PIM in Systems

Passive intermodulation becomes a system-level concern when the aggregate PIM from all sources in an RF path affects network performance. While individual components may meet specifications, the combined effect of many components in a complete system can exceed acceptable levels. Furthermore, system architecture, installation environment, and operational conditions all influence PIM behavior in ways that component-level testing cannot fully predict.

This article examines PIM from a system perspective, addressing how different system architectures are affected by PIM, how to troubleshoot PIM problems in operational systems, and how system-level decisions influence overall PIM performance. Understanding these system-level aspects is essential for network designers, installers, and operators who must deliver reliable wireless service.

Antenna Systems

Antennas are often the largest contributors to system PIM because they contain multiple junctions and are exposed to environmental stresses. The antenna is also where PIM has its most direct impact, as PIM generated anywhere in the transmit path radiates from the antenna and can re-enter through the receive path.

Antenna PIM Sources

Multiple elements within antenna systems can generate PIM:

Feed network: The corporate feed network that distributes power to antenna elements contains numerous junctions, solder joints, and connectors. Each of these is a potential PIM source. High-quality manufacturing with attention to material selection and joint quality is essential.

Radiating elements: The antenna elements themselves may have mechanical joints where element sections connect. Some antenna designs use welded or one-piece elements to eliminate these joints.

Phase shifters: Electrically or mechanically adjustable phase shifters for beam tilt contain moving contacts or switching elements that can generate PIM.

Diplexers and filters: Internal filtering in multiband antennas adds components that must meet PIM requirements.

Radome: While the radome itself is typically non-metallic and does not generate PIM, metal hardware used in radome mounting can create PIM sources.

External PIM Sources

Objects in the antenna's near field can generate PIM that couples back into the antenna:

Tower and mounting structure: Rusty bolts, corroded joints, and metal-to-metal contacts on towers and mounts can generate PIM when illuminated by the antenna's radiated field.

Other antennas: Adjacent antennas, especially those with PIM problems or those using different frequencies that might generate in-band products, can couple PIM into the victim antenna.

Metallic objects: Guy wires, cable trays, safety climb systems, obstruction lighting, and other metallic objects near the antenna can generate PIM.

Ice and debris: Ice bridges between metal parts, bird nests with metallic debris, or accumulated metallic contamination can create unexpected PIM sources.

Identifying external PIM sources often requires systematic investigation, potentially including reorienting or relocating the antenna to determine if external objects are contributing.

Multiband and Multi-Operator Considerations

Modern antenna systems often support multiple frequency bands and may serve multiple operators:

Interband PIM: Transmit signals from one band can mix to create PIM products in another band's receive frequencies. This is particularly problematic when multiple operators share an antenna.

Wideband antennas: Antennas covering wide frequency ranges must maintain low PIM across all operating frequencies, a more demanding requirement than narrowband designs.

Shared feed systems: When multiple bands or operators share feed cables and combiners, PIM from any source affects all users of that path.

System planners must carefully analyze frequency plans to identify potential PIM interference between bands and operators.

Distributed Antenna Systems

Distributed antenna systems (DAS) present unique PIM challenges due to their complexity, the large number of components, and the shared infrastructure approach that is fundamental to their architecture.

DAS Architecture and PIM

A typical DAS includes:

Signal sources (base station equipment)
Head-end equipment (combining, conditioning)
Transport (fiber, coax, or hybrid)
Remote units (active distribution points)
Passive distribution (splitters, cables)
Antennas (typically many)

Each element and every connection in this chain is a potential PIM source. The aggregate PIM is the combination of contributions from all sources, which can add constructively or destructively depending on phase relationships.

Passive DAS Challenges

Passive DAS architectures distribute RF signals without active electronics beyond the head-end:

High power at splitters: The first splitters after the head-end handle the full transmit power and are subject to high stress. PIM here affects all downstream paths.

Many connections: A typical passive DAS may have hundreds of connectors. Even if each meets specification, the aggregate effect can be significant.

