Post on 12-Apr-2015
description
Company Confidential 10/4/99 1
Interference in GSM systems are classified into three major categories .
Co-Channel Interference
Non-Co-Channel Interference
Other Interference
InterferenceInterference
Company Confidential 10/4/99 2
Interference on a channel caused by another cell/mobile using the same frequency.
C/Ic is the measure of co-channel interference GSM specifies the C/Ic threshold of 9dB for a service
quality of 0.4% BER on Type II bits. 9db also includes 2db implementation margin 9db is decided considering the implementation of SFH Without SFH, the preferred threshold is 12 dB.
C / Ic 9 db
CIc
Co - Channel interferenceCo - Channel interference
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Co-Channel Interference can be Downlink as well as Uplink
Downlink If caused by BCH carrier, will be present always. Non_BCH carrier interference will be traffic dependent. During peak traffic hours, the interference will be high.
Uplink Interference
Uplink Co-Channel Interference , will never be continuos for a long period, since Mobiles always have bursted transmission.
So interference will be high during peak traffic hours.
Co-Channel InterferenceCo-Channel Interference
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Causes Distant Cells due to tight frequency re-use patterns. Distant Cells due to errors in frequency planning. Mulitpath from Distant cells( strong reflector, Water). C/Ic will degrade the Ec/No, so if Noise floor itself is high, then even
a high value of C/Ic can deteriorate quality.
C Ic
Noise
ARFCN "N" ARFCN "N"
Co-Channel InterferenceCo-Channel Interference
Company Confidential 10/4/99 5
Interference caused when wanted and unwanted GSM RF channels co-exist.
C
Ia
GSM receivers are designed for an Adjacent Channel Suppression of minimum 18db at an offset of 200 Khz, 50db at 400 Khz and 58db at 600 Khz.
Adjacent Channel InterferenceAdjacent Channel Interference
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C/ Ia1 = - 9 dbC/ Ia2 = - 41 dbC/ Ia3 = - 49 db
Thresholds
ACS = 18db & C/c = 9db This means if Ia is 9db above C, then with 18db ACS, it equals C/Ic.
N N+1N-1
9db
N+2N-2
41 db
49 db
N-3 N-3
Adjacent Channel InterferenceAdjacent Channel Interference
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Causes
Adjacent ARFCN's in same cells Adjacent ARFCN's in adjacent cells Distant Cells due to tight frequency re-use patterns. Distant Cells due to errors in frequency planning. Mulitpath from Distant cells( strong reflector, Water). Improper Receiver filters ( low ACS ) C/Ia will degrade the Ec/No, so if Noise floor itself is high, then
even a low value of C/Ia can deteriorate quality.
Adjacent Channel InterferenceAdjacent Channel Interference
Company Confidential 10/4/99 8
Interference coming on a GSM signal from an undesired source, i.e neither a co/adj channel cell or MS.
Malfunctioning or Maladjusted Transmitters Base station malfunction, rogue mobile
Paging, broadcast, etc. Intermodulation Products
Strong signals in adjacent channels Harmonics from Other Bands
Sources
Other InterferenceOther Interference
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Sources
Radar
Big problem near harbors
Industrial
Lumber dryers, welders
Illegal Transmissions
Common in developing world ( Cordless Phones, etc)
Other InterferenceOther Interference
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Malfunctioning or Maladjusted Transmitters
GSM has ARFCN of 200 KHz bandwidth
0.3 GMSK gross modulation bandwidth is 270 KHz
This means there is some spill of energy into adjacent channel
This leakage is 30 dbc for the 1st adjacent channel*
If this spill is not within specs, then this will result into co-channel interference to other cell using this adjacent ARFCN ( next page ).
Broken Transmitters may also cause similar problems , but the effect will only be present , when the Mobile goes on call, which is not a continuos process.
But malfunctioning of a batch or a particular model can pose serious issues.
Other InterferenceOther Interference
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ARFCN 1
C/Ia1 = - 9 dbC/I = 9 db
ARFCN 2
ARFCN 4
ARFCN 3
43 dbm200 KHz : 20 dbm
39 dbm
39 dbm
A
B
C
D
BTS broken Transmitter
Other InterferenceOther Interference
Company Confidential 10/4/99 12
Spurious Emissions
GSM has a spec of -36dbm for generation of spurs
High level spurs when generated from the transmitter will result into interference.
The interference may not that severe to the cells of the same network since the level of spurs would be quite low and propagation loss will be large
Spurs generated by a BTS of one Operator can very well become quite serious to other Operator BTS cell in the same area.
Other InterferenceOther Interference
Company Confidential 10/4/99 13
Spurious Emissions
Operator "A" Operator "B"
925 - 940 MHz940 - 955MHz
Spur of -20 dbm at 942.0 MHz
ARFCN 35( 942.0 MHz )
Other Interference Other Interference
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TRX f1 COMBTRX f2
f1 f22f1-f2 2f2-f1
Intra-cell Intermodulation
Non-linearities in the combiner can result into intermods Intermods as GSM spec should be higher than -70dbc or -36dBm
Other InterferenceOther Interference