Basic Of GSM
Which is needed
IN Drive Test
Written By
Shekh Jafar
RF ENGINEER
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) Handover is not defined
2) Due to distance b/w two BTS
3) Due to not availability of TCH
4) Interference
5) Overshoot
6) Congestion
7) Proper handover is not defined
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:
It is the received signal strength of serving
cell.
Unit = dbm
Range= -0 to -120
Good= -50 to -75
Avgas = -75 to -90
Bad= -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%
6
6.4to12.8%
7
greater than 12.8%
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
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
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
Channels in GSM:
1)Physical
Channel
2)
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.
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.
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
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.
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):
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:
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 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.
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 “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)
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.
Gud
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