Cellular communications towers contain a wide range of antennas, cable assemblies, and other components that can contribute to the system's passive intermodulation (PIM) distortion. Typically, PIM distortion is caused by problems with the metal-to-metal junctions in the RF signal path, which are aggravated by the advancing age of components and systems in the field.1 Although it sounds routine to control, minimizing PIM in wireless communications systems can be quite a challenge. The secret to avoiding problematic PIM is extremely careful installation combined with parts that are designed to minimize distortion in the first place. Fortunately, a growing number of portable tools can help measure PIM, and an expanding number of components are being developed with improved PIM performance.
"PIM is one of the biggest concerns in the market today," asserts Dave Bolan, Product Marketing Manager, Anritsu Co.. PIM does not discriminate by frequency band, but it does greatly affect the receive band and, consequently, decrease network capacity. It is aggravated by higher operating power levels, so it is a growing problem as service operators share base stations, operate antennas across multiple bands to avoid tower loading limitations, and install multi-band feed systems. (The additional frequencies in the feed system increase the opportunity for PIM to effect a receive band.)
Even though components may pass PIM testing in the factory, they can still be easily damaged during transit or installation. Portable PIM test equipment has been on the market for years, and one of the most recent products to hit the market is Anritsu's PIM Master series, which not only helps locate sources of PIM distortion but also their distance from the test equipment. Boonton Electronics offers the PIM21 series of PIM testers for accurate evaluation of PIM levels at customer-specified test signal frequencies.
Kaelus (formerly Summitek Instruments) has been selling factory PIM test equipment to equipment manufacturers since 1996 and mobile units for field testing since 2005 (Fig. 1). Tom Bell, the company's Vice-President of Marketing and Business Development, notes that PIM field testing has unearthed recurring problems with field-terminated coaxial connections. "Workmanship issues caused by dull tools (or sometimes the wrong tool), improper installation torque, metal chips falling in the RF connections, etc. have been identified," says Bell, "These defects are not visible using the test equipment quickly identifies these problems, allowing operators to maximize the RF performance of their system."
Local ordinances and zoning restrictions are making it difficult or impossible for service operators to add antennas to towers when they expand their systems to handle more frequencies. As a result, networks are constrained to a limited number of antenna ports. According to Bell, "this means that PIM generated by the transmit tones at the cell site are in the same feed line used for reception." More specifically, the original cellular towers transmitted on one antenna and received on another completely separate antenna, with more than 40-dB isolation between the two. This meant that PIM generated by the transmit signals would be attenuated by 40 dB before entering the receive lines. Now that extra isolation no longer exists, which greatly aggravates the problem.
One way around all of these PIM issues would be to use frequency combinations where PIM does not fall in the range of the receive signal path. However, wideband systems, such as Third-Generation/Fourth-Generation (3G/4G) cellular Long Term Evolution (LTE), make this approach essentially impossible. Christopher K. Horne, Ph.D., PE, and Chief Technical Officer for the LBA Group, Inc., points out that third-order PIM presents the highest level of interference of PIM mixing products, followed by fifth- and seventh-order PIM signal products. Since third-order products can spread across as much as three times the channel bandwidth, a 10-MHz LTE signal, for example, can cause interference across as much as 30 MHz of the operating bandwidth. "In the past," says Horne, "careful frequency planning could eliminate concerns about PIM, but in LTE and 4G networks, the probability of interference is greatermaking PIM a larger concern." LBA offers RF engineering design and performance consulting, including PIM evaluations. Forward Link is another source of PIM consultation.
Clearly, operators should be worried about PIM. But according to Peter Jackson, Director, Europe, Communication Components, Inc. (CCI), they might not be worried enough: "While some operators are clearly defining their network PIM strategies, others have become distracted by service delivery, apps, and value-added services and need alerting to this overlooked area. In many cases, they've left their original core businessthe networkin the hands of third-party infrastructure providers in fully managed scenarios. And while, in some markets, this can be a good thing, in others, simple issues relating to final signal integrity and installation at the cell site may result in the occurrence of PIM for simple engineering reasons."
PIM elevates the noise floor, making it hard to separate in-band noise from the desired mobile signals. It exists in all passive systems. However, in the worst-case scenario, it can degrade data rates and quality of service (QoS), two concerns that are top of mind for wireless network operators looking to keep and attract subscribers. "PIM has become the new industry benchmark in determining the health of a cell site. It can reduce both an operator's coverage provided by a base station as well as the operator's capacity to handle voice and data traffic," reports Jackson.
