basic of gsm by pawan

Basic Of GSM

Which is needed

IN Drive Test

Written By PAWAN KUMAR

RF ENGINEER

Call Drop Reasons:

1) Interference2) Bad Rx level

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3) High VSWR4) Bad Rx Quality5) High BER6) Handover is not defined7) When RLT= 0 , call drop occurs

Handover Reasons:

1) Rx Level2) Rx Quality3) Interference4) Power budget5) Timing Advance

Handover Failure Reasons:

1) Handover is not defined2) Due to distance b/w two BTS3) Due to not availability of TCH4) Interference5) Overshoot6) Congestion7) Proper handover is not defined

Beam width & Tilt:

Antenna beam width & tilt improves the coverage of the planned area.

Beam width= 65 Dbi

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Reduce co-channel interference:

1) Proper frequency planning2) Proper reuse pattern3) Physical parameters like tilt ,orientation

EIRP= Effective isotropic radiated power

CCSR= Call complete success rate

GSM 900Band

Uplink= 890-915 MHz

Downlink= 935-960 MHz

GSM 1800Band

Uplink= 1710-1785 MHz

Downlink= 1805-1880 MHz

WCDMA Band

Uplink=1920-1980 MHz

Downlink=2110-2170 MHz

RX Level:

It is the received signal strength of serving cell.

Unit = dbm

Range= -0 to -120

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Best= -0 to -65

Good= -65 to -75

Avgas = -75 to -85

Bad= -85 to -120

RX Quality:

It is the quality of voice which is measured on the basis of BER.

Range: 0 to 7

Good = 0 to 4

R.Q BER

0 0to 0.2 %

1 0.2to 0.4 %

2 0.4 to 0.8 %

3 0.8to 1.6%

4 1.6to 3.2%

5 3.2to 6.4%

6 6.4to12.8%

7 greater than 12.8%

BER: Bit Error Rate

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It is the ratio of number of error bits to the total number of bits Transmitted on a given time interval.

RX Quality depends on BER.

FER: Frame Erasure Rate

It is the ratio of discarded frame to the total number of frames transmitted on a given time interval.

Range: 0 to 13 %

Best: 0

Good: 0 to 4 %

RL Time Out Counter:

This defines the maximum value of the radio link counter expressed in SACCH blocks.

Range= 4 to 64 in step size 4 when RLT = 0 , then call drop occurs.

SQI: Speech Quality Index

It is the quality of voice. It depends on BER & FER.

Range= -20 to 30

Good= 20 to 30

Interference:

It is the unwanted signal.

Two types of interference

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1) Co-channel Interference2) Adjacent channel Interference

Co-channel Interference:

Several cells use same set of frequency. These cells are called co-channel cells & interference produced on these cells is called co-channel cells.

Adjacent Channel Interference:

This is produced due to the adjacent cells.

+ 1 & - 1 Difference

C/I: carrier to interference ratio

It is the ratio of signal strength of serving cell to the ratio of signal strength of undesired cells.

Range: 9 to 30

Good = 15 to 25

For Hopping: 9 to 11

For Non Hopping: 12 to 30

BASIC: Base Station Identity Code

It is the combination of NCC (Network Color Code) & BCC (Base station color code).

NCC= 0to 7

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BCC= 0 to 7

Link Budget In GSM:

It is used for path balance b/w uplink & downlink.

Reduction of C/I:

1) Interference2) Proper handover is not defined3) Check Mechanical & Electrical Tilt4) Due to Power

Channels in GSM:

1) Physical Channel

2) Logical Channel

Logical Channel:

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1) Control Channel2) Traffic Channel

Control Channel:

1) Broadcast Control Channel2) Common Control Channel3) Dedicated Control Channel

Traffic Channel:

1) Half Rate2) Full Rate

BCCH (Broadcast Control Channel):

It is Downlink channel.

This channel contains system parameters needed to identify the network.

This parameter includes LAC (Location Area Code), MNC (Mobile Network code) & frequency of neighboring cells.

FCCH (Frequency Correction Channel):


This channel contains frequency correction bursts.

SCCH (Synchronization Channel):


This channel is used by the MS to learn the BSIC & as well as TDMA frame number.

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Cell Broadcast Channel:


It is used to broadcast specific information to n/w subscribers such as weather, traffic, sports, stocks.

CCCH (Common Control Channel):

1) Random Access channel2) Paging channel3) Access grant channel

RACH (Random Access Channel):

It is only Uplink channel.

This channel is used by MS to request an initial dedicated channel from the BTS. This would be the first transmission by MS to access the network.

AGCH (Access Grant Channel):


This channel is used by the BTS to notify the MS of the assignment of an initial signaling.

