Articles and Technical Papers

License-Free Spread Spectrum and FSK Wire Industrial Data Communications

Paper presented at
The 1996 IEEE Conference
Michel E. Maes and James R. Steffey
Data-Linc Group
2635 151st Place NE
Redmond WA 98052
U.S.A.
(425) 882-2206

Introduction

Spread spectrum technology continues to gain acceptance as the wireless technology of choice for industrial automation applications. For the major automation industry manufacturers of Process Logic Controllers (PLCs), Remote Terminal Units (RTUs) and other data acquisition, control and processing equipment, data communications reliability and integrity are paramount for the often challenging environments encountered in real world Distributed Control System (DCS) and extensive Supervisorial Control and Data Acquisition (SCADA) installations. The principal industrial automation manufacturers are often dependent upon wireless modem communications to transmit data between their equipment and other devices such as PLCs, RTUs and PCs in multi-point applications at very long range. Special protocol, timing, interfacing and other issues must be addressed in the modems intended to provide the appropriate connectivity, compatibility and interoperability with their equipment. In addition, with the increasing difficulty in obtaining a site license, particularly in population centers, the availability of license free operation in the 902-928MHz and 2.4-2.48GHz license free bands can be most advantageous.

Spread Spectrum Overview

A wide variety of radio modems have been employed for the purpose of wireless data communications. Typically, a site license was required and, for the requirements of the modern industrial automation era, range capability, baud rate and turnaround delay (response time between the Master and successive Remotes) were often inadequate. Today, with the recent introduction of advanced spread spectrum techniques, even extensive DCS and SCADA requirements can now be addressed and satisfied.

Spread spectrum technology employs two separate techniques for data transmission, referred to as Frequency Hopping and Direct Sequence (see diagram). For the industrial data acquisition and control market, including SCADA applications, the frequency hopping technique is particularly useful where reliability and data integrity are more important than the timeliness of data transmission, especially when the difference is measured in milliseconds. With advanced technology spread spectrum modems, full duplex uncompressed data rates at 115.2 Kbaud are available for line-of-sight range capability of 20 to 30 miles at 1 watt power output using a standard omni directional whip antenna. Significantly increased range capability is also achieved with the use of Repeaters and external Yagi antennas. As a rule of thumb, for distances of up to one-half mile, the line-of-sight requirement does not apply. At distances greater than one mile, a line of site limitation can be expected with the interim distance dependent upon local conditions.

For multipoint communications between a Master radio and multiple remotes, the vast majority of installations will achieve net throughput approaching the 115.2 Kbaud point-to-point capability under normal operating conditions. Even where interfer-ence problems are encountered, however, a significant percentage of multidrop control and monitoring functions will typically require data throughput of no more than 19.2 Kbaud. Frequency hopping spread spectrum radios are more than capable of supporting such data rates while also overcoming interference and other multipath fading problems. In addition, the hopping technology facilitates the use of multiple pairs in close proximity.

Because frequency hopping technology relies on a sophisticated pre-set hopping algorithm to maintain synchronization between the Master and remotes, rapid response time (typically on the order of a 5 to 15 millisecond turnaround delay) and the 115.2 Kbaud full duplex uncompressed data rate capability are especially important to maintain high data throughput. In addition, where the spread spectrum radios are expected to operate in particularly adverse environments, they can be pre-set to automatically retransmit data packets up to 10 times, each at a different frequency within the 902-928 MHz band. Because they are narrow band transmitters, frequency hoppers can transmit at more than 100 distinct frequencies in accordance with the pre-set hopping algorithm. Since interference could obscure the entire band only under the most extraordinary circumstances and each frequency hop is at least 6 MHz, it is virtually certain that the transmission will be received even if multiple retries are required. Such techniques are intended to assure significantly improved data reliability and integrity. Data trans-mission rates, however, are reduced accordingly, making the 115.2 Kbaud maximum data rate and the impressive response time capability invaluable in the maintenance of desirable net data throughput where interference, contention, multipath selective fading and collision problems are encountered.

