UMTS Interference Investigation ZTE University
Content
Internal interference External interference Interference Investigation Process
Interference Classification
UMTS is an interference limited system, network quality, capacity and coverage are associated with the background noise. Internal interference
Internal interferencenormally defined as interference generated by faulty equipment. And the equipment is located between RF TX port and antenna.
External interference
external sources of interference. Interaction between the external system.
Internal interference
The antenna connector is screwed too tight or too loose. The antenna feeder is bent too much, and the load connections are loose, etc. Antenna, feeder line, joint and load quality problems caused by the device itself, such as feeder line damaged, antenna isolation is worsen and so on. There is a small part for the base station caused itself. Power amplifier is spurious, inter-modulation indicators worsen and duplexer is lack of isolation etc.
Inter-modulation Linear Dynamic Range Linear Component S IN
P OUT (dBm)
a
S OUT
S OUT = a S IN
Linear Dynamic Range
linear range
non-linear ideal
3 dB
real
1 dB
Noise Floor C1 dB C3 dB
P IN (dBm)
Inter-modulation
Products Generation S IN
S OUT
Non-Linear Component
S OUT = a S IN + b S2IN + g S3IN +...
Two Tone Inter-modulation Products S IN = cos(F1) + cos(F2) a S IN = a cos(F1) + a cos(F2)
LINEAR
b S2IN = b cos2(F1) + b cos2(F2) + 2b cos(F1).cos(F2)
S OUT =
cos(2F1)
cos(2F2)
2ND ORDER
cos(F1 ± F2)
g S3IN = g cos3(F1) + g cos3(F2) + 3 g cos2(F1)cos(F2) + 3 g cos(F1)cos2(F2) cos(3F1)
cos(3F2)
cos(2F1 ± F2)
cos(2F2 ± F1)
3RD ORDER
Inter-modulation Third Order Inter-modulation Products
F1 F2
P OUT (dBm)
2F1 + F2 2F2 + F1
F1 + F2
2F1 -F2
2F2 -F1
Inter-modulation Products Spectrum F1 - F2
2F1
2F2
3F1
3F2
Third Order Intercept point
linear range
P LINEAR = P TOI
3rd order IM products
P IN (dBm)
Noise Floor IP3
Inter-modulation
GSM/UMTS Collocation Aspects GSM / UMTS Inter-modulation Analysis
• 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
The internal interference has following characters:
The main path and the diversity path RTWP are not relevant. But 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.
Content
Internal interference External interference Interference Investigation Process
External interference
An external source of interference may be the existing GSM systems, repeaters, mobile phone jammers, microwave transmission equipment and the illegal use of the UMTS 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.
External interference(BBU,IP switch etc)
In UMTS, RTWP requirement is -105dBm/5MHz on antenna connecter port. So antenna specially high gain antenna must keep enough distance between electric element (5m)
Source B8200+ switch
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
A large number of irregular burst, particularly short existing. fixed-cycle interference, the existence of time is typically a few seconds.
External interference(TV)
TV signal amplifier is very close to antenna. Interference normally is a wide band signal(1-5MHz). TV user has complain for bad TV signal.
Interference from Other System
The existing wireless communication systems (such as GSM, DCS, PHS, TD-SCDMA, CDMA2000, etc.) may interfere with UMTS system. Forms of interference between the systems are mainly spurious, blocking and inter-modulation.
Barrage Jamming
When the interference signal is too strong, it will block UMTS receiver, exceed the working scope of the amplifier and mixer, make the receiver can not demodulate, interfere the receiver. UMTS receiver blocking requirements for:
≤-40dBm (in-band) ≤-15dBm （out-of-band） ≤16dBm （GSM、DCS in-band）
Comparing the out-of-band interference signal amplitude by test and the UMTS receiver blocking indicators, we can determine whether there is barrage jamming.
Barrage Jamming
Operating Band
XIII
Center Frequency Interfering Wanted Minimum Offset Type of of Interfering Signal mean Signal mean Of Interfering Interfering Signal power power Signal Signal 880 - 915 MHz -40 dBm -115 dBm ±10 MHz UMTS signal * 860 - 880 MHz -40 dBm -115 dBm ±10 MHz UMTS signal * 915 - 925 MHz 1 MHz - 860 MHz -15 dBm -115 dBm CW carrier 925 MHz-12750 MHz NOTE *: RTWP normal value is about -106.5dBm.
