Passive Intermodulation (PIM) scenarios in Public Safety Network Rollout

Passive Intermodulation (PIM) scenarios in Public Safety Network Rollout
One of the most common problems for every wireless network – commercial or private, is the Passive Intermodulation (PIM). The issues caused by PIM have become more important as the rollout of the Public Safety in the 700 MHz is in progress. Nowadays, it is very critical to understand the potential PIM factors and how these can affect the general performance of the network.

What is PIM?

Common issues of passive components in RF systems are:
• Loose connectors (due to vibrations, inadequate torque, poor alignment)
• Cable bending
• Galvanic corrosion at dissimilar metal joints
• Printed Wire Boards (PWB) non-linearities in antennas
• Dielectrics in antennas
• Objects in antennas proximity (such as metallic structures)
• Acidity corrosion
All those issues lead to appearing in RF systems places seen by RF signals as non-linearities of passive components. Non-linearities in RF systems act like a mixer, for example a device able to combine two or more Tx signals into a wide spectrum of signals of different amplitudes and frequencies called Intermodulation (IM). Intermodulation caused by non-linearities of passive components is called Passive Intermodulation (PIM).




Figure 1: The 700 MHz Band

Possible PIM Scenario

In the figure 2 below we can see a typical self-interference issue. In this scenario Public Safety DL subcarriers cause 3rd, 5th and 7th order intermodulation (IM) and can create interference in the UL. IM3 and IM7 have the most critical impact, while IM9 cannot be considered as critical.

Figure 2: Self-interference issue

In figure 3 we have a different scenario where IM3 and IM5 products can interfere with commercial and Public safety UL. In this scenario commercially used carriers are mixed with Public Safety carriers. We mostly emphasize to 3rd order intermodulation products as its effect can create significant degradation to network’s performance.

Figure 3: Interference between commercial and Public Safety

Potential Impact of PIM on Public Safety network

The noise levels are increased by the intermodulation products, which can affect network’s performance. The reason is because higher noise levels will create lower SINR (Signal to interference plus Noise Ratio). As a result, the CQI (channel quality indicator) will be reduced and then the cells will be forced to move to lower order modulation schemes, resulting to lower data throughput. Other issues may observed can be increased drop calls, increased access failures, reduced system coverage and capacity.

Figure 4: PIM impact

While PIM has obvious effects from the network side, it also creates issues for the user equipment (mobile devices) used by police, fire, EMS, and other responders. The higher noise levels at the base stations will cause the mobile devices to drain their batteries faster because the mobile devices will transmit at higher power to compensate for the reduced sensitivity at the tower.

PIM Sources

Internal PIM. PIM can be caused by internal system factors such as:
• Antennas
• TMAs
• Diplexers
• Duplexers
• Cables
• Connectors
• Loose connections
• Rusty surface
External PIM. PIM can also be caused by external system factors such as:
• Tower modules and components
• Bolts
• Brackets
• Nearby metallic objects and obstacles

Figure 5: PIM source by structure road and connectors

Figure 6: PIM source rust

Figure 7: PIM source at frame joint

Methods to Minimize PIM

It is obvious that during the rollout of Public Safety the PIM effect needs to be minimized. The goal is to reduce it to a level below the noise floor or low enough that performance is not impaired. One way is to improve linearity because it reduces the magnitude of all IM products. There are some fundamental steps that can be taken, including:
• Visually inspect and clean the connectors and tight them
• Use factory terminated and PIM tested RF jumper cables where possible
• Verify that RF mating surfaces are clean and free of mechanical damage prior to assembly
• Wipe mating surfaces with a lent-free wipe, moistened with alcohol to remove dirt & oils
• Face coaxial cables downward while cutting so that any metal flakes produced fall out rather than into the coaxial cable
• Always use sharp cutting tools when preparing the ends of coaxial cables
• Use the correct cable preparation tools for the type and size coaxial cable you are working with
• Remove any metal burs from the cut edges of coaxial cables prior to connector attachment
• Prevent foam dielectric material from getting trapped between metal contacting surfaces
• Remove all adhesive residue from the mating region of the coaxial cable center conductor
• Properly align RF connectors prior to assembly to prevent damage to mating surfaces
• Apply the manufacturer’s specified torque to all mated pairs of RF connectors.
• Do not over-torque RF connectors as this may cause damage to contacting surfaces
• Prevent excessive vibration and shock to RF components when transporting them to the site
• Prevent RF components from impacting the tower while hoisting
• Leave protective caps on RF connectors until you are ready to attach the mating cable
• Avoid loose metal objects within the ½ power beam widths of base station antennas
• Cable trays, vent pipes, air conditioning units, metal flashing, guy wires, etc.
• Avoid loose metal objects anywhere within one wavelength of base station antennas
• Loose mounting hardware, safety chains, etc.

TTS Wireless has the appropriate personnel and equipment to identify PIM sources, reduce the PIM effect and provide recommendations for eliminating it.

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