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5/18/2014 TELECOM 2G,3G,4G,RF,IPv6 Study Materials: GSM
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Mobile Communication,Question related to GSM,CDMA,WCDMA &
LTE,Short description about
RF Deive Test,RF Planning,RF Optimization,IPv4 & IPv6
TELECOM 2G,3G,4G,RF,IPv6 StudyMaterials
Friday, January 18, 2013
GSM Interview Question -Answer
1. What is the function of SDCCH & SACCH?
Ans. (a) SDCCH---- Slow Dedicated Control Channel.
Function----
a) Location updates
b) SMS
c) Ciphering Initiation
d) Equipment Validation
e) Subscriber authentation
f) Call set up signaling
(b) SACCH---Slow Associated Control Channel.
Function:
(a) Timing advance data
(b) Transmit power control
(c) transmission of signaling data
(d) radio link supervision measurements
2. What are the reasons for Hand Overs?
(Ans) . (a) Signal Strength (RX LEVEL)
(b) Signal Quality (RX Qual)
(c) Power Budget
(d)Timing Advance.
(e) Interference
2. What are the shortcomings in handovers?
a) Call Dropping.
b) Ping- Pong handover
c) Far- Away cell effect
4. What is intelligent hand over?
Ans)Fuzzy logic
a) Neutral networks
5. What are internal & external hand over?
Ans
(1)INTERNAL HANDOVER
a) ( INTER BTS) ---- Transfer between two
channels (time slot) in same cell.
b) ( INTRA BSC ( BTS BTS)-----1 . transfer between BTS under
control of same Bsc
2 Measuring the quality of radio connection
3 Power levels
2. EXTERNAL HANDOVER.
( a) INTER BSC (BSC_ BSC)---- 1. Transfer between BTS under
the control of diff
BSC
2. BSC TO BSC
3 . NSS to attends the hand over
4. MSC controls.
Dharmendra singh
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5/18/2014 TELECOM 2G,3G,4G,RF,IPv6 Study Materials: GSM
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( b) INTER MSC ( MSC- MSC) ----1. transfer between cell
under
the control of diff MSC
.What is the frequency Hopping its imp?
(Ans)
It is defined as sequential change of carrier frequency on
the radio link between mobile & base station.
Two types of freq hopping----- 1. Base band freq hopping.
2. synthesized frequency hopping.
7. Explain the major diff between BBH & SFH?
Ans
In BBH the no of hopping freq is same as no of TRX.
In SFH the no of Hoping freq can be in the range of 1to 63.
8. what are the advantages of Frequency Hopping?
1. Frequency Diversity
2. Interference Averaging
3. capacity
9. How in frequency hopping there is enhancement of network
capacity?
Freq hopping implement will enable more aggressive freq
reuse pattern, that leads to better spectrum efficiency.
It can add more transceiver in the existing sites , while
maintaing the net work quality/
Freq hopping compressing the available spectrum to
make room for extra capacity
.
10. Define the freq. hopping parameters?
Frequency Hopping Parameters
GSM defines the following set of parameters:
Mobile Allocation (MA): Set of frequencies the mobile is allowed
to hop over.
Maximum of 63 frequencies can be defined in the MA list.
Hopping Sequence Number (HSN): Determines the hopping order used
in the
cell. It is possible to assign 64 different HSNs. Setting HSN =
0 provides cyclic hopping
sequence and HSN = 1 to 63 provide various pseudo-random hopping
sequences.
Mobile Allocation Index Offset (MAIO): Determines inside the
hopping
sequence, which frequency the mobile starts do transmit on. The
value of MAIO ranges
between 0 to (N-1) where N is the number of frequencies defined
in the MA list.
Presently MAIO is set on per carrier basis.
Motorola has defined an additional parameter, FHI.
Hopping Indicator (FHI): Defines a hopping system, made up by an
associated
set of frequencies (MA) to hop over and sequence of hopping
(HSN). The value of FHI
varies between 0 to 3. It is possible to define all 4 FHIs in a
single cell.
Motorola system allows to define the hopping system on a per
timeslot basis. So
different hopping configurations are allowed for different
timeslots. This is very useful
for interference averaging and to randomize the distribution of
errors.
11. What are the effects of freq hopping?
(Ans)
1 Handovers:
2 Call setup:
3 Frame Erasure Rate (FER):
12 .Explain in brief what is FER.
Ans FER= Number of erased blocks\ total no of blocks *100
It is the right measure of voice quality.
FER is performed on speech& signaling frames
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FER------- 0 to 4%, GOOD.
4 to 15% , slightly degraded
Greater than 15%, useless
13. What happens when speech frames discarded in FER?
System will interpolate.
14. What happens when signaling frames discarded in FER?
MS is instructed to resend.
15. What is TCH_ TCH Interference? How it is measured.
When TCH carries are reused that leads to co-channel
interference.
When TCH carrier have call activity.
This is measured by delta measurement. --- 1. BCCH carries
are
diff
2. TCH carriers in both cell 1& cell2 are same AFRCN TCH
16. Define the terms?
BER--- The number of erroneous bits received
Total no of bits received.
RBER---1 Residual bit error rate
2 It is performed on demodulated speech frames that
are not mark corrupt
.BFI -- Bad frame indication.
17. Explain the parameters in TEMS POCKET mobile.
1.
2. Llcell BCCH ARFCN
3.
4.
5.
L1. Logical channel.----- BCCH
L2. Logical channel ----- TCH
BC-- serving cell BCCH AFRCN.
BS-- base station identity code.
RXLEV- recieved signal strength
TC-- traffic channel
TS - time slot number.
