By Paul Kruczkowski, Editor
For years, leading mobile carriers have been laying claim to the fastest data rates, largest coverage area, or highest network reliability. The trend has persisted through the 4G-LTE buildout, as carriers continue to build new cells sites and add capacity to existing sites with these goals in mind. However, a phenomenon called passive intermodulation (PIM) can seriously hinder such efforts, dramatically affecting data rates, coverage, and system reliability.
What Is PIM?
PIM occurs when multiple high-power signals encounter nonlinearities in the transmission line system, creating unwanted products of significant levels to degrade the performance of a base station. The use of older transmission equipment, improper equipment installation, and poor component selection can all lead to PIM problems.
The formulas nF1 – mF2 and nF2 – mF1 are used to calculate the frequencies of third-, fifth-, and seventh-order PIM products created by two carriers where n and m are positive integers and F1 and F2 are the fundamental frequencies of the carriers. Spreadsheets or software can be used to calculate the PIM products produced by more than two fundamental carrier frequencies.
PIM becomes an issue when these products fall within or close to the frequency band of a collocated or nearby receiver. The likelihood of interference increases as the bandwidths of the fundamental frequencies increase. Third-order products are 3x the bandwidth of the fundamentals, and the fifth-order products are 5x as wide, so 5 MHz downstream signals would produce third-order PIM products that are 15 MHz wide.
These unwanted products raise the noise floor of the receiver, reducing its dynamic range and sensitivity. Weaker signals are no longer received, resulting in reduced coverage. If a receivers sensitivity is -110 dBm, but PIM raises the noise floor to -100 dBm, the receiver is 10 dB less sensitive, resulting in reduced coverage by whatever distance (in miles) the 10 dB (10x) reduction equates to. PIM also causes higher bit error rate (BER), leading to more error protection bits in data transmission and resends, resulting in lower data rates. Extreme levels of PIM can effectively create a hole in coverage.
Sources Of PIM
Connectors are the primary culprit of high PIM levels in a base station transmission system because they are mechanical connections. Poor seating to a connector’s mate or improper center pin depth can generate PIM through micro-arcing or intermittent spot contact. Aging connectors develop oxidation layers or corrosion from humidity and salt air, which can result in electron jumping and tunneling that also generates PIM. Something as simple as connector selection can greatly influence PIM levels. For example, connectors made with ferromagnetic metals like stainless steel and nickel can add as much as 40 dBm of PIM to a signal, so non-ferrous base metals with white bronze, silver, or gold plating are ideal.
But connectors are not the only components that can cause PIM issues. Antennas, like connectors, are mechanical in nature and so can suffer from corrosion and fatigue failures from wind, vibrations, or inadequate mounting. Cables can also generate PIM, but typically only when damaged or poorly terminated — for instance, if the cables shielding is compromised or a metal filing is left in the transmission line. Components like switches, splitters, and diplexers can introduce PIM products for many of the same reasons connectors do, while circulators and isolators contain ferrite material that can create PIM.
PIM can’t be controlled by filtering since it is created in the transmission system after the mask filter. As such, the best remedy to PIM is to test the system, locate the nonlinearity, and eliminate it. In addition, it is also prudent to use low-PIM products on the antenna system for new installations or for site upgrades and repair.
There are several PIM testing solutions on the market, one of which is the PIM Master from Anritsu. This analyzer applies two CW tones of 20, 30, or 40W, measures the noise floor of the receiver, and identifies the third, fifth, and seventh order PIM products in the receive band. It also has a proprietary feature called “Distance-to-PIM (DTP)” that locates the fault causing the PIM product, which is important to resolving the problem. Once the faulty component is identified it can be repaired or replaced.
A typical PIM test measurement would apply two 20 W (43 dBm total) continuous wave (CW) signals to the antenna system, and the desired PIM levels would be between -150 dBc and -170 dBc, which is a maximum PIM level of -107 dBm. As a practical matter, the PIM level should be better than the receiver’s sensitivity level — -107 dBm / -150 dBc is in line with that expectation, and measurements of -125 dBm / -168 dBc are commonplace. However, it is worth noting that a standard figure used worldwide is -97 dBm / -140 dBc, a figure that isn’t realistic for antenna systems installed over a decade ago, since they were not designed to reduce PIM.
Now that PIM is a recognized industry problem, RF manufacturers that serve the mobile communications industry are taking great care in selecting materials and employing manufacturing processes to create low-PIM product lines. These products eliminate design elements that can create PIM, making them better suited for high-power multi-carrier applications. Some examples include:
Antennas from Bird Technologies Group
Reactive splitters from Huber + Suhner
Low-PIM RF switches from Dow-Key Microwave with typical third order PIM levels of -125 dBm/ -168 dBc
PCB material using rolled-annealed (RA) copper from laminate manufactures like Taconic and Arlon Materials for Electronics has better PIM performance than electro deposited (ED) copper foil, because RA copper has a much smoother surface.
PIM is a problem that wireless carriers need be concerned about, because it can seriously impact the performance of a base station and, in turn, the customer experience. The only real solution to PIM on existing sites is PIM testing, followed by repairing or replacing antenna system components that generate PIM products. Fortunately, component vendors recognize the importance of designing low-PIM products, and carriers can select these products and implement good installation practices to reduce their exposure to PIM.
To learn more about PIM and PIM testing check out these additional resources: