Site Survey Basics for Industrial Wireless Applications

The Wireless Site Survey is the precursor to a successful wireless implementation. Why?

Simply because, every industrial wireless application is unique in regards to the RF waves propagation space available and the conditions of this space. These conditions will determine how easy or how difficult is to transmit data wirelessly and they are related to distance, terrain, physical obstacles, electromagnetic noise, meteorological conditions etc.

Every single factor mentioned above has the potential to be critical to the success of the wireless installation, almost to the point where it can make the installation unfeasible. Such potential can only be determined during a site survey and this is the reason why the site survey is usually a smart investment.

For industrial applications, a particular challenge is represented by the nature of obstacles such as the presence of heavy equipment, large machinery and a lot of metal surfaces that can reflect RF signals. Another challenge is the electromagnetic noise typically present in industrial facilities and generated while switching or operating heavy electrical loads.

As a result, it’s absolutely necessary to perform a wireless site survey to fully understand the behavior of radio waves within the operating environment before installing any wireless I/O’s.

Site Survey goals

The main goals of a wireless site survey is to determine first feasibility and secondly the right placement of the wireless devices and other accessories such antennas, cables that provides good signal coverage throughout a facility or other designated area. In most cases, “good coverage” means support of a minimum data rate or throughput.

The need and complexity of a wireless site survey will vary depending on the facility. For example, an indoor application over a small area 50-100 feet may not even require a site survey.

A larger facility or an application involving large distances as is the case with Water and Wastewater applications generally requires an extensive wireless site survey.

Critical steps for a site survey

1. Understand the application type

   1. Peer-to-peer (wire replacement)

   2. Peer-to-multi-peer 9Wireless Modbus) application

2. Determine the hardware requirements

   1. Number of access points (number of I/O, radios, gateways)

   2. Their user-desired location

3. Evaluate the distance involved and LOS (line of sight) potential

   Based on distance and LOS and given the hardware specs and basic formulas (see “Critical Concepts for Successful Industrial Applications”) we can calculate the Power at Receiver to determine the installation feasibility and/or determine if any repeaters are needed and also the type and height of the antennas.

4. Obtain a facility diagram

   If possible locate a set of building blueprints or facility maps. For outdoor applications consider a good free tool such as Google Earth software

5. Identify major physical obstacles and their nature.

   Inspect the facility before performing any testing to verify the accuracy of the facility diagram, if any were provided. This is a good time to note any potential obstacles and their nature that may affect the propagation of RF signals. Some typical attenuation levels for common obstacles are provided below for reference:

   Plasterboard wall

   3dB

   Glass wall with metal frame	6dB
   Cinder block wall	4dB
   Office window	3dB
   Metal door	6dB
   Metal door in brick wall	12.4dB

    

6. Determine the best location for remote I/O’s and antennas

   1. Determine the signal strength using RSSI indicator.

      The signal strength can be determined in a very basic way by examining the RSSI indicator which is a feature built-in any Wi-ModPak device. The RSSI led will change colors from RED to YELLOW to GREEN depending on the strength of the signal.

b. Determine the signal strength using 2 Wireless Modbus gateways.

A more accurate measurement can be performed by using 2 Wireless Gateways (2 x WPM-1W115K for 900MHz applications or 2 x WPM24-50MW96K for 2.4GHz applications) and the RSSI test tool available in Wi-ModPak software. For a quick test, both gateways can be battery powered using 2 x 9V batteries (18V series). In this case, one wireless gateway will be connected to a laptop running the Wi-ModPak software and the other wireless gateway can be remote, just battery powered and having the TD and RD terminals shorted in order to act as “mirror” for the test string

data.

c. Determine the signal strength and /or any potential interference sources.

There are more advanced third-party hardware and software solutions which allow more precise measurement of the signal strength (in dB). These solutions also allow the identification of the interference sources, their signal strength, frequency channel and even their location using a “Geiger” type utility.

Because of the it’s wide acceptance there are more spectrum analysis tools for 2.4GHz applications than 900MHz or 5GHz. These third-party software based tools along with laptop Wi-Fi cards make the spectrum analysis a very easy process. You can just upload this software on a laptop and test the coverage of each I/O point location by walking around the facility. These are available from several different companies, such as AirMagnet, Berkeley Varitronics Systems, and Ekahau. Some of them also offer spectrum analyzers and software tools for 900MHz and 5GHz applications.

   7.    Document result.

Once the best location of I/O and/or gateways is established it’s important to document the findings so that the actual equipment installers can use this information at the time of installation.