In contrast, where adverse conditions are encountered, a direct sequence spread spectrum radio can actually fail to maintain communications. As a wide band transmitter, it obviously does not have the ability to access multiple distinct narrow band frequencies for the purpose of optimizing avoidance of interference or multipath problems. Although its net throughput will always equal or exceed the frequency hopping transmitter, it is at the expense of data reliability in conditions where hostile environment problems are encountered. The wide-band transmitters utilized for direct sequence data communications will in fact fail to maintain communications when the level of interference at the receiver exceeds the received signal power by even a small margin. Frequency hopping technology, on the other hand, will sacrifice the highest possible data rates to assure the maintenance of communications even in the face of the severest interference, although at a lower net throughput. Clearly, worst case transmission speeds of 9600 baud to 19.2 Kbaud are typically a small price to pay for insuring data reliability and integrity in critical operations. Nevertheless, direct sequence spread spectrum technology does have its place, particularly where the highest possible data rates are required and interference or other data coomunications problems will not be encountered at present or in the future.

As with existing data transmission over wire, Packet Protocol Acknowledgment, supported by CRC 32 bit error checking, is designed to insure that the data received by any spread spectrum modem is not delivered from its buffer to the attached device until it is acknowledged as a correct transmission, guaran-teeing that what is received is identical to what is sent. In addition, Packet protocol extensions and radio link control techniques support the assignment of specific addresses to individual modems, allowing the radios to function as Repeaters; the establishment of a "listen before transmit" feature serves to eliminate contention problems. Factory preconfiguration of the modems and cables provides simple installation and transparent operation. In addition, the end user still retains total control over the full range of menu driven options available in the event that future field reconfiguration is required.

In summary, wireless data transmission continues to benefit from rapid advancements in the technology. The commercial market is now enjoying the results of an extensive research program conducted by the military over an extended period in its effort to insure reliable data integrity even under the most challenging battlefield conditions where the ability to maintain communications in the face of purposeful attempts at jamming is clearly essential. Additional applied research by the private sector, designed specifically for the requirements of the industrial automation customer, have added features and benefits appropriate for such end users.

Finally, it should also be recognized that legitimate concerns have recently been raised that rapidly increasing usage of the 902-928 MHz band can eventually lead to saturation and, thereby, deterioration in data transmission capability. It is also evident that the frequency hopping technology provides a means of dramatically enhancing avoidance of the problems, such as interference, that result from saturation. As the 902-928 MHz band becomes increasingly crowded, additional radio link control techniques will serve to maintain the viability of data transmission reliability and integrity as well as the data throughput rates required for typical industrial data acquisition and control applications. Implementation of the technology in the 2.4-2.48 GHz band, intended to be license free virtually worldwide, will serve to alleviate saturation in the lower band.

Spread Spectrum Technology

Spread spectrum technology "spreads" a transmitted radio signal over a frequency band wider than the minimum bandwidth required to transmit the information. Narrow band signals from standard radio transceivers see only a slight increase in the noise floor while a spread spectrum receiver does not detect the narrow band signals because it is listening to a far wider bandwidth at a prescribed code sequence.

Direct sequence spread spectrum systems modulate a narrow band carrier by a code sequence utilized to change the carrier phase of the transmitted signal. A fixed length pseudo-random generator produces the code sequence which repeats itself following a specific number of bits. The speed of this code sequence, measured in "chips" per second, is referred to as the chipping rate. The spreading of a direct sequence signal is a function of the chips per bit and the information transmitted by a spread spectrum radio utilizing this technique is recovered by multiplying the received signal with a copy of the code sequence.

In frequency hopping spread spectrum systems, the carrier frequency of the transmitter changes in accordance with the pseudo-random code sequence. A pre-set algorithm is employed to maintain synchronization between the radios as they hop sequentially through a set pattern of frequencies.

Time hopping, pulsed FM and hybrid systems are other alternatives utilized depending on the desired characteristics and communications requirements. Time hopping is achieved by varying the period and duty cycle of a pulsed RF carrier in a pseudo-random manner controlled by the code sequence. TDMA spread spectrum systems combine time division multiple access and frequency hopping spread spectrum techniques.

Pulsed FM systems, often referred to as chirp systems, modulate the RF carrier with a fixed period/fixed duty cycle sequence. The carrier frequency of each transmitted pulse is frequency modulated resulting in additional spreading of the carrier. For example, the spreading function can be a linear chirp sweep, either up or down in frequency. The most common hybrid system combines direct sequence and frequency hopping, providing certain characteristics not available with either technique when used alone.

