Top five selection criteria for industrial wireless technologies

22 July 2009

Direct Sequence Spread Spectrum.
Direct Sequence Spread Spectrum.

Jim Davis looks at five of the top variables to compare existing wireless solutions and their adequacy in an industrial environment.

Wireless applications clearly present significant benefits to the industries they serve, such as enabling new monitoring and control capabilities, adding flexibility to existing capabilities and reducing cost of operations and process management. In response, many different wireless solutions and applications are sprouting up across the spectrum to address this growing industrial demand. That said, however, the industrial space presents a unique set of challenges that many traditional wireless solutions are just not specifically designed for, such as high reliability, low system power consumption and the ability to survive in an RF-unfriendly physical environment, all while being cost-effective.

The growing usage of wireless technologies also becomes a challenge as there are many different applications fighting for the same RF space, thus leading to an overly crowded spectrum and compounding the existing industrial challenges. With these unique challenges in the industrial space as well as the challenges that wireless applications present, how does one compare existing solutions and ensure they are adequate in addressing their environments?

Reliability
Reliability, in this context, is the ability of a wireless solution to communicate despite the many obstacles that the industrial space presents. Wireless systems contain specific characteristics that can help qualify how well they will respond reliably in a given system. One such characteristic is RF spectrum usage—where, physically, in the RF spectrum do they communicate? Another is the receive sensitivity of the technology—how little do the transceivers need to hear in order to make out the communications? A third is output power—how loud can the technology communicate? A fourth is RF agility—the measure of the ability of a technology to move and avoid interference in the RF spectrum. A fifth is interference immunity—an RF technology’s ability to communicate in a given channel despite interference.

RF spectrum usage is highly dependent on environment due to the physical nature of RF waves. The lower the frequency, the larger the wavelength and thus the less prone to absorption by typical manufacturing materials such as liquids and reinforced concrete. At the 2.4 GHz portion of the unlicensed ISM band most widely used in the industrial arena, however, the small wavelengths are quickly absorbed by the hostile RF environment, thus requiring even more focus on the remaining variables for measuring reliability.

Receive sensitivity, output power and interference immunity can be combined to form a larger, more important variable for defining reliability: link budget. Link budget is the absolute value of the receive sensitivity plus output power and interference immunity—the better the receive sensitivity, the larger the output power and the more interference immunity a solution has, the larger the link budget. The larger the link budget, the less likely the wireless solution will be impacted by RF absorption and interference, leading to greater potential for reliability. Transceiver-receive sensitivities and output power tend to be component-level discriminators that can be easily evaluated and compared.

Interference immunity, however, is largely a function of the types of technologies a wireless transceiver implements in order to improve its survivability. One of the best technologies in use today that directly improves this capability is Direct Sequence Spread Spectrum (DSSS) modulation.

DSSS modulation is essentially a method of performing forward error correction to the transmitting signal to minimise the impact of data loss due to signal interference. Specifically, DSSS encodes a set of data into a larger bit-stream based on a pseudo-random noise code shared by the transmitter and receiver (see Figure 1). Even with demodulation errors due to signal noise or interference, the original data can still be recovered.

Lastly, RF agility improves reliability by hopping or moving within the RF spectrum. The more freedom a solution has to move around, the better able the solution will be to find an RF-quiet environment and receive less interference. Different RF-agile technologies in use today include pseudo-random or algorithm-based hopping schemes that continuously hop around the spectrum in the hope of minimising interference as well as more intelligent schemes that only move when necessary (see Figure 2). The problem, from a reliability perspective, with the first agility scheme is that in a busy RF spectrum you may inadvertently continue hopping into portions of the spectrum, which contain high interference. By contrast, intelligent systems find a quiet location and stop moving. Regardless of the agility scheme, though, RF agility is equally a function of RF spectrum usage and channel size. Depending on RF spectrum usage, you may have more or less room for this agility.

