01 June 2004
Industrial wireless system design
By Eric Marske
Radio site analysis can be broken down into three phases. First is the initial site work; second is the radio frequency (RF) data analysis; and third is the on-site testing.
Let's focus on the first two phases of this process, because the final on-site testing phase is a validation of the calculated values that came in the first two phases.
The on-site radio survey requires RF test equipment that few people outside the radio world have. The tool used by RF system designers in the site analysis phase is a computer software program that calculates all radio parameters and allows the user to evaluate how the radio equipment will operate in the supplied conditions. There are many computer programs on the market, ranging up to thousands of dollars.
Elevation of this lowest site
Before beginning to use the site-design software, one needs to gather some information about the radio site.
- Simple site layout: All radio systems are a series of point-to-point links, and each of these links needs to undergo evaluation. Begin by making a simple diagram of all locations in the radio system. Include all locations where radio communication is required.
- Line of sight: Mark all areas on the diagram where there is no line of sight (LOS) between the antennas.
- Distances: Mark all distances between sites on the diagram that have a line of sight or will need communication between them. Distances can come from site maps, global positioning system (GPS) data, or from maps that contain longitude and latitude information. Site-design software has a tool for calculating the distance between two points given the longitude and latitude of each point.
- Site data chart: Make one.
- Elevation: Find the elevation above sea level for each node in the system, and put the values in the data chart. Site elevation can come from a topographical map or GPS data.
- Antenna height: Estimate the antenna height above ground level required to achieve LOS to the designation site, and mark the values in the data chart.
- Cable length: Estimate the feedline length from the antenna to the equipment cabinet, and mark the length in the data chart.
- Elevation differential: The elevation differential relates all antenna heights to each other in single system. To calculate the elevation differential for all sites in the system, find the site with the lowest elevation, and make its value zero in the data chart. Subtract the elevation of this lowest site for all other elevations in the system, and mark their values in the elevation differential block.
- Adjusted antenna height: To calculate the adjusted antenna height, add the value from the antenna height block to the elevation differential block. All the data one needs for using the RF design program is now available in the site data chart and the simple site layout.
Estimated antenna height and cable length
RF site design program
The RF site design tool we use here (an Esteem model) is a Microsoft Excel spreadsheet that analyzes all the data we have gathered on the site during the initial site work. The program automatically calculates the system gains, losses, and heights, and displays these values on a screen. Through comparisons from on-site measurements to calculated values, the calculated values tend to be more conservative.
One should analyze all communication paths in the system, such as the master to remote, master to repeater, or repeater to remote. These point-to-point link tests will build the backbone for the entire RF system.
As an example, look at this radio site. Let's select a radio that will provide a high-speed wireless Ethernet system.
All communication paths in a radio system need evaluation. Here we will analyze the link from the control room to pump site number one. Using the information gathered during the initial site work, one enters all information into the data entry keyboard section of the program.
Once all the site data has entered the system, the RF path analysis will display. Note that all the calculated radio site values such as effective radiated power (ERP), feedline loss, TX power, expected RX signal strength, receiver sensitivity, and maximum distance come on screen for each end of the radio system. The key site data such as fade margin, height warnings, and Fresnel zone warnings will appear in the center of the data field and will turn red if there is a problem in that particular link.
The fade margin for this radio path is 6.5 decibels for the equipment we specified. This number will increase or decrease with any hardware change in the system. Remembering that the fade margin level we are looking for during analysis is greater than 3 decibels, this communication link will work reliably at 11megabits per second over the distance of 6 miles. W
Behind the byline
Eric Marske has an AAS in electrical engineering. He is an industrial automation manager at Electronic Systems Technology, Inc. (www.esteem.com) in Kennewick, Wash.
Reflect, absorb, and scatter
Radio manufacturers use the term line of sight (LOS). But what does it mean, and why is it of such concern?
All frequencies used for industrial wireless communication require a clear, unobstructed path between antennas to make any predictions on system performance. If anything, such as trees, buildings, or hills, is in the direct radio path between the antennas, the RF energy will reflect, absorb, or scatter, and signal loss will occur. The amount of these losses will depend on the frequency and the composition of the reflecting material. In all cases, if there is no LOS between the antennas, predictions on system performance cannot project accurately, and on-site testing is required.
If evaluating a radio system's performance over a distance of miles, one must consider the curvature of the Earth's surface and its effect on required antenna heights. As we have learned, line of sight is critical to making predictions on system performance. As the distance between two radio sites increases, the minimum height to clear the radio horizon will also increase. To maintain a clear line of sight, adjustments to the antenna height on either end of the system may be necessary.
Signpost ahead: Fresnel zone
With the use of higher frequency transmitters—2.4 gigahertz and above—in high-speed wireless networks, another radio phenomenon called the Fresnel zone is an important consideration.
Radio waves do not radiate out in straight lines. The Fresnel zone is the ellipsoid spread of the radio waves around the visual line of sight after they leave the antenna. This area must be clear of obstructions, or the signal strength will decrease due to signal blockage.
Typically, a blockage in the first 20% of the Fresnel zone height introduces little signal loss to the link. Beyond 40% blockage, signal loss will become significant. The height of the Fresnel zone will be calculated and displayed in the RF design tools when using frequency bands that are suffering because of these blockages.