WCDMA-P&O-A-EN-WCDMA Interference Investigation-201012
WCDMA-P&O-A-EN-WCDMA Interference Investigation-201012
WCDMA-P&O-A-EN-WCDMA Interference Investigation-201012
Investigation
ZTE University
Content
real
1 dB
Noise Floor P IN
C1 dB C3 dB (dBm)
Inter-modulation
Products Generation
Non-Linear Component
S IN S OUT
S OUT = S IN + S2IN + S3IN +...
2F1 + F2
2F2 + F1
2F2 - F1
2F1 - F2
F1 + F2
F1 - F2
P IN
Noise Floor
(dBm)
IP3
Intermodulation
4 - 2G/3G Collocation Aspects
• 2nd order IM products from GSM900 Tx fall into UMTS uplink band
• 3rd order IM products from GSM1800 Tx fall into UMTS uplink band
• No 2nd or 3rd order IM products of UMTS Tx fall in any of the GSM bands
GSM / UMTS
Collocation
• extra Tx chain filtering
• use of diplexer/triplexer
Internal Interference
Normally the internal interference has following characters:
Generally, the main path and the diversity path RTWP are not relevant.
But if they are, they must have special reasons, e.g. they are jointly
somewhere.
there is a certain correlation between interference and the traffic. When
the traffic is little, there is high probability that the interference doesn’t
occur.
RTWP generally has high volatility, and the volatility in the range of about
10 dB or greater than 10 dB;
Once the interference occurs, it has a certain duration without mutable
change generally, which is very different from the external interference;
From the time characteristics of RTWP, RTWP has no significant changes
in law generally;
Internal interference (PA problem)
After PA power off, the interference disappear also;
Scanning result is normal;
RTWP no change when replace antenna;
Long term RTWP statistics is random;
RTWP of different frequency is similar, while RTWP of main path and diversity
path has large difference for same frequency.
External interference
External interference
An external source of interference may be the existing 2G systems,
repeaters, mobile phone jammers, microwave transmission
equipment and the illegal use of the WCDMA system band
equipment. 。
Possible External interference source
External Interference
If have the following characteristics, we can judge to be External interference :
Under normal circumstances interference is relevant to both the main set and
the diversity, which means the interference has a similar trend and little
difference ( e.g. less than 5dB );
External interference also affects cells with geographical proximity;
From the time characteristics of RTWP, external interference generally has
discontinuity, and there is a regularity of the interference time of occurrence,
duration and interval with an exception of such as microwave interference,
repeater gain setting error.
External interference(BBU,IP switch etc)
In UMTS, RTWP requirement is -105dBm/5MHz on BTS antenna
connecter port. So antenna specially high gain antenna must keep
enough distance between electric element (5m)
Indoor system
External interference(BBU,IP switch etc)
B8200/IP switch is installed in the same room with antenna;
Multiple narrow pulse interferences;
Interference normally is on
890MHz 、 891MHz 、 900MHz 、 907.5MHz 、 910MHz 1720MHz
、 1750MHz;
External interference(repeater)
The interference caused by repeater, trunk amplifier usually have
following reasons :
Poor stability, easy to self-excitation;
Gain setting unreasonable, leading to the interference to the donor
base station is too serious;
The characteristics of interference caused by repeater, trunk amplifier
are:
Long time stability of interference or unexpected interference
Long time stability of interference is generally caused by the gain
setting problem of the repeater or the trunk amplifier
Repeater interference is generally a large area of directivity
disturbed while trunk amplifier interference is rounded.
External interference(repeater)
RTWP is high for All frequency used in repeater;
RTWP of main path and diversity path has large difference for same
frequency;
RTWP is stable.
Interference caused by wrong gain setting
Interference caused by Self-excitation
≤-15dBm ( out-of-band )
M a r k e r 1 [ T 1 ] R B W 3 0 0 k H z R F A t t 0 d B
R e f L v l - 5 0 . 3 5 d B m V B W 1 M H z M i x e r - 2 0 d B m
- 1 0 d B m 1 . 9 2 2 4 0 0 0 0 G H z S W T 5 s U n i t d B m
- 1 0
A
- 2 0
- 3 0
- 4 0
1 M A
1
- 5 0
- 6 0
- 7 0
- 8 0
- 9 0
- 1 0 0
- 1 1 0
S t a r t 1 . 9 G H z 8 M H z / S t o p 1 . 9 8 G H z
D a t e : 2 6 . N O V . 2 0 0 4 0 8 : 5 2 : 1 3
Microwave Transmission Interference
The microwave transmission interference has following characteristics :
Long basic stability ;
Strong directivity ;
Interference effects of a wider range;
Spectrum is a broadband spectrum;
Other electro magnetic interference.