For example, lower frequency solutions will have less room than higher frequencies due to frequency allocation constraints. 2.4 GHz solutions contain approximately 100 MHz of available spectrum while 900 MHz solutions only contain approximately 26 MHz of room. Channel size is also a major factor in determining RF agility. The smaller the channel size and the larger the room for agility in the spectrum, the greater your RF agility and ability to avoid interference, and fit between interferers. For example, in the 2.4 GHz set of wireless solutions, 802.15.4-based solutions are 5 MHz wide and contain only 16 available channels while 1 MHz-wide solutions typically have 80 available channels, giving them more available places to move to in order to avoid interference.

Reliability, therefore, is the sum of link budget plus RF agility with respect to RF spectrum usage. The greater the link budget and the greater the RF agility, the more reliable a given wireless solution will be across the same RF spectrum.

Simplicity
The test for a simple wireless solution in the industrial space is that it performs and is as easy to implement as its wired counterpart. Two perspectives that need to be addressed in terms of simplicity that lead to this ultimate goal are the equipment designer’s perspective on the one hand, and user’s perspective on the other.

From the design engineer’s perspective, simplicity is defined as the ease with which the wireless solution is developed and implemented into an end product. Simplicity, in this regard, is a function of the ease-of-use of the components involved, the tools available to aid design and development, as well as the availability of existing certified components to eliminate or minimise the daunting task of a local wireless certification process.

From the user’s perspective, simplicity relates to ease-of-commissioning—placement and activation of the wireless solution in its intended environment; as well as its impact on business processes. For example, technologies that minimise the commissioning impacts of wireless solutions may be directly related to the reliability and range capabilities of a system.

Power efficiency
Measuring typical power consumption of the components in use in the system is the traditional means of comparing wireless solutions, but this does not tell the complete story of how well a particular solution minimises system power consumption. For example, a highly reliable system that spends most of its time in the lowest power consumption state, sleep mode, will typically be more power-efficient than other systems that may tout lower transmit and receive states but are less reliable. These less reliable systems will spend less time in sleep mode. Reliability, therefore, is a crucial factor in determining how power-efficient a system truly is.

In addition to reliability, other system-level actions that minimise power consumption and boost power-efficiency include system behaviours, such as active power management, that control dynamic output power levels. A solution that continually focusses on minimising its output power to ensure only the lowest level necessary to communicate is used, will not only be reliable but power-efficient. This form of power-efficient technology, while not necessarily new to radio technologies, is new in terms of ensuring a system focusses on truly minimising system power consumption.

Range
Range is the distance a radio signal can travel and still be interpreted as data by the receiver. Taking into account the constantly changing and RF-hostile environment in the industrial space, the best measure to use in determining what solution will yield the best range is to compare link budgets and reliability. In addition, wireless solutions can also increase their link budget by means of on-chip and off-chip power amplifiers. But again, only a highly reliable solution will yield the greatest range, assuming these same power amplifiers can be used in any solution. Also, the most power-efficient solutions will only use these power amplifiers, a source of high power consumption, when absolutely necessary.

Cost
Finally, cost is the last of the top-five variables to use when comparing solutions. The intent of listing cost here is not to say the lowest cost is always the best solution. Cost should be calculated across the complete system. For example, if costly work-arounds are employed due to low reliability—such as increasing the number of power amplifiers to boost range because of low link-budgets—then these costs should also be included in the wireless solution comparisons.

Conclusion
Reliability, simplicity, power-efficiency, range and cost are the top five variables when comparing and selecting a wireless solution for the industrial space. Each measure contains a unique perspective in the benefit they provide and must be reviewed individually when conducting comparisons to ensure the best wireless solution for the given application is selected. The call-to-action for those developing wireless solutions is to provide technology that equally addresses each of these five variables and can meet the challenge of the industrial space. Both consumers and developers demand solutions that meet these criteria.

Jim Davis is the Global Marketing Manager (CyFi) at Cypress Semiconductor.


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