Basic Of GSM Which is needed IN Drive Test
Written By GURU , 9473656410 8 1
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Call Drop Reasons: 1) Interference 2) Bad Rx level 3) High VSWR 4) Bad Rx Quality 5) High BER 6) Handover is not defined 7) When RLT= 0 , call drop occurs
Handover Reasons: 1)Rx Level 2)Rx Quality 3)Interference 4)Power budget 5)Timing Advance
Handover Failure Reasons: 1) 2) 3) 4) 5) 6) 7)
Handover is not defined Due to distance b/w two BTS Due to not availability of TCH Interference Overshoot Congestion Proper handover is not defined
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Beam width & Tilt: Antenna beam width & tilt improves the coverage of the planned area. Beam width= 65 Dbi
Reduce co-channel interference: 1) Proper frequency planning 2) Proper reuse pattern 3) 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: 3
It is the received signal strength of serving cell. Unit = dbm Range= -0 to -120 Good= -50 to -75 Avgas = Bad=
-75 to -90 -90 to -105
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%
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6.4to12.8%
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greater than
12.8% 4
BER: Bit Error Rate 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 5
Interference: It is the unwanted signal. Two types of interference 1) Co-channel Interference 2) 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
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BASIC: Base Station Identity Code It is the combination of NCC (Network Color Code) & BCC (Base station color code). NCC= 0to 7 BCC= 0 to 7
Link Budget In GSM: It is used for path balance b/w uplink & downlink.
Reduction of C/I: 1) Interference 2) Proper handover is not defined 3) Check Mechanical & Electrical Tilt 4) Due to Power
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Channels in GSM: 1) 2)
Physical Channel Logical Channel
Logical Channel: 1) Control Channel 2) Traffic Channel
Control Channel: 1) Broadcast Control Channel 2) Common Control Channel 3) Dedicated Control Channel
Traffic Channel: 1) Half Rate 2) 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): It is Downlink channel. This channel contains frequency correction bursts. 8
SCCH (Synchronization Channel): It is Downlink channel. This channel is used by the MS to learn the BSIC & as well as TDMA frame number.
Cell Broadcast Channel: It is Downlink 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 channel 2) Paging channel 3) 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): It is Downlink channel. This channel is used by the BTS to notify the MS of the assignment of an initial signaling.
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Dedicated Control Channel (DCCH): 1) SDCCH 2) SACCH 3) FACCH
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 as uplink & downlink channel. It is bi-directional 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 is used as uplink & downlink channel. It is bi-directional channel. It associates with TCH only. It is used for handover.
Full Rate Traffic Channel: Voice= 13kbps 10
Data= 9.6kbps
Half Rate Traffic Channel: Voice= 6.8kbps Data=4.8kbps
Frequency Hopping: Frequency hopping is the change of frequency on a particular time interval. 1) Improved coverage capacity. 2) Reduce Interference.
Two Types: 1) Baseband hopping 2) 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.
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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): 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 Swapping 3) Diversity Swapping
Sector Swapping: 12
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 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 Tilt 2) Electrical Tilt
Mechanical Tilt: Mechanical Tilt is used for horizontal direction. It is used to increase the coverage. There should be less mechanical 13
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 Code 2) 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
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“Beacon” constantly transmitting the scrambling code of the cell. 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 Handover 2) Soft Handover 3) Hard Handover 4) I- RATE Handover
Softer Handover:
Handover occur b/w two sectors
of same Node B (BTS) 16
Soft Handover:
Handover occur b/w two different
Node B (BTS).
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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