1. Mitigating external sources of Passive Intermodulation (PIM)
Tom Bell Sr. Product Manager
Anritsu Company
Slide 1

2. Agenda What is Passive Intermodulation (PIM)
What has the industry done so far to mitigate PIM
Work that remains: External PIM sources
Factors we think are important
Mitigating PIM (avoidance)
Roof-top sites
Indoor sites
Small cell sites
Distance-to-PIM (DTP) – tool for identifying external PIM sources
Conclusion
Acknowledgements

3. Passive Intermodulation (PIM)
TX signals at site
Passive Intermodulation (PIM)
Passive Intermodulation (PIM) F1 F2
2*F1–1*F2
2*F2–1*F1
3*F1–2*F2
3*F2–2*F1
4*F1–3*F2
4*F2–3*F1
5*F1–4*F2
5*F2–4*F1
IM 9
IM 7
IM 5
IM 3
IM 3
IM 5
IM 7
IM 9
PIM = interference
PIM = new frequencies generated by Tx signals at a cell site when they encounter non-linear junctions or materials in the RF path
PIM falling in an operator’s uplink can elevate the noise floor:
▪ Dropped calls ▪ Access failures ▪ Slower data rates

4. Mitigating PIM in RF components
IEC was developed to define PIM test equipment / procedures (First release )
PIM test equipment generates test tones and measures the magnitude of the IM products generated
Measuring IM3 “characterizes” the linearity
Reducing IM3 reduces all higher order products
Using this equipment, RF equipment manufacturers have learned how to produce low PIM components
IM3 levels < -150 dBc with 2x 20W test tones are achievable
Commercial PIM test system (Summitek Instruments)
PIM test system built using separate components

5. Mitigating PIM in the field
High power, portable PIM test equipment first introduced in 2005 (Australia)
High power, battery-operated, portable PIM test equipment first introduced in 2012
With this equipment, installation contractors are learning assembly skills required to produce low PIM systems
No burrs, jagged edges, loose material
No metal flakes inside connections
Prevent dielectric materials from being trapped between mating surfaces
Correct assembly torque
IM3 levels <-140dBc with 2x 20W test tones are achievable
Portable PIM test system
Typical workmanship issue

6. PIM sources beyond the antenna
Low PIM components + assembly skill does not guarantee success!
Non-linear junctions in the RF path beyond the antenna can be the source of PIM
Metal flashing for waterproofing
Sheet metal vents
Metal safety chains
Rusty / corroded surfaces
Loose mounting hardware
External PIM sources present a serious challenge for installers & network operators

7. Near field / far field impact
Reported last year in 2013 IBTUF presentation (Kaelus + Anritsu)
PIM sources near an antenna have a stronger impact than PIM sources far away from an antenna
Energy couples directly into non-linear junctions rather than radiating

8. Near field / far field impact
2D2/λ used as an approximation to calculate far field, (D>>λ)
Near field region extends:
10’s of meters on macro sites
1’s of meters on small cell sites
1/10’s of meters on indoor sites

9. Mitigating PIM - Rooftop macro sites
65° beamwidth, directional panel antennas typical
Tx power level is high ( > 43dBm)
Avoid metal objects within half power beam widths
Place antennas as close to the building edge as possible
Avoid loose metal objects within 1 wavelength of the antenna (in any direction)
Metal safety chains
Loose mounting hardware

10. Mitigating PIM – In-building sites
Omni-directional, single element designs
Very wide beam widths
Impossible to avoid metal objects
Ceiling tile frames
Light fixtures
Metal duct work
Fire suppression pipes
Fortunately:
Power levels are very low (< 20 dBm)
Near field region is very small (<< 1m)
Antennas flush mounted to ceiling

11. Mitigating PIM - Indoor DAS
PIM caused by external sources can often be reduced significantly with small antenna movement
Move dominant non-linearity from near field to far field
Pre-installation PIM tests can be used to determine optimum antenna placements for low PIM
Movement within 1 m of design location typically has negligible impact on coverage
Small movement can have large (>50 dB) impact on PIM!

12. Mitigating PIM - Small cell sites
Many potential PIM sources will be in the antenna near field
Omni-directional patterns typical
Antenna location is not adjustable
Need new method to mitigate PIM

13. Quasi-omni antenna evaluation
Will a quasi-omni antenna provide the missing adjustment mechanism to reduce PIM?
Rotate antenna on the mount
Steer pattern null at PIM source to reduce PIM
Omni pattern
PIM
Quasi-omni pattern

14. Quasi-omni antenna evaluation
PVC support frame
Test conducted inside anechoic chamber
Rotate antenna
PIM
Antenna Centerline
20 FT. LDF4-50A jumper cable
Adjustable PIM source height
Adjustable distance from antenna
Anritsu PIM Master™ MW82119A-0700

15. Quasi-omni antenna evaluation

16. Consistent / repeatable external PIM source
Diode
Germanium diode used as PIM source
Soldered to printed circuit dipole
Very stable
Able to control polarization

17. Quasi-omni antenna evaluation1 3 5 1 6 2 2 4 6 3 5 4
Far field radiation pattern
External PIM source in antenna near field
>30 dB PIM reduction achieved by rotating the antenna !

18. Distance-to-PIM (DTP)
Memory DTP
Active DTP
Relative distance
DTF
DTP
Relative distance
Test mode available from both Kaelus & Anritsu to identify the location of PIM sources
Overlay functions available from both manufacturers to show location of PIM relative to a known feature

19. Kaelus example: Rooftop site
Antenna
PIM Sources beyond the antenna

20. Anritsu example: Tower site
Green trace = DTP with steel wool on antenna radome
Yellow trace = DTP with steel wool removed
Both peaks occur at the same location (∆ distance = 0 ft)
PIM source is at the antenna aperture
Steel wool as “PIM marker”

21. Steel wool on antenna (green trace) Steel wool moved 1 m (blue trace)
Anritsu example: indoor antenna
Steel wool on antenna (green trace)
Steel wool moved 1 m (blue trace)

22. Conclusion
External PIM sources remain a challenge for network operators
“Avoidance” is the primary strategy
Keep loose metal-to-metal contacts out of 3 dB beamwidths
Re-position antennas to move external PIM out of near field
When avoidance does not work… try null steering
Take advantage of nulls in antenna patterns
Rotate antenna to steer null to direction of PIM source
Distance-to-PIM makes identifying external PIM much easier

23. Acknowledgements
Nicholas Cordaro at Verizon Wireless for proposing quasi-omni antennas as a solution for reducing PIM in small cell deployments
CSS Antennas for providing the antennas and the measurement range to enable evaluation of the concept.
Phillip Chan, John Beadles and Marc Beranger at Rogers Communications for developing the antenna location PIM survey as a means to optimize PIM performance in indoor wireless systems.

24. Tom Bell Sr. Product Manager
Questions
Tom Bell Sr. Product Manager
Anritsu Company