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Background | What is Spread Spectrum? | Direct Sequence or Frequency Hopping? | Installation Planning | Why Not Go Wireless? | Glossary of Terms GE MDS is pleased to present this overview of Supervisory Control And Data Acquisition systems (SCADA) communications. OverviewThe wireless revolution is having a major impact on how we set up and communicate between nodes in a supervisory control and data acquisition system (SCADA). GE MDS is a world leader in wireless data networking solutions with applications in the SCADA, telemetry, telecommunications, and on-line transaction processing markets. In the past, customers needing wireless communication systems would be required to obtain an FCC license. In some localities, licenses are becoming difficult, or even impossible, to obtain. A popular solution to the licensing problem is provided by the emergence of Spread Spectrum (SS) technology. This new digital technology provides most of the capabilities and performance of a licensed radio system with a license-free approach. For long distance data communications in a control or data gathering application, Frequency Hopping Spread Spectrum (FHSS) radios provide a robust solution. Spread Spectrum typically uses the frequency band of 902 to 928 MHz. Today's new digital radios employ frequency hopping within this range to ensure maximum performance and maximum reliability for SCADA communications. Sophisticated hopping sequences and forward error correction reduces interference from unwanted sources. These radio enhancements provide very low bit-error rates and greater range of use. In addition, use of Digital Signal Processing (DSP) technology allows systems to communicate at speeds of 19.2 kbps, and higher, even with weak signals and in the presence of interference. The added intelligence created by the use of microprocessors in the radio allows for improved diagnostic facilities. These facilities allow for the monitoring and control of radio parameters, locally at the radio, as well as via the master base station. This paper briefly discusses the historical background of spread spectrum radio, FCC rules and regulations, and the definition and application of direct sequence and frequency hopping techniques. Also covered are system architecture options, design, and installation considerations. BackgroundMore than 12 years ago, the Federal Communications Commission (FCC) first allowed spread spectrum operation to be used in the commercial sectors of radio communications. More recently, on April 3, 1997, the FCC amended its Part 15 rules to further recognize and insure the future of spread spectrum operation. Part 15 unlicensed devices, such as spread spectrum systems, now enjoy significant protection of their right to operate in the 902 to 928 MHz band with respect to the licensed (Part 90) Location and Monitoring Services (LMS) that work on fixed frequencies in the same 900MHz band. The FCC is the agency assigned in the United States to authorize and grant permission to operate radio transceivers in both the licensed and unlicensed frequency bands. We find the use of both licensed and unlicensed technology for SCADA systems using radio communications in the U.S. In some instances, there are systems integrating licensed and license-free radios to provide a complete wireless solution. Part 15 authorizes the unlicensed operation of spread spectrum transmitters within the 915 MHz, 2450 MHz and 5800 MHz frequency bands at higher power levels than would normally be permitted for other unlicensed devices. The current regulations limit spread spectrum systems to a maximum peak transmitter output power of 1 Watt. When operating at that power level, the maximum directional gain of the associated antenna may not exceed 6 dBi, resulting in a maximum Equivalent Isotropically Radiated Power (EIRP) of 4 Watts. Direct sequence systems must employ a minimum bandwidth of 500 kHz with a processing gain of at least 10 dB. Frequency hopping systems in the 915 MHz band must use at least 50 channels. The rules at 2450 MHz and 5800 MHz are somewhat different. The FCC has recognized the extensive applications being developed for Part 15 devices. These Part 15 devices are used for a variety of consumer and business oriented applications, including wireless LANs, automatic meter reading systems, SCADA systems, telephones, broadband access to Internet services, video conferencing, health care monitoring, and traffic light control. What is Spread Spectrum?The term Spread Spectrum (SS) describes a communications technique whereby a radio frequency signal is modulated (spread) a second time so as to generate an expanded bandwidth wideband signal. Spread Spectrum is usually used for data transmission. The two most popular types of Spread Spectrum modes are Frequency Hopping and Direct Sequence.