Barrage Jamming
The out-of-band blocking signals are close to 0dBm, tested from the first stage LNA output of UMTS base station. And as the receiver’s band blocking indicator is -15 dBm, the big margin of the non-linear distortion signal ( >-50dBm ) generated in 1922.4 MHz frequency is caused by barrage jamming, while the link is already saturated.
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
Internal interference External interference Interference Investigation Process
Interfere Investigation Process High RTWP Power off PA RTWP OK?
Y
RRU replaced
N Check: RTWP value of last 10 days( all frequency neighbor cell) NW design plot Neighbor cell info, repeater info
Repeater exist? Y
N
Power off Repeater, Y RTWP is ok?
Repeater problem
N BBU & antenna in same room?
N
Y
Only F1 RTWP increase?
N
Y
BBU or some other element Nearby antenna
Interfere Investigation Process(continued) RTWP of main path and diversity path is high correlative
Y
N RTWP of Neighbor RRU Increase ?
Y
N RTWP increase for All N frequency
Y
Inter-modulation
Wide band interference
TV signal amplifier
Interference detect
Two ways commonly used to detect interference.
Using sweep signal generator is able to detect a certain frequency bandwidth. Observation network performance indicators in the "average RTWP" is the best way to discover the network uplink interference. Under normal circumstances, the cell is empty and the RTWP should be around -105dBm. If the average RTWP of some cells is up to about -95dBm, that is, 10dB more than the empty cell, then we can conclude that these cells have been interfered on uplink."The biggest RTWP" is recommended only as a reference when making judgments, it is also possible caused by the access to peak.
Interference detect
Interference Data Acquisition
Track the cell to be located a week (at least 3 days), 24 hours of its RTWP data. Track the cells (located adjacent to the cell) near the cell to be located a week (at least 3 days), 24 hours of their RTWP data.
Analysis of the Interference level
The interference must be determined berfore process
Interference between -105 to -95 dBm ====> mild level. between -95 to -85 dBm ====>moderate level. greater than -85 dBm ====> serious level.
The NW interference situation also need to be determined:
If all cells from large area are interfered strongly, the interference may be from outside source. If only some cell is interfered, it may be due to hardware problems caused by internal interference or interference from a smaller external interference.
Internal Interference (stand-alone antenna) Start Disconnect Point A Connect match load to UMTS base station Is RTWP back to normal?
No
UMTS base station equipment error
No
Antenna error from point A to antenna
Yes Connect jumper at point A to VSWR tester Is RTWP back to normal? Yes External Interference
Internal Interference Locating(sharing antenna) Start Check if it is caused by UMTS base station equipment
Disconnect Point A Connect match load to UMTS base station Is RTWP back to normal?
No
UMTS base station equipment error
Yes Reconnect point A Check if the interference is
Turn off the power amplifier of GSM base station Is RTWP back to normal?
caused by GSM output signals No
Internal Interference Locating(sharing antenna)
Check if it is caused the jumper from point C to D
jumper error from point C to D
No
Disconnect point C and D
Disconnect point A and B
Connect jumper of point C to VWSR tester
Connect jumper of point A to VWSR tester
Connect jumper of point D to match load
Connect jumper of point B to match load
Is RTWP back to normal?
Is RTWP back to normal? Yes
Disconnect point E Connect jumper of point E to VWSR tester
Check if it is caused the jumper from point A to B
No
jumper error from point A to B
Yes
Check if the interference is caused by GSM output signals
Internal Interference Locating(sharing antenna) Is RTWP back to normal?
No
antenna error from point E to the antenna
Yes Replace the combiner Check if it is caused the combiner
Reconnect point A, B, C and D Turn on the power amplifier of GSM base station
Is RTWP back to normal? Yes
combiner error
No
external interference
External Interference Locating Process External Interference Investigation Data Acquisition Data Analysis On-site Investigation and Test Interference Source Verification Interference Source Confirmation End
External Interference Locating Process
Data Acquisition
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. GSM&UMTS 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
Data 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 Locating Process
Interference Source Verification
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, 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 Locating Process
Interference Source Confirmation
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 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.
PHS Interference Source Determination
UMTS 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.
Summary
For the external interference, common solutions are as follows:
Adjust UMTS 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 UMTS 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 UMTS receive band. Modify the parameters of the interference source device, or add the filter.
