An important part of the business case for metrocells is that they need to have a relatively long, trouble-free working life, with minimal maintenance. This leads to demands for plenty of processing power "headroom" to allow more complex software to be remotely downloaded in the future, RF front ends that can cope with either 3G or LTE, and long-life components that won't age prematurely or fail quickly.
Warranty periods aren't long enough yet
But these factors alone won't stop equipment located in harsh outdoor environments from failing over a period of time. We all know the frustration when our latest electronic gizmo (camera, computer, TV, smartphone etc.) stops working just a few days after the warranty expires. This is a far more serious and costly issue for operators, who might have tens of thousands of metrocells carrying the majority of their data traffic in a few years time. The replacement cost and customer impact of even a few percent failing each month would cause irreparable damage to both the reputation and bottom line of the operator.
Why the IP67 standard and other environmental testing doesn't go far enough
The industry has adopted a standard to address the water ingress issue. Products conforming to IP67 specifications will have been tested to ensure that they continue to work when temporarily submerged in a meter of water for thirty minutes. In addition, manufactures of outdoor devices will conduct temperature cycling tests in a laboratory environment for a period of time. Most devices will meet both the IP67 requirements and the temperature requirements; however, these tests don't quite match real world conditions.
The reason is that the tests are done separately, not in sequence and not under real world conditions. In live deployment, you'll find the worst weather can change from hot to cold with rain/sleet/ice all happening at the same time. Even in relatively cold places, solar heating from direct sunlight might heat up the box one minute, shadows and rain might cool it down the next.
Hermetically sealing the box doesn't solve the problem
Fluctuations in temperature create failures in hermetically sealed boxes due to pressure differentials. Such pressure differentials can be caused by altitude (transporting cargo on an airplane), rapid changes in temperature (using a portable bar code scanner indoors and then outdoors), or internal temperature changes within a device (internal heat source). This constant heating and cooling cycling causes repeated pressure build-up (expansion) and vacuums (collapsing).
For example, a sudden rainstorm or high wind can cause a drop in temperature creating a 200 mbar (3 psi) vacuum inside an enclosure.
This frequently changing pressure differential between the inside and out, stresses the seals and gaskets of any box, and over a period of time – say 1-2 years – the liquid water will search out the weakest point and be drawn in. Once that happens, it's pretty much downhill from there, leading to intermittent and/or permanent equipment failures, performance problems and resulting outages.
Regardless of how good the seals and gaskets are fitted to telecom equipment, this symptom has been observed in many countries and many networks.
Coming up for air
A solution to the problem is to fit a vent that allows the equipment to breathe, but keeps out moisture and dust. This is where we can turn to other industries that have already solved the problem.
W. L. Gore & Associates, who are best known for GORE-TEX® fabric used in outdoor sports clothing, have found many other uses for their advanced fluoropolymer technology. In fact, the company has a $3 Billion turnover, which includes products developed for medical, industrial and electrical industries to name a few.
The company has developed a micro porous membrane made from ePTFE, which is utilized in a variety of product forms and fits directly into the equipment. These vents enable air to flow in and out of the enclosure, while keeping out water, dust, dirt and debris.
An alternative engineering approach is to design a "tortuous path", where the air flows through a labyrinth to reach the inside and outside of the sealed enclosure. The trouble with this approach is that moisture and dirt can eventually find their way into the enclosure through the open pathway.
Widely used in telecom networks today
This isn't particularly new technology and has been adopted widely by many leading telecom equipment vendors worldwide. Gore has shipped over 35 million protective vents into outdoor telecommunication devices to date, where they are fitted to many types of equipment including wireless backhaul, routers, and base stations – indeed almost any outdoor electrical equipment.
Although they can't divulge a list of their customers for commercial reasons, it is understood that most OEM vendors use them today. Likewise, several small cell cellular, Wi-Fi and backhaul vendors have already incorporated the technology into their designs.
Designing in longevity from the outset
Jason Zambotti, product specialist at W. L. Gore & Associates, tells me that they often get panic phone calls from operators who are suffering high equipment failure rates in the field. Despite equipment being properly sealed when initially manufactured, micro-leaks form over time due to the impact that pressure differentials place on seals and gaskets. These micro-leaks can allow water and environmental contaminants to enter the sealed enclosure, causing device failure.
To avoid costly problems like this in the longer term, he would recommend incorporating a venting solution from the outset of product design. Some (more enlightened) operators already have this on their supplier checklist and use this when evaluating new telecom equipment. He'd like to see more awareness of the potential problem throughout the industry, and testing revised to match real-world scenarios.
Where venting doesn't apply
Residential, enterprise and indoor public access small cells typically don't operate in such harsh environments. Typically they reside inside weather protected buildings, often air conditioned, and aren't exposed to moisture or precipitation. Venting wouldn't really be appropriate in these situations.
Venting also doesn't address heat transfer/dissipation, so external heat sinks will still be required. There wouldn't be a flow of air inside the device, just a slow pressure equalisation to match temperature changes.
As small cells move outdoors to the street and rural communities, the requirement for long lasting, low maintenance equipment in harsh exposed environments creates new design constraints.
The IP67 standard and other environmental testing on its own doesn't adequately test for the real world conditions, because it checks for temperature change tolerance and resistance to water ingress separately and not in sequence.
Equipment vendors and operators need to consider designing in a venting solution for both Metrocell and wireless backhaul products to avoid reliability consequences and costly problems in the long term.
Disclaimer: W L Gore & Associates are sponsors of ThinkSmallCell. Our thanks to Jason Zambotti for his explanation of the technology when preparing this article.
For more information about Gore's venting solution, visit their website
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