- TECH

Report
Best Practices for Achieving
Low PIM Performance
in DAS Networks
Slide 1
Agenda
 What is Passive Intermodulation (PIM)
 Sources of PIM in a DAS environment
 Reasons why PIM is particularly problematic in DAS
 How a PIM tester works
 Best practices for mitigating PIM
 Anritsu MW82119A product features
Slide 2
Passive Intermodulation (PIM)
TX Signals at site
Passive Intermodulation (PIM)
F1
F2
2*F1–1*F2
Passive Intermodulation (PIM)
2*F2–1*F1
3*F1–2*F2
3*F2–2*F1
4*F1–3*F2
4*F2–3*F1
5*F2–4*F1
5*F1–4*F2
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
Slide 3
▪ Slower data rates
What does PIM look like at the site?
PIM
Mobile uplink signal
Slide 4
What does PIM look like to the
operator?
QUIET
BUSY
QUIET
High average
noise level
BUSY
PIM Repair
Lower average
noise level
Slide 5
What is a non-linear junction?
Linear junctions
Current
 Current increases linearly with applied
voltage
 High pressure, metal-to-metal contacts
 Welded or soldered connections
Voltage
Non-linear junctions
 Current does not increase linearly with voltage.
 Low pressure, metal-to-metal contacts
Current
 Oxide layers on metal surfaces
 Arcing across small air gaps or cracks
Voltage
What is non-linear in a DAS?
Damaged / poorly made components
 Broken cracked solder joints
 Loose internal fasteners
 Braided cables with crimp connectors
 Nickel plating
 Shipping damage
Slide 7
What is non-linear in a DAS?
BTS #1
Input
Out
Out
BTS #2
Input
Slide 8
High power RF terminations
 Notorious for poor PIM
 Impact depends on frequencies
being combined
What is non-linear in a DAS?
Metal flakes inside RF
connectors
 Worse when touching inner or
outer conductor
Slide 9
What is non-linear a DAS?
Poor cable termination
 Loose metal flakes
 Rough / irregular metal
edges
Slide 10
What is non-linear a DAS?
Metal flakes inside cables
 Plenum rated cables have no foam
to prevent metal flakes from
falling inside
Slide 11
What is non-linear in a DAS?
Loose RF connectors
 Must be torqued with a torque
wrench
Slide 12
What is non-linear in a DAS?
Type-N connectors
 Easy to damage contacting surfaces if
over torqued
 Tend to loosen over time
 7/16 DIN connectors better
 4.1/9.5 or 4.3/10 “mini DIN” connectors
also better
Slide 13
What is non-linear in a DAS?
External PIM Sources
 Fire suppression pipes
Antennas
flush
mounted
to ceiling
 Air handling ductwork
 Metal hangers
 Lighting fixtures
 Ceiling tile frames
 Etc., etc., etc.
Slide 14
And… there are many opportunities
for PIM!
Floor 3
Radio 1
Radio 2
Floor 2
Floor 1
In this simple example:
 63 RF connections
 14 power dividers
 31 cable assemblies
 1 hybrid combiner
 15 antennas
 1 RF termination
Slide 15
Where does PIM occur?
F1
F2
2*F1–1*F2
2*F2–1*F1
3*F1–2*F2
3*F2–2*F1
4*F2–3*F1
4*F1–3*F2
5*F2–4*F1
5*F1–4*F2
IM 9
IM 7
IM 5
IM 3
IM 3
BTS Uplink
IM 5
BTS Downlink
 PIM occurs at mathematical combinations of the Tx frequencies
F1 = 1930 MHz
F2 = 1945 MHz
•
IM3 = (2*1930) – (1*1945) = 1915 MHz
IM3 = (2*1945) – (1*1930) = 1960 MHz
•
IM5 = (3*1930) – (2*1945) = 1900 MHz
IM5 = (3*1945) – (2*1930) = 1975 MHz
•
IM7 = (4*1930) – (3*1945) = 1885 MHz
IM7 = (4*1945) – (3*1930) = 1990 MHz
•
IM9 = (5*1930) – (4*1945) = 1870 MHz
IM9 = (5*1945) – (4*1930) = 2005 MHz
Slide 16
IM 7
IM 9
Where does PIM occur?
(the easy way)
F1
F2
15 MHz
15 MHz
15 MHz
15 MHz
15 MHz
15 MHz
15 MHz
15 MHz
15 MHz
IM 9
IM 7
IM 5
BTS Uplink
IM 3
IM 3
BTS Downlink
Slide 17
IM 5
IM 7
IM 9
Where does PIM occur?
(the easy way)
F2
F1
30 MHz
30 MHz
30 MHz
30 MHz
IM 5
30 MHz
IM 3
BTS Uplink
IM 3
BTS Downlink
 Wide frequency spacing moves IM products farther apart
 Increases probability that low order IM product will fall in uplink band
Slide 18
IM 5
Neutral Host Systems Increase Probability
that IM Products Will Fall in Rx Band
Single PCS operator:
 FCC did a good job!
 IM 11 first product that
falls in A-block uplink
Multiple operators / bands:
 Different story!
 IM 3 falls in E / F block
uplink
 IM 5 falls in A-block
Slide 19
What is the bandwidth of
PIM signals?