Content
Power off PA Y
RRU replaced
RTWP OK?
N
Check:
RTWP value of last 10 days( all frequency neighbor cell)
NW design plot
Neighbor cell info, repeater info
Repeater exist? Y
N
Interfere Investigation Process(continued)
RTWP of main
path and diversity path
is high correlative
N
Start
Disconnect Point A
Yes Yes
Disconnect point E
Check if the interference is
Connect jumper of point E caused by GSM output signals
to VWSR tester
Internal Interference locating(sharing
antenna)
antenna error
Is RTWP back No
from point E
to normal?
to the antenna
Yes
Is RTWP back No
external interference
to normal?
Yes
combiner error
Internal Interference locating(sharing
antenna)
Disconnect the WCDMA base station and antenna feed system’s main set;
Set a match load on the transmit port of a base station to shield external
interference; if the base station is located in room under closed environment,
we can also set a small antenna on the transmit port of a base station;
Check if RTWP is back to normal;
If normal, WCDMA base station equipment problem is excluded, go to step 4;
If not normal, the interference comes from WCDMA base station equipment
itself, location progress ends.
Reconnect the WCDMA base station and antenna feed system’s main set
( jumper A );
Close GSM base station;
Check if RTWP is back to normal: If normal, the interference is relevant to GSM
base station’s transmit signals, their inter-modulation products fall on the
WCDMA receive band, resulting in uplink interference. Interference sources
could come from GSM base station transmitter channel (from the launch port of
GSM base station to the antennas), it also may be inter-modulation results from
GSM signals of current site and other sites.
If not normal, the interference has nothing with GSM base stations transmit
signals . And the interference sources may come from the WCDMA base
station transmit path (from WCDMA base station transmitter to the antenna
port), or from an external system.
Internal Interference locating(sharing
antenna)
Disconnect GSM base stations and antenna feed system ( C point
); disconnect the jumper and the combiner (D point ); start to test
standing wave ratio of the jumper from point C to point D.
Connect standing wave ratio test equipment to jumper at point C;
If normal, the antenna feed system path below point D is OK, we need
to continue investigation;
If not, jumper from point C to D doesn’t work, end the progress.
Disconnect GSM base stations and antenna feed system ( A point
If normal, the antenna feed system path below point B is OK, we need to
continue investigation;
If not, jumper from point A to B doesn’t work, end the progress.
Disconnect the main feeder and the combiner (E point ); start to test
If normal, the antenna feed system path from point E to the antenna is OK; If
not, the interference comes from the antenna feed system path from point E
to the antenna, we can locate fault location in detail according to the tips from
the standing wave ratio test equipment. Positioning process ended.
Internal Interference locating(sharing
antenna)
Replace the current combiner with a good one.
Reconnect WCDMA & GSM base stations and the antenna feed system ( Point
A, B, C, D, E )
Open the GSM base station power amplifier.
Check if RTWP is back to normal:
If normal, combiner fault is concluded. Positioning process ends.
If not, the interference comes from outside of the system. Go to external
interference location process flow.
Auxiliary diversity receiver is simple receiver path, and has transmission signal.
So interference location is relatively simple. We can first investigate in the
antenna feed system failure, refer to the main set’s investigation. If the antenna
feed system is OK, the interference comes from outside of the system; Or a
detailed fault location is required according to the tips from the standing wave
ratio equipment tips.
In actual networking environment, connections of the antenna feed system
vary widely, and some design of the antenna feed system with sharing sites is
very complicated. The alignment progress for internal interference is to
investigate the signal source first, and then the antenna feed system
equipment and the connection quality. If no interference is found in the first 2
steps, the problem is caused by external interference.
Internal Interference locating(sharing
antenna)
If certain nodes in the antenna feed system path have poor connection or failure, the
amplifier‘s output signals through the antenna feed system produce intermodulation
with other system’s signal while sharing sites. The intermodulation products fall into the
WCDMA receive band and cause interference. This is the main reason for internal
interference and can be resolved by improving the antenna connection, replace the
device or change the system frequency configuration.
If certain nodes in the antenna feed system path have poor connection or failure, the
other system’s (e.g. DCS system) output signals through the antenna feed system
produce intermodulation while sharing the antenna feed system with WCDMA. The
intermodulation products fall into the WCDMA receive band and cause interference.
This is the main reason for internal interference and can be resolved by improving the
antenna connection, replace the device or change the system frequency configuration.
If DCS1800M system is combined with the WCDMA path, we need to confirm its
frequency configuration with operators, check if three-order intermodulation (2f1-f2,
2f2-f1) of DCS frequency has fallen into the WCDMA receive band (1920M ~ 1980M).
