Coexistence and Diversity Techniques for Reliable Wireless

by | Oct 31, 2007 | Industrial IoT, Technologies | 0 comments

Dr. José A. Gutierrez is Corporate Director of Technology at Emerson. As such, he not only advises our wireless experts in Emerson Process Management, but also across the other Emerson businesses.

At the recent ISA Expo, he presented the paper, Reliable Wireless: Mitigating the coexistence Challenge. His key point is that through a number of communications diversity techniques, high communications reliability on the order of 99.9% or higher is achieved. These diversity techniques are supported the IEEE communications standards and are used in the new wireless field network standard, WirelessHART.

José had quite a bit of expertise to share and has a long history of participating with many standards bodies. Some of these include IEEE LAN/MAN, editor-in-chief of the IEEE 802.15 Working Group-Task Group 4, program manager of the ZigBee Alliance, board of directors’ member of the Wireless Industrial Network Alliance, and chairman of the Networking Working Group of the ISA SP100 committee.

He began his presentation by defining the term coexistence from the IEEE 802.15.2-2003 Part 15.2 standard:

The ability of one system to perform a task in a given shared environment where other systems have an ability to perform their tasks and may or may not be using the same set of rules.

Collisions and coexistence issues can happen when two or more packets overlap in both time and frequency with sufficient energy to interfere with one another. Coexistence can be measured by the end-to-end message delivery success rate overall all operational conditions.

Different country’s governmental regulations address the sharing of the radio frequency (RF) spectrum in different ways. The common approach has been in assigning different bands for applications such as TV, AM and FM radios, cell phones, toys to name but a few. You can get an idea of how these frequencies are divided with the U.S. frequency band allocation chart. A personal aside–it also makes for a great eye chart!

José discussed the unlicensed bands referred as ISM bands, short for industrial, scientific and medical bands. These bands are allowed for usage in a variety of applications and in some cases with worldwide availability. Only device certification is required for use in this band. Limits are imposed on the radiated power of devices transmitting at these frequency and they require governmental certification for the country in which they operate. These bands include 900MHz (902-928 MHz), 2.4GHz (2.4-2.4835GHz), and 5.7 GHz (5.725-5.875Ghz). For you non-electrical engineers, an Hz or Hertz is one frequency cycle per second. These unlicensed bands are very crowded.

The good news for process manufacturers is that the responsibility rests with automation suppliers to get their wireless devices certified for use.

The presentation covered the various ways information could be transmitted on these frequencies. It’s enough for a future post, but I’ll list some of the methods here: narrow band, frequency hopping, direct sequence spread spectrum (DSSS), orthogonal frequency division multiplexing (OFDM) and ultra wide band (UWB).

Tying all this back to coexistence, the IEEE 802 standard committee is the authority on wireless coexistence and ensures that these technologies will effectively coexist with all previous technologies.

José posed the question about what wireless suppliers can do to eliminate the coexistence challenge. The solution is to apply techniques that create diversity to mitigate this coexistence challenge. These diversity techniques include:

  • Path Diversity: Mesh Networking
  • Frequency Diversity: Channel Hopping
  • Time Diversity: Time Division Multiplexing
  • Power Diversity: Power control over multiple communication links
  • Space Diversity: spatial location of sensing devices (not practical for WSNs)
  • Coding Diversity: Use of advanced DSSS technology

The key for wireless field networks is to use a combination of these techniques to deliver the necessary high end-to-end message delivery success rate for reliable wireless operation. These diversity solutions used in IEEE-based standards applied in industrial applications including DSSS, OFDM, and UWB and used in the WirelessHART standard help eliminate coexistence issues as one of your considerations.

You can learn more about wireless basics, the technologies, cases for how they can be applied in plant applications and IT considerations by visiting the on-line wireless courses at PlantWeb University.

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