Although spread spectrum technology is bandwidth inefficient and quite complex in its implementation, it offers decided advantages where data communications reliability and integrity are paramount. Not only is interference, including multi-path selective fading, significantly mitigated but the ability to share the same frequency band with other users and dramatically enhanced data security represent additional benefits that result from the pseudo-random code sequence and the wide signal bandwidth. Codes with low cross correlation properties enable individual groups of transceivers to operate side by side without resultant interference problems. More than one signal can be transmitted at the same time on the same frequency. Anti-jamming capability (referred to as processing gain) is a function of the wide bandwidth used for signal transmission.

Demodulation of the spread spectrum signal is accomplished, first, by removal of the spectrum spreading (defined as "correlation") followed by demodulation of the signal. Synchronization of the spreading code of the receiver with the transmitter must be maintained to result in despreading of the spread spectrum signal. Ongoing research efforts have resulted in dramatic developments in digital signal processing ASICs (Application Specific Integrated Circuits) which are responsible for the cost effective solutions to initial acquisition and tracking, the two components of synchronization critical to the success of spread spectrum technology and central to the complex tasks of correlation and synchronization. FCC Part 15 spread spectrum devices, limited to frequency hopping and direct sequence spreading techniques, operate in three frequency bands: 902-928MHz, 2.4-2.4835GHz and 5.725-5.85GHz. Maximum peak power output is limited to one watt.

In summary, the most important features of spread spectrum technology include:

Principal Characteristics of Spread Spectrum Technolory:

Spread Spectrum Conclusion

The principal industrial automation companies spend considerable time, effort and expense insuring that their PLCs, RTUs and other equipment perform as flawlessly as possible. Nevertheless, the need not only to collect, distribute and process data but also to transmit it between such devices as well as PCs in DCS and SCADA installations makes the selection of data communications equipment, including modems, particularly important toward the maintenance of data integrity. Spread spectrum technology serves to significantly improve data transmission reliability and frequency hopping techniques further increase the assurance, in combination with error checking, that data will not be lost. Finally, meticulous attention to the issues of interconnectivity, compatibility and interoperability with industrial automation equipment, including factory configuration of modems and cables, can literally provide a "no brainer" installation (no modem field settings, programming or connec-tivity problems) as well as insurance against potential operational problems.

FSK Data Communications Over Wire

Frequency Shift Key (FSK) technology supports data transmission on existing in-place AC/DC power lines, instrument loop wires, PBX wiring or any other two conductor wire for a wide variety of applications, such as office/business datacom, process control, data acquisition, monitoring, energy management, access control, etc. FSK modems may be used for point-to-point, multi-drop, polling/addressing, multiplexing or full LAN applications transmitting discrete, analog (4-20 mA, 0-5 volt, etc.) or digital (RS232, 422, or 485) data. Data rates are available to 9.6 Kbaud and at distances up to 8 miles on any two conductor wire without Repeaters or line drivers.

Typical applications for the technology include: Security system and access control monitoring; automated materials handling and warehousing (i.e. transmitting data from control computers to stacker cranes over the AC/DC lines supplying such devices, across the sliding contacts); remote display in driver/electronic sign control; mine safety data gathering; waste water treatment remote monitoring/control tasks; and multi-station process control or monitoring. In addition to data communi-cations on any in-place wiring, other advantages include:

In summary, the ability to move any data signal (digital, analog, discrete or pulse stream) on any two conductor already existing or installed wiring can often result in substantial savings over the expense of traditional cable installation, particularly for difficult applications or in hostile environments.

The FSK Advantage

Traditionally, data is transmitted between computers and/or peripheral devices such as Process Logic Controllers (PLC's) and Remote Telemetry Units (RTU's), either by direct multi-wire cable or through a modem. Direct multi-wire cable is large (9-25 wires), bulky, cumbersome, and frequently difficult and expensive to install. It has a range limited to about 100 feet without line drivers. A modem is a device that converts digital signals into a form that can be transmitted much longer distances over single or double wire pairs, or telephone lines. Most modems on the market are made to transmit data over the public telephone lines, and the telephone cannot be used for voice when data is being transmitted. Special modems are also available, however, that allow voice and data to be transmitted simultaneously without interfering with each other over private, in-facility PBX telephone lines, or to transmit data over facility 4-20 mA instrument loop wires or power lines. Other versions can send full duplex data over any two conductor wire at distances of up to 20 miles. In addition, add-on signal coupling modules permit data to be easily transferred from one wire pair to another such as from PBX telephone lines to AC power wiring. Other modules allow the modems to be used in multi-drop mode so that data can be obtained from hundreds of monitoring locations over any two conductor wire network.

