Cure found for cell-site deafness – CTIA preview

All around the world, mobile cell-sites suffer from inexplicable dropped calls and degrees of uplink degradation “deafness”.  Many engineers know in their hearts that the cause is more than likely PIM (passive inter modulation). PIM just lies hidden in the cell’s RF infrastructure but until now there has been no way of diagnosing the level of the problem on site in order to set about curing it.

Summitek has finally brought to market two user friendly truly portable PIM “diagnostic measurement” tools iMT and iQA for use in the field.  Both have the same high-power and effectiveness as their benchtop model which is the standard for laboratory PIM testing. It’s being demonstrated at CTIA (Central Hall C3, Booth 6221).
PIM often only shows up under full-power transit conditions. It is caused by a variety of factors such as arcing between surfaces that are not correctly prepared or where mating surfaces are not 100% clean or terminations not 100% torqued.
The route from transmitter to airwaves is a complex one – filters, diplexers, combiners, feeders, connectors, jumpers, and the antenna itself – and not only is every single component a candidate for causing  PIM, but so also is the way it was installed one, three maybe five or ten years ago.
VP Sales Interconnect technologies, “PIM is insidious, it slowly grows and the cell becomes more damaged, more ‘deaf’ – but no-one could investigate and explain why – until now.”
Conventional VSWR and ‘distance to fault’ testing are robust measurements in themselves, but incapable of finding PIM distortion and its causes; and even the low-power portable PIM testers are inadequate because they don’t have the power levels to simulate the system environments which cause the real-life PIM issues.
Summitek iMT and iQA field test units are both full 20w + 20w testers. Both are fully field ruggedized, capable of being dropped and stood upon (not that Summitek actively recommend such treatment) and can be easily taken by technician-in-van to any site and used to test and diagnose PIM issues, and once the suspect element has been replaced/repaired to test for a full “clean bill of health” on the entire RF system. There is even a small back-pack tester (iHA companion) which can be taken to the tower-top during identification and cure operations.
Engineers know that gaining an extra few dB of signal to noise ratio on the uplink receive path saves on lost calls and improves data thru-put. PIM cures effected with Summitek iMT and iQA portable field PIM testers have instances improving the noise floor by a massive 5dB delivering an environment which allows optimal receiver performance
Summitek forecasts a frightening rise in PIM issues as existing infrastructure is re-used for new technologies like 3G/UMTS and LTE.
Says Skinner “Feeders, filters and the like have been in-situ for years and they were built for either analogue or GSM single frequency operation. UMTS and LTE use wideband approaches so old empirical PIM solutions like using a different frequency are no longer an option. What’s more, because they are wideband instead of discrete transmission frequencies, the wideband carrier being transmitted creates a type of spreading effect of the third, fifth and seventh order intermods causing significantly raised noise floors for the uplink receivers.”
Although this is a potential minefield says Skinner, now with the Summitek iMT and iQA field testers it is possible to confirm end-to-end RF interconnection quality through the analysis of the presence of PIM – problems can be identified and rectified so that existing infrastructure can be re-used with the certainty of high quality operational use.

PIM-free RF components
From its leading position as the manufacturer of the laboratory standard Summitek PIM bench tester, the companies sister brands will also be on the CTIA booth (Central Hall C3, Booth 6221) showing:
Triasx HSPA proven Filters, Diplexers, triplexers, combiners and splitters
Triasx Sole supplier of RF products (Active solutions and Tower Mount Amplifiers) to the world’s fastest national mobile network
Allrizon radio receiver conditioning solutions, (filtering and amplification)
Allrizon Microwave broadband RF filter solutions from 2.4GH to 38GHz

About MBNL

Mobile Broadband Network Limited (MBNL) is a 50/50 joint venture company owned by 3 UK and T-Mobile (UK). MBNL was formed following the ground breaking network consolidation agreement that was completed on the 18th December 2007. MBNL manages and delivers the combined 3G access networks of the two companies. 

