01 September 2002
Wireless takes control
By Davis Mathews and Randy Klassen
New spread-spectrum technology makes gains in market
New technology is enabling greater use of spread-spectrum radio for monitoring and control in industrial environments. Today, spread spectrum can securely move small amounts of sensor and control information, even transmitting mission-critical data through heavy interference. Distance between transmitter and receiver reaches 300 feet to 15+ miles while maintaining reliability and information integrity.
History books show consumers and corporations have used wireless radio technology in the form of fixed frequency radio in homes and cars and to transmit data in industrial applications. Operation requires a government license that theoretically prevents other broadcast signals inside the "bandwidth" and territory covered by that license. This high-power output allows transmission across great distances and "blasting" through obstacles. The downside is an almost immediate drop in performance if interference (man-made or environmental) moves into the allocated bandwidth. Limited available frequencies also means users, particularly in urban areas, must often wait years for a license.
To allow greater access and utilize new radio technologies dealing with interference, in 1987 the Federal Communications Commission (FCC) allocated industrial, scientific, medical (ISM) spread-spectrum bands.
Instead of using costly long-run cable, the telemetry world has used radio technology for years. Licensed radios and even spread-spectrum radios are commonplace in the wide-open spaces of the oil fields and outlying municipal water systems across North America. Here, reliability depends on the FCC maintaining an end user's exclusive rights to that portion of the bandwidth. Reliability is purchased or, in the case of spread-spectrum radios, maintained often simply because there aren't other radios competing for the bandwidth in the same area.
A typical dilemma tank farms face today illustrates the technology. Scattered I/O from multiple tank levels on one side of a highway must relay to a distributed control system or similar system across the road. Digging trenches, laying conduit, and pulling cable makes acquiring these signals costly, not to mention the costs of engineering and inspections and the time needed to acquire right of way prior to implementing the solution. Wireless I/O interfaces are less expensive, in some cases costing tens of thousands of dollars less.
An industrial wireless I/O interface can send analog and discrete signals from a sensor to a programmable logic controller (PLC) or from a PLC to a pump-in this case, reliably reporting levels, pressure, flow, and alarms to control pumps, valves, and switches by updating data far more often than required.
Reliability maximizes through frequent sampling of small data packets. Small information packets are a critical component to designing a reliable industrial wireless interface. Whereas traditional telemetry SCADA requires lots of information to go through the air, cable replacements for industrial I/O require only bytes of information to move. Because errors occur when bits are received incorrectly, the smaller the packet, the less chance for error.
Applications such as alarms are essentially one bit of information that is either ON or OFF (4-20 mA current loop output is usually transmitted in 1 or 2 bytes). Data checks values to detect changes. Each packet is an independent update, eliminating the need to include networking information. Sampling more often than needed provides real-time data but also allows for the chance to lose data if heavy interference clutters the radio environment.
Small packet size can also yield more power per bit. Given a pristine radio environment, there is a clear relationship between speed and distance (here, speed equates to rate). In a setting with no interference, if you apply 1 watt of transmit power to a transmitter sending information at a slow speed, that radio will fire its signal farther than a radio sending out information at a high speed. The more power per bit, the better able you are to penetrate walls, bounce around tanks, and propagate through mazelike metallic structures. The flip side is that more bits per second in transmission reduces power per bit. Therefore, in applications where I/O is moved 300 feet to 15 miles, a small number of bits stands a better chance of making it to the receiver than a large number.
The FCC allows two methods for building a license-free spread-spectrum radio: Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS). Differing physical mechanisms for dealing with and rejecting interference means DSSS and FHSS behave differently in industrial settings.
|DSSS radio transmitter function||DSSS radio function with encountered interference|
Interference and how DFSS and FHSS address it are vital to understand. Wireless radios encounter interference through electromagnetic interference or radio frequency interference from industrial equipment, from other licensed users (even in ISM bands), or from unlicensed radios (especially in ISM).
In DSSS radios, a data packet starts out as "narrow data." It then generates a random code word for every bit in that packet. These code words spread the narrow data sent and "widen" it across a much wider bandwidth.
Signal strength at any one frequency reduces as the energy spreads.
The spreading code reapplies at the DSSS receiver. The signal then despreads, and data can be retrieved. Canceling spreading leaves data in its original state.
Narrowband interfering signals pass through only one spreading code generator (entering after the transmitter but before the receiver), and spreading occurs by the same code despreading the original data.
Reliability depends on the signal strength and signal-to-interference ratio to get by the widened interferer. In short, as the power of the interfering signal increases, it reaches its threshold, and the radio fails.
With FHSS radios, the complexity lies with the hopping synchronization of the narrow data signal that remains unaltered. Data remains narrow from transmitter to receiver.
The FHSS radio is a narrowband, fixed frequency radio-but only for an instant-before it hops to another fixed frequency radio on another channel, and then another, and another, and so on. And it has plenty of room to hop. The 902-928 megahertz ISM frequency band is wide enough to hold 1,000 licensed narrowband radios.
Small packets of data go to the receiver with hops in a pseudo random pattern to more than 50 different frequencies around the band, before repeating the hopping sequence. Encounters with a significant interfering signal on a frequency generate error detection and discard the packet. The hopping sequence continues, and data updates resume. Interfering signals can knock one packet out of a FHSS radio's hop pattern, but the rest of the updates get through, no matter how powerful the narrowband interference.
This is very different from DSSS, which maintains error-free transmission of its data until the interferer goes over the top of its jamming margin, at which point the throughput of the DSSS quickly drops to zero-not appropriate for mission-critical industrial I/O.
FHSS radios do not "avoid" interference, they "tolerate" it. Each packet checks, and when it encounters interference, the bad packet does not get processed. As the hopping pattern continues, the radio moves along its sequence looking for the next packet to get through cleanly, at which time the good data is output. Slow and steady, FHSS radios are the industrial I/O tractors.
FHSS also has the advantage of being a small moving target. Throughput does not cease until the entire ISM frequency band theoretically plugs at any one location. This enables the FHSS to reliably get small redundant messages through areas of heavy interference, even as interference increases.
In a low to medium interference environment-one in which the interfering signal strength is below the jamming margin of the DSSS-100% of the DSSS message will get through, while the FHSS will experience packet losses due to the interference. In this case, the DSSS radio is a good choice for large packet messages.
In heavy interference environments, where interfering signals exceed the jamming margin of the DSSS, the DSSS radio will cease to work. The FHSS continues to function until the entire ISM frequency band is jammed (a very unlikely scenario). FHSS is the perfect choice for small packet redundant data-alarm and emergency stop signals-because even though packets are lost, others get through. WBJ
Behind the byline
Davis Mathews is product marketing manager for Phoenix Contact, responsible for industrial signal conditioning, data conversion, and hazardous area products. Randy Klassen is marketing and industrial OEM account manager for Omnex Control Systems Inc.
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