F1
F2
200 KHz
200 KHz
600 KHz
600 KHz
1 MHz
1 MHz
1.4 MHz
1.4 MHz
1.8 MHz
1.8 MHz
IM 9
IM 7
IM 5
BTS Uplink
IM 3
IM 3
IM 5
BTS Downlink
 PIM bandwidth increases as carrier bandwidth increases
 PIM bandwidth increase with PIM order
Slide 20
IM 7
IM 9
Can one wide band carrier
generate PIM?
5 MHz
5 MHz
5 MHz
5 MHz
5 MHz
5 MHz
5 MHz
5 MHz
IM 3
IM 5
IM 7
IM 9
BTS Uplink
BTS Downlink
 YES !
 Signal is composed of individual sub-carrier frequencies
Slide 21
5 MHz
What is the solution?
LTE
Eliminate non-linearity so that multiple
frequencies can peacefully co-exist!
CDMA
Slide 22
How do we measure linearity?
F1, F2
Load
Test lead
IM products
PIM
PIM tester
PIM
 With a PIM tester!
 Inject two CW test signals at a known magnitude
 Measure 3rd order intermodulation product (IM3)
 IM3 “characterizes” the linearity of the system
 If IM3 is low = linearity is good
 If IM3 is high = linearity is poor
Slide 23
Reducing IM3 Reduces all
IM Products
F1
F2
IM 3
IM 3
A
IM 11
Noise
Floor
A
IM 9
IM 7
IM 5
IM 5
BTS Uplink (Rx)
BTS Downlink (Tx)
 Low IM3 = good linearity
 Low IM3 drives higher order products below the noise floor
Slide 24
IM 7
Best Practices for Achieving Low PIM
performance in a DAS
Slide 25
1) Start with high quality
components
Only use RF components that:
 Are designed for low PIM performance
 Proper material selection
 No loose metal-to-metal junctions
 IEC-62037-1 provides more extensive list of design guidelines
 From companies that:
 Own PIM test equipment
 Have their manufacturing processes under control
 100% test to verify performance (-150dBc with 2x 20W test tones )
Slide 26
2) Use crews that are PIM certified
 Understand the importance of cleanliness
 Use alcohol swabs to clean mated pairs of connectors
 Vacuum end of cables before attaching connectors
 Understand the importance of proper cable preparation
 Cut plenum rated cables with razor blade cutter (not saw)
 Use sharp cutting tools
 Use the correct tools for the type of cable in use
 Understand the importance of tight connections
 Always use a torque wrench (DIN)
 Do not over/under tighten (Type-N)
 Have experience operating PIM test equipment
Slide 27
Radio 2
Radio 1
3) PIM test as you build
Antenna location tests
Sector test
Branch test
System test
Floor test
Slide 28
3) PIM test as you build
(con’t)
 Problems are identified and corrected before being replicated
throughout the DAS
 Faster to locate PIM sources when testing smaller sections
 Easier access to antennas / splitters to troubleshoot and correct
problems
 Tools, materials, man-lifts, still on hand
 Reduced re-work at antenna locations (when antenna locations are pretested)
Slide 29
Antenna Location Testing
 Place antenna as close to design location as
possible using low PIM pole
 Perform PIM test
 Low test power (15 - 30 dBm typical)
 Lowest frequency only (700 or 850 MHz)
 Pass/Fail limit (depends on test power)
 Find passing location within 1 m of design
location
 Small movements = big changes in PIM!
PIM tester
Slide 30
Things to Avoid Around Antennas:
 For every rule there is an exception
 This is why it is important to test!
 Try to avoid placing antennas within 1m of:
 Metal seams (ductwork, etc.)
 Concrete with obvious signs of rust
 Lighting fixtures
 Electronic devices
 Products containing RF ID tags
Slide 31
PIM Master MW82119A Features
Slide 32
Key Specifications / Capabilities
for DAS
Parameter
Specification
Small size
350 x 314 x 152mm
(13.8 x 12.4 x 6.0in)
Light weight
9 kg to 12 kg *
(20 lb to 27 lb)
Battery operation
> 3.0 hours
Wide power range
25dBm to 46dBm
(0.3W to 39.8W)
Low residual PIM
-117 dBm @ 2x 43dBm
Distance-to-PIM™
YES
PIM vs. Time
YES
Remote Control
YES
* - Weight varies by frequency option
Slide 33
Battery Operation
= Enhanced Portability
Slide 34
PIM vs. Time Capability
 PIM magnitude vs. time
Tapping on RF
connections
Limit Line
 Tap on all RF connections while
performing the PIM test
 Visual indication of PIM stability
 Peak PIM held for Pass/Fail
Slide 35
Distance-to-PIM (DTP) Capability
 The fastest way to locate PIM
 PIM magnitude vs. distance
 Eliminates guess-work
 Able to see PIM beyond antenna
 6 markers + marker table to identify
multiple sources
Slide 36
Wide Range of Test Accessories
2000-1745-R - Backpack Accessory Kit
760-259-R – Transit case
(Holds MW82119A only)
760-265-R – Transit case
(Holds MW82119A plus accessories)
(accessories not included)
2000-1746-R - Hard Case Accessory Kit
Slide 37
Conclusion
 PIM is a challenge in DAS environments
 Many potential sources
 More harmful frequency combinations
 Best practices for mitigating PIM include:
 Using high quality components
 Using trained installation crews
 PIM testing as you build (starting with antenna locations)
 Anritsu PIM Master is well suited for PIM testing in a DAS environments
 Exceptional portability
 Fully integrated, Distance-to-PIM (DTP) technology
Slide 38

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