This collaboration will lead to almost complete population coverage for 3G services across Britain by the end of 2008 with significant improvement to dense urban in-building coverage in 2009. As the world's largest known active 3G network consolidation agreement, this has significantly increased both operators' 3G network quality and coverage, accelerating the provision of new high-speed mobile broadband services and delivering substantial cost savings as well as environmental benefits by de-commissioning 5000 radio base station sites. By combining their 3G access networks (the mobile masts and infrastructure that connects to each operator's separate core network) T-Mobile and 3 UK will create Europe's most extensive high-speed downlink packet access (HSDPA) network. The agreement offers customers comprehensive mobile coverage in the first stage of joint development in 2008 and further improvements to quality of service in 2009. It furthers both companies' joint commitment to deliver true mobile broadband internet access in addition to reliable voice and text services

Passive Intermodulation

Passive Intermodulation
Common Misconceptions – Sweep vs PIM:

• Sweep tests measure efficiency of signal propagation
• PIM tests measure ability to propagate signals without generating interference.
• Both tests are important and necessary to ensure quality site construction 

Measurable impact on network capacity
Correcting PIM issues on a site will have a measurable impact on the RF network. Not only quality but also capacity (especially in ‘noise floor dependent technologies like W-CDMA)
Root causes of PIM:

• Loose RF connectors
• Poorly prepared RF cable terminations
• Improperly mated / misaligned parts
• Deformed / rough mating interfaces
• Metallic debris (flakes, chips)
• Cracked / cold solder joints
• Loose mechanical fasteners (screws, rivets)

Quotes from operators conducting regular PIM Testing:

“PIM testing helps to find RF plumbing failures that are masked or not detected during sweep testing.”
“We spent extra time on every connector so you wouldn’t make us look bad.”
“We thought we were good before [PIM Testing]. Now, we have had to become even better.”
“PIM Testing changed everything. This [PIM testing] is my report card.”
“You can lie to me, but you can’t lie to the PIM Tester.”
“Two bad [PIM] reports, they get a warning. On the third [bad PIM Report], I send them home.”

Summary:

• Passive Inter-Modulation (PIM) degrades system performance.
• Multi-band networks are susceptible to PIM internal and external to the infra-structure.
• Multi-operator, passive DAS networks are susceptible to PIM problems.
• Portable PIM test equipment is readily available to identify performance problems in both outdoor and indoor cellular infrastructure.
• PIM testing and VSWR testing are necessary to verify site performance and installation quality.

PIM Generator, Bad factory connectors on jumpers.

PIM Generator

Poor workmanship on this connector was the cause of failing PIM.

Bad factory 

connectors on jumpers.

5 bad jumpers that were brand new. They all had connector issues.

Bad connector.

Today we were having an issue with this connector and this is what we found when it was opened.

Brand new jumper.

Today we tested jumpers prior to sending them up the tower. Brand new out of the bag -121 dBc.

Missing washer in connector.

While testing we found the jumper to the radio was failing PIM testing on all lines. The crew was using the existing jumpers instead of replacing them with new factory jumpers. When we opened the connectors to see if there was anything wrong and we found that all connectors were missing the internal washer. This was determined to be the cause of the failing connectors. All the old connectors were replaced and all lines passed PIM testing. Prior to PIM testing, all lines were sweep tested and passed.

The first picture is of the old connector and the second is of what it should look like

Sweep Testing vs. PIM Testing

Sweep Testing vs. PIM Testing
PIM testing and sweep (VSWR) testing are not the same. Each test has a specific function. VSWR testing is measuring the frequency response of the Device Under Test (DUT) in terms of return loss and insertion loss. PIM testing is measuring the level of inter-modulation [interference] created within the DUT. A device or antenna system can pass VSWR/Sweep testing and fail PIM testing, and vice-versa.Static vs. Dynamic Testing
Static testing is, well, static. In a static test, the test equipment is connected to the DUT and the tech begins the testing cycle. Everyone stands very still while the test results are recorded. This is an easier test to pass and it is also invalid. PIM testing is a dynamic test.

