Part7 GSM Interference Analysis and Optimization

GSM Interference Analysis and Optimization NPO Refresher Course July, 1st to 3rd 2010 Vodafone MS – RoB

Jignesh Parmar [email protected] Nokia Siemens Networks National NPO, Ahmedabad, India NSN Internal Document 1 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

What is interference? • Unwanted signal to the receiver – Internal System Interference
Resulting from tight frequency re-use
Multipath
Hardware Fault
Spurious transmission
Repeater malfunction – External Interference
Radar
Other Communication system
Jammer

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Classification of Interference Sources • Natural Noise – Atmosphere noise – Galaxy noise – Solar noise (quiet period) • Man-made Noise – Interference of ignition systems of vehicles or other engines – Interference of electronic communication system – Interference of power line – Interference of scientific research, medical and household appliances

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Main Interference Sources Affecting Mobile Communication • Internal interference – TRX fault: if the performance of TRX is reduced due to manufacture –

– – –

cause or application, which will cause self-excitation of TRX amplification circuit resulting in interference. Spurious emission and inter-modulation: if out-band spurious emission index of BTS TRX or amplifier exceeds the limit, or isolation between TX and RX antennas is not enough, all these will form interference on the receiving channel. Inter-modulation may also occur in passive equipment such as passive antenna and feeder. Improper frequency planning Co-frequency interference Adjacent frequency interference

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Main Interference Sources Affecting Mobile Communication • Repeater interference – If the installation of repeater is non standard, causing insufficient isolation between the donor antenna and the serving antenna, so selfexcitation is caused. This affects normal working of BTS that the repeater belongs to. – For the repeater adopting wideband non-linear amplifier, the intermodulation index far exceeds the requirement of the protocol. If the power is comparatively high, the inter-modulation component will be large, and this will cause interference to the BTS around. – If the repeater is malfunctioning due to some fault it will go in oscillation mode results in interference.

NSN Internal Document 5 © Nokia Siemens Networks

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Main Interference Sources Affecting Mobile Communication • External interference – Interference of other communication equipment with high power. – Radar station: From 70s~80s of the 20th century, the frequency used by the decimeter wave radar was similar to that of GSM, and its transmitting power was very high, which generally reached tens and hundreds of kilowatts, so the out-band spurious emission is comparatively large. Thus, it easily causes interference to the BTS. – Analog BTS: The frequency band used by the analog mobile BTS overlaps with the GSM frequency band in certain segment. – Communication equipment at same frequency band: As the types of communication equipment are so many, some manufacturers maybe adopt the frequency band but doesn’t comply with the current communication standards. As the GSM frequency band is occupied by the equipments, interference will be caused within the GSM system coverage area.

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Impact of Interference • When there is interference in the network, the subscribers usually encounter the following phenomenon:

– The subscriber cannot hear the voice , and the background noise is

too loud. – When fixed telephone subscriber calls MS subscriber, or MS subscriber calls fixed telephone subscriber, call drop occurs after “du, du , du” is heard. – The conversation cannot be carried on smoothly, and call drop often happens. – When interference exists in the network, from the result of traffic statistic, there are some characteristic as following.
There are Level 4~Level 5 interference band in TCH measurement function ,






and the measurement value is more than 1. Congestion rate is comparatively high. The call drop rate is higher than other cells. The handover success rate is low. Through Drive Test, it is found that: • It is difficult to handover . • The Rx level is high, but the quality is bad. Through tracing the Abis interface signaling with signaling analyzer, it is found that the bit error rate is higher than other cells.

NSN Internal Document 7 © Nokia Siemens Networks

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Carrier to Interference Ratio Interference and reuse/reuse distance

Cellular network

3

• partial overlap of cells

4 2

• only a few frequencies per cell • frequency re-use distance

1

5

7

6

3 4

re-use distance

2 1 7

5 6

NSN Internal Document 8 © Nokia Siemens Networks

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Receiver Sensitivity Level impacts Interference & Noise Level [dBm ]

20

0 20

-104 dBm min. sensitivity GSM (BTS)

-97 dBm min. sensitivity GSM (BTS)

normal BTS

-88 dBm min. sensitivity GSM (BTS)

micro BTS

pico BTS

-40

-60

-80

C/N

-100 C/N

NSN Internal Document 9 © Nokia Siemens Networks

C/N

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The necessary CNR increases Bandwidth

Carrier to Interference Ratio Requirements Following GSM 05.05:

Reference interference ratio for all BTS and MS types: • For co-channel interference:

C/Ic =

9 dB

• For (first) adjacent channel interference:

C/Ia1 = - 9 dB

• For (second) adjacent channel interference:

C/Ia2 = - 41 dB

• For (third) adjacent channel interference:

C/Ia3 = -49 dB

At these values, the so called reference interference performance in terms of (maximum) frame erasure rate, bit error rate or residual bit error rate must be met for the different type of channels in different specified propagation conditions (TU3 no FH, TU3 ideal FH, TU50 no FH, TU50 ideal FH, RA250 no FH).