Mixed ages and sources: Buildings often have DAS infrastructure installed over many years, with components from different manufacturers and vintages. Older components may not meet current PIM standards.

Access difficulties: DAS cabling often runs through building infrastructure where access is limited. This complicates both installation quality and troubleshooting.

Active DAS Considerations

Active DAS uses remote electronics to distribute signals, typically over fiber:

Reduced passive RF path: The RF path is limited to the connection from the remote unit to the antenna, reducing the number of passive elements that can generate PIM.

Lower power levels: Each remote unit transmits at lower power than a centralized system, reducing PIM generation and impact.

Remote unit PIM: The remote unit itself must have low PIM, including its internal components and output connector.

Local troubleshooting: PIM problems are localized to individual remote units rather than affecting the entire system.

Active DAS generally provides better PIM performance than passive DAS for large systems, at the cost of greater complexity and power requirements.

DAS PIM Testing

Testing PIM in DAS requires a systematic approach:

Section testing: Test individual sections of the DAS separately to isolate PIM sources. This may require disconnecting portions of the system.

Progressive testing: During installation, test each section before proceeding to the next. This catches problems before they are buried behind other infrastructure.

Frequency considerations: Test at frequencies representing all bands that will be deployed, as PIM behavior varies with frequency.

Distance-to-PIM: Use DTP capabilities to locate PIM sources within the distributed architecture, recognizing that cable velocity factors and branch points complicate interpretation.

Small Cells

Small cells present both challenges and opportunities for PIM management. Their lower power levels reduce PIM severity, but their deployment in uncontrolled environments and the pressure for compact, low-cost designs create new concerns.

Small Cell PIM Characteristics

Several factors differentiate small cell PIM from macro sites:

Lower power: Small cells typically transmit at watts rather than tens of watts. Since PIM power depends on the product of carrier powers, lower transmit power dramatically reduces absolute PIM levels.

Shorter paths: The RF path from radio to antenna is much shorter, with fewer connectors and less cable. This reduces the number of potential PIM sources.

Integrated designs: Many small cells integrate the antenna with the radio unit, eliminating external RF connections entirely. This removes major PIM sources.

Tighter sensitivity requirements: Despite lower absolute PIM levels, small cells may still require attention to PIM because their receivers are designed for lower signal environments.

Deployment Environment Challenges

Small cells are often deployed in environments that are difficult to control:

Street furniture: Mounting on lamp posts, utility poles, and street furniture means exposure to traffic vibration, weather, and potential vandalism.

Building facades: Facade-mounted units must coexist with building structures that may include metallic elements creating external PIM sources.

Uncontrolled surroundings: Unlike macro sites where the RF environment is somewhat controlled, small cells are surrounded by metallic objects (vehicles, signs, fences) that can contribute to PIM.

Maintenance access: Small cells in public spaces may be difficult to access for maintenance and troubleshooting.

Design Trade-offs

Small cell economics pressure designs toward lower cost, which can impact PIM:

Connector choice: Lower-cost connector types may be specified to meet price targets. These must still meet PIM requirements.

Antenna integration: Integrated antennas eliminate external connections but may use lower-grade internal components.

Materials: Cost pressure can lead to material substitutions that may affect PIM. Careful specification is required.

Despite these pressures, small cell PIM specifications have become more stringent as operators deploy denser networks and rely more heavily on small cell coverage.

Base Stations

Traditional macro base stations, with their high power levels and complex RF infrastructure, require particular attention to PIM. The combination of high transmit power and sensitive receivers makes base stations highly susceptible to PIM interference.

Base Station RF Architecture

The typical macro base station RF path includes:

Radio equipment: Transmitters, receivers, and associated electronics. Active equipment can generate intermodulation but this is typically characterized and managed separately from passive PIM.

Jumper cables: Short cables connecting radios to main feeders. These experience the full transmit power and contain connectors at both ends.

Main feeder cables: Long cables running from equipment shelters to antenna locations, often up towers. These cables contain splices and connectors.