Definition of Frequency HoppingFrequency Hopping SS involves the application of a pseudorandom code which causes the transmitter to periodically hop or jump to a new frequency, transmit information on the frequency for a defined period of time, then hop to the next frequency and repeat the process. In order for the receiver to recover the transmitted information, it must hop to the same frequencies as the transmitter. Thus, the pseudorandom code and some synchronization information must be known at the receive end of the link. Frequency Hopping Spread Spectrum usually uses narrow-band transmit and receive techniques, thus providing long distance communications with excellent noise immunity and interference rejection capabilities. Definition of Direct SequenceDirect Sequence SS also involves the application of pseudorandom codes known to both ends of the link, but the code is used to cause a fixed frequency transmitter to spread its power more or less evenly across a wide band of RF spectrum, usually many Megahertz. Pseudorandom codes are selected to give the spread signal a noise-like character, which when detected by a conventional receiving device, looks very much like random noise. The receiver must be wide enough to recover all of this bandwidth in order to recover the transmitted signal, and then, using the same pseudorandom code as the transmitter, de-spread the signal to its original data component. Direct Sequence systems also have good immunity to noise and interference when used with highly directional parabolic antennas in relatively short range applications. Direct Sequence or Frequency Hopping?The different modulation techniques of broad (direct sequence) and narrowband (frequency hopper) have specific advantages for specific purposes. The frequency hopper has the advantage of spreading its signal across the a wide bandwidth using maximum allowable transmit power and high receiver sensitivity (system gain) but at the same time using traditional narrowband filtering techniques to reject noise and interference with brute force filtering. It is by far, the optimum technique for a SCADA system requiring long range communications in a shared radio spectrum. Interference Issues - The Good NewsOne of the common concerns to potential Spread Spectrum users is the possibility of interference to or from other services occupying the same band of channels. Prior to recent FCC rules changes, there was particular concern that interference caused by Part 15 devices to the Location and Monitoring (LMS) receivers could possibly lead to the Part 15 users being required to cease operation. Herein lies the good news. Part 90.361 of the FCC rules, addresses the operating conditions under which a Part 15 spread spectrum transmitter in the 902-928 MHz band is presumed not to cause interference to the wideband Part 90 Location and Monitoring Services (LMS). These provisions were developed in conjunction with the LMS rule making, and any changes to the regulations would be associated with that radio service. Furthermore, and most important, placing these provisions in Part 90 of the rules serve to alert LMS operators that they cannot claim harmful interference has occurred from most spread spectrum operations. The FCC does not require that limitations be placed on spread spectrum signals occupying the LMS sub-bands, provided you comply with height/power restrictions when necessary. There are areas within the U.S. where the wideband LMS channels are not being used or are only partially used. As indicated above, Part 15 spread spectrum transmitters complying with the conditions in Section 90.361 of the rules are already presumed not to be a source of harmful interference to wideband LMS systems. As a consequence, there is no basis for prohibiting the operation of spread spectrum systems in the wideband LMS channels. Benefits of Spread Spectrum Combined with DSPThe beneficial effect of the digital signal processing (DSP) in the spread spectrum radio allows improvements in data recovery using forward error correction, and improved signal path distances as a result of improved receiver sensitivity. Data turnaround times (system latency) have also been reduced resulting in greater system throughput. High performance radios can offer data turnaround times of less than 10 milliseconds. DSP allows radio hardware functions that have historically been implemented in analog hardware to be implemented in digital hardware with mathematical precision and repeatability. Benefits of DSP include:
When looking at technology to apply in SCADA or other telemetry needs, make sure to consider the flexibility the system offers. Does it allow you to mask off certain parts of the 902-928MHz band to avoid potential cellular or pager interference? Are there hundreds of channels to chose from and thousands of network addresses available? Does it allow you the ability to use it as an extension of an existing MAS system? Do you have to modify your system in any way to accommodate the type of radio you are considering? (i.e., Is it transparent ?) Does the company you are considering have its main focus in radio communications and do they have a field service and engineering department with radio and data experience to assist you through system installation and start-up? While considering the questions above, also find out if the product or system has network connectivity. In this day and age of enterprise-wide access to company information, seek a level of comfort that the product has a migration path that will allow expandability into network solutions of tomorrow. With fewer personnel being assigned to maintenance, consider having not only access to SCADA information but also the diagnostic data of the radio system. Above all else, consider the intent of the product design you are using! Is it designed from the board up for SCADA communications? Installation PlanningAs with a licensed system, the installation of the SS transceiver is not difficult, but it does require some planning to ensure communication reliability. This section pro- vides tips for selecting an appropriate system architecture, site location, choosing an antenna system, and minimizing the chance of harmful interference. General RequirementsThere are three main requirements for installing the transceiver: adequate and stable primary power, a good antenna system, and the correct interface between the transceiver and the end device. Site SelectionFor a successful installation, careful thought must be given to selecting proper sites for the master and remote stations. Suitable sites must provide:
These requirements can be quickly determined in most cases. A possible exception is the last item verifying that an unobstructed transmission path exists. Microwave radio signals travel primarily by line-of-sight, and obstructions between the sending and receiving stations will affect system performance. If you are not familiar with the effects of terrain and other obstructions on radio transmission, the discussion below will provide helpful background. Terrain and Signal StrengthMost SS transceivers operate in the 900 MHz frequency band. While this band offers many advantages over VHF for data transmission, it is also more prone than VHF to signal attenuation from obstructions such as terrain, foliage, buildings, and other things in the transmission path. A line-of-sight transmission path between the master radio and its associated remote site(s) is highly desirable and provides the most reliable communications link. A line-of-sight path can often be achieved by mounting the station antenna on a tower or other elevated structure that raises the antenna to a level sufficiently high to clear surrounding terrain and other obstructions. The importance of a clear transmission path relates closely to the distance to be covered by the system. If the system is to cover only a limited geographic area, for example, 1 to 3 miles, then some obstructions in the transmission path can usually be tolerated with minimal impact. For longer range systems, any substantial obstruction in the transmission path may reduce communication reliability, or block the transmission entirely. Much depends on the minimum signal strength that can be tolerated in a given system. Although the exact figure will differ from one system to another, a Received Signal Strength Indication (RSSI) of -D90 dBm or stronger should provide acceptable performance in many spread spectrum systems. While the radio will work at lower signal strengths, it is good practice to provide a fade margin to account for variations in signal strength which may occur from time-to-time. To assist in path analysis in the field, RSSI should be a parameter measurable directly from the SST radio. Conducting a Site SurveyIf you are in doubt about the suitability of the radio sites in your system, it is best to evaluate them before a permanent installation is begun. This can be done with an on-the-air test (preferred method), or indirectly using computer generated path surveys. An on-the-air test is preferred because it allows you to see first hand, the factors involved at an installation site and to directly observe the quality of system operation. If time is short and a site survey is impractical, a computer path study is a good alternative. Factors such as terrain, distance, transmitter power, receiver sensitivity, and other conditions are taken into account to predict the performance of a proposed system. Why Not Go Wireless?Frequency hopping spread spectrum radios provide an excellent economic alternative to traditional hard wired and licensed radio solutions. They are designed to operate in harsh conditions and can be scaled for progressive growth and unlimited coverage using repeater links. Frequency hopping is also an ideal solution for avoiding interference. With a typical system payback of two years, (based on leased line rate comparisons, and total control over installation and maintenance), a wireless spread spectrum SCADA system can provide you with years of reliable data communications. Glossary of TermsIf you are new to Spread Spectrum radio, some of the terms used in this paper may be unfamiliar. The glossary below explains many of these terms and will prove helpful in understanding the operation of the transceiver. Bit The smallest unit of digital data, often represented by a one or a zero. Eight bits (plus start, stop, and parity bits) usually comprise a byte. |
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