Currently, the vast majority of data transmitted in industrial/commercial monitoring and control applications is in digital form, typically either RS-232, 422 or 485, except where extensive ISDN type installations involving Computer Integrated Manufacturing (CIM) networks are required. Nevertheless, much of the equipment is often analog in which A/D and D/A conversions and instrument loop wire installations are a typical necessary expense in moving the data from where it originates to where it is needed. Analog to digital conversion for the sole purpose of data transmission to other locations (where additional D/A converters must be installed if sensor outputs are being sent, for example, to analog recorders or totalizers) can prove to be more expensive and less reliable than special modem devices providing a number of distinct advantages. The FSK Advantage depends upon the ability to convert ASCII data streams, or any other digitally encoded information, into frequency equivalents (see diagram), thereby reproducing space and mark (the 1's and 0's of the digital format) as discrete frequencies prior to transmission from point-to-point. Each device, therefore, acts as a combination converter/transceiver supporting full duplex data communications between the modems on any two conductor wire. 100/106.5 KHz and 150/156.5 KHz frequency pairs are employed, for example, to provide simultaneous bi-directional data transmission. The FSK Advantage is ideally suited to conditions where the transmission of digitally encoded data streams compromises data integrity as a result of the sensitivity of digital data to hostile environments. In addition, the technology can be utilized not only to transmit discrete and digitally encoded data as frequency equivalents, but to move analog data as a variable pulse rate without the traditional A/D and D/A conversions. The technology can also provide an alternative to cable installation by superimposing discrete, digital, analog or pulse stream data on in-place power, PBX, instrumentation or other existing wiring to permit datacom simulta-neously with power, voice or instrumentation signals.

Industrial data acquisition and process control challenges in high value-added operations are such that equipment reliability and dependability are essential in preventing costly down time incidents. In this digital era, equipment malfunctions often require extensive and lengthy troubleshooting efforts before a problem can be detected and corrected. Too often, the inability to rapidly identify data transmission reliability as the culprit can significantly compromise quality or delay resumption of operations. The care taken in the selection of computer hardware, software and peripheral equipment such as PLC's or RTU's should also be applied to the recognition that data communications between devices is every bit as critical not only in controlling installation costs but also to insure efficient, reliable on-going operations.

Summary

The technology provides a family of low cost data communications devices available for a broad range of industrial and commercial applications, capable of solving the most difficult data communications problems with exceptional versatility and ease. The products feature data rate transparency, universal interfacing, and the ability to perform comprehensive monitoring and control over widely dispersed networks at a fraction of the normal installation cost for conventional hard wired data systems. Modular units are available for digital, discrete and analog data communications which substantially reduce cabling requirements and permit reliable operations in difficult operating environments.

Operate on Virtually Any Wires Available

High Data Rates/Long Range

Excellent Immunity to Harsh Environments

Universal Interfaces - Digital, Analog, Discrete, Voice

Multiple Operating Modes: Point-to-Point, Multi-point, Multidrop, Polling, Multiplexing

Transparency

FSK Data Communications Conclusion

For decades, modules based on the inherently rugged, robust FSK technology have been plugged into the AC electrical outlets at home for the purpose of remote operation of appliances. Advanced FSK technology can now provide important and critical advantages where reliable data communications are essential. Clearly, any significant improvement in the inherent immunity to EMI, noise, surge, ground improvement in the inherent immunity to EMI, noise, surge, ground loop or shifting ground plane headaches and (and their impact upon data integrity) provides present and future freedom from the problems that can plague instrumentation technicians and other operations personnel in their efforts to insure on-going data reliability and integrity. In summary, while many additional features and characteristics of the technology serve to facilitate dependable, reliable and economical data communications even under harsh conditions and in hostile environments, the important "bottom line" advantage of this technical approach to industrial data communications is the ease of installation, ease of operation and ease of reconfiguration.

Michel E. Maes, President, and James R. Steffey, Vice President, both graduated in Physics from the University of Washington in the late 1950s. They founded Data-Linc Group in 1988 following many years of research and development efforts devoted to modem communications.

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