Ensuring coexistence between automation wireless systems

Under the umbrella of ZVEI, the German electrical and electronic trade body, a number of member companies have jointly investigated problems of wireless coexistence around industrial applications. Measurements from hundreds of practical and simulated test situations were carried out at the Institut f¨¹r Automation und Kommunikation (IFAK), Magdeburg, using currently available automation components. The data from these tests were then used as a basis for assessing the practical implications for radio systems operating in physically adjacent and service-overlapping situations.

WIRELESS SYSTEMS such as WLAN or Bluetooth have been used in the home and in offices for many years. They are also being increasingly installed in plant and machine automation systems. However, the requirements associated with such applications are often more demanding. For example, applications may require defined response times with very high availability.

A trend towards using the 2.4GHz frequency band can be observed since it is both licence-free and available worldwide. Furthermore it offers a high bandwidth and sufficient range, and is not influenced by the typical industrial sources of interference. Such extensive frequency band usages means that it is not possible to rule out completely a mutual influence of wireless systems operating in parallel proximity and may lead to a restriction in the required availability of individual systems. Wireless resources should thus be carefully considered before use.

This paper offers guidance for differentiating wireless systems, their behaviour during parallel operation, and measures for coexistence management. Above all, it will hopefully demonstrate how simple it is to avoid mutual wireless interference and to arrange interference-free parallel operation.

In addition to savings on cables and connectors, radio-based systems offer increased mobility and flexibility as well as the wear-free transmission compared to cabled systems. These advantages of wireless are particularly apparent for HMI and where sensors and actuators are used on moving plant components. For example, some tasks can be handled far more efficiently by using wireless networking for data acquisition terminals or AGVs (automated guided vehicles). Of course each of these applications has different operational needs. Since no wireless radio system can satisfy all requirements simultaneously, it may be necessary to operate several systems in parallel for different tasks.

The same medium is used by all wireless systems for radio transmission: the electromagnetic ether which surrounds them. This medium is a limited resource since only certain radio frequencies are available for transmission. Coordinated use of this resource is thus required. Personnel responsible for the IT and building infrastructure, logistics, and the automated production systems must therefore work together closely when planning wireless applications. It makes sense to obtain an overview of frequency usage, wireless systems and their characteristics1.

International and national regulatory authorities determine how the limited frequency resource can be used for radio transmission using electromagnetic waves (Fig. 1). They provide a frequency allocation plan in which licence-free frequencies and those requiring a Licence are defined. Only a few frequency bands are used in the automation technology sector; they are presented briefly here. (See Table 1).

Fig. 1 (above): Licence-free frequency bands in the electromagnetic spectrum

Table 1: Frequencies used in Europe for wireless automation technology

Licence-free ISM bands
So called ISM bands (Industrial, Scientific and Medical) have been approved by the regulatory authorities for radio applications. The main advantage of the ISM bands compared to other frequency bands is that devices using them be used without any further limitation if they comply with the respective statutory directives. The disadvantage is quite clear: these frequency bands are often used because they are licence-free, and mutual influence can result from radio systems (known and unknown) operating in close proximity. Typically approved ISM bands are at 433MHz, 2.4GHz and 5GHz. These are used in spatially limited networks such as WLAN, ZigBee or Bluetooth for instance. But microwave ovens, wireless thermometers, and vehicle remote controls also use ISM band frequencies.

In addition, the frequency band between 868 and 870MHz can be used licence-free in Europe. Different output power, bandwidth and duty cycle sometimes apply to different applications; the safety sector (alarm systems and fire detection systems) and building automation. RFID systems also frequently use this band.

Outside Europe for instance in the USA the frequency band from 902 to 928MHz is available as an ISM band for similar applications. An exclusive frequency band from 1880 to 1900MHz is reserved for the DECT standard, whereas in the USA, for example, DECT is operated in the 2.4GHz ISM band (referred to as upbanded DECT). The frequency band from 5.1 to 5.7GHz can be used licencefree by WLAN systems, but requires additional techniques such as dynamic frequency selection and power regulation.

In the automation technology sector, additional frequencies can be used following licensing by the regulatory authorities. Examples are 448MHz and 459MHz where limitations are applied on the duty cycle. Ranges of up to 10km can be covered as a result of the higher transmitter power which is permissible.