Lightning protection: Surge suppressors in the RF path must be designed for low PIM. These are often overlooked as PIM sources.

Tower-mounted amplifiers: If present, TMAs include connectors and components that can contribute PIM.

Antenna systems: As discussed above, a major contributor to system PIM.

Power Level Considerations

Macro base station transmit power (typically 20-60 watts per carrier, sometimes more) drives PIM concerns:

PIM power scaling: Third-order PIM power is proportional to the product of carrier powers (P1 times P2 squared for the 2f1-f2 product). Doubling all carrier powers increases PIM by 9 dB.

Multi-carrier systems: Modern base stations may transmit many carriers simultaneously. Each pair contributes its own PIM products.

Power control: Base station power varies with traffic load. PIM may be most severe during peak traffic when all carriers are at high power.

MIMO configurations: Multiple-input-multiple-output systems may combine signals in ways that affect PIM generation and observation.

Sensitivity and Capacity Impact

PIM impacts base station performance through receiver desensitization:

Noise floor elevation: PIM products in the receive band add to the noise floor, reducing the signal-to-noise ratio for received signals.

Uplink capacity: Elevated noise floor reduces the distance from which the base station can receive signals, effectively shrinking the cell coverage area.

Cell edge performance: Subscribers at cell edges experience the most impact because their signals arrive at lower levels and are more affected by noise floor increases.

Carrier aggregation: Systems using carrier aggregation may have PIM products from one band affecting reception in another aggregated band.

The impact of a given PIM level depends on the traffic conditions, subscriber distribution, and interference environment, making it difficult to establish universal acceptable limits.

Multi-Technology Sites

Many base station sites support multiple technologies and frequency bands:

2G/3G/4G/5G coexistence: Different technologies may use different frequency bands with different duplex spacings, creating various PIM product possibilities.

Shared antenna systems: Consolidating multiple technologies onto shared antennas reduces infrastructure but concentrates PIM sources.

Band combinations: Some band combinations create PIM products that fall into other bands' receive frequencies. These interband PIM scenarios require careful frequency planning.

In-Building Systems

In-building wireless systems (IBWS) provide coverage inside buildings where outdoor signals do not penetrate effectively. These systems have their own PIM considerations related to the building environment.

In-Building System Architectures

Several approaches are used for in-building coverage:

Passive DAS: Coaxial distribution from a central location. PIM from the entire passive network affects all coverage.

Active DAS: Fiber-fed remote units with short coaxial runs to antennas. PIM is localized to each remote unit's coverage area.

Small cells: Standalone small cells with independent backhaul. Each small cell is an independent PIM domain.

Repeaters: RF repeaters amplify outdoor signals for indoor distribution. PIM in the repeater or distribution system can affect coverage.

Building Environment Factors

The building environment creates specific PIM concerns:

Ceiling space: Much DAS infrastructure runs in ceiling plenums where it is exposed to dust, temperature variations, and potential damage from other trades.

Metallic ceilings: Metal ceiling panels and grid systems can create external PIM sources when illuminated by antennas.

HVAC equipment: Metal ductwork and equipment near antennas can contribute to PIM.

Building vibration: Mechanical equipment creates vibration that can affect connector reliability.

Renovation impacts: Building renovations may disturb or damage existing DAS infrastructure.

Installation Quality Challenges

In-building installations often face quality challenges:

Mixed installer experience: DAS installation may be performed by low-voltage contractors with limited RF experience.

Schedule pressure: Construction schedules often compress DAS installation into limited windows.

Access limitations: Working in occupied buildings limits access and work methods.

Component availability: Last-minute substitutions of components may not maintain PIM performance.

Detailed specifications, installer training, and thorough testing are essential to achieve consistent quality in building systems.

Shared Infrastructure

Modern wireless networks increasingly share infrastructure among operators. This sharing creates PIM challenges beyond those faced by single-operator deployments.

