interference.ppt

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


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


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


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


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


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



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.



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


Sources

Radar

Big problem near harbors

Industrial

Lumber dryers, welders

Illegal Transmissions

Common in developing world ( Cordless Phones, etc)



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.



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



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.



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


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


Documents

interference.ppt