Spread Spectrum - A Brief Tutorial
By Randy Klassen, OMNEX Control Systems Inc.
Spread Spectrum is a phrase that's being heard more and more in the sensor marketplace, the industrial
process and control world, and in telemetry Supervisory Control and Data Acquisition (SCADA). Those who
investigate Spread Spectrum radios quickly discover that they are license free and moderately powered.
Beyond that, little seems to be known.
FIXED FREQUENCY RADIO
Before we begin our discussion of spread spectrum radio, let's back up for a moment and focus briefly on the
fixed frequency radios we all listen to in our homes and cars. The process by which we do this is relatively
simple. We "tune" our receiver to a specific frequency and listen to the information being sent.
Where do these signals originate? From licensed transmitters sending out signals on regulated fixed
frequencies. The laws governing the use of these frequencies (FCC regulations) dictate that the person with
the transmitter must purchase a license from a government organization that oversees the allocation of
frequencies. Once a license is granted, theory holds that no one else will be able to legally broadcast signals on
the purchased frequency within the territory covered by the license, and any receiver capable of decoding the
signal on that frequency can listen in. In other words, fixed frequency radios work when users comply with the
rules and regulations, and when illegal use of those frequencies can be policed.
An example of a licensed radio in the mobile control industry would be one that functions at an allocated
frequency of 457.8250MHz, has a bandwidth of 25KHz and is restricted to a power output of 5 Watts. The
"licensee" pays for this license on an annual basis and expects that no one else will interfere with the operation
of his/her machinery. The licensee is expected not to overpower his/her radio, or move it off this frequency.
The benefits of a licensed fixed frequency radio include:
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High power output that allows for sending the signal a great distance and the ability to "blast" through
obstacles. This gives the average person the ability to listen to some radios stations well beyond the
limits of the outlying suburbs, and enables police and emergency vehicles to communicate from deep
within concrete and underground structures.
A third party governing body (FCC) regulates licensing and coordination of radios in a given area.
The drawbacks of a licensed fixed frequency radio include:
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An almost immediate drop off in performance if interference moves into your allocated bandwidth. In
the industrial work place it is important to remember that interference does not only come from
someone illegally broadcasting on your frequency, it often comes from environmental interference that
cannot be controlled. For example, EMI generated by power sources or other equipment.
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Ongoing fees and bureaucratic hassles with the regulatory organization(s).
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Limited access to the number of frequencies you may need if your system requires multiple transmitting
radios. Sometimes companies wait for months or years to get three to five frequencies in their local
area that will work with the radio equipment they need for their specific application.
All in all, the difficulties with licensed radios in industrial environments (where interference from power lines
and machinery cannot be governed) have outweighed the benefits, and no amount of money spent on a
license can protect a fixed frequency radio from the many sources of interference generated on the plant floor
or in the refinery.
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Spread Spectrum - A Brief Tutorial
By Randy Klassen, OMNEX Control Systems Inc.
SPREAD SPECTRUM RADIOS
In response to the dwindling supply of available licensed frequencies, and to industrial users' requirements for
radios with the ability to overcome or tolerate high levels of interference, the FCC made the ISM bands
(Industrial, Scientific, Medical) available to commercial users in 1987 in order to combat the lack of available
licensed bands by offering more bandwidth and providing access to radios that could overcome and/or
tolerate industrial interference.
These ISM band radios are known as spread spectrum radios and their history began during WWII when RF
technologies were being evaluated for torpedo control and battlefield communications. (There is an interesting
aside here regarding Hedy Lamar, but we'll leave that for you to chase down on the web if you're so inclined.)
As these technologies developed within the military, fixed frequency radios that could be jammed or rendered
inoperable by heavy interference were replaced and spread spectrum became commonplace in restricted
circles. Now, as more and more companies come on line with spread spectrum expertise, these radios are
becoming commonplace in the every day world as their commercial appeal broadens. And though spread
spectrum radios are often views as one "type" of radio, few people realize that under FCC regulations, two types
of spread spectrum radios are allowed - Direct Sequence Spread Spectrum Radios (DSSS) and Frequency
Hopping Spread Spectrum Radios (FHSS).
Sorting out the differences between these two types of spread spectrum radios will hopefully help the reader
clarify why each type plays a different role in terms of application and function.
DIRECT SEQUENCE SPREAD SPECTRUM (DSSS) RADIO
Just like a fixed frequency radio, a DSSS radio starts with a data packet that it is required to send. But instead of
sending that data as a "narrow band" signal, the DSSS generates a 15, 63 or 127 bit pseudo random code word
for every bit in that packet and then combines these code words with the packet by multiplying them together.
These code words spread the "narrow" data being sent across a much wider bandwidth than would normally
be required. The result is a signal with lower power density, stretched across a wide bandwidth waiting for a
receiver to find it.
