IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007111Optimizing Wireless LAN for LongwallCoal Mine AutomationChad O. Hargrave, Jonathon C. Ralston, and David W. HainsworthAbstract—A significant development in underground longwallcoal mining automation has been achieved with the successfulimplementation of wireless LAN (WLAN) technology for com-munication on a longwall shearer. WIreless-FIdelity (Wi-Fi) wasselected to meet the bandwidth requirements of the undergrounddata network, and several configurations were installed on op-erating longwalls to evaluate their performance. Although theseefforts demonstrated the feasibility of using WLAN technologyin longwall operation, it was clear that new research and de-velopment was required in order to establish optimal full-facecoverage. By undertaking an accurate characterization of thetarget environment, it has been possible to achieve great improve-ments in WLAN performance over a nominal Wi-Fi installation.This paper discusses the impact of Fresnel zone obstructions andmultipath effects on radio frequency propagation and reports anoptimal antenna and system configuration. Many of the lessonslearned in the longwall case are immediately applicable to otherunderground mining operations, particularly wherever there is ahigh degree of obstruction from mining equipment.Index Terms—Automation, coal, longwall, mining, WIreless-FIdelity (Wi-Fi), 802.11b.I. INTRODUCTIONA. ProblemTconducive to WIreless-FIdelity (Wi-Fi)-based communication.Fig. 1 shows the primary machinery used on a longwall coalmine face, namely the roof support system, the armored faceconveyor (AFC), and the shearer. The shearer moves backand forth across the longwall face along the AFC rail, cuttingcoal from the face. As the shearer moves along, the individualroof supports move forward to keep the mining area secure,while behind the moving wall ofsupports the unsupported roofcollapses. A typical longwall face is about 250 m across; theface will rise and fall and deviate in and out across this fullwidth.The key design challenges to overcome from a Wi-Fi com-munications viewpoint are the narrowness ofthe tunnel formedby the roof support system, the prevalence of metallic struc-tures, the fact that the face is not straight, and the limitedoptions for robust antenna mounting on the shearer body. ThisHE environment on the coal face of an operating long-wall is very harsh and does not appear immediatelyPaper PID-06-06, presented at the 2005 Industry Applications Society An-nual Meeting, Hong Kong, October 2–6, and approved for publication in theIEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining IndustryCommittee of the IEEE Industry Applications Society. Manuscript submittedfor review October 15, 2005 and released for publication August 7, 2006.The authors are with Exploration and Mining, Commonwealth Scientific andIndustrial Research Organisation, Pullenvale, QLD 4069, Australia.Digital Object Identifier 10.1109/TIA.2006.885892Fig. 1.Longwall shearer, roof supports, and AFC.nonideal propagation channel produces “dead-band” regionsacross the face, resulting in communications drop outs with theshearer. In addition, the data rates achieved during the systemoperation were often lower than required for the growing band-width demands (including streaming video) of the automationproject.B. TechnologyThe current wireless LAN (WLAN) system used for thelongwall automation project is the mature IEEE 802.11b stan-dard. This standard employs the direct sequence spread spec-trum (DSSS) transmission method on various channels in the2.4-GHz range. The use of this modulation technique is oneof the factors that make 802.11b so attractive for use in anunderground environment with many metallic surfaces, sinceDSSS has a certain degree of inherent resistance to multi-path interference. DSSS spreads each information bit acrossa range of frequencies using a variety of coding techniques(corresponding to the standard bandwidth capacities of 1, 2,5.5, and 11 Mb/s) to create a range of symbols for informationencoding. When a reflection (due to multipath interference)causes a particular symbol transmission to overlap with thenext bit/symbol being transmitted, the despreading applied tothe correct symbol to isolate the correct bit value will tend tonot despread the undesired (reflected) symbol. However, ifthereare sufficient reflections in the environment, the multipath delaywill be long enough that the more complex encoding techniqueswill be compromised by misinterpreted bit patterns in adjacentsymbols, leading to data corruption and packet retransmission.In this case, the wireless equipment may drop the data rate0093-9994/$25.00 © 2007 IEEEAuthorized licensed use limited to: CHINA UNIVERSITY OF MINING AND TECHNOLOGY. Downloaded on March 08,201 0 at 02:41 :58 EST from IEEE Xplore. Restrictions apply.
