by Dan Paladino
As recently as five years ago the telemetry solutions in the water industries were primarily licensed systems. However, the scarcity of available licenses and improved technology has made spread spectrum radio the technology of choice. While acceptance of spread spectrum technology is increasing, a number of “myths” about the technology have circulated through the industry.
With the introduction and advancement of any technology, misconceptions and misunderstandings always surface. Spread spectrum, like any technology, can be an extremely valuable tool when deployed with a thorough understanding of its capabilities. The objective of this article is to address the “myths” and provide a better understanding of how spread spectrum works.
Security
Spread spectrum was invented in 1941 and was initially used by the U.S. Navy during World War II to prevent the Germans from “jamming” American radio transmissions for radio guided torpedoes. These original radios contained a roll of paper slotted like a player piano to cause channel switching. This original system used only 88 frequencies. Today, the switching is controlled by embedded software code that enables a radio to change frequencies in excess of 200 times per second and use more than 100 channels.
Spread spectrum technology is complex enough that anyone trying to intercept a signal would have to match more than 186,000 possible parameters to be on the same channel with the radio and then would only be in sync for about 1/100th of a second. In addition to matching parameters, today’s radios use advanced encryption protocols to further secure the data.
Saturation
A common misconception is that as more and more users “share” this frequency, it will become saturated. Although saturation is a possibility, technology advances designed to counter this condition far outpace the theoretical state. Throughout the country spread spectrum radios are delivering data to multiple users without significant conflict or data loss.
Over the years, continued improvements have been made allowing spread spectrum technology to be deployed into applications once thought to be beyond the technology’s capability. Now, spread spectrum can be used over long distances, in close proximity and as high-speed backbones. Numerous radio systems can be deployed without impacting the effectiveness of any given network. This is done by modifying timing, hopping patterns and various other programmable options allowing numerous systems to share the same frequency range.
Range
Another “myth” is that spread spectrum can only be used for short range applications. To the contrary, spread spectrum can be deployed to meet long or short distance requirements. By federal regulation, a spread spectrum radio’s output power is limited to one watt of radiated power at the radio and four watts at the antenna. Licensed radios by contrast can have between two and five watts at the radio and 20 watts at the antenna.
Based on power alone, a licensed radio generally will win the distance test. However, power is not the only factor in achieving range. Spread spectrum radios can easily establish links of 20 miles and they have even been able to link at distances greater than 60 miles. At distances in excess of seven to 10 miles, however, curvature of the earth becomes an obstacle to overcome.
Increasing the range of spread spectrum systems typically involves the use of multiple repeaters. Repeaters can be installed in parallel or in a series to increase range and also to work around obstacles or line of site concerns. Some complex networks have documented the use of more than 100 in a single system.
Some manufacturers produce radios that can operate as a slave and repeater simultaneously. This feature offers network extension and a cost-reduction benefit. The slave/repeater function eliminates the need for dedicated repeaters and allows the PLC (Programmable Logic Controller), RTU (Remote Terminal Unit), or other intelligent device to multitask as both a slave unit − sending data back to the host − and as a repeater for other devices further down the network hierarchy.
Compatibility
Many people believe that if they install a base of licensed radios, they must use the same manufacturer and model of radio they originally purchased. However, it is possible to mix spread spectrum radios into an existing licensed radio system to create a “hybrid” solution. Such a network can be accomplished by placing a repeater in the existing system. This hybrid approach allows the user to take advantage of the benefits offered by each manufacturer and/or technology.
It is also possible to create hybrid systems by combining CDPD (Cellular Digital Packetize Data), satellite, cell phones, and landline telephone modems individually with spread spectrum.
Interference
Another common misconception is that spread spectrum and other radio communications will interfere with each other. The most common spread spectrum band in the U.S. is 902 to 928 Megahertz. This frequency band is set aside by the federal government and is allocated for spread spectrum devices and the rules are structured to allow the band to be shared by multiple users. The official designation for this band is ISM, which stands for industrial, scientific and medical usage.
Licensed radios use frequencies outside of this band. No licenses are granted for any frequencies inside the ISM band. Consequentially, there will be no overlap between licensed systems and spread spectrum systems. The two technologies will always broadcast on separate frequencies and thus cohabitate without negative results.
In the event of interference between bands, installation of a band pass filter usually solves the issue.
Obstructions
“Line of Sight” is a commonly misunderstood term. Line of site is what a radio wave can “see,” not what the human eye can see. Each frequency band has varying abilities. It is true that spread spectrum radios are more restricted by line of sight than some licensed radios. While line of sight is always preferred, spread spectrum radios will indeed pass data through obstacles such as buildings, trees, and in many cases over hills. What happens to a radio signal in these environments is that the obstacles introduce “attenuation” into the signal path. Attenuation is a resistance that reduces the strength of the signal. Attenuation occurs over a distance -- the greater the distance the greater the attenuation. Attenuation also increases with the presence of tree branches and foliage. A radio may transmit for 20 miles with clear line of sight, but it may not be able to do so if obstacles exist throughout the 20-mile path. This is where the ability to use repeaters comes in handy!
Installation
The key to any successful network deployment is an understanding of both the capability of your radio and knowing the terrain in which you will be communicating.
Performing a “path study” prior to starting a project will create a network design that allows you to work around any obstacle to ensure a solid and robust communication system, regardless of “line of sight” in the area.
Conclusion
Spread spectrum is a very reliable and robust technology which can be used in the most demanding of environments. The acceptance of this technology is proof that “myths” are simply that and that there are few limitations on how you can use spread spectrum radios. As with all radio products, thoroughly research and field test performance specifications claimed in marketing literature to ensure the resulting network will meet your communication requirements.
About the Author:
Dan Paladino has more than 10 years of experience with wireless communication and control in utilities markets including electric power, water/waste water, and metering solutions. In his current role, Paladino is responsible for business-to-business and OEM growth and development for Freewave Technologies. Prior to joining the company, he was an independent consultant to organizations and providers using wireless technology to communicate in metering and sub-metering electric, water, and natural gas metering deployments.