If yes, we need to communicate with the operator, propose operators to refine the
unreasonable frequency configuration.
After the above three steps of investigation, if interference still exists, we need to the
go to the scene for further processing:
Start LMT of Node B, test the RTWP of the alignment cell, so we can observe the
RTWP change status in the follow-up alignment investigation.
Internal Interference locating(sharing
antenna)
If there is DCS combined with WCDMA, we need to check the carrier of DCS in detail
(what carrier there is on each channel, its frequency, BCCH in which channel) and mark
the channel of BCCH.
If there is DCS combined with WCDAM, according to the results of interference , we
need to propose operators to change BCCH to the channel to be located for the
interference (GSM may have all launched in both channels, but only on one channel
BCCH is launched ). If we don’t do so, as the interference is related to the DCS traffic,
there is no guarantee that the interference is reproduced when on-site location.
In the channel one by one tap every RF connector ( especially jumper connectors, load,
as well as jumpers and antenna connector ), check RTWP value, if RTWP changes
correspondingly, larger or less, there is problem with the connector, we need to have
operators to conduct joint projects with the relevant personnel fastening, redo the work
of quality improvement and other projects, please note to close off the power amplifier
of the cell in order to avoid radiation damage before carrying out engineering work.
If the combiner is OK but the interference exists, we can use YBT250, filter and
directional antenna to check if there is interference at WCDMA antenna ( Refer to
"electromagnetic interference testing guide book“ for filters and directional antenna
device requirements. Under special circumstances, filters need to be customized in
accordance with local WCDMA receive bands and other wireless systems’ emission-
band), If interference is not observed by YBT250, change the base station antenna to
make sure if it is caused by the antenna. If it still exists after changing the antenna, go
to the "interference type found" session.
Internal Interference locating(sharing
antenna)
If interference signal is received using YBT250, filter and directional
antenna, go to "external interference location" session.
If interference alignment is still failed after investigations, go to
“interference type found" session. On-site position progress ends and
restore the original settings of the base station.
All of the above position steps will be recorded in detail and outputted
as “XX interference location detailed record“. If the problem has been
positioned, besides “XX interference location detailed record“, collate
relevant information on cases and send to the headquarters staff for
archiving. Otherwise, send “XX interference location detailed record"
to the headquarters support specialists for further help.
External Interference Locating Process
External Interference
Investigation
Data Acquisition
Data Analysis
End
External Interference Locating Process
Data needed for external interference locating are:
24-hour RTWP data for one week ( 3 days at least ) of the
target cell and its surrounding cells
MapInfo for the site distribution, the relative position and
distance information of sites
cell antenna azimuth and antenna height;
Survey photo of all sites;
If the cell to be located is provider of the repeater;
2G&3G repeater distribution around the cell to be located;
PHS&CDMA1.9G distribution around the cell to be located;
structure chart of antenna feed system of the cell to be located;
External Interference Locating Process
Analyze the long-time and short-term characteristics of RTWP of the cell to
be located in different time, so as to locate the interference when it is mostly
active.
Analysis:
the long-time and short-term characteristics of RTWP of cells around the cell
to be located at the same time;
According to the cell distribution map, take environmental analysis on the
cell to be located using survey photo and such resources
Analyze the relevance of main set and diversity of the cell to be located,
according to the antenna’s structure chart
Integrate RTWP value of the cell to be located and around cells using AOA
method, forecast the interference source with antenna direction and height;
AOA Method
According to multiple
base station cells’
antenna, determine the
direction of interference
source in relation to
current cell. Draw on a
map, the intersection of
the antenna direction
shall be the location of
interference sources.
External Interference Locating Process
On-site investigation and testing
Check the actual situation around the antenna, whether there is metal
shielding or reflective objects;
Check the antenna and antenna distribution of other operators;
Check carefully if there is objects of suspicion, such as PHS base stations,
CDMA1.9G base station, repeater, microwave transmission, etc.;
Use YBT250, filter and antenna to test interference strength, direction and
spectral properties;
Test and analyze the general location of the interference source;
Determine the suspected interference source;
External Interference Location Process
After investigated the source of interference, you need to verify. Common
methods are as follows:
When the suspected interference source is a block of metal objects or
other reflectors, you can remove it or adjust the antenna position to
verify.
When the suspected interference source is an active device (such as
other systems’ base station, repeaters, microwave transmission and
other electronic equipment), you can turn on and off the device at a
proper time to verify.
Observe if RTWP value changed before and after the operations. If
RTWP value is back to normal, the interference source is found. If
RTWP value has no change or little change, we need to continue
investigation.
External Interference Location Process
When the interference source is confirmed, we should timely make a
record of it (including its spectrum intensity, spectrum information,
GPS information, interference source entity’s photo), and positioning
steps in detail, then output the interference test report.