The 2.4GHz ISM band
Based on the development of cheap yet sophisticated radio components, and driven by the home and office sectors, radio technologies in the 2.4GHz band are making their way into automation technology. These exhibit many different and proprietary standards. One reason for the keen activity at this frequency is its agreed worldwide allocation as an ISM band with only minor variations. It can also deliver a high bandwidth. This can translate to offering either a high data rate, or enhanced immunity to influences from other radio systems. Furthermore, the interference spectrum in an industrial environment, originating for example from arc welding machines or power electronics, does not extend up to frequencies of 2.4GHz. Therefore no interference originates from such devices (see also Fig. 2).

Fig. 2. Interference spectra of other devices typically found in plant and industrial facilities

However, difficulties may occur if several different radio systems overlap in frequency.

Radio systems: the differences
Radio technologies encountered in automation technology may differ with regard to the frequency band used, the bandwidth requirement, and the number of channels, among other features. As far as users are concerned, the data rate, cycle time, the maximum number of network nodes, and the coexistence performance are also significant for the choice of a technology. When choosing a radio system in the 2.4GHz band, the application should be initially classified in order to ensure parallel operation with systems which may already exist.

• The transmission of safety-related data (e.g. emergency stop) imposes maximum requirements with regard to fail safety and reliability;
• Closed loop controls and also the control of machines mainly impose high demands on the response time;
• If only data for visualisation or recording is transmitted, this is less time critical but may require a high data rate.

The channel quality for radio communication in an industrial environment is subject to intense variations. A direct line-of-sight frequently does not exist because of obstacles (sometimes moving) between the radio units. Multipath reflection from surfaces results in superposition of the radio waves and therefore greatly variable received signal strength. Radio systems designed for use in such conditions therefore demand appropriate technologies for media access.

Comparatively large communication zones may be required in big plants. Hence wireless networks with several radio nodes, each operating at a relatively low transmitting power, are used rather than the running of a single node at high power. In process automation, for example, less time-critical sensor data can be transmitted via several radio nodes to the control receiver. The network management then not only supports the simple transmission of data, but also the search for alternative transmission paths within the network (referred to as routing).

In addition to transmitter output power, receiver sensitivity and the ambient conditions, the communication range of a radio system is also dependent on the antennas used with the system. According to its specific design, efficiency in certain directions may be increased or reduced. Every antenna type has its characteristic directivity diagram. The maximum gain is usually specified for the direction of maximum sensitivity. This antenna gain applies to both the transmitter and receiver directions. Therefore the orientation of the antennas to each other must be observed when installing a radio system.

Wireless system overlap
As soon as several radio systems of the same or different types are used, it is possible that they will influence (perhaps adversely) each other. But exactly when and how does this influence occur in parallel operation? Influencing of radio operation can only occur if several systems are transmitting (Fig. 3):

• at the same location;
• at the same time, and;
• at the same frequency.

The cause is therefore always the simultaneous use of a frequency, but the practical effects depend on the immunity to radio interference of the wireless systems involved.

The effects of spatial and frequency overlap depend on the selection of the technology used and the associated radio planning. Overlap with regard to time depends on the application itself. For instance:

• On the frequency of radio use: How and how often are telegrams sent, cyclically or only onevent?
• On the duration of the individual frequency use: How long does the transmission of a telegram take? (Will depend on bit transfer rate and data quantity...)

One refers to the coexistence of several systems if fault free operation, relevant to the respective applications, still exists despite mutual radio overlap1.

Fig. 3. The graphic shows the most important influencing factors of radio systems, and defines the terms used

Effects of mutual interference
When applied to automation applications radio interference mainly has an effect on telegram transmission delay: interference may cause a hold up on a telegram until it has been completely and correctly transmitted. This transmission delay is measured at interfaces accessible to the user, for example the duration from the triggering of a sensor until the availability of the signal on a fieldbus, or from scanning of a barcode up to successful transfer to a network (see Fig. 4).

Fig. 4. The transmission delay is (and can only be) measured at interfaces accessible to the user

Measurement of transmission delay over a longer period, and an analysis of how frequently delays occur (see Fig. 5) therefore permits rapid assessment of whether a radio influence is present, and whether this radio influence is still permissible. What can still be considered as permissible depends on the application (see Table 2, column 3). The transmission delay and error or frame loss rate must be below a tolerable maximum value (See red bar in Fig. 5).

Fig. 5. Example of system response time without (top) and with radio interference (bottom)

The bit error rate is significantly higher in wireless applications compared to wired networks. Therefore all wireless systems use internal mechanisms for error correction and telegram repetition. Some technologies initially listen to the radio medium and wait with their transmission for a clear space if necessary. Radio transmissions therefore always have a certain time response jitter which can sometimes amount to a multiple of the minimum transmission delay. Jitter resulting from radio influences is more or less significant depending on the immunity to radio interference of the system considered.