TX - transmit power
C/I -- Carrier to interference ratio in db
RQ -- Receive bit error rate
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Interview question of GSM
1) Which channel is used to transmitrandom access signals? BCCH
CCCHCorrect! SDCCH TCH 2) Which oneof the following is the
combinat...
L BC BS RXLEV
L TC TS TX
C/I RQ FE
TA DSC CHM
RH CiMd
MCC MNC LAC
RA CI
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FE frame erasure rate.
TA -- Timing advance
CHM --C hannel Mode
RH -- cell reselction Hystresis
CiMdCiphering mode
RAC Routing area code.
LACLocation area code.
18. Explain the analysis behind RX Qual.?
RX Qual is the basic measure. It reflects the average BER
over
the certain period of time(0.5s)
RX QUAL done over 104 TDMA frames.
Limitation of RXQUAL---- 1. The distributions of bit error
over
time.
2. Frame erasure
3. Hand over.
19. What are type of interference occur?
1. Co- channel interference.
2. Adj-channel interference.
3. Near end- Far end interference.
20. What is ERLANG?
Unit of telephone traffic intensity is called Erlang.
One ERLANG is one channel occupied continuously for one
hour.
1E = 64Kbps.
21. what do you mean by GOS?
It is the probabity of having a call blocked during busiest
hour.
Ex GOS=0.05 means one call in 20 will be blocked call during
busiest hour because of insufficient capacity.
22. What are the technique GSM offers which combat Multipath
fading?
Equalization
Diversity
Freq Hopping
Interleaving
Channel coding
23. What are control &traffic channels?
CONTROL CHANNEL.----1 BCH
2. CCCH
3. DCCH.
TRAFFIC CHANNEL-- Half rate
Full rate
EFR == Enhanced full
rate.
24. What are BCH, CCH, DCCH channels?
BCH-- 1. BCCH
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2 .FCCH
3. SCH
CCCH. --- 1.PCH
2. AGCH
3. RACH
DCCH---- 1.SDCCH.
2. SACCH
3. FACCH
25. What are types of bursts?
Normal Burst
Frequency Correction Burst
Synchronization Burst.
Dummy Burst
Access Burst.
26. What is adjacent channel separation in GSM?
Urban Environment-------- 200khz
Sub Urban Environment ---- 400khz
Open environment ----- 800khz
27. What is the watt to dBm conversions?
Power in dBm = 10 log( watts *100)
0 dBm= 1mili watt
1watt = 30dbm
28. What are the optimizations you have done during Drive
Test?
What are samples in gsm?
Which modulation take place in GSM
In one TRU how many frames are there?
What is the value RXLEV of neighboring cell?
What do you mean by VAD?
What is BFI, where it is use?
2929. Define the hopping parameters in detail?
The MA is a list of hopping frequencies transmitted to a mobile
every time it is
assigned to a hopping physical channel. The MA-list is a subset
of the CA list. The MA-
list is automatically generated if the baseband hopping is used.
If the network utilises
the RF hopping, the MA-lists have to be generated for each cell
by the network planner.
The MA-list is able to point to 64 of the frequencies defined in
the CA list. However, the
BCCH frequency is also included in the CA list, so the practical
maximum number of
frequencies in the MA-list is 63. The frequencies in the MA-list
are required to be in
increasing order because of the type of signaling used to
transfer the MA-list.
The Hopping Sequence Number (HSN) indicates which hopping
sequence of the
64 available is selected. The hopping sequence determines the
order in which the
frequencies in the MA-list are to be used. The HSNs 1 - 63 are
pseudo random sequences
used in the random hopping while the HSN 0 is reserved for a
sequential sequence used
in the cyclic hopping. The hopping sequence algorithm takes HSN
and FN as an input
and the output of the hopping sequence generation is a Mobile
Allocation Index (MAI)
which is a number ranging from 0 to the number of frequencies in
the MA-list
subtracted by one. The HSN is a cell specific parameter. For the
baseband hopping two
HSNs exists. The zero time slots in a BB hopping cell use the
HSN1 and the rest of the
time slots follow the HSN2 as presented in Error! Reference
source not found.. All the
time slots in RF hopping cell follow the HSN1 as presented in
Error! Reference source
not found..
When there is more than one TRX in the BTS using the same
MA-list the Mobile
Allocation Index Offset (MAIO) is used to ensure that each TRX
uses always an unique
frequency. Each hopping TRX is allocated a different MAIO. MAIO
is added to MAI when
the frequency to be used is determined from the MA-list. Example
of the hopping
sequence generation is presented in Error! Reference source not
found.. MAIO and HSN
are transmitted to a mobile together with the MA-list. In Nokia
solution the MAIOoffset
is a cell specific parameter defining the MAIOTRX for the first
hopping TRX in a cell.
1.1 Hopping Sequence Number
1.2 Mobile Allocation Index Offset
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The MAIOs for the other hopping TRXs are automatically allocated
according to the
MAIOstep -parameter introduced in the following section.
30. What is the effect of frequency hopping in RXQual?
Frequency hopping causes some changes in the RXQUAL
distribution. Also,
there are some differences in a way the RXQUAL distribution
should be interpreted.
The Frame Erasure Ratio (FER) is a ratio of discarded speech
frames compared to
all the received speech frames. A speech frame is generally
discarded if after the
decoding and error correction process any of the category 1a
bits is found to be changed
based on the three parity bits following them in a speech
frame.
FER is a measure of how successfully the speech frame was
received after the
error correction process and it is thus a better indication of
the subjective speech
quality compared to the RXQUAL which gives an estimate of