Infrastructure Sharing Models

Several sharing models are common:

Site sharing: Multiple operators use the same site but maintain separate antenna systems. PIM from one operator can affect another through external coupling.

Antenna sharing: Multiple operators share antenna systems with internal combining. PIM in shared components affects all operators.

Active sharing: Operators share active radio equipment with logical separation. PIM in shared RF paths affects all operators using that path.

Neutral host: A third party owns and operates shared infrastructure. PIM responsibility falls to the infrastructure operator but affects all tenant operators.

Multi-Operator PIM Issues

Sharing creates specific PIM complications:

Interoperator PIM: Signals from one operator mix with signals from another to create PIM products. These products may fall in bands used by either operator.

Responsibility allocation: When PIM affects one operator, determining whether the source is in that operator's equipment, another operator's equipment, or shared infrastructure is challenging.

Coordination requirements: Changes by one operator (power levels, frequencies, equipment) can affect PIM experienced by others.

Testing complexity: Testing multi-operator systems may require coordination to turn off or isolate different operators' signals.

Contractual and Operational Considerations

Shared infrastructure requires clear agreements on PIM:

PIM specifications for shared components
Testing requirements and responsibilities
Remediation procedures when problems occur
Coordination requirements for changes affecting PIM
Access provisions for testing and maintenance
Liability allocation for PIM-related issues

Clear agreements established before deployment prevent disputes when problems arise.

Co-Location Issues

When multiple antenna systems are installed in close proximity, whether at shared sites or in dense urban environments, interactions between systems can create PIM beyond what either system would exhibit in isolation.

Antenna-to-Antenna Coupling

Closely spaced antennas can couple energy to each other:

Near-field coupling: When antennas are separated by less than a few wavelengths, strong near-field coupling can occur. Signals from one antenna excite the other, where they can mix with that antenna's signals.

Side lobe coupling: Even well-separated antennas couple through their side lobes. This coupling is typically weaker but can still transfer significant power.

Induced currents: RF energy from one antenna induces currents on nearby metallic structures, including other antennas. These currents can couple into the other antenna's feed system.

The coupled energy experiences any nonlinearity in the receiving antenna's structure, generating PIM that appears to originate from that antenna.

Frequency Planning for Co-Location

Careful frequency planning minimizes co-location PIM:

PIM product analysis: For each pair of transmit frequencies at the site, calculate the frequencies of potential PIM products. Identify any that fall in receive bands of any system.

Frequency separation: Where possible, select frequencies that place PIM products outside all receive bands.

Band isolation: Use antennas with good front-to-back ratio and directional separation to reduce coupling between systems on different bands.

Filter placement: Filters at antenna ports can attenuate coupled signals, reducing the excitation that generates PIM.

Structural PIM at Co-Located Sites

Dense installations create many potential external PIM sources:

Tower hardware: The tower structure itself, with its many bolted and welded joints, can generate PIM when illuminated by multiple transmitters.

Cable congestion: Dense cable runs with many crossings and contacts create potential nonlinear junctions.

Mounting hardware: Antenna mounts, brackets, and supports contain numerous metal-to-metal contacts.

Weatherproofing materials: Metal tape, cable hangers, and similar materials can create unexpected conductive paths.

Maintaining co-located sites for low PIM requires attention to all metallic structures, not just the RF system itself.

Troubleshooting Methods

When PIM problems occur in operational systems, systematic troubleshooting identifies the source and guides remediation.

Characterizing the Problem

Before troubleshooting, clearly define the problem:

Symptoms: What is the observed impact? Receiver desensitization, coverage loss, dropped calls?

Frequencies: Which bands are affected? What transmit frequencies are present that could create the observed PIM products?

Time dependence: Does the problem correlate with traffic patterns, weather, or other time-varying factors?

Recent changes: Have there been any changes to the system, site, or surrounding environment?