Portable test solutions, such as Kaelus' iQA series of PIM test instruments with range-to-fault technology, CCI's PIMPro analyzer family (Fig. 2), and Anritsu's Distance-to-PIM solution (available on the firm's PIM Master portable analyzers), are designed for use at cellular sites. (While PIM is an issue for military applications as well, those designers tend to use individual instruments and components to build a customized test set that addresses particular specifications.) One issue with the growing availability of portable test equipment for PIM is the need for proper training by operators of the equipment. Companies like pimtesting.com aim to offer consultation and training on field testing for PIM. Providers of equipment also traditional return loss sweep instruments, but PIM offer training and certification. These include Anritsu's PIM Certification course, which builds off of its online Line Sweep course, and Kaelus' two levels of PIM test-equipment training.
To avoid PIM, the mobile operator needs to ensure that the PIM signals remain below the base station receiver's sensitivity. "The amplitude of these undesired signals is directly influenced by the fidelity of the transmission line path, including all components and junctions along that path, which might introduce a nonlinear effect to the signals passing through them," notes Jackson. He also cautions engineers to use a spectrum analyzer to compare spectrum responses between sectors. This makes it possible to discriminate between PIM energy and external interference signals coming from outside the antenna, such as from adjacent cell sites, old TV transmitters, or metallic structures nearby.
BACK TO BASICS
One low-tech way to avoid PIM is to take care when installing coaxial cable connectors, avoiding debris and ensuring a clean connection. A crowded cell site has many coaxial cables, and each must be properly attached to its connector (Fig. 3). PIM distortion can also be caused by corroded joints, which is an unavoidable problem for cellular infrastructure and towers that have been in the field for some time. Antennas must also be free of internal and external corrosioneven rust on the tower.
At the manufacturing level, avoiding PIM distortion means attention to quality control during the assembly process, as well as to the quality of work by subcontractors and vendors. Unlike systems that include active components, passive or linear components cannot be actively filtered. Bell suggests eliminating inconsistent metal-metal contacts from the design altogether, replacing them with capacitively coupled connections, soldered or welded connections, or by using more fasteners in regions that carry RF current.
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Component manufacturers are designing products specifically to address the concerns of PIM. For example, K&L Microwave and Dow-Key Microwave offer a series of filter solutions for monitoring broadband emissions. Cable companies, such as Times Microwave Systems and W.L. Gore & Associates, as well as connector companies like Pasternack Enterprises, have also developed products to address the concerns of PIM distortion.
Unfortunately, PIM cannot be completely avoided, so the next best solution is to detect it, find its source, and make the repair or necessary modification, all as quickly as possible. Bolan explains the urgency: "PIM testing requires taking the sector off the air and climbing the tower.So it takes time and costs money, as it often must be done with an expansive tower crew rather than an RF technician."
How can RF designers avoid PIM issues in their customers' end products? Kaelus' Bell suggests that PIM be treated as a critical requirement early in the design process. He advises designers to select vendors who understand PIM and have tight control over their manufacturing processes. What are the most important parameters to test? In 1997, the industry defined IEC 62037. Known more popularly as the PIM testing standard, the standard's scope is to address intermodulation distortion in RF connectors, cable assemblies, and cables. The standard defines test procedures for PIM distortion. many tones to transmit during the test.
"Looking back at the original discussions, the technical reasons for selecting IM3, two test tones, 20 W test power, and CW testing are still valid today," observes Bell. He cautions designers to be wary of marketing language that he sees creeping into the discussionnamely, that PIM testing should "simulate the real base-transceiver-station (BTS) environment." " sounds good but is technically incorrect," claims Bell. "If we were trying to simulate the BTS environment we would be transmitting multiple 100-W tones, using LTE waveforms and measuring IM5, IM7, IM9, or higher based on the actual frequency bands in operation at the site."
So, what's most important when selecting a PIM testing solution? Anritsu's Bolan notes that since PIM is power sensitive (the more power in the tower, the more likely there will be distortion), it is important to use a test solution with real-world output power.
By far the biggest development for PIM avoidance in the last few years has been the availability of portable equipment to test for PIM distortion in the field. As more field testing takes place, component vendors are learning what the critical failures are for deployed parts, and can work on addressing these issues in next-generation parts. Moving forward, portable equipment manufacturers must continue to optimize equipment for precision as well as ruggedness and weight. We can expect these portable testers to evolve toward including more functionality and diagnosis capability in single units. And as QoS degrades, we can expect carriers to start putting concerns about PIM distortion front and center.
1. "Understanding PIM 1.0," Application Note, Forward Link.