception of risk on a continuum from everyday to extreme events and a critical view of urban risk governance as a project that implicates multiple formal and informal actors at difference scales. Out of this focus emerges a research frontier that demands sustained, detailed studies of the links between multi-faceted and multi-scalar development processes and risk but also the re-thinking of scale and jurisdiction as ordering concepts; a stronger understanding of the linkages between environmental / public health risks and small and extreme disasters, and relative changes in manifestations of these forms of risk and in their social differentiation; and better theorisation of governance innovations. For practice, the issue stresses the over-riding need to move beyond a narrow focus on hazard or disaster events and the immediate actors involved to engage a much wider set of actors in integrated planning processes; to develop data to enable holistic policy-making and to build on the emergence of demand-led planning to re-frame the practices of risk-sensitive and resilient urban development.

Introduction
As the Introduction to this Special Issue outlined, the relationship between urbanisation and urban growth, urban governance, poverty and inequality, and ecological degradation in sub-Saharan Africa is transforming in complex ways. The nexus this creates is altering the landscape of urban disaster and climate change risk in the region. At the same time, new external policy agendas, including the Sustainable Development Goals (SDGs), Sendai Framework for Disaster Risk Reduction (SFDRR) and the UNFCCC Paris Agreement, as well as new resilience framings and initiatives, are shifting expectations of what urban risk management and adaptation can and should do, towards more integrative approaches that link policy domains across scale, sector and time. However, the extent and depth of this shift in practice, including the extent to which multiple actors share a common narrative and vision about the concepts and challenges of urban risk, resilience and development, is inevitably contested both in the present and into the future. Building a scientific evidence base that can draw on lived and professional experience to highlight the long-term and indirect linkages and consequences of development decision-making on risk and its management is needed. This does not override the importance of addressing the pressing needs of human security and inclusive economic growth in sub-Saharan Africa, but does call for a joined-up agenda, a message at the heart of many of the papers presented in this Special Issue. 1 Overseas Development Institute, London (corresponding author: a.fraser@odi.org.uk) 2 King's College London 3 University of Cape Town 4 King's College London 5 University College London 6 Makerere University, Kampala, Uganda In this context, this Special Issue identifies three (inter-related) elements that open up a new research frontier for future work on sub-Saharan Africa's risk and resilience. First, understanding the process of risk accumulation in Africa's urban development, which needs local research that focuses on complex interactions that produce risk and inform how it can be mitigated. Second, understanding the linkages between everyday risks (in particular environmental / public health risks), small disasters and extreme events and relative changes in their nature, scale and distribution. Third, research that can inform reform of the governance of the urbanisation-risk nexus. Across these elements, smaller urban centres are greatly under studied, with local actors lacking capacity to raise the visibility of such contexts, despite the demographic growth and risk that will potentially accumulate in the coming decades. This overall recasting of the scope of research work in the region -and beyond -has implications for practice and policy in reducing risk and building resilience to a range of hazards (everyday, small and large) across urban sub-Saharan Africa -though we are cognisant of the profound diversity of African cities and the imperative of avoiding simplification.

Understanding processes of risk accumulation in urban development
The case for 'risk-sensitive' development is now established in the academic literature and international policy frameworks. Within the climate change arena this extends to the interaction of mitigation and adaptation goals as part of urban development strategy and planning (Simon, 2014 andParnell, 2015).
The synthetic papers by Dodman et al, Satterthwaite and Douglas in this volume prompt us to reflect on the multiple and complex ways in which risk and urban development are intertwined in the region, warning of the need for rich and sustained scientific enquiry. Dodman et al. identify core gaps in understanding the distinctive characteristics of the relationship between risk and vulnerability trends and other dimensions of human development and well-being in African urban contexts, including specific facets such as the implications of the demographic profile of urbanisation (in particular youth and gender), the linkage between violence and disaster risk, the relationship between household mobility and transport and risk reduction. Satterthwaite points to the striking dearth of specific and systematised knowledge about urban infrastructure and services development and risk, in particular in small and medium-sized urban centres. In turn, the paper by Wisner should be read as a reminder of the context-specific nature of particular urban development trajectories, in the case of Goma pointing to disaster recovery processes as sites of changing urban form and function, and the dysfunctional implications for urban risks of 'developmental' responses in the context of conflict and institutional fragility and fragmentation.