Systematic Isolation

A systematic approach isolates the PIM source:

Test system residual: Verify that test equipment is not contributing to measured PIM by testing with a known low-PIM load.
Section testing: Test portions of the system in isolation. Disconnect the antenna and test the cable run; disconnect the cable and test the antenna.
Distance-to-PIM: Use DTP measurements to identify the distance to PIM sources.
Component substitution: Replace suspected components one at a time and retest.
Environmental investigation: Examine the physical environment for external PIM sources.

Common PIM Sources by Symptom

Certain symptoms suggest particular sources:

PIM that varies with temperature: Suggests thermal expansion affecting connections, or temperature-dependent material properties.

PIM that varies with wind: Indicates a mechanical issue, possibly a loose connection or cable movement.

PIM that varies with rain: May indicate water ingress into connectors or cable, or external PIM from wet metallic objects.

Intermittent PIM: Usually indicates a marginal connection that makes and breaks intermittently.

PIM that appeared suddenly: Often caused by damage, contamination, or environmental change.

PIM that increased gradually: Suggests aging or progressive degradation.

Remediation Verification

After corrective action, verify the fix:

Retest PIM at the same frequencies and power levels as initial testing
Compare to initial test results and specifications
Monitor over time to confirm the fix is stable
Document the problem, cause, and resolution

Performance Impacts

Understanding how PIM translates to network performance helps prioritize mitigation efforts and justify investments in low-PIM infrastructure.

Receiver Sensitivity Degradation

PIM products in the receive band add to the noise floor:

Noise figure degradation: If the PIM product power approaches the thermal noise floor, the effective noise figure increases. For example, if PIM products are 3 dB above thermal noise, the effective noise floor increases by 4.8 dB.

Sensitivity calculation: The sensitivity reduction in dB equals 10*log(1 + P_PIM/P_noise), where P_PIM is the PIM power and P_noise is the thermal noise power.

Wideband versus narrowband: PIM products are narrowband (if excited by CW carriers) or have bandwidth related to the modulation. The impact depends on whether PIM falls within the receiver's channel bandwidth.

Coverage and Capacity Effects

Receiver desensitization translates to network-level impacts:

Coverage reduction: Reduced sensitivity means signals from distant users are not received, effectively shrinking cell coverage.

Cell edge throughput: Users at cell edges experience reduced data rates due to lower signal-to-noise ratio.

Uplink capacity: The number of simultaneous users the cell can support decreases because each user's signal must be relatively stronger.

Handover behavior: Coverage reduction can cause handovers to occur at unexpected locations, potentially affecting call continuity.

Quantifying PIM Impact

Translating PIM levels to network metrics requires analysis:

Link budget impact: Include PIM-induced noise floor increase in uplink link budget calculations to determine coverage impact.

Traffic modeling: Use traffic models to estimate the capacity impact of coverage changes.

Interference analysis: Consider PIM in the context of other interference sources to determine its relative importance.

Economic analysis: Weigh the cost of PIM mitigation against the value of improved coverage and capacity.

In general, tighter PIM requirements cost more to achieve but provide better network performance. The optimal balance depends on the specific deployment scenario.

Specification Setting

System-level analysis informs component specifications:

Budget allocation: Allocate total acceptable PIM among components based on their relative contributions and the difficulty of achieving low PIM in each.

Margin: Include margin for aging, environmental effects, and variation among production units.

Test conditions: Ensure test conditions (power, frequency, environment) are representative of operational conditions.

Specification review: Periodically review specifications based on field experience and evolving network requirements.

Conclusion

Managing PIM at the system level requires understanding how individual component PIM combines to affect overall performance, how system architecture influences PIM behavior, and how to troubleshoot problems in complex installed systems. Different system types, from macro base stations to small cells to distributed antenna systems, present their own challenges and require tailored approaches.

Key system-level principles include: testing complete systems rather than relying solely on component specifications; considering external PIM sources in the deployment environment; carefully managing shared and co-located infrastructure; maintaining systematic troubleshooting processes; and relating PIM measurements to meaningful network performance impacts. By applying these principles, network engineers can deliver wireless systems that achieve their performance potential without degradation from passive intermodulation.