Table 2: Typical applications in factory and process automation: Time response requirements and telegram loss rates

The telegram loss rate can be very different across various applications (see Table 2, column 5). When using a wireless system to control a machine, for example, downtime through violation of a delay limit should never occur (very small error rate required) whereas when transmitting the data measured by a vibration sensor it is even possible to tolerate occasional data loss or the violation of a time limit (relatively high error rate permissible).

The path toward coexistence
Radio frequencies must be considered a scarce resource and thus used carefully, something which can be achieved without great effort through sensible measures. Experience has shown that coordination of interests and priorities is not always carried out at an early enough stage, since radio systems are not introduced everywhere at the same time due to the different availability of various solutions for specific applications. Personnel with a project stake in using radio technology should participate in the planning at a sufficiently early stage so that individual interests can be considered for the various departments within a company. Furthermore, the term 'wireless' is often considered by IT departments or other user groups to be equivalent to standard office network connections and data transmission over WLAN; this sometimes results in problems of understanding. Only if these aspects are taken into consideration can the actual coexistence management of radio systems be carried out.

Coexistence management mainly comprises the following steps:
1. The registration of all radio applications in all departments of the company according to the following criteria: Where is each radio system used and in which frequency range does it operate?; Who is responsible?; What is the exact application?; How is the radio spectrum used in time?
2. Assessment of the coexistence situation leading to...
3. Minimisation of radio influences (if this should be necessary).
4. Continuous checking following installation for operational changes across users (regular checking for compliance with the requirements of frequency management, etc).

These steps, in particular minimisation of radio influences, should be carried out with the support of an expert. The minimisation process will be considered in more detail. The aim is to decouple the wireless systems in at least one of the sectors location, frequency or time so that desired operation of the systems involved is guaranteed.

The minimisation process
Spatial decoupling. The transmitted power of a wireless system determines the spatial coverage of the radio cell since the received signal strength at a given point is (generally) inversely proportional the cube of the distance from the transmitter antenna. Therefore the frequency band as a resource can be used again by other systems at a safe distance, but this depends on the receiver sensitivity of the radio systems involved. The transmitted power should therefore be set as small as possible to permit reuse of frequency bands.

Receivers with higher sensitivity result in expansion of the range, but also simultaneously increase the potential for being influenced by other wireless systems. The spatial coverage can be additionally influenced by selection of the antennas. Antennas are available with various directional characteristics and the associated antenna gains. Optimisation of the wireless systems can also be achieved through appropriate positioning of antennas.

Users can normally predict the radio field using planning tools, e.g. for WLAN or Bluetooth systems. This includes the setting of transmit power, the selection, location and orientation of antennas among other factors.

Figure 6 shows an example of radio propagation planning in which radio cells 1 to 3 have each been assigned a different, non overlapping frequency band A to C. Radio cell 4 is located sufficiently far away from radio cell 1 such that radio cell 4 can use the frequency band A again without influencing radio cell 1.

Fig. 6. Example of radio field propagation planning with spatial decoupling of several radio cells and with reuse of radio frequencies

Frequency decoupling. The frequency occupation of a system is defined by the radio technology chosen and its settings. A basic differentiation is made between fixed frequency systems, to which a frequency band is assigned by specific configuration (e.g. WLAN) and variable frequency systems (e.g. Bluetooth), which uniformly occupy various channels of the total available frequency band by means of hopping sequences. Both types of frequency occupation facilitate decoupling in the frequency domain: Fixed frequency radio systems remain separate simply by exclusively reserving a frequency band for each system (see Fig. 7). Variable frequency radio systems do not require exclusive reservation. They permanently change the frequency with different hopping patterns so that, in the event of a collision, the transmission is repeated at a different frequency.

Fig. 7. Frequency decoupling with different frequency bands for fixed-frequency radio systems

In order to decouple fixed and variable frequency radio systems operating at the same location, variable frequency systems can work with a so called blacklist, i.e. they avoid the frequencies which are used by the fixed frequency systems. Measurements have shown that the generation of such a blacklist for automation applications should not be carried out autonomously by the radio system itself, but should be planned. Automatic mechanisms can often only recognise other frequency users if these transmit frequently and for a long enough period which does not necessarily occur in the case of typical industrial automation applications. For example, Bluetooth only recognises other radio systems if these load a radio channel by 10 % or more.