External Interference Location Process
The so-called single site test is referring to the direction of interference,
starting from the interference area to the mostly interfered position, and
eventually found the interference source.
First, we pick up a test point with interference. Perform 360-degree test with
Yagi antenna to find out the most interfered direction, and then continue
along this direction to find the interference source. Here we can make use
of binoculars, compass and also other equipment.
This method is used when facing towards the interference sources.
However, it is a little bit of luck, provided that the suspicious interference
source is obviously on the direction of the strong interference. Otherwise,
we need to use the three-point position analysis.
External Interference Locating Process
three-point locating is the most commonly used methods. The so-
called three-point locating is testing 3 places using the spectrum
analyzer with directional antennas, according to cross point of the
interference source direction, approach from the multi-direction
gradually to the most strongly interfered location, and eventually find
out the interference source.
External Interference locating Process
We first locate a large area interfered, and then find point A where is
interfered seriously, conduct tests to determine the direction of the strongest
interference source.
Along the strongest interference source direction, find the second point,
point B. For point B, the interference is stronger on the same direction with
point A than the opposite. So the interference source is basically right
between point A and B.
we also need to pick up another point, which is point C. Point C should be
located where point A and B are non-collinearity, we can select according to
the local topography, and not in the area where the 2 points are collinearity,
as if they are collinearity, there is no cross point to be selected for testing.
Then sweep in the area of point C. The test area is within point C and point
A&B’s crossing angle. The interference source is located within the cross-
section of the first 2 points and where the interference is mostly strong.
Finally, in cross-regional take on-site investigation to determine the
interference source.
PHS Interference Source Determination
China Unicom‘s WCDMA network, is subject to external interference with the
greatest suspicion of PHS (except cities with CDMA1.9G network). As PHS
base stations are distributed intensively, it is hard to determine quickly and
accurately which PHS base station leads to interference with the spectrum
scanning. With the help of PHS field strength tester, we can learn quickly
which PHS base station cause interference.
Take an example of PHS35C, the test result below shows the signal field
strength (dBuV) from each PHS base station;
PHS Interference Source Determination
When the measured signal strength of PHS base stations is below
65dBuV, PHS base stations won’t interfere WCDMA base stations;
When there are more than one measured signal strength of PHS base
stations are above 65dBuV, PHS base stations will interfere WCDMA
base stations;
When the measured signal field strength of PHS base stations is above
75dBuV, PHS base stations severely interfere WCDMA base stations.
Summary
To remove the interference caused by internal system, common ways are
as follows:
Check the antenna device. Tighten the loose connectors, change the
broken or aging devices, reduce the curvature of feeder lines that
bend too large, and replace the devices of low performance with high
ones.
Modify the frequency. In a share antenna system, as one 2G cell may
use multiple frequencies, which cause the inter-modulation
interference. When the inter-modulation interference is found out as
the improper setting of the frequencies, we can consult to modify the
2G frequencies.
If the problem is caused by the base station equipment itself, then
replace the components of corresponding base station.
Summary
For the external interference, common solutions are as follows:
Adjust WCDMA system antenna’s position, azimuth angle, tilt and height to have the
antenna deviated from the interference source, and to increase the space isolation
between WCDMA system and the interference source.
If it is confirmed that the surrounding metal objects has led to strong inter-modulation
products, we need to remove these objects. However if these objects can not be moved,
we can move the antenna to see if the problem is worked out.
Modify the frequency to avoid the third-order products fall within the WCDMA receive
band. As the frequency of the external signal is generally not changeable, So we can only
change the system frequency, such as sharing sites. And when the current operator's 2G
frequency generates the inter-modulation with other operators, which falls on the current
operator’s 3G-spectrum, we can modify the current operator's 2G frequency. Of course, it
is troublesome when changing the 2G frequency.
Add the filter. When the interference source is PHS base stations or CDMA1.9G base
stations, The interference to WCDMA base stations are mainly spurious interference. We
need to install the filters on the interference source, consulting the relevant operators.
When necessary, we can ask for mediation from a local non-commissioned organization.
Modify the parameters of the interference source device. When the interference source is
a repeater or power amplifier devices, we can modify its uplink and downlink gain to
improve the self-excitation, reduce the interference to donor and the surrounding base
stations.
Turn off the source of interference.
Use a special algorithm of Node B receiver, e.g. when the uplink receiver frequency
bands are interfered one or several fixed frequency points, and the interference is very
narrow and strong resulting in poor receiver signal to noise ratio, we can use a special
algorithm to remove the interfered frequency points of the 5M band, and to restore the
signal with the remaining energy.