Spread Spectrum: A Viable Solution

Kevin Carey, Microwave Data Systems

In the wireless environment, obtaining a license for a specific radio channel can be difficult, especially in urban areas where hundreds of users must vie for their “turf” in the RF spectrum. Overcrowding is likely to be a growing problem in densely populated areas as more utilities take advantage of wireless technology in their SCADA/telemetry networks. When licensed frequencies are not available—either directly or through existing license holders—practical alternatives exist for many users.

Recognizing the need for better access to the spectrum, the Federal Communications Commission and the authorities of many other nations have established bands where users can take to the air without formal licensing or notification. Spread spectrum technology is the key to this authorization, and involves distributing a transmitted signal over a wide swath of radio frequencies. By limiting the duration and level of signals on any one frequency, many users can be accommodated on a single band with a low risk of interference.

Originally developed for military communication during World War II, spread spectrum has been adapted over the past decade for wide use in low power, license-free applications—including telemetry monitoring and control. It has become a proven resource for modern data communication, and many utilities now are taking a close look at the technology as an alternative to traditional licensed networks.

Misconceptions about spread spectrum abound, with the chief concerns being range and interference potential. These points need to be clarified in light of new technological developments that have made spread spectrum a practical choice for today’s SCADA/telemetry applications.

The notion that spread spectrum radios have a range of only a few miles over a line-of-sight path needs to be compared with the facts. While range is dependent on many factors, including antenna type/height, terrain, and receiver sensitivity, the majority of well-designed systems will have little difficulty achieving an eight- to 10-mile range, even in urban areas. A modest amount of effort put into optimizing an installation can increase this range considerably. Many spread spectrum networks operate today at ranges of 15 miles or more.

Frequency band plays an important part in range. Of the two bands commonly used for spread spectrum—900 MHz and 2.4 GHz.—both offer useful range for most telemetry applications, although 900 MHz takes the edge in range and reduced susceptibility to natural or man-made obstructions. Although clear, line-of-sight paths always provide the best performance, a 900 MHz system will often tolerate moderate blockage with only minimal degradation.

Another factor related to range is the spreading technique used. Today’s radios employ either Frequency Hopping or Direct Sequence techniques. The difference relates to how the transmitted signal is spread across the spectrum. Direct sequence distributes a small amount of RF energy across an entire band simultaneously, while frequency hopping is a narrowband technique that switches the carrier frequency several times-per-second in step with the receiving station. While both are valid techniques, experience has shown that narrowband systems provide more reliable coverage over long-range paths, or in the presence of weak signals.

Interference is rarely a problem with well-designed spread spectrum radios. They employ coding schemes to minimize data “collisions” and will issue a “re-send” request when a transmission is blocked. These requests will continue until the data is received correctly. Channels plagued with continuous interference (such as a constant carrier) can be deleted from the spreading sequence altogether, effectively “sidestepping” the interference. These schemes, along with unique addressing for all units in a network can provide virtually error-free communications in even harsh RF environments. As a bonus, data security is enhanced because of the thousands of addressing and hopping combinations that are possible.

The misconception that spread spectrum networks require a complex array of transmission towers with high maintenance requirements also needs to be addressed. In the majority of cases, spread spectrum antennae can be mounted atop existing structures, or secured to a simple pole at the radio site. The maintenance requirements can be as simple as performing a visual inspection once a year.

The ease of installation, reliability and low maintenance of today’s spread spectrum devices make them worth considering for utility data communications. Although licensed radio continues to offer the best assurance of reliability, spread spectrum presents a viable alternative in heavily loaded areas where frequencies are held at a premium.

Kevin Carey is a Technical-Marketing Writer employed by Microwave Data Systems (MDS) in Rochester, N.Y. He has 17 years of experience writing for the communications and electro-mechanical industries, with an emphasis on end-user guides, technical manuals and associated product literature.

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