At the DSSS receiver, the same spreading code is reapplied to the spread received power signal once it is found
and the wideband signal is narrowed. The data is retrieved intact because the spreading process is independent
of the data and when the spreading is cancelled; the data is left in its original state. The data has moved from a
narrow state, to a wide one, and back to narrow. The nature of this process enables data to be moved through
the DSSS radio very quickly, and in terms of security, the radio signal essentially hides amidst the "background
noise," undetectable.
But what happens when a narrowband interfering signal enters the wide low power signal's bandwidth?
Because it enters the system after the transmitter, but before the receiver, it passes though only one spreading
code generator and is therefore spread by the same code that is de-spreading the original data. Is this a problem?
If the original data signal is stronger than the interference, the spreading of the interfering signal by the
receiver's code generator is rejected in favor of the more powerful narrowed data signal and 100% of the data
gets through. The DSSS has overcome its interference. But when the strength of the interfering signal exceeds
that of the original data signal by some margin (depending on the radio's process gain or upper jamming
margin), errors occur repeatedly and data throughput of the DSSS radio ceases.
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Spread Spectrum - A Brief Tutorial
By Randy Klassen, OMNEX Control Systems Inc.
The process gain of the original data signal can be increased somewhat by bumping up the length of the
spreading code. But in the end, as the power of the interfering signal increases, a threshold is reached and the
radio fails immediately and without warning. This can obviously be troublesome in an industrial workplace full
of powerful interferers. It should be noted that this process applies to narrowband interferers and it is
interesting to point out that a DSSS system has no effect on wideband interferers or common broadband
noise. The de-spreader in the DSSS simply rearranges the random wideband signals with the result being
equally wide and equally random. So then, how does the other spread spectrum radio overcome or tolerate
interference, and what, if any advantages does it have over the DSSS which dies when the interference in it's
bandwidth exceeds what it can filter out, similar to the fixed frequency radio?
FREQUENCY HOPPING SPREAD SPECTRUM (FHSS) RADIO
With a FHSS, the complexity and ability to tolerate interference does not rely on the spreading and despreading
of the data signal (like it does with a DSSS), but with the hopping synchronization of an unaltered, narrow data
signal. In a nutshell, the FHSS is a narrow band fixed frequency radio - but only for an instant - then it hops to
become 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 MHz ISM frequency band, for example, is wide enough to hold the
equivalent of approximately 1000 separate, licensed narrowband radios.
An OMNEX FHSS HopLink hops in a pseudo-random pattern around the frequency band, updating the receiver
with its small packet of data over at least 50 different frequencies before repeating its hopping sequence. On
those occasions when the HopLink does encounter a significant interfering signal on a frequency it has hopped
to, it detects an error and that transmitted packet is discarded. The hopping sequence continues and the data
updates resume. In other words, an interfering signal can knock one packet out of a FHSS radio's hop pattern,
but the rest of the updates get through, no matter how powerful that narrowband interferer becomes.
Hopping in this fashion, each FHSS packet has the unique characteristic and advantage of being a small moving
target, and herein lies the key to the FHSSs ability to tolerate interference. The radio accepts the fact that some
packets will be knocked out, but because it's hopping across such a wide bandwidth, throughput will not cease
unless the entire ISM frequency band is jammed. This enables the FHSS to reliably transmit small redundant
messages (level, flow, pressure, temperature, alarm, etc.) through areas of heavy interference, even if the
strength and size of individual interferers increases.
This is quite different from the DSSS, which maintains error-free transmission of its data until one interferer
exceeds its jamming margin, at which point the throughput of the DSSS quickly drops to zero… not at all
appropriate for mission critical industrial I/O.
TUTORIAL SUMMARY
In summary, what are the effects of interference on the various forms of radios available to the industrial user?
Narrowband licensed radios cease to operate when interference, typically in the order of one-tenth the
strength of the original desired signal, enters the frequency band this radio operates in. The radio "dies"
because it cannot move away.
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Spread Spectrum - A Brief Tutorial
By Randy Klassen, OMNEX Control Systems Inc.
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 direct sequence message will get through, whereas the frequencyhopper will experience packet losses due to the interference. In this case, the direct sequence radio is a good
choice for large packet messages, similar to those sent by LAN radios used in offices to link PC's.
In heavy interference environments - one in which the interfering signals exceed the jamming margin of the
DSSS - the direct sequence radio will cease to work, whereas the frequency-hopper will continue to function
until the entire ISM frequency band is jammed, a very unlikely scenario. Here the frequency hopper is the perfect
choice for small packet redundant data, such as alarm and emergency stop signals, because although a few
packets are lost, the rest get through.
To dispel a common misconception, FHSS radios do not avoid interference, they tolerate it. Each packet is errorchecked and when interference is encountered, the affected 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. Slow and steady, FHSS radios are the obvious choice for moving short packet,
redundant industrial I/O, for example, temperature, level, pressure and flow.
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copyright 2003 OMNEX Control Systems Inc.
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1-800-663-8806