NSN Internal Document 10 © Nokia Siemens Networks

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Carrier to Interference Ratio Requirements Co-channel interference: 9 dB Quality (Co Ch related on reuse distance; cs-service)

20

0

V LE Rx

20

Rx LE V

Leve l [dBm ]

-40

+ 9 dB

CIR CoCH

-60

-80

Neighbour cell -100 Distance

Server cell NSN Internal Document 11 © Nokia Siemens Networks

Distance

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Adjacent Channel Interference Adjacent channel related quality (CS)

Quality limit of serving cell

-20

ur ed ea s

Rx LE V

f

2

f

m

1

V LE Rx

0

Rx LE V

f

at

2

f1

Level [dBm] 20

- 9 dB CIR AdCH

-40

Adjacent channel Interference BTS and MS

CIR CoCH + 9 dB 60 80 100

Distance

Serving cell Distance

NSN Internal Document 12 © Nokia Siemens Networks

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Neighbour cell

Adjacent Channel Interference Adjacent Ch. Interference; cs -service PWR Level [dBm ]

CIR AdCH

10

+ 9 dB

0

21 dB 20

-40

-60

-80

-100

200 kHz F1 NSN Internal Document 13 © Nokia Siemens Networks

200 kHz F2

Frequency range

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

30 dB

Carrier to Interference Ratio : Exercise • Carrier = - 79 dBm • Interference = - 70 dBm • Carrier to Interference Ratio (C/I) = - 9 dB • Interfering signal is 9 dB higher than Wanted Signal • Carrier = - 80 dBm • Interference = - 104 dBm • Carrier to Interference Ratio (C/I) = 24 dB • Interfering signal is 24 dB lower than Wanted Signal

NSN Internal Document 14 © Nokia Siemens Networks

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Adjacent Channel Interference 200 KHz

9 dB

Surface below black curve from fC - 100 KHz to fC + 100 KHz -> serving power

30 dB

Surface below red curve from fC - 100 KHz to fC + 100 KHz -> interfering adjacent power

fC - 100 KHz

NSN Internal Document 15 © Nokia Siemens Networks

fC + 100 KHz

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Must be at least 9dB smaller than surface below black curve

Locating Interference

NSN Internal Document 16 © Nokia Siemens Networks

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Discovering Interference via OMC Traffic Measurement Discovering potential interference via traffic statistics result • Check the “Average TCH busy time (second)” in TCH measurement function of each cell, the reason is that this index can show the TCH mean occupied time (s”), which is usually called “TCH mean holding time” in the BSC of other manufacturer”, within the measurement period. If it is found that the Average TCH busy time (second) of certain cell is comparatively short (such as less than 10s), then maybe there is strong interference in the cell, causing that handover/call drop happens due to bad quality after TCH channel occupied to MS. • Certainly, if hardware fault occurs in certain TRX (non-BCCH or non-SDCCH carrier) of a cell, the case mentioned above will also appear. NSN Internal Document 17 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Quality Interference analysis DL /UL Rx Quality x Rx Level

Same level – quality distribution for both UL and DL HW Problem: Bad Quality

Good Quality

for all Rx Levels Coverage Problem: Bad quality and Low Rx Level

HW Problem All samples below 100dBm CL10  Measurements of the SACCH NSN Internal Document 62 © Nokia Siemens Networks

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DL - C/I and Interference Matrix (BCCH) • Detection of interferers: The RX level and RX quality measurements can be performed on per TRX basis. Non hopping network -> with affected TRX single affected frequency recognized Hopping network -> affected MA list recognized only Often there are several nearby cells using the affected frequency or one of the frequencies of the affected MA list • Manual search for interferers: The potentially interfering base stations have to be switched off temporarily - Very time consuming. Especially in hopping networks hardly possible to find out the interferers • Search for interferers on basis of traffic: If the interference level increases with the traffic in a potentially interfering cell, than very probably this is indeed an interfering source

NSN Internal Document 63 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

UL Receive Level of Unused Time Slots Measurement of uplink receive level on idle channels = uplink interference

Averaging over interferenceAveragingProcess (AP) = 1..32 SACCH periods Classification into interference bands based on interferenceAveragingProcess (BO1..BO4) = -110..-47 dBm

BSC tries to allocate TCH from best interference band (can be requested by MSC) If not available, BSC tries to take TCH from next band

BO5 –47 (fixed) rxLevUL = -75 dBm BO4 -90 BO3 -95 BO2 -100 BO1 -105 BO0 –110 (fixed) NSN Internal Document 64 © Nokia Siemens Networks

0 1 2 3 4 5 6 7

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Interference • Detection of interference: •UL and DL RX level and RX quality statistics for busy TCH: For a RX level guaranteeing a stable connection (for outdoor higher than about -90 dBm) most of the RX quality measurements should indicate either quality 0 or 1 It is an indicator for interference if speech quality is bad and level high. • UL RX level of un-used TCH: The BTS can measure the RX level of unused time slots. Any signal detected there indicates interference directly.