Dedicated Control Channel (DCCH):

1) SDCCH2) SACCH3) FACCH

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SDCCH (Stand Alone Dedicated Control Channel):

It is used as uplink & downlink channel. It is bi-directional channel.

This is signaling channel. It is used for call setup b/w the MS & BTS.

It is used for call setup, authentication, location update, SMS on idle mode.

SACCH (Slow Associated control Channel):


It is used to transfer signal when MS have ongoing conversation on traffic or while SDCCH is used.

It used for updating of RX level, RX Quality, BCCH.

FACCH(Fast Associated Control Channel):


It associates with TCH only.

It is used for handover.

Full Rate Traffic Channel:

Voice= 13kbps

Data= 9.6kbps

Half Rate Traffic Channel:

Voice= 6.8kbps

Data=4.8kbps

Frequency Hopping:

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Frequency hopping is the change of frequency on a particular time interval.

1) Improved coverage capacity.2) Reduce Interference.

Two Types:

1) Baseband hopping2) Synthesizer Hopping

Baseband Hopping:

Call is transferred from TRX to TRX.

In this method, there are several TRX & each have a fixed frequency within the hopping plan.No. of TRX = NO. Of frequency for hop.

Each TRX would be assigned a single time slot within a TDMA frame.

Synthesizer Hopping:

Call is transferred from time slot to time slot.

This requires to TRX itself to change frequency according to the hopping sequence.

So, One TRX would hop b/w multiple frequencies on the same sequence that the MS is required to.

Hopping Parameter:

Mobile Allocation:

Set of frequency in which mobile is allowed to hop.

HSN (Hopping Sequence Number):

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It determines the orders of hopping.

Range= 0 to 63

0= Cyclic Hopping

1= Pseudo Random Hopping

MAIO (Mobile Allocation Index Offset):

It is the number which tells from which frequency, frequency hopping has to be started from given MA list.

Swapping:

It means interchange.

Three types of swapping occur in drive test.

1) Sector Swapping 2) Cyclic Swapping3) Diversity Swapping

Sector Swapping:

BCCH of 1st sector occurs in 2nd sector & BCCH of 2nd sector occurs in 1st sector. Then is the sector swapping b/w 1st & 2nd sector.

Cyclic Swapping:

BCCH of all sectors are interchanged i.e

1-2

2-3

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3-1

Diversity Swapping:

It occurs due to interchange of TX/RX cable.

Handover:

Handover is a process in which call is transferred from one cell to another cell without disconnecting the call.

There are two types of handover.

1) Intra cell handover: B/W two sectors of same BTS.2) Inter cell handover: B/W two different BTS.

Tilt:

1) Mechanical Tilt2) Electrical Tilt

Mechanical Tilt:

Mechanical Tilt is used for horizontal direction. It is used to increase the coverage. There should be less mechanical tilt in the city; eg.-2, if we will give 2 MET then we will get good coverage.

If we will give 4 or 5 MET in the city than back lob will get produced which will affect the back sites. In the rural area MET is given to get coverage for a long distance.

Electrical Tilt:

Electrical tilt is used for vertical direction. It is used to spread the coverage. We can use 4 or 5 electrical tilt in the city.

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WCDMA OR 3G

Wide band code division multiple access

Frequency Band: 2100

UPLINK: 1920 to 1980 MHz

DOWNLINK: 2110 to 2170 MHz

Duplex Scheme:

Frequency division duplex (FDD)

Time division duplex (TDD)

Channel Spacing: 5 MHz

25 times the channel spacing of GSM (200 KHz)

Codes Used In WCDMA:

1) Channelization Code2) Scrambling Code

Channelization Code:

Downlink: Separates different users within a cell.

Uplink: Separates physical channel of one user.

Scrambling Code:

Downlink: Separate cells in same carrier frequency.

Uplink: Separate users.

The common pilot channel (CPICH) is broadcast from every cell. It carries no information & can be thought as a “Beacon” constantly transmitting the scrambling code of the cell.

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WCDMA cells are identifies by their SC.

It’s like a BCCH in GSM.

RSCP: Received signal code power.

It is the signal strength after spreading. It is the received power of a particular CPICH.

Range: -30 to -120

Good: -50 to -75

RSSI: Received signal strength indicator. It is the signal strength before spreading.

Range: -30 to -140

Good: -60 to -90

EC/Io: It is the ratio of energy per chip to the total noise spectral density.

Range: 0 to -13

Good: 0 to -7

Handover Types:

1) Softer Handover2) Soft Handover3) Hard Handover4) I- RATE Handover

Softer Handover: Handover occur b/w two sectors of same Node B (BTS)

Soft Handover: Handover occur b/w two different Node B (BTS).

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Hard Handover: Handover occur b/w different RNC.

I-RAT Handover: Handover occur b/w two different band, or handover occur b/w 3G to GSM.

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basic of gsm by pawan

Engineering