Passive Intermodulation (PIM)

Passive Intermodulation Fundamentals

John MillerApplications Engineer

Kaelus(office) +44 (0)1383 437920(mobile) +44 (0)7800 542843

AMERICAS : EMEA: APAC :

Global Technical Service & Support

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.

f1

f2

f1+f

2f

2-f

1

2f1-f

2

2f1

2f2

2f2-f

1

3f2-2f14f

2-3f

1

3f1-2f

24f

1-3f

2

InterferenceInterference

Desired

PIM order

Frequency

Am

plitu

de

f1

f2

f1+f

2f

2-f

1

2f1-1f2

2f1

2f2

2f2-1f1

3f2-2f14f

2-3f

1

3f1-2f

24f

1-3f

2

3rd Order

5th Order

7th Order

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 (typicall y)

Odd products are most likely to fall in the Rx band.

PIM is getting more complex with

Networks evolution towards :

Indoor or outdoor complex DAS systems (shared by several operators)

Site sharing with multiple transmit carriers (same RF infrastructure for multiple operators)

Multi-technology sites (GSM 900 MHz, GSM 1800 MHz, UMTS 2100 and 900 MHz, CDMA 450, ...LTE)

Combining UnitGSM 900GSM 1800WCDMA

OP

1B

TS

OP

2B

TS

OP

3B

TS

OP

1B

TS

OP

2B

TS

OP

3B

TS

OP

1N

ode

B

OP

2N

ode

B

OP

3N

ode

B

Load

1800/WC

DM

A

900/1800

Triplexer

RF Output ports

PSPS

PSPS

PSPS

PSPSOutdoor

area

AntAnt

AntAnt

AntAnt

PSPS

AntAnt

AntAnt

Ant

PSPS

PSPS

AntAnt

PSPS

AntAnt

AntAnt

AntAnt

PSPSAntAnt

PSPSAntAnt

PSPSAntAnt

PSPS

AntAnt

IndoorArea 1

IndoorArea 2

IndoorArea 3

IndoorArea 4

IndoorArea 5

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)

Incident signals

Reflected signals

Ideal Infrastructure

Transmitted signals

Minimum loss

Minimum

Reflections measured by sweep test

Measures the quality of impedance matching in the system.

Linear Infrastructure

(No new frequencies)

Incident signals

Reflected + interference signals

Transmitted signals

Non-ideal Infrastructure

Reflections still measured by sweep test

Interference measured by PIM tester

Non-Linear Infrastructure(Creates new frequencies)

PIM Severity

Repairing IM3 reduces ALL IM products.

Metal flake

11th Order-93 dBm

9th Order7th Order

5th Order3rd Order-60 dBm

Outside PCS Rx band for

F1 & F2 selected

Possible to have Same Band intermodulationissues

3rd order

5th order

7th order

935 MHz 960 MHz890 MHz 915 MHz900 GSM Rx band 900 GSM Tx Band

935 MHz 960 MHz890 MHz 915 MHz900 GSM Rx band 900 GSM Tx Band

1805MHz 1880MHz1710MHz 1785MHz1800GSM Rx band 1800GSM Tx Band

1805MHz 1880MHz1710MHz 1785MHz1800GSM Rx band 1800GSM Tx Band

2110MHz 2170MHz1920MHz 1980MHz2100 UMTS Rx band 2100 UMTS Tx Band

2110MHz 2170MHz1920MHz 1980MHz2100 UMTS Rx band 2100 UMTS Tx Band

Also, Cross Band intermodulation issues involving UMTS

DCS1800 into UMTS2100

1805MHz 1880MHz 1920MHz 1980MHz

1800GSM Tx band UMTS2100 Rx Band

3rd order

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

CDR data and voice

RRC Success rate%

RAB Establishment Success % Voice Traffic Erl

PS Traffic (Mb)

Average Throughput RTWP

RSSI

UMTS

PIM Source

PIM

PIM

TX 1

TX 2

TX 1 TX 1

TX 2 TX 2

PIM Test Equipment

PIM Low PIM Load

How does a portable PIM tester work?

-100 dBm

0dBm

0dBc

-143 dBc=

dBm dBc

Power relative to the carrier levelAbsolute power relative to1mW

PIM level is expressed in either dBm or dBc

CA

RR

IER

PIM

2 x 20 Watts

CA

RR

IER

+43 dBm

PIM

PIM level is meaningless without also stating the carrier power level.

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.

-100 dBm

CA

RR

IER

PIM

+43 dBm

20 Watts

0dBm

CA

RR

IER

PIM

2 Watts

-130 dBm

+33 dBm

Noise Floor

PIM test process at 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

PIM Sources at the cell site:

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 repai rs.

Find problems before Mother Nature does!

Wind induced vibration Temperature variations

Dynamic versus Static testing:

Fault locations are found by gently tapping on con nections 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

Incident signalsTransmitted signals

Reflected + interference signals

Linear Infrastructure

PIM sources can be external to the system!

External PIM Sources

Typical 65 Beamwidth Antenna Patterns

4 ft. (1.2m) 800MHz

8 ft. (2.4m) 800MHzEnd View(Azimuth Pattern)

Side View(Elevation Pattern)

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.

PIMPIM

PIM

SKY

SKY

Empty cardboard

boxes (NO STAPLES)

Large PVC pipe

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 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

PIM Survey in EMEA by KAELUS

70.37%

3.09%

29.63%

Sites or sectors with PIMproblems

Sites or sectors w ithVSWR problems

Sites or sectors w ithoutPIM Problems

47

37

27 25

5 5 3 2

GS

M90

0/G

SM

1800

/UM

TS

2100

GS

M90

0

GS

M18

00

GS

M90

0/U

MT

S21

00

UM

TS

2100

GS

M90

0/G

SM

1800

GS

M18

00/U

MT

S21

00

CD

MA

450

On sites with PIM problems, different site technology feeder lines

were tested.

PIM Survey in EMEA by KAELUS

The PIM defects found were primarily due to workmanship issues at the RF connections.

Many lines had multiple defects

KPI improvements reported on the majority of sites that PIM was detected and repaired at site

PIM Survey in EMEA by KAELUS

65.35%

28.95%

13.60% 14.04%

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

Connectors Jumper Cables Other PassiveDev ices

Antennas

GSM 900 Greenfield Site

GSM 900This 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 Greenfield Site

!!"#$%&!%

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.

After all PIM generating connections were reworked the receiver noise floor was reduced to -112dBm.

Rx Noise Rx Noise floor before floor before PIM testingPIM testing

Rx Noise floor Rx Noise floor after after

reconstruction reconstruction using PIM using PIM

testertester

Here Here --a 5dB gaina 5dB gain

at the noise floorat the noise floor

GSM 900 + WCDMA 850 Indoor DAS

UMTS2100 Rooftop Site

3G RooftopUrban dense site

Surge Arrestors were the source of PIMAfter their replacement

Drops in Voice and Packet improved

After PIM changes

PIM test equipment: iQA Series

!" #$ %&( )(( *( +((%(,

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