Receiver limitations should also be observed, which in some cases may not be able to suppress adjacent frequencies sufficiently sharply.

Fig. 8. Example of blacklisting: a Bluetooth system leaves space for severalWLAN channels

Figure 8 shows an example in the 2.4GHz band used by a Bluetooth system. A blacklist has been transferred to the system, causing the frequency bands of three WLAN systems (on channels 1, 6 and 11) to be skipped and leaving the 'gaps' available for use. As a result of the large number of wireless components and possible applications in the 2.4GHz band, this is a highly valuable resource. While the bandwidth is comparatively large, it is finite. To achieve adequate decoupling for a system, it may be advisable to use the 5GHz band as an alternative, particularly for applications generating large data quantities or high duty cycles. The 5GHz band, which in some countries has a bandwidth many times that of the 2.4GHz, is not used very much at the moment.

Time-based decoupling. If various radio systems use the same frequencies at the same location, they can nevertheless all fulfil their respective tasks reliably. This is possible if spectrum usage time occupancy by all systems is low and if appropriate error correction mechanisms are used. Data can then be transmitted by one system during the transmission pauses occurring in the other systems. The use of spectrum time occupancy is usually defined by the application, e.g. by the number of data transmissions per unit of time or the inactive interval between two data packets.

Spectrum time occupancy also depends directly on the data rate of the system used. If the data rate of a system is larger, less time is required to transmit a data package than for a system with a smaller data rate.

If the duty cycle of a typical data transmission for the control of a plant is examined, it may be seen that the radio medium is only lightly loaded by an individual system, and that capacity is available for further radio systems. Wireless systems in industrial applications frequently have a low time occupancy. For instance, a system may only transmit data during a specific event such as triggered transmission of sensor status or the wireless connection of scanners for logistics.

Most current applications are fairly relaxed in their response time requirements. Acceptable values can then be significantly more than 100ms, similar to the response time for simple data transmission associated with an IT network. With such systems the bottom line is to avoid lengthy interruptions resulting from standing interference, even if relatively short lived.

However, not all industrial applications have such undemanding time requirements or short duty cycles. For example, the fast data exchange between a central controller and distributed I/O modules may require response time in the range of single milliseconds, but may still present a low duty cycle on average. On the other hand, video transmission from a camera might occupy the radio medium for almost the whole time, and is also time critical.

To permit better understanding, the three dimensional view in Fig. 9 clarifies the relationship between signal strength or transmitted power, frequency use and time dependent behaviour. It shows the occupation of the 2.4GHz band with a number of typical wireless systems in parallel operation. The volumes of telegrams are shown in different colours and represent the actual radio resource usage of the various wireless technologies, illustrating the term 'use of spectrum in frequency and time'.

Time decoupling of several radio systems with more than one time-critical application is difficult to assess without specialist knowledge, particularly if one of the radio systems has a low immunity to interference. An expert should be consulted in such a case.

Summarising coexistence
Although statements on coexistence must always be considered together with the specific applications, the following can be derived from practice and from the measurement:

• The radio influences of the systems investigated are low in typical industrial applications, since the data telegrams are typically very short in automation applications and also occur staggered in time.
• Coexistence management should be carried out with the assistance of an expert in the case of parallel operation of different systems with more than one time critical application.

The following procedure can be recommended: Forward planning of radio propagation should generally be carried out in the planning and configuration phase. Even at this early stage, positioning and selection of antennas, and assignment of frequencies have an influence on the later integration into an existing plant.

However, if spatial and frequency overlapping are a fact of life, it is necessary to consider the time utilisation and frequency characteristics of individual systems as well as the reliability requirements of the applications.

If wireless systems with low immunity to interference are used in time-critical applications, all associated systems must be checked in order to guarantee coexistence. If it is impossible to separate such systems spatially or in the frequency bands, expert advice will be needed.

Prospects
Positive experience with current wireless technologies will lead to further distribution and establishment of radio systems. Coexistence management will thus remain a prerequisite for successful systems operation. Meanwhile industrial wireless system manufacturers will continue working on improvements for interference immunity. New technologies will also be added as time passes, and these are likely to lead to even greater demand for spectrum rather than leading to a reduction in potential interference.

New frequency bands released by the regulatory authorities could in the future result in more bandwidth. However, it can also be expected that these will be increasingly used and that the present coexistence situation will remain regardless. Therefore coexistence management is always going to be of prime importance for system operators.