PIM testing is dynamic! Really. Dynamic testing means every component, connector, and connections in the DUT must be tapped and/or wiggled. If you are not tapping, you are not testing.The PIM Test Process

Typically, the technician will have the tower hand start tapping at the antenna and work his way back to the main feed line connector. Then the same process is completed from the bottom main feed line connector back to the carrier’s radio. During all the tapping and moving, the PIM tech monitors the PIM levels. If PIM levels remain under the specified threshold, results are recorded and the DUT passes. If PIM levels exceed acceptable levels, the tech will work with the crew to isolate the bad component(s).PIM Testing is a Team Sport

PIM testing is a team sport. Preferably, there are 3 or more people on the team. The team consists of the PIM tech, at least one tower hand, and one ground person/installer. Each member of the team has responsibilities during the testing process.On the Tower

The person at the top of the tower has three main responsibilities: safety, communication, and perfecting the art of the tap. First and foremost, the tower hand needs to be safe at all times. During PIM testing, especially during troubleshooting, there is the added safety consideration of protecting against an RF burn. Most approved PIM test sets have a transmit power of 20 watts or more. Although an RF burn is unlikely, it is extremely important that the tower hand and the tester are in constant communication. The person on the tower must let the tester know when the load is attached and confirm that the RF has been turned off prior to breaking any connections or removing the load. During the testing process the tower hand needs to communicate clearly where he is tapping/wiggling to the tester. The tester has no way of knowing where the tower hand is tapping without open communication between the two. Each tester will have a preferred method/technique for dynamic testing at the top of the tower. The tester and tower hand should discuss expectations and answer any questions prior to anyone climbing the tower. The person on the tower must keep safety in mind and communicate constantly with the tester to keep the test process moving forward.What to Expect

For the person on the tower, PIM Testing can seem like a dog chasing his tail. As long as the tech behind the test equipment has been trained and has the appropriate experience, trust that there is a method to the madness. During the testing process, if the system is passing, you will be moving between the antenna and main feed line connector, tapping and/or wiggling every component and connector as go.
During troubleshooting the tech will ask you place the load at a given juncture to help isolate any PIM issues. The tech may have you move to one point, say the end of the main feed line, run a test, and then ask you to move to the end of the top jumper. After another test, you may be asked to move back to the main feed line connector. Anytime you are more than six feet off the ground, this moving back and forth may seem like the tester is chasing his shadow. If you begin to get frustrated, ask the tech for a little insight into what is going on during the process. Remember: breathe, be patient, and resist the urge to make any strategic “accidental” tool drops. [Should be “patient,” not patience] When testing is going well, if you are the ground support, PIM testing is fast, efficient, and just another day in field. If the system is passing, the person on the ground will be tapping from the main feed line connector back to the test jumper during each test. It’s not difficult, but you can expect to be up and down the ladder repeatedly. When PIM issues arise and troubleshooting and repair begins, the person on the ground can expect to ride the same emotional roller coaster as the person on the tower. The tech will have you place the load strategically to isolate any PIM issues in the line. The tech may ask you to rework a connector (sometimes repeatedly) to resolve a PIM issue. Even after reworking a problem, a PIM issue may still exist in the same location. PIM testing is highly sensitive and can detect workmanship and manufacturing issues that are not visible to the human eye. During the testing, remember: breathe, be patient, and resist the urge to throw that wrench, even though you have good aim and a clear shot at the tech.The Tech
Despite appearance sometimes, the PIM tech/tester has a lot going on during the testing process. In addition to collecting, interpreting, and recording the data, the tester is responsible for coordinating the team. In addition to the technical aspects of testing, the tech has three additional obligations to the testing team. Like the tower hand and the installer, the tech has to communicate constantly with the team to make sure he never turns the RF on before the team is ready. He should let everyone know prior to turning RF on and off. There is beauty in clarity. The tester should make a conscious effort to explain the process and explain what he doing especially during troubleshooting. There are limitations to the PIM test set. Let the crew know what they are and how they affect the troubleshooting process. Finally, troubleshooting and PIM testing in general can be frustrating for all involved. The tech needs to be mindful of the demands on the people on the tower and on the ground and do all that is reasonable to ease the pain of testing while insuring the validity of the test.What to Expect
The PIM tech/tester is usually either a member of the installation crew or at least works for the same company or a third party tester. Either way, when the troubleshooting gets thick and the day long, the tech becomes the center of attention. The tech can expect to be put under a good deal of pressure to relax the standards or allow a static test to slide for the sake of goodwill. You cannot. As the technician, you are responsible for the validity of the test. That means sticking to the test protocol and customer specification. And, once a problem is identified, sticking to your guns until the problem is resolved. While in the middle of troubleshooting, concentration focused, it’s easy to forget to communicate with the rest of the team. When you feel the frustration level begin to rise, take a minute to let everyone know exactly what you’re doing. Constant communication helps keep everyone engaged and frustration levels low. When you are the PIM tech/tester, you may feel like the bad guy, but remember integrity is everything.
After hundreds of PIM tests, we have seen the value of PIM testing, especially as an integral part of a quality assurance program. However, PIM testing can be tedious and frustrating. Testing often takes longer than the allotted time. And a lack of understanding by the crew or a lack of training for the PIM tech only compounds matters. Yet, when the crew is informed, the tech trained, expectations are communicated, and the team understands their roles: PIM testing becomes one more way for a tower crew to prove just how good their workmanship really is.Recommendations for Improving PIM Test Results
PIM testing helps to ensure the installation is of the highest quality. Here are a few tips to improve the PIM test results and testing experience. On the Ground
The responsibilities of the installer on the ground with the PIM tester are the same as those of the tower hand’s, with one addition. Like the tower hand, the installer should always keep safety in mind. The installer should not break any connection without confirming the RF power has been turned off. The installer must also communicate with the tester and follow any instructions regarding the dynamic testing (the tapping). Finally, we generally look to the installer to help keep the testing momentum going.What to Expect

