Comment

1 September 2005

Spread spectrum monitoring and control

New technology will make wireless I/O commonplace in harsh industrial environments.

Wireless radio technology as fixed frequency radio in our homes, cars, and factories is common.

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 off in performance if interference (manmade 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 FCC allocated Industrial, Scientific, and Medical (ISM) spread spectrum bands.

Radio technology has worked in the telemetry world for years instead of costly long run cable. Licensed radios and even spread spectrum radios are commonplace in the wide-open spaces of the oil fields and outlying municipal water systems around North America. Here, reliability depends on the FCC maintaining the end user's exclusive rights to that portion of the bandwidth. One claims reliability, or in the case of spread spectrum radios, often maintains reliability simply because there aren't other radios competing for the bandwidth in the same area.

New technology is enabling greater use of spread spectrum radio for monitoring and control in industrial environments. Today, we can securely move small amounts of sensor and control information including transmission of mission critical data through heavy interference. Distances between the transmitter and receiver of 300 feet to 15-plus miles are possible while maintaining reliability and information integrity.

A typical dilemma faced by tank farms illustrates the technology. Scattered I/O from multiple tank levels on one side of a highway must somehow reach a DCS 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 PLC or from a PLC to a pump. It is capable of reliably reporting levels, pressure, flow, and alarms to control pumps, valves, and switches by updating data far more often than required.

Pristine radio environment

Maximizing reliability happens 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 transmit through the air, cable replacements for industrial I/O require only bytes of information to travel. Since errors occur when bits arrive 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 (4-20mA current loop output is usually transmitted in one or two 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 and allows data to be lost if the radio environment is cluttered by heavy interference.

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 baud rate). In a setting with no interference, if one watt of transmit power is applied to a transmitter sending out 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 maze-like metallic structures. The other side is more bits per second in transmission reduce Power per Bit. Therefore, in applications where I/O travels 300 ft to 15 miles, a small number of bits stand a better chance of making it to the receiver than a large number.

Spreading code generator

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.

It is important to understand interference and how DFSS and FHSS address it. Wireless radios encounter interference through EMI or RFI 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 that is transmitting and widen it across a broader bandwidth.

Signal strength at any one frequency goes down as the energy spreads.

The spreading code reapplies at the DSSS receiver. The signal contracts back and data recur. 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 spread by using the same code de-spreading 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, passing of the threshold happens, 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 narrow band 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. Indeed, it has plenty of room to hop. The 902-928MHz ISM frequency band is wide enough to hold approximately 1000 licensed narrowband radios.

Small packets of data convey 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 the system discards 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 is checked, and when interference shows up, the bad packet is not 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. cFHSS also has the unique advantage of being a small moving target. Throughput does not cease until the entire ISM frequency band at any one location is full. 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 like alarm and emergency stop signals because even though packets are lost others get through.

The redundancy of data transmission and small packet size make FHSS the preferable choice for industrial wireless I/O applications such as simple analog and digital signals. The technology provides reliability even in confined environments where many FHSS radios operate.

FHSS industrial strength radios including DIN rail mount wireless I/O, are rapidly gaining market acceptance. New technology will continue to improve application usage making wireless I/O commonplace in harsh industrial environments in the years to come. W

Nicholas Sheble (nsheble@isa.org) edits wireless reliability features. Content for this piece is from Phoenix Contact (www.phoenixcon.com/wireless/).