Coexistence is possible through decoupling in at least one of the sectors location, frequency and time:
Spatial decoupling

• Adaptation of transmitted power
• Antenna selection
• Antenna positioning and orientation

Decoupling in the frequency domain

• Channel selection
• Blacklisting of frequency ranges or channels

Minimisation of frequency use over time

• Average loading of all individual systems as low as possible

FAQ on coexistence
Can the coexistence of industrial radio solutions be configured?

"Various parameters for the coexistence of radio solutions may be supported depending on manufacturer and radio solution. Typical parameters include transmitted power, channel, access procedure or blacklisting. Does a coexistence indicator exist?

"Coexistence is not the property of a radio system but a state in which different wireless systems fulfil their correct functions despite the existence of other radio applications (according to VDI Guideline 2185) and influences. A specific indicator or a simple measured value in the form of a number, is not available.

Does the type or orientation of the antenna play a role?

"Selection of the antenna influences local propagation and reception of radio signals. Antenna manufacturers provide diagrams which describe the radiated signal strength of an antenna in free space. To achieve an optimum connection, all antennas should be used with optimum directivity and have the same orientation. It should be noted that the range of interference can be many times the operational range.

Who evaluates coexistence?

"VDI Guideline 2185 suggests, in addition to introduction of coexistence management, that a named 'frequency representative' should be responsible for evaluation in the application environment.

Do typical industrial processes and devices influence radio transmission?

"Widely used devices such as motors and power converters typically have no influence. Depending on the wireless solution used, some industrial applications such as microwave heating (2.45GHz) or arc welding (up to 1GHz) may cause interference to wireless systems. The radio system manufacturer should be able to offer information on potential interference sources and suggest appropriate measures to ensure successful coexistence.

How can coexistence be guaranteed permanently?

"To guarantee the coexistence of radio solutions, it is advisable to organise and monitor all operational radio systems in a continuing programme.

Why is coexistence a topic for industry but not for home?

"Coexistence is actually a topic wherever several radio systems are used. However, industrial radio is usually subject to significantly higher availability demands than applications in the private sector. Therefore coexistence in industrial environments is of greater significance.

Is enterprise WLAN disturbed by radio in automation technology?

"WLAN is fine for use alongside current automation radio components as long as the automation system does not require time-critical (ms) dependency. The enterprise WLAN must be taken into account in the coexistence management.

Can mobile phones, laptops, walkie-talkies, handsfree units or measuring equipment be brought into halls with wireless automation solutions?

"If industrial applications in the 2.4GHz ISM band are used, it is inadvisable to use mobile phones, laptops, PDAs etc. The influences from these potential RF sources on existing industrial solutions cannot always be predicted.

Is a standard mobile phone a potential source of interference?

"Mobile phones have their own frequencies and should not interfere with automation applications. However Bluetooth and WLAN functions on mobile handsets should be deactivated. What should be the role of the enterprise IT department?

"The IT department should be part of the coexistence management process if it operates radio systems in the vicinity of wireless plant.

Finally...
Comprehensive measurements made under simulated factory and plant conditions show that wireless works! The following statements can be made:

• Due to the small data quantities common in automation applications, significantly lower radio interference can be expected than is generally the case.
• However, in order to achieve the reliability and availability required, industrial components must be used which differ significantly from consumer and office equipment.
• Coexistence is possible, but must be planned. It always depends on the application, and can also change during the life cycle of a plant, e.g., through the addition of further wireless systems.
• Planning requirements increase with the number of wireless systems operating in parallel. In this context 'parallel' means: At the same location, at the same frequency, and at the same time.
• In difficult cases, coexistence management must be carried out with the assistance of a specialist, for instance if several different systems with strict real time requirements but with low immunity to radio interference have to be operated in parallel.
• It is important that the finite resources for wireless communication are used with consideration, and that for the various fields where wireless technology is to be used, interests are balanced and prioritised sufficiently early.

References
1. Guideline VDI / VDE 2185: Radio based communication in industrial automation. The VDI / VDE Society for Measurement and Automatic Control (VDI / VDE GMA Gesellschaft Mess und Automatisierungstechnik); Beuth Verlag, September 2007

Edited from Coexistence of Wireless Systems in Automation Technology: Explanations on reliable parallel operation of wireless radio solutions

ZVEI Automation Division, Frankfurt-a-Main, Germany www.zvei.org/automation