Responding to this research agenda requires detailed analysis that can capture the underlying drivers of risk across spaces, sites and networks that are rural and urban, formal and informal, biophysical and institutional and local, national, regional and global. As Pharoah and Zweig exemplify in this volume, risk is both 'de-localised' in its causes and intrinsically 'local' in its manifestations. In its abstract form resilience and systems thinking embodies aspects of these socio-ecological and multi-level systems interlinkages (Bai et al 2016), but understanding causative effects and feed-backs across systems and scales is challenging to analytic practice. Issues of scale and jurisdiction must also be nuanced -for, as Pharoah and Zweig remind us in this volume, risks cascade across rural-urban spaces in often hidden ways.
This Special Issue tests conceptual approaches that can elicit new explorations of these integrated relationships. Frick et al, this volume, draw on a scholarly history of landscape approaches to the political ecology of environmental problems as a way to explore both their biophysical and social construction (Reed et al. 2016). They apply the adapted lens of a riskscapes to the context of rapidly-urbanising spaces to allow the inter-play between multiple institutional processes and risk creation to come to the fore (Frick et al, this volume). This layered understanding not only creates an integrative picture of risk and vulnerability that reflects resilience and systems dynamics, but for policy makers it points to the imperative of action on multiple fronts. Safeguarding and strengthening locally appropriate urban risk management practices, supports global development frameworks which have placed the imperative of a stronger focus on risk as central to collective progress.
Learning and analysis about such dynamic relationships are, however, often weakly owned by those practicing risk and urban development for sub-Saharan Africa, as many papers in this volume illustrate. A systematic and extensive empirical agenda for each place must be linked to capacity building that will ensure the necessary rebalancing of development policy and practice based on the locally relevant risk related evidence. The question of the relationship between urban development and future risk accumulation or reduction also begs questions about the practical entry points for addressing underlying root causes. Analytically, adopting a risk and resilience framing for urbanism in sub-Saharan Africa opens scope for a reconsideration of development structures, practices and underlying priorities including those related to persistent poverty, public health, urban growth and climate change. Further, as reflected by Dodman et al (this volume), risk and resilience practice can change the underlying political relationships between urban actors and development pathways of urban areas, while conversely interventions to address unmet urban development challenges can also reduce risk (Dodman et al, this volume, and Dobson, this volume). There is, however, greater scope for learning from a wider variety of urban interventions about this two-way relationship as it plays out in sub-Saharan African contexts, in an institutional landscape which includes a heavy international donor dominance, the importance of traditional authorities and new forms of private capital and investment. Key questions include:  What is the evidence base of successful (and failed) attempts at integrated planning and practice? In such initiatives, how do different actors negotiate the co-benefits and tradeoffs between priorities and interventions, over short and long-term time horizons?  What methods and approaches allow for city actors to take ownership of learning processes in support of integrated, risk-sensitive development pathways?

Understanding linkages between everyday risks (especially environmental/public health risks), small disasters and extreme events
The second inimically-related frontier for research is the development of thinking about socalled everyday risks -chronic risks related to poor urban planning and management and inadequate infrastructure and services -and their relationship to disaster events in urban contexts. As highlighted in the Introduction and papers in this volume, not only are such risks the most immediately relevant manifestation of vulnerability in peoples' lives, but they are strongly inter-linked with small-scale, extensive disaster events and have cumulative effects for larger-scale disaster events. Articles by Songsore, Pharoah and Zweig and Wanda et al. (this volume) exemplify the two-way interplay between everyday and more extreme disaster risks in different urban contexts in the region, the compounding effects of one on the other and differentiated social landscape of risk that they present. Echoing the previous section, a focus on everyday risks opens up opportunities to explore integrated approaches to risk reduction involving urban planning and public health -against the backdrop of critical and growing debates about the perceived failure and role of planning in African cities and importance of urban public health in Africa (ACC 2010; Oni et al. 2016). Planning and public health were professions that were co-established to address the relationship between health and the urban environment (Corburn 2009). However, existing knowledge on this relationship is based overwhelmingly on North America and Europe, highlighting the need for research that is grounded in the reality of diverse, rapidly changing and risk-prone Southern urban contexts.
