Mitigating external sources of Passive Intermodulation (PIM) Tom Bell Sr. Product Manager Anritsu Company Slide 1
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
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
Mitigating PIM in RF components IEC 62037 was developed to define PIM test equipment / procedures (First release - 1999) 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
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
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
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
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
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
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
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!
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
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
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
Quasi-omni antenna evaluation
Consistent / repeatable external PIM source Diode Germanium diode used as PIM source Soldered to printed circuit dipole Very stable Able to control polarization
Quasi-omni antenna evaluation 1 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 !
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
Kaelus example: Rooftop site Antenna PIM Sources beyond the antenna
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”
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)
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
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.
Tom Bell Sr. Product Manager Questions Tom Bell Sr. Product Manager Anritsu Company Thomas.bell@anritsu.com