Passive Intermodulation (PIM)
Passive Intermodulation Fundamentals
John Miller Applications Engineer
Kaelus
(office) +44 (0)1383 437920 (mobile) +44 (0)7800 542843
Global Technical Service & Support
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EMEA:
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What is Passive InterModulation (PIM)
PIM = Interference
PIM = Noise generated by Tx signals interacting with materials in the RF path.
PIM = Reduced BTS Coverage and Capacity.
The result
MANY unwanted frequencies are generated.
Desired Interference
f1 2f1-f2 f2 2f2-f1 3f2-2f1 4f2-3f1 2f1 f1+f2 2f2
Interference
f2-f1
4f1-3f2
3f1-2f2
PIM order
3rd Order 5th Order 7th Order 2f1-1f2 f 2-f1 4f1-3f2 3f1-2f2 f1 f2 2f2-1f1 3f2-2f1 4f2-3f1 2f1
Amplitude
f 1+f2 2f2
Frequency
The order of an intermodulation product m*f1 n*f2 = m+n. 2*f1-1*f2 = 3rd order product 4*f2-3*f1 = 7th order product. The lower the order the higher the level (typically) Odd products are most likely to fall in the Rx band.
PIM is getting more complex with
Indoor Area 1 1800/WCDMA Ant Ant Ant Ant
900/1800 Indoor Area 2
PS PS Ant
PS
PS
Ant PS Ant
Networks evolution towards: Indoor or outdoor complex DAS systems (shared by several operators)
Triplexer
PS Indoor Area 3 PS Ant PS
Ant
PS
Ant
Ant
Outdoor area
Indoor Area 4 PS
PS
Ant PS Ant Ant Indoor Area 5 Load
RF Output ports
Site sharing with multiple transmit carriers (same RF infrastructure for multiple operators)
Combining Unit
WCDMA GSM 1800 GSM 900
OP 3 Node B
OP 2 Node B
OP 1 Node B
OP 1 BTS
OP 3 BTS
OP 2 BTS
OP 3 BTS
Multi-technology sites (GSM 900 MHz, GSM 1800 MHz, UMTS 2100 and 900 MHz, CDMA 450, ...LTE)
OP 2 BTS
OP 1 BTS
What are the main PIM Sources
Non-linear metal-to-metal contacts:
Loose RF connectors. Poorly prepared RF cable terminations Improperly mated / misaligned parts Cracked / cold solder joints Loose mechanical fasteners (screws, rivets)
Ideal Infrastructure
Minimum loss
Incident signals
Linear Infrastructure
(No new frequencies)
Transmitted signals
Minimum Reflections measured by sweep test
Reflected signals
Measures the quality of impedance matching in the system.
Non-ideal Infrastructure
Incident signals
Non-Linear Infrastructure
(Creates new frequencies)
Transmitted signals
Reflections still measured by sweep test
Reflected + interference signals
Interference measured by PIM tester
PIM Severity
5th Order 11th Order -93 dBm 9th Order 7th Order 3rd Order -60 dBm
Outside PCS Rx band for F1 & F2 selected
Metal flake
Spectrum Analyzer Mode
Repairing IM3 reduces ALL IM products.
Possible to have Same Band intermodulation issues
GSM900
890 MHz 900 GSM Rx band
915 MHz
935 MHz 900 GSM Tx Band
960 MHz
GSM1800
3rd order
5th order
1710MHz 1800GSM Rx band 1785MHz 1805MHz 1800GSM Tx Band 1880MHz
7th order
UMTS2100
1920MHz 2100 UMTS Rx band
1980MHz
2110MHz 2100 UMTSTx Band
2170MHz
Also, Cross Band intermodulation issues involving UMTS
DCS1800 into UMTS2100
3rd order
1805MHz 1800GSM Tx band
1880MHz
1920MHz UMTS2100 Rx Band
1980MHz
5th order
And with LTE there are multiple combinations nowadays
Where to look for PIM on statistics?
Main search focus on:
General CDR, CSR, CSSR
GSM
GSM Mean interference on Idle (IOI) TCH Completion Rate and Drop % TRAFFIC RATE Erlangs TCH Assignment Rate SDCCH Assignment rate and drops HO Success Rate UL Quality Erlang Minutes Per Drop DROP_AFTER_TCH_ASSIGN SDCCH_RADIO_FAIL
UMTS
CDR data and voice RRC Success rate% RAB Establishment Success % Voice Traffic Erl PS Traffic (Mb) Average Throughput RTWP RSSI
How does a portable PIM tester work?
PIM Test Equipment
TX 1 TX 2
TX 1 TX 2
TX 1 TX 2
PI M
PI M
PI M
Low PIM Load
PIM Source
PIM level is expressed in either dBm or dBc
dBm
2 x 20 Watts +43 dBm 0dBm
CARRIER
dBc
0dBc
-100 dBm
PIM
CARRIER
-143 dBc
Absolute power relative to1mW
Power relative to the carrier level
PIM
PIM level is meaningless without also stating the carrier power level.
20 Watts
PIM level is VERY dependent on test power level. ~3dB change in PIM level for every 1dB change in test power. 2W not enough power to certify a site. IEC 62037 recommends 2x 20W (+43dBm) test carriers.
