In the world of wireless, engineers attempt to provide an environment that is the most conducive to providing an excellent wireless connection with AP placements, coverage cells, controller configs, etc. The goal is to provide the highest possible SINR to all clients, thus increasing performance of the entire network. Yet, in warehouses that tends to be very difficult. Things like warehouse shelving and even the inventory itself create heartburn for the best WLAN design engineer. Sometimes things literally just get in the way of our signal! So how do we provide adequate coverage in tough environments?
I’ve worked in a lot of warehouses over the years and one issue I commonly face when designing a warehouse solution is finding where I can put the AP/antennas to provide the coverage desired. It’s tough. Especially in environments that are subject to workflow optimization and equipment relocation. So let’s look at some design options.
Design #1 – Overhead omnidirectional antennas
Some warehouse locations it’s relatively simple to use this method as they have open areas and/or high ceilings with plenty of clearance above the shelves. Thus, slapping up some internal, omnidirectional antenna APs is fairly quick and painless. But what if the location has minimal ceiling clearance, tight aisle spacing, etc? If you look at the image above, you’ll see the results of doing just that. In this particular case, the inventory levels were very high (year round) and the makeup of the items were extremely heavy amounts of metal and cardboard. I would argue one could not begin to create an environment more harsh to the absorption of 5GHz signal than existed in that space. Putting more omnidirectional APs into a situation like this to resolve coverage gaps would lead to an immense amount of juggling in order to provide ubiquitous coverage in both bands. I’m not even sure the best could minimize the amount of ACI/CCI/CCC in the space that would result from such an attempt. While it might be possible, I’d argue the end game just isn’t worth the struggle. You may be thinking, use a different AP/antenna and that’s just what I was thinking leading us to the next option.
Design #2 – Overhead/End of Aisle patch antennas with smaller beamwidths
Doing a pico cell is a good thought, this would allow you to maintain respectable levels of ACI/CCI/CCC as the spots would have minimal overlap and little LOS. This improves our coverage but again, clearances above the shelving come into play since there could be very little propagation into the adjacent aisle from the AP location. So how could we help fix this? If you’re thinking move the antennas to the end of aisles shooting down them, I like how you think.
By moving the AP to the end of the aisle as seen in the picture, you greatly improve reception down the entire aisle. An added benefit is with inventory in the shelves absorbing signal, it minimizes the contention created from placing APs down each aisle since there is little propagation between aisles. Yet even this approach can have drawbacks. For one, I’ve never seen an AP defeat a forklift. Don’t get me wrong, I’m sure to an extent it’s possible, but not sure I want to be the one to leave things to chance.
With each of these designs there are pros and cons. For example, with design #1 we’ve added so many APs that can hear each other, we’ve increased contention immensely. Even though most warehouses are only sending small amounts of information at a time (ex. barcodes) we still have the potential for issues. If a forklift is moving at high speed, it needs to be able to send that data quickly to avoid issues with roaming. If it has to wait, the chances of retransmits increases greatly. With design #2 a drawback is you are depending upon signal being able to penetrate anything between the client and the AP. What happens when a forklift is in between? Or maybe additional inventory is stacked in the way, even if it’s temporary? This defeats our goal of providing as high an SINR as possible since impediments are blocking our signal. So how do you provide the best chance for clear LOS, yet minimize APs hearing each other?
Yes, I’m a big fan of antennas. One has to be when you’ve worked in places like stadiums and cruise ships. Thinking back to approaches used inside arenas with antennas mounted high overhead to provide the greatest SINR due to clear LOS got me to thinking about trying this in a warehouse. Now they are completely different spaces, yet the goal is the same. While I appreciate the ease of design afforded by internal, omnidirectional antenna APs, they just can’t do everything that antennas can. Plus, who wants EASY?
Design #3 – Specialized antennas FTW!
AccelTex recently introduced a special antenna with a 15 (H) x 120 (V) beamwidth that is absolutely FANTASTIC for meeting warehouse coverage needs. The antenna I was working with was a 4 lead, RPSMA connector antenna found here (other options for connectors available via website). As you can see in the image taken from a passive survey, the beam characteristics are perfect for keeping coverage within the aisle. This allowed me to meet my goal of increasing coverage without increasing contention from neighboring APs. For reference, the aisles shown here are approx. 300′ (~100m) long and for testing the antenna was placed mid aisle, perpendicular to the ground. Even at the aisle edges, the signal level was measured at -53dBm by the Ekahau SideKick. Even with the high sensitivity of the SideKick, most environments should still see sufficient coverage in those areas even from weaker handheld scanners. For example, the scanner used in this environment reported a -64dBm strength which was more than capable of a quality WLAN connection.
The reason for this approach was I felt this was the best location to avoid LOS issues within the warehouse racks. Not only that, but the location being high overhead (~33′), it limits the chances a passing forklift will clip the antenna. While the signal does fall off a bit near the aisle edges, that’s ok for this design. The perimeter of the warehouse will be covered with omnidirectional APs that will help supplement the aisle edges if needed, to ensure a quality roam between APs.
I hope that these three design options help spur some creative ideas when you are doing your next WLAN design project, whatever the type of facility may be. After all, it’s not for my health that I get on scissor lifts with all this gear for testing. Especially when it’s the middle of July with 103 degree weather outside in Texas. Let me know if this helps! I love to see how others tackle similar projects!