3 rf interference

8/13/2019 3 RF Interference

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15/7/05 1Company Confidential

Interference in GSM systems are classified into

three major categories .

Co-Channel Interference

Non-Co-Channel Interference

Other Interference

Interference


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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 interference


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Frequency reuse implies that in a given coverage area there

are several cells that use the same set of frequencies.These

cells are called co-channel cells,and the interference between

signals from these cells is called co-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 along period, since Mobiles always have bursted transmission.

So interference will be high during peak traffic hours.



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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"



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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 Interference


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Interference resulting from signals which are adjacent in frequency to

the desired signal is called adjacent channel interference. Adjacent

channel interference results from imperfect receiver filters which allow

nearby frequencies to leak into the pass-band.The problem can be

particularly serious if an adjacent channel user is transmitting in very

close range to a subscribers receiver,while the receiver attempts to

receive a base station on the desired channel.

Adjacent channel interference can be minimized through careful

filtering and channel assignments.By keeping the frequency separation

between each channel in a given cell as large as possible, the adjacent

channel interference may be reduced considerably.Thus instead ofassigning channels which form a continuos band of frequencies within a

particular cell,channels are allocated such that the frequency separation

between channels in a given cell is maximized.


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C/ Ia1 = - 9 db

C/ Ia2 = - 41 db

C/ 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



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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.



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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 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 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 isnot a continuos process.

But malfunctioning of a batch or a particular model can pose

serious issues.

Other Interference


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ARFCN 1

C/Ia1 = - 9 db

C/I = 9 db

ARFCN 2

ARFCN 4

ARFCN 3

43 dbm

200 KHz : 20 dbm

39 dbm

39 dbm

A

B

C

D

BTS broken Transmitter

Other Interference


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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 wellbecome quite serious to other Operator BTS cell in the same

area.

Other Interference


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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


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TRX f1 CO

MBTRX 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 Interference

3 rf interference

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