PIM
2 Watts +43 dBm +33 dBm 0dBm CARRIER CARRIER
-100 dBm
-130 dBm
Noise Floor PIM
PIM test process at cell site:
PIM Sources at the cell site:
Site Guy Wires Steel Tower Other Sites IMD RF In-Line Surge Arrestors Low Grade Antennas Roof Flashing/Vent Hoods Installation Poorly torqued connectors Scratches Stress Fractures Contamination On conduction materials Dielectric material Materials Rust Ferromagnetic materials Poor Quality Plating Test Equipment Poor Quality Loads Low quality or damaged jumpers, adapters and connectors Misc. Cracked / Cold Solder joints Contact Pressure
Resistive loads generate PIM
90 connectors typically not as good as straight connectors.
RF Connector Selection:
Avoid using Type-N 7-16 DIN connectors are more robust and produce less PIM
Poor cable preparation
Dirt / trash
Poor cable preparation
Poor cable preparation
Cable damage
PIM testing should be dynamic
Passing a static PIM test does not necessarily insure long term performance. Lightly tap RF connections. Flex cables while holding connector fixed. If PIM level jumps out of specification make repairs. Find problems before Mother Nature does! Wind induced vibration Temperature variations
Dynamic versus Static testing:
Fault locations are found by gently tapping on connections or components Tapping on a bad junction causes PIM to spike Dynamic PIM testing is very important! Finds the location of static PIM failures Stresses all connections to make sure that the feed system is robust and will not fail prematurely due to environmental exposure (Hot, Cold, wind loading, vibration, etc.)
PIM sources can be external to the system!
Transmitted signals Incident signals
Linear Infrastructure
PIM
Reflected + interference signals
External PIM Sources
Typical 65 Beamwidth Antenna Patterns
4 ft. (1.2m) 800MHz
End View (Azimuth Pattern)
Side View (Elevation Pattern)
8 ft. (2.4m) 800MHz
If external PIM is suspected, rotate antenna on mast.
PIM PIM
45 Rotation
Elevation tilting may be more effective to identify external PIM sources.
PIM
PIM
15 Rotation
Antenna Testing
Make sure there are no PIM sources within the field of view of the antenna! Recommendations: BE SAFE! Point antenna toward the sky. No metal objects within the half-power beamwidths of the antenna in both Azimuth and Elevation. Antenna should be placed on non-metallic supports during test. Test equipment and test operators should be located off the end of the antenna; not the side. Use a known good antenna to verify the test environment before testing. SKY
PIM
PIM
SKY
PIM
Testing Antennas
Empty cardboard boxes (NO STAPLES)
Large PVC pipe
PIM Tests
Summary
PIM = reduces site performance PIM sources can be eliminated / minimized through: Careful construction techniques Use of low PIM components. Careful site design. PIM testing should be dynamic (not static) PIM testing AND VSWR testing are needed to verify system performance.
KAELUS field trial evaluation
PIM Survey in EMEA by KAELUS
KAELUS trials: >319 feeder lines surveyed. 26 Operators and OEMs, 17 countries Many PIM problems recorded Few VSWR problems found The sites were originally commissioned with Sweep test equipment so it is no surprise that the VSWR results are good
Sites or sectors with PIM problems 3.09% Sites or sectors with VSWR problems Sites or sectors without PIM Problems 70.37%
29.63%
PIM Survey in EMEA by KAELUS
On sites with PIM problems, different site technology feeder lines were tested.
47 37 27 25
5 GSM1800 GSM900 UMTS2100 GSM900/UMTS2100 GSM900/GSM1800/UMTS2100
5 GSM900/GSM1800
3 GSM1800/UMTS2100
2 CDMA450
PIM Survey in EMEA by KAELUS
The PIM defects found were primarily due to workmanship issues at the RF connections.
70.00% 60.00%
65.35%
Many lines had multiple defects
50.00% 40.00% 30.00% 20.00% 10.00% 0.00%
28.95% 13.60% 14.04%
KPI improvements reported on the majority of sites that PIM was detected and repaired at site
Connectors
Jumper Cables
Other Passive Devices
Antennas
GSM 900 Greenfield Site
GSM 900 Greenfield Site
GSM 900
This site has suffered repeated intermittent ULQUAL issues and been visited many times. On the 22/10 with the riggers changed the antenna because of bad PIM measurements. PIM detection and Correction
GSM 900 + WCDMA 850 Indoor DAS System
GSM 900 + WCDMA 850 Indoor DAS
A carrier operating a WCDMA850 network overlaid on a GSM900 network had an in-building distributed antenna system suffering from high Dropped Call Rates (DCR).
The DCR was logged over a period of time and plotted in. There was a significant increase when the CDMA equipment at the site was switched on.
GSM 900 + WCDMA 850 Indoor DAS
After all PIM generating connections were reworked the receiver noise floor was reduced to -112dBm.
Rx Noise floor before PIM testing
Rx Noise floor after reconstruction using PIM tester
Here a 5dB gain at the noise floor
UMTS2100 Rooftop Site
3G Rooftop Urban dense site
Surge Arrestors were the source of PIM After their replacement
Drops in Voice and Packet improved
After PIM changes
PIM test equipment: iQA Series
High Power Premium Adjustable power (2 20W) Adjustable frequencies Rugged construction Integrated transit case Panel PC with touch screen interface, on-board reporting
Contact Us: John Miller Applications Engineer john.miller@kaelus.com Mob:+447800542843
2011 Kaelus Inc. All Rights Reserved
www.kaelus.com
Thank you
2011 Kaelus Inc. All Rights Reserved
www.kaelus.com