NSN Internal Document 65 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Tests

NSN Internal Document 66 © Nokia Siemens Networks

Example drive test

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Tests Example drive test

NSN Internal Document 67 © Nokia Siemens Networks

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Drive Test - Exercises Case study • Cell 11 is...

Cell 11

showing high drop rate in NMS statistics. Customer complaints point to a possible problem area around a particular road crossing.

(high call drop rate)

Cell 11 • Cell 11 is ... covering part of a suburban city area including a road crossing. There are other cells in the area (see sketch), but Cell 11 is dominant in and around the road crossing.

NSN Internal Document 68 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

(dominant in road crossing)

Drive Test - Exercises Information High traffic ?

• No frequency assignment problems on neighboring cells • Drive tests confirm that the road crossing is causing drops on very many calls • Cell 11 has signal level around –85dBm at the road crossing • Downlink RXQUAL values are showing worse quality than other cells in the area • Uplink RXQUAL is showing same characteristic as other cells in the area (no problem)

- 85 dBm

High interference (fading) ?

NSN Internal Document 69 © Nokia Siemens Networks

Cell 11 (dominant in road crossing)

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Call drops

Drive Test - Exercises

Cause of the problem Problem is caused by a competitor site at the building in the upper right side of the road crossing. The competitor cell provides around –35 dBm at the road crossing, which creates signal differences in "our" network of around 50dB between the serving level and the interfering level.

Competitor Cell - 35 dBm

Cell 11 (dominant in road crossing) - 85 dBm

NSN Internal Document 70 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Test - Exercises

Solution The only solution is to provide a higher signal level at the road crossing, so that the signal difference gets lower than the current 50 dB.

Adding New Site - 40 dBm

- 35 dBm

• The probable solution is to add a new site close to the road crossing. • Plus frequency reallocations

Cell 11 (dominant in road crossing) - 85 dBm

NSN Internal Document 71 © Nokia Siemens Networks

Competitor Cell

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Test - Exercises 10

GSM specs requires that a mobile must overcome this kind of interference when the signal difference is around 40dB even if the serving level is close to the sensitivity level of the mobile. There is generally no exact specifications for this kind of interference, but it is commonly known, that this phenomenon is happening. Note: The competitor channel is not adjacent to the serving channel, it might be separated by several MHz!

0

Rx Level [dBm]

20 -35 dBm -40

-60

-80

-100

CIR (competitor server) max. 40 dB allowed

-85 dBm

Cell 11 (dominant in road crossing) 200 kHz F1

NSN Internal Document 72 © Nokia Siemens Networks

Competitor „ channel“

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

200 kHz Fn

Frequency range

Drive Test - Exercises

New BTS

A new BTS with a 3+2+3 configuration has been implemented in the network during night-time. The implementation engineer standing next to the BTS has made successful test calls on all timeslots of all TRX's.Using a test mobile a short drive test has been done at approx. 500m from the site and the following verifications have been done :

· · ·

The directions of all 3 sectors by looking at the BCCH frequencies The functionality of the handover relations between the sectors The uplink cabling of each TRX by making one outdoor test call on each TRX

The implementation engineer parks his car to make some phone calls to arrange the rest of his work. The first call is successful. Another call is also successful. The 3rd call he has to make fails during the setup. Again he makes some phone calls to check the new site and get's the following results : 1. OK 2. OK 3. NOK 4. OK 5. OK 6. NOK 7. OK All the time he is standing in the area between sector 1 and 2 and making phone calls on sector 1. NSN Internal Document 73 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Test - Exercises Description of the actions • The BSS Technician went on site for checking the hardware, and the operator's OMC controller checked the alarms. Nothing was found. The frequency plan was checked, it was clear that too many frequencies were involved here and that interference can't easily be avoided. Different parameter fine tuning trials were implemented, but they didn't improve anything. Base band hopping was deactivated for some hours to have statistics per TRX. But nothing came out of this test. • Some frequencies were swapped and changed, but no real improvement was seen. The problem should come from an external source or from a problem with the hardware itself (intermodulation problem was proposed as it was seen on other cells already). • The operator went back on site to measure the UL interference, and concluded that UL interference is coming from MS of other cells. It was reported that the MS were coming from the back.

NSN Internal Document 74 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Drive Test - Exercises Cause of problem It turned out that the side and back lobes from the antenna were picking-up UL signals from MSs on cells behind

Solution Change the antenna

• •

Replace the antenna with one which has less back and side lobes Move the antenna on the roof, so that the building is a screen for the MS on the cells behind and the back/side lobes. The MSs operating on the cells behind (co-channels) are not seen by the antenna's back/side lobes anymore

Solution 1

Move the sector Solution 2 NSN Internal Document 75 © Nokia Siemens Networks

GSM Interference Analysis/JP/NNPO/ 1st to 3rd July 2010 @ VF RoB

Thank You

NSN Internal Document 76 © Nokia Siemens Networks

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