Do not assume your hardware is to blame if you experience poor wireless connectivity

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Sunstate Cement, an Australian-owned supplier of cement products to Queensland and northern New South Wales, sought to improve its productivity through wireless communications.

The company had completed work recently on an $85 million expansion program that increased capacity to 1.5 million tonnes per year of its bulk and bagged cement products.

Part of that expansion included the installation of wireless LAN network between three massive ship unloaders and four corresponding access points.

And that's where the problems began. From day one of the installation, the WLAN didn't function properly, meaning the loaders couldn't communicate interrupt free back to the main controller. This posed huge safety and productivity issues. 

"These three shiploaders needed to work as a team but without a reliable communication link, this was impossible," said Siemens Australia PLC and Networks Product Manager Falk Hohmann.

At first, Sunstate Cement thought the problem was with the Siemens equipment that had been installed. The company had paid to have installed a high quality wireless LAN network that didn't function as expected.

There were many collisions because the traffic was not coordinated between the four WLAN access points.

There were many collisions because the traffic was not coordinated between the four WLAN access points.

They had very low signal strength, even from only a couple of meters away from the WLAN access points and there were several communication drop outs between the main controller and the three 'RTU' PLCs which were installed on those unloaders.

When contacted about the problems, Hohmann flew to the company's site to investigate first hand. What he found was a comedy of errors. But understandably, no one at Sunstate Cement was laughing.

"The first thing I found was that the installer of the system hadn't conducted any WLAN channel planning. WLAN is a shared media, not like switched Ethernet, and there were so many collisions/interferences because the traffic was not coordinated between the four WLAN access points," he said. 

This illustrates the connection between strong signal strength and low quality.

This illustrates the connection between strong signal strength and low quality.

The images above illustrate the connection between strong signal strength and low quality. 

He explained that access collision could be avoided by conducting space division multiple access (SDMA) or frequency division multiple access (FDMA). WLAN installations like this usually show strong signal strength but a very poor signal quality. This is like a class room where everyone starts to talk at the same time.

The result is a very noisy classroom but the so called "signal to noise ratio" or also called signal quality is so small that nobody could actually follow a conversation.

To avoid overlaps and interferences, WLAN simulation software like Siemens Sinema E can be used to improve the communication performance even before the actual installation starts.

"We also discovered that the engineer had installed a 5 GHz antennae but the network was configured for a 2.4 GHz frequency range.

Also, the access points antennae were not only installed below structures, they were too low and adjacent to solid concrete walls," he said.

"So, after selecting the 5 GHz range for that network, we reallocated channels to each of the access points and the corresponding loaders."

Hohmann said channels available for the network ranged between 149 and 165 (5745 MHz – 5825 MHz) for this outdoor application.

But because there is a 2 MHz overlap between channels, Hohmann arranged the channels in an adhoc order to prevent any interference as illustrated in the figures below.

The 2 MHz overlap between channels allowed them to arranged in an adhoc order to prevent any interference.

The 2 MHz overlap between channels allowed them to arranged in an adhoc order to prevent any interference.

Using a Cisco spectrum analyser, the second round of tests showed a slightly weaker signal strength (-70 dBm or 48 percent) but much greater signal quality.

"Unfortunately," recalls Hohmann, "we still experienced communication drop outs. So, what else was wrong and why was it only 48 percent?" Hohmann looked to the client antennae, which were installed on each of the hoppers. 

"The installer had used omni directional antennae, which can have high gain but a very small vertical lobe. When the loaders were in the parked position, it was impossible for the antennae to have line-of-sight communication due to a big difference in the height of the mounting positions," he said. 

The antenna pattern should always be considered in order to determine the antennae opening angle.

"Therefore the antenna pattern (shown above) should always be considered in order to determine the antennae opening angle. In order to do so the -3 dB curve is the most interesting section," said Hohman.

"And we noticed that the antennae were mounted too close together. In order to use antenna diversity (which we would always recommend in industrial applications) properly in a 5 GHz radio cell, we recommended to use 20 times the wavelength as the clearance."

The wavelength of a 5 GHz electromagnetic wave is ~5.5 cm. Therefore, the resulting distance between the two access point antennae should be around 1.1m.

Hohmann said the antennae were also mounted too close to the wall (around 4 cm), causing strong interference due to reflecting waves, resulting in signal cancellations.

The antenna was incorrectly located underneath a belt conveyor.A minimum distance would be twice the wavelength (2 x 5.5 cm = 11 cm). And being located underneath a belt conveyor (as shown alongside) certainly didn't help matters, either! 

Raising the antennae from its current height of 2m to around 5m proved very worthwhile.

Another 'hidden' issue Hohmann uncovered within the cable installation was the amount of redundant cable.

"Here we found 5m of excess cable stuffed into a 1m channel," recalls Hohmann.

"As we all know, buckled or wound up cables cause a very strong cable attenuation problem. Luckily, we were able to utilise the additional cable when the aerials were relocated away from the overhead structures and raised higher."

After making the changes, and using Siemens iPCF (rapid roaming functionality) the system is now operating as it was originally designed to.

It has been running without interruption for more than 12 months without any replacement or additional hardware.

More information.