Are you kidding? LMR cable has been a staple to the fixed wireless industry for years. Why can’t we use it with our LTE products? I want to give credit to Cameron Kilton, the newest member of the Baicells North America Team, for bringing this to our attention.
First, let’s define PIM. PIM stands for Passive Intermodulation. PIM is defined as the unwanted signal or signals generated by the non-linear mixing of 2 or more frequencies, but what does it really mean to the consumer? High PIM means poor reception and limited bandwidth to the end user, which in turn means lost customers. Low PIM means strong signals with more bandwidth for more users, which means happy customers and higher revenues. From a hardware perspective, it means that each and every connection must be designed to minimize PIM and tested to ensure it is installed properly
Extensive testing by LTE providers discovered that legacy LMR braided cables may test perfectly in a Return Loss or VSWR test, but generally possess only average PIM performance. The braided outer conductor can act like hundreds of loose connections that behave poorly when tested for PIM, particularly as they age. For permanent installations, braided cables are not recommended. Times Microwave has recently introduced the LMR-SW Low-PIM cable to the market.
PIM lowers the reliability, capacity and data rate of LTE systems. It does this by limiting the receive sensitivity. As LTE usage grows, licensed and unlicensed spectrum has become crowded. Engineers must often select less desirable RF carrier frequencies and accept potential PIM issues. Compounding this problem, existing antenna systems and infrastructure are aging, making any PIM that does occur stronger.
When PIM interference falls within the receive band of a LTE base station, it makes the receiver less sensitive to weak signals that limits receive coverage. This increases the block-error-rate (BLER = No of erroneous blocks / Total no of Received Blocks. Normal BLER is 2% for an in-sync condition and 10% for an out-of-sync condition). If the connection is for data, interference from PIM creates more error protection bits and resends, which causes a lower overall data rate. In some cases, PIM can even cause receiver blocking, shutting down the sector.
PIM shows up as a set of unwanted signals created by loose or corroded connectors, nearby rust, medium or high PIM braided cable products and other variables listed below. Other names for PIM include the diode effect and the rusty bolt effect.
Some of the more common components that can cause PIM include:
Connectors
Connectors on the antenna run are the first suspects in any PIM hunt. Connectors are a likely cause of PIM and subject to a number of problems. First, if the mating surfaces have small gaps, a “voltage potential barrier” can be formed that allows electron tunneling (a diode effect) or microscopic arcing to take place. Either will cause PIM in the presence of strong signals.
Damage caused by over-tightening, insufficient contact pressure, distorted contact surfaces, foreign material in the mating surfaces, or corrosion can cause small gaps. In addition, corrosion may create crystals, which also have a nonlinear effect on RF signals. Corrosion is a particular problem in coastal areas where humidity and salt air are prevalent. In this case, connectors may need cleaning on a regular basis.
While it’s not a common problem with connectors designed for LTE service, it is worth mentioning that manufacturers make low PIM connectors with nonferrous materials. Ferrous materials have a nonlinear effect when used with RF signals. For example, stainless steel can add 10 to 20 dB of PIM to the signal.
Connectors with nickel plating, or gold over nickel, can add 20 to 40 dB of PIM to the signal. Connectors made for LTE usage are non-ferrous and plated with coatings such as silver, white bronze, and gold.
Cutting the cable at installation time may create metal particles or debris. If some of these particles remain in the cable, or get into the finished connector, they can cause PIM when they touch a current carrying surface. Contaminates can be a source of intermittent PIM if the cable assembly is flexing with temperature or from the wind.
The center conductor depth is important. If it is set too far back, the resulting poor contact may cause PIM under use. If it sticks out too far, it may cause physical damage when connected. This damage may lead to gaps the next time it is connected. One way of dealing with this problem is to use connector-clamping tools that set the center pin depth properly. If pin-depth becomes a common problem, special gauging fixtures are available to measure the center pin depth.
Careful cleaning, proper assembly, good weather wrapping, and proper connector torque are the best solutions to connector caused PIM. Tooling must be kept clean, sharp and well adjusted. Unfortunately, the first instinct when a bad connection is identified is to over tighten the components, which nearly always results in damage by deformation.
Cables
Cables do not typically cause PIM, but poorly terminated or damaged cables can and do cause problems. Beware of cables with a seam in the shielding. As the cable ages, this seam can corrode, causing PIM. The cable’s center conductor may also be faulty because plated copper does not always adhere well to the aluminium core. The copper can flake off if poorly manufactured, creating metal particles and poor connections that have the potential to create intermittent PIM.
Cables can change their physical configuration as temperature varies. For instance, sunshine can warm cables, changing their electrical length. A cable that happens to be the right length to cancel out PIM when cool may show strong PIM after changing its length on a warm day, or, it can work the other way around, good when hot and bad when cold. In addition, the physical change in length can make a formerly good connection into a poor one, also generating PIM. Finally, water in the cable run is not beneficial when trying to reduce PIM.
Here are some datasheet links for Low PIM cables from various manufacturers:
Antennas
Antennas are a critical part of any transmission system. Antennas are subject to fatigue breaks, cold solder joints and corrosion. They take the full power of the signal, or signals, and if they create any PIM, it will be broadcast along with the rest of the signal. If also used for reception, the PIM is already in the conductor, with no transmission loss, ready to cause harm to reception.
Nearby Corrosion
Any nearby corrosion can cause PIM. Look for rusty fences, rusty roofs, rusty mast bolts, and so forth.
Keeping rust away from the tower will pay dividends in reliability and let maintenance personnel sleep better at night.
Lightning Arrestors
While lightning arrestors do not purposely cause PIM, they are a source of micro arcing. As they age, their breakdown voltage gets lower until finally RF power peaks can cause them to arc in a very similar manner to antenna or connector micro arcs. If one of the connectors becomes damaged, it will cause PIM in the traditional sense. These products have suffered incredible price pressures and are a good example of an item that is not made as well as it once was.
With this information, the smart wireless operator/engineer will use quality Low-PIM components when designing and building their infrastructure. In this competitive RF environment, the smart operator reduces design flaws, construction errors, inspects components on a regular basis and monitors performance on a daily basis to pinpoint when PIM may be secretly rising within the system. These techniques will result in a competitive advantage and will yield happier, better connected customers.