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InTech, Jun 2004 by Schmidt, Garrett

Going wireless helps clean up a landfill.

Operators of the Roanoke, Va., regional landfill had a problem. The solid waste disposal facility needed to monitor the leachate storage tank level at a remote point of the landfill. The problem was operators had to constantly monitor the tank because the runoff of leachate is a primary concern due to Environmental Protection Agency monitoring. In addition, the runoff could cause considerable damage to the nearby property.

Several times a day an operator had to drive a half mile to the tank, take a manual reading, and log the data manually, because an old buried line had failed.

Because of the distance and constantly changing landscape of the landfill, laying conduit and running wire was expensive and impractical.

The landfill operators thought wireless, but they had to educate themselves on the various forms of wireless they needed for their operation.

The word "wireless" evokes several different emotions from people in an industrial context. Some get excited about the opportunity to employ a new technology, while others believe a copper wire is the only way to get information from point A to point B in a reliable manner. Trust it or not, wireless is here to stay. The questions are, "is wireless technology right for my application?" and if so, "which wireless technology is right?" The first step toward answering these questions is to determine exactly what the application is. The radio frequency (RF) environment, distance you need to transmit, type of information, amount of information, and alternatives all affect the final decision about the appropriate technology.

Options

Several mediums exist for the transmission of data, including copper, fiber optics, and wireless. Users often look first to traditional cabling or fiber for their solutions, finding that it may be entirely too expensive, time consuming, labor intensive, or even impossible to implement. Once users make the decision to use wireless, they face a whole new set of choices:

As the number of wireless device manufacturers grows, so do the choices for wireless technology. The first big decision is whether to use a licensed or an unlicensed system. A licensed system gives the user a dedicated bandwidth to transmit on, with no interference from other radios, as well as a high-powered radio. Unfortunately, these features come with a steep price tag. The user must renew the license with the Federal Communications Commission (FCC) yearly to maintain rights to the frequency. An unlicensed system is much less expensive, but the user must accept limitations on transmit power and interference.

New license-free radio transmission techniques and standards are emerging almost daily, giving the user a host of choices for the radio output power (up to the FCC limitation of 1 watt), RF baud rate, transmission distance, transmission type, operating frequency, and proprietary or standard protocol. Most industrial wireless applications fall in the license-free frequency ranges of 902-928 megahertz or 2.4 gigahertz, which are allocated by the FCC for Industrial, Scientific, Medical (ISM) use. In the ISM bands, two transmission methods are typically used.

The first is frequency hopping spread spectrum (FHSS). Frequency hopping radios function by constantly changing their transmit frequency, moving in a pseudorandom pattern around the frequency spectrum at a high rate of speed. This transmission method has a high interference tolerance, guaranteeing that information will pass between the radios until 100% of the frequency band clogs with RF noise. Frequency hopping is also inherently secure; the moving target is extremely difficult to track and lock onto. Typically, users who employ this technique require a relatively low baud rate, which allows for a great amount of energy per data bit, improving the probability of successful radio transmissions. Many proprietary radio systems implement FHSS to move serial data or I/O updates over the air. Bluetooth devices (802.15.1) also use frequency hopping.

Direct sequence spread spectrum (DSSS) is another transmission method employed in the ISM bands. DSSS is able to move large packets of information, but is more prone to interference. Direct sequencing starts with the transmitter replacing every bit of user data with a random code word. The code word then spreads out across the RF spectrum to occupy a much wider bandwidth for transmission. At the receiver the code word is de-spread, leaving the original bit intact. Interference that enters the packet suppresses at the receiver when the code word is de-spread, but a certain jamming margin exists. The DSSS radio will function perfectly until the jamming margin is exceeded, at which point the radio will shut down completely. For this reason, a user is better off if the DSSS is in low-to-medium interference environments; 100% of the data will pass, and larger packet sizes can go out at higher baud rates. Wireless Ethernet (802.11) devices, as well as ZigBee devices (802.15.4), employ direct sequencing.