Top Five Selection Criteria For Industrial Wireless Technologies
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? Let’s take a look at five of these top variables to address this question.
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—a 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. However, RF spectrum and its usage is a highly, governmentally regulated area of wireless communications to minimize interference with other wireless communications technologies. Only a few areas of the spectrum are reserved either locally or internationally for unlicensed use for these forms of communications; these are known as the Industrial, Scientific and Medical (ISM) band. Within this band, the predominant frequency that’s accepted and used is the 2.4-GHz portion of the ISM band. In this frequency, the small wavelengths are quickly absorbed by the industrial space’s hostile RF environment, thus requiring even more focus on the remaining variables for measuring reliability.
Receive sensitivity, output power and interference immunity can all be quantified together 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 against. Interference immunity, however, is largely a function of what 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 minimize the impact of data loss due to signal interference. Specifically, DSSS encodes a set of data into a larger bit-stream based on a pseudorandom noise code shared by the transmitter and receiver. For example, in figure 1, 8-bits of data are encoded into 32 chips—in this case 4 chips are equivalent to 1 bit. The chips are then modulated onto the RF signal and transmitted. The receiver demodulates the chips off of the received signal and then reverses the DSSS encoding scheme. Even with demodulation errors due to signal noise or interference, the original data can still be recovered.
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