A dedicated set of papers in this volume seek to further ground perspectives on everyday and disaster risk in an African reality, but at the same time derive new conceptual treatments that can advance global research and policy agendas in this area. The paper by Pharoah and Zweig most pointedly challenges the term everyday risk as potentially masking the complexity, severity, diversity and changing nature of chronic risks, and its association with local realities, rather than the relationship between the 'local' and multi-scale causation processes. These are points echoed by Songsore who draws on two decades of research in Ghana to show the scalar trajectories of environmental burdens and differentiated vulnerabilities created in the intersection between everyday risks and disasters. Further research that rethinks conceptualisations of the everyday, the linkage between everyday risk and larger-scale risk and the ways in which this relationship is re-shaped by new socio-economic and political processes at different scales is undoubtedly necessary if the full spectrum of risk faced by Africa's urban citizens is to be understood and addressed. The papers in this volume underscore the need for such research to be fully embedded in peoples' own understandings and experiences of the risks they face.
Moving from theory to practice, Osuteye, Johnson and Brown, in this volume, point to the need for methodological innovation in research and practice to develop data distinguished between everyday, small-scale and large-scale disasters in urban areas, and to disaggregate such data to account for differences in the social and spatial distribution of urban risk and loss drivers both within and between urban centres of different sizes (small, medium and large) and geographic locations. They emphasise the potential to combine different data sources (e.g. Demographic and Health Surveys, census data, hospital records, police reports, newspaper archives, etc.) to provide a more detailed and comprehensive picture of urban risk -but more work is needed to establish what systematic data collection services exist and might be built to capture this disaggregated view of risk, and how such data can be owned and used by relevant policy actors.

Key questions include:
 What changes are occurring in the relative nature, scale and distribution of risk between everyday, small and extreme events, how is relative change mediated by policy and practice and what are the implications for equity?  Which organisations, and what legislation and policy agendas, dictate and mediate institutional boundaries between environmental/public health risks and everyday disasters, and between everyday and extreme events? What opportunities are there for transition and transformation at these interfaces?

The governance of the urbanisation-risk nexus
At the centre of the issues raised by this Special Issue lie questions about the governance of urbanisation and risk that demand continued attention from both researchers and practitioners. Urban risk governance is understood in its broadest and most meaningful sense, as the institutions that affect the occurrence of risk, and not just formal administering of disaster risk management. The over-riding picture of the urban risk governance landscape presented in this volume is one of fragmentation, localised agendas and unchecked risk accumulation, sometimes exacerbated by the involvement of external actors (exemplified in the papers by Mwanda et al. and by Wisner), yet tempered by a rising demand-led orientation in policy and practice (as Dobson attests). As many of the papers recognise, holism and partnership are necessary to bring risk and development together in ways that address multiple everyday risks and the linkages across levels and scale that define urban risks for people. This is an ambitious agenda that moves beyond a narrow focus on hazard or disaster event and the immediate actors involved to one that implies a much wider set of actors, including urban planners and public health officials but also officials at different levels of government and non-state actors, who must work together to engage with the politics of urban risk and development.
In research terms, the papers in the Special Issue draw in a novel breadth of social theory to move us to the conceptual forefront of understanding this governance landscape in sub-Saharan Africa (a project already set out in Lawhorn et al., 2014, and pursued in different directions here) as well as beyond. Previous volumes concerned with risk in African cities have explored the importance and policy relevance of conceptual approaches such as urban political ecology, human security and livelihoods and 'teleconnectedness' as well as approaches to governance systems such as multi-level governance as a way of unpacking the dynamic scalar dimensions of risk and resilience governance (Simon and Leck 2015; Simon and Leck 2013). The papers in this volume inject a new suite of critical approaches that are perhaps more revealing of the power and knowledge dynamics underpinning contemporary struggles over the nature of urban resilience and inclusive urban futures in the region. Deploying concepts such as bricolage to explore the relationship between formal and informal institutions and approaches such as actor-network theory to understand narrative constructions underpinning contests over risk management solutions in the new context of an informal, urbanising landscape, Frick et al and Leclerq reveal the complex institutional interactions and power structures that shape risk for people in the present, and structure their options into the future. Ajibade in turn brings an Urban Political Ecology lens to focus on processes of capital accumulation in relation to resilience projects and to highlight the interests promoted and forged through modernist adaptation and urban development schemes -and those left out. The final paper by Wisner also points to the need for wider research at the intersection of violence, conflict and disaster risk in the region, the institutional dynamics in such fragile urban contexts where states have ceded or lost authority to non-state actors and the spatial complexity of life in such contexts. These are core themes underpinning the overall Urban ARK Programme but also take us to the frontier of global research into urban resilience, as a field grappling with the normative implications of resilience thinking, and the expression of subjective and political dynamics in resilience concepts and frameworks.
While deeply illuminating of real-world dynamics, these critical views of risk governance often appear intractable when applied to real-world decision-making, where it asks 'what should we do, or do differently?' The Special Issue has also sought out practitioner experiences and examples that highlight innovation and opportunity in urbanising sub-Saharan African regions: the innovative practices of those citizens living under the new normality of informal settlement, the experience of innovative partnerships between slum dwellers' organisations and local governments and research innovations to support better data collection on multiple forms of risk. The ongoing challenge is both how to theorise these African innovations as well as build on them in practice (Castán Broto, 2014 Finally, in thinking about implementation, the suite of papers in this volume provoke rereflection on the pathways and entry points for transformative resilience-building given the reality of local governments unwilling or unable to tackle the vast service and infrastructure deficits currently facing the region's urban areas, reinforcing risk exposure and accumulation. We suggest that acknowledging and building on the multiplicity of actors involved in urban riskscapes appears to offer the most promise. A flexible, context-sensitive approach that is less wedded to institutional blueprints and best practices and more pragmatic to finding institutional best fit (Lockwood 2013) also seems appropriate. Perhaps most central to this emerging agenda is the observation that demand led urban planning -planning that brings decision-making power back to the city (from international donors and consultants) and from the city to the local -offers a distinctive reorganising of power, accountability flows and priorities for the city. Surely given the trajectories of urban demographic growth and risk faced by cities and towns in sub-Saharan Africa (and elsewhere) it is this kind of core shift in the framing and doing of development that requires careful consideration and supported experimentation. The potential to build from the bottom and learn from small-scale experiments (Ziervogel et al. 2017; Castán Broto and Bulkeley, 2014) also needs to be married with the imperative to align with interventions and systems with similar objectives. Here, strengthening the role of local government to absorb and enable multiple interventions is key.

Conclusion
The Special Issue presented here speaks not only to regional agendas for tackling risk and building resilience, but also to global agendas. As the global development and disaster risk community moves to the implementation of the Paris Agreement, the Sendai Agreement and the Sustainable Development Goals, embodied in part in the New Urban Agenda, it is clear that coherence between these endeavours is essential if the gains to one agendas are not to be undermined by the co-occurring losses on another. The SDGs offer a framing narrative for this coherence, albeit one that is complex to localise and, if reductive in practice, risks marginalising our understanding of causal relationships and processes in the search for targets and metrics. Urban development that drives up risks will further undermine development, just as disaster risk interventions that do not factor in underlying processes of development will be undermined. Moving forward to ensure that this does not happen will necessarily entail examining all spectrum of risks and their accumulation, in the context of the urban and economic development pathways of urban areas and their respective governance.