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Tutorial on Wireless Communications and Electronic Tracking Part 2: Advanced 1.0 Introduction

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Tutorial on Wireless Communications and Electronic Tracking Part 2: Advanced 1.0 Introduction NIOSH Mining Safety and Health Content Tutorial on Wireless Communications and Electronic Tracking Part 2: Advanced 1.0 Introduction In 2006, three major underground coal mine accidents in the United States claimed the lives of 19 miners and prompted lawmakers to pass the Mine Improvement and New Emergency Response Act (MINER Act) of 2006. One of the requirements of the MINER Act is to provide wireless two-way communications and location information between underground workers and surface personnel following an underground accident. At the time of the accidents in 2006, most coal mine communications systems consisted of either leaky feeder systems or pager phones. A few mines had electronic brass- in/brass-out systems or zone-based tracking, but location tracking throughout the entire mine was not a common practice. Since then, efforts to develop new radio communications and personnel tracking technology have resulted in many new systems on the market for underground mine applications, and new systems continue to be introduced. New communications technologies include radio node network systems, such as mesh and Wi-Fi; improved leaky feeder systems; low-frequency, through-the-earth systems ; medium frequency radios; and combinations of these technologies. New personnel electronic tracking technologies include radio frequency identification (RFID) and radio ranging techniques. Due to the increasing availability of new systems, the Mine Safety and Health Administration (MSHA) requires underground coal mines to have compliant communications and electronic tracking (CT) systems installed by June 15, 2011. The new CT technology that is available may be unfamiliar to mining professionals who need to purchase, install, and use this technology. To provide a better understanding of CT technology, a two-part tutorial has been developed by the Office of Mine Safety and Health Research (OMSHR), a division of the National Institute for Occupational Safety and Health (NIOSH), which consists of an overview of the available technologies (Part 1) and advanced details on how available systems operate (Part 2). The reader should review the Tutorial on Wireless Communication and Electronic Tracking - Part 1: Technology Overview online before studying this document - Part 2: Advanced. Readers of this tutorial are assumed to be associated with coal mining, and therefore familiar with coal mining operations and terminology, and to have a technical background in electronics and/or communications systems. This tutorial is meant for those who need detailed information to compare systems and whose job responsibilities require advanced knowledge. For example, the mine’s communications expert will need to understand how the underground environment influences the performance of CT systems, how different CT systems work, and their advantages and disadvantages. This tutorial provides an advanced discussion of the available CT technologies and operating characteristics starting with general communications system performance considerations in Section 2.1. This is followed by descriptions of specific technologies used for both primary communications (daily use, high bandwidth) and secondary communications (emergency use, low bandwidth). A comparison of the available technologies is presented in Section 2.6. Chapter 3 provides details on the operation and performance of the available miner tracking systems. A critical question related to CT technology is, "What happens after a major accident?" Chapter 4 discusses issues related to survivability and reliability. Chapter 5 discusses safety issues such as MSHA certification of electronics used in explosive atmospheres, safe battery designs, and concerns related to electromagnetic fields. Chapter 6 discusses considerations related to the mine operations center (MOC) on the surface. There are also two appendices; Appendix A provides CT systems engineering specifications, and Appendix B gives basic wireless CT theory including link budget analysis and electromagnetic interference (EMI). A list of technical references and standards are also included at the end of this tutorial for further information. The italicized words in the text of this document are hyperlinked directly to the Mine Communications and Tracking Glossary. 2.0 Communications System Performance 2.1 General Performance Considerations The discussion of wireless communications systems begins by considering the general characteristics of the systems and addressing the following questions: What is necessary to establish communications between two radios? What frequency or frequencies are appropriate for use in a mine? What frequency or frequencies must not be used to avoid potential interference? How much radio frequency (RF) power is allowable for use in underground coal mines? How much bandwidth is needed? What is bit error rate (BER) and how is it related to reliability? How are communications components interconnected to form a network? Why is a network necessary? How does a network configuration or topology affect the ability of the network to survive an accident? On what basis are different systems or technologies compared? What are the appropriate metrics for measuring performance? Another important consideration is the basic type of wireless communications system, i.e., primary or secondary. Primary communications systems are those used by miners for providing daily underground and surface communications throughout their shift. These systems are typically hand-held devices operating in the conventional radio bands (e.g., very high frequency (VHF), ultrahigh frequency (UHF), 2.4 GHz, 5.8 GHz). Leaky feeder and node-based systems are examples of such primary systems. Secondary communications systems are those which operate in nonconventional frequency bands (100 Hz to 1 MHz) and are not readily portable, but they may be more likely to remain operational following a mine accident or disaster. Medium frequency (MF) and through-the-earth (TTE) systems are examples of secondary systems that may provide survivable alternative paths to primary communication systems. All of these systems will be discussed in more detail in the following sections of this chapter. 2.1.1 Physical Communications Link The essence of communications between two radios is the establishment of a physical communications link between the devices. Figure 2-1 shows the factors that contribute to the simplest communications link between a transmitter (Tx) and a receiver (Rx). The radio frequency (RF) power flows from the sender (transmitter) to the receiver along this link. For example, the power applied to the Tx antenna travels down the cable connecting the transmitter to the Tx antenna, then to the Tx antenna, through the medium in which the electromagnetic (EM) signal travels, through the Rx antenna, and through any cable that might be used to connect the Rx antenna to the receiver. At this point in the communications link, the power is referred to as the receiver power. Figure 2-1. Components of a simple wireless communications link. A link budget analysis is the quantitative evaluation of the factors that contribute to RF power gain or loss in establishing a communications link between a transmitter and receiver. The purpose of a link budget analysis is to calculate the allowable path loss (Lp). The allowable path loss is the maximum energy that can be dissipated in the transmission medium before the communications link is no longer possible. Because the path loss increases with distance, the maximum allowable path loss can be used to estimate the maximum possible separation distance between the transmitter and receiver, which is referred to as the transmission or coverage range. The link budget analysis can also be used to compare the performance of different systems and system configurations. The path loss is calculated as follows: (1) Equation 1 shows that the allowable path loss (Lp) is dependent on the Tx power (Pt), Rx signal level threshold or minimum received power (Pmr) which accounts for noise, Tx antenna gain (Gt), and Rx antenna gain (Gr). Any additional losses, such as cable losses, are categorized as a miscellaneous term (Lmisc). All terms are in decibel (dB) units; the antenna gains are in Decibel (dBi); and Tx and Rx powers are in dBm or dBW (see Appendix B.1.1). To establish the communications link, the received power has to be above the receiver signal level threshold; otherwise, the signal may be too weak, which means that the receiver cannot process the signal and the link cannot be created. Most of the terms in Equation 1 that contribute to establishing and maintaining the communications link are fixed by the equipment being used. The values of those terms can be obtained from the manufacturers, except for the Pmr term which includes natural and manmade noise and is a site-specific (mine-specific) consideration. The equation yields the allowable path loss or propagation loss (Lp). Propagation is the common term used for describing electromagnetic waves (or energy) traveling through a medium. The propagation loss is largely a function of the transmission medium characteristics and the wavelength of the electromagnetic energy, as will be discussed in Section 2.1.4. At very low frequencies (less than about 10,000 Hz), EM waves can propagate directly through the earth. At somewhat higher frequencies (100-1,000 kHz), EM waves couple to, and are transported by, metallic conductors. At even higher frequencies (greater than about 100 MHz), the waves may
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