• Use connector tools.
• Over or under torque on connectors affect PIM levels: Use a torque wrench.
• Make sure connectors are clean. Use alcohol swabs and canned air (like those used to clean computer keyboards) to remove dirt, metal shavings, and other contaminates prior to sealing the connector.
• Make sure connectors are clean. Not a typo, it is that important. Clean connectors can dramatically improve PIM levels.
• Pay attention to detail. In the field, there appears to be a direct correlation between time and attention to detail during the installation and the amount of time, energy, and money spent PIM testing and troubleshooting. A fast and furious installation usually results in slow, tedious, PIM testing.
• Experience and training matters: whether through on-the-job training or manufacturers certification courses. Generally, the more experience and training crews have, the better the PIM testing goes. The quality of the installation is better and the lower PIM levels prove it.

PIM stands for Passive Inter-Modulation

 PIM stands for Passive Inter-Modulation

“Passive IM, similar to Active IM but occurring in passive devices, is present whenever RF signals at two or more frequencies are simultaneously present in a conductor of RF energy. Every passive RF device generates passive IM products when more than one frequency is present in the device. The signals are mixed by the non-linear properties of junctions between dissimilar materials. Typically, it is the odd-ordered products (e.g. IM3=2*F1-F2) that can be very problematic should they fall within an uplink, or receive band of the base station because they appear to the receiver as interference. The result can be a receiver desensitization which is independent of the receiver's random noise floor.”
PIM occurs/is generated when two or more signals are passed through a passive device. Examples of passive devices are antennas, coax, jumpers, connectors, and diplexers. If there is a manufacturing defect, workmanship issue, or an issue with a connection, say in the mating of a jumper and a hard-line connector (i.e., the mating of the center pin of a connector and its receptor), a third signal is created. That third signal is “PIM” and is what the test set measures

There is a great deal of misinformation about PIM, its causes, and PIM testing circulating out in the field these days. According to some crew accounts, wind, trains, low flying planes, the wrong color sunshine, and the occasional scurrying squirrel cause PIM.
In the field, there are two main categories for PIM causes.

Manufacturing defects: weak soldier joints, contamination, mismatched materials, etc.

• These usually result in a pass/fail for a component.
• Failing components are replaced when spares are available.

Workmanship issues: improper torque, loose connectors/connections, dirty connectors, poor terminations (poor connector installation), and “tuning” to pass VSWR specifications.

• These are the tower crews’ nightmare and result in countless hours of troubleshooting, rework, and frustration.
• These issues are probably the source of most myths and misinformation surrounding PIM and PIM testing.

PIM FOR TOWER CREWS AND INSTALLERS

PIM FOR TOWER CREWS AND INSTALLERS
PIM testing is a superior quality control tool that is gaining prominence in the United States wireless industry. More and more carriers are requiring, or exploring, PIM testing as part of the contractor’s closeout requirements. However, tower crews often cringe at the mere mention of PIM testing. This page is meant to help the installers and tower crews better understand PIM testing and what to expect during the testing process.

Distance to PIM

There is a lot of buzz recently about Distance to PIM. There are discussions on blogs, websites, and even on a few sales materials. In a recent OnPoint/Summitek training class, minutes after the instructor began, a student asked, "when will we have Distance to PIM?" This was before he even knew what PIM actually is, or even how a PIM test set works. “What about Distance to PIM” is a common question posed by installers and new technicians alike, especially those whose troubleshooting training is limited to a Site Master. These questions seem to be driving the demand for a distance to PIM feature. Be careful what you wish for.

Although distance to PIM (DTP) is still in the developmental stages, the demand for the feature seems to be increasing. That demand appears to be centered around the ideas that the Distance to PIM feature will reduce the time it takes to PIM test a new installation, make PIM testing easier, and make passing a PIM test easier. DTP is cool, really cool. But, will it actually save time? Or, will it lead to an unnecessary test requirement and an overall decline in the integrity and validity of the test results?

There are potentially great advantages to having a DTP option available, but there are serious doubts that quantifiable time savings is one of them. In theory, when a tech comes across a line that has a static failure of, say, a solid -120 dBc, the DTP would tell the technician where to start troubleshooting. This would be a legitimate use of DTP as a troubleshooting aid. For Site Technicians working for a carrier there is potential for even greater value in DTP. A site technician could use DTP on an existing site to determine if a tower crew needs to be brought in to do repairs or if he could do the repairs himself. It seems pretty cut and dry that DTP will save site technicians time. How can that not have value? But, there is a difference between isolating a PIM source at an existing site and certifying a new installation or upgrade.

On a new installation or site upgrade, given the same solid -120 dBc failure, times savings reaped through the use of DTP would be marginal. Assuming there were no fluctuations in PIM levels during dynamic testing (dynamic testing is the first step in PIM source isolation), DTP could save the technician the time it takes to jump the load to isolate the failure. However, if the technician has to take the time to calculate and input the electrical lengths (which could vary from sector to sector, if not line to line), it is probably a wash, time-wise. In fact, DTP could drastically increase the amount of time required to PIM test a site, and consequently ultimately lead to an overall degradation of the integrity of the results.

PIM is a measure of installation quality. And, PIM testing is a dynamic test. Without dynamic testing, the site certification is invalid; the test results are invalid. The concern is that when DTP becomes a standard feature on PIM test sets, untrained techs and/or tower crews will use the DTP to cheat the dynamic test standard. It is well known that crews cheat the test when they become frustrated and can't isolate a PIM source and conveniently "forget" the dynamic standard when facing deadlines. DTP will give those crews and technicians inclined to cheat justification to skip all that tapping and wiggling. "Nothing showed up on the DTP. The line is good." So much for test validity. So long to quality control.

Once the RF and Performance Engineers know that DTP is available, if they don’t understand that DTP is only a troubleshooting aid, they may require another "mandatory test." That means they will soon require a DTP test on each line, just like it happened with the VSWR Distance to Fault test requirement. Over time, carries, engineers, and even the techs will forget that DTP is a troubleshooting aid, not a test standard. Once that happens, test teams will have to unnecessarily break the connection between the antenna and jumper to run the DTP test. It's never a good idea to break connections unnecessarily during PIM testing. Worse, there would be no real value added by the additional test. By leading to a new test requirement, DTP could ultimately increase the amount of time needed to test a site, for what would amount to be no real reason.

There is little doubt that DTP will be available soon. The demand is too great for test equipment manufacturers to ignore. Unfortunately, the potential misuses of DTP outweigh any time savings that might be gained. And, it is doubtful that DTP will significantly decrease PIM testing time. There is one way to significantly reduce the time it takes to perform and pass a valid PIM test, however.

A well-trained technician and tower crew can dynamically test, troubleshoot, and make necessary repairs on an 18-line site in 2 to 4 hours. We do it regularly. 18 months ago we allowed 6-8 hours for testing a new site. Now, we estimate no more than 4 hours for those crews that have embraced the "new" standard of quality brought on by PIM testing. The best way to reduce testing time is to improve the quality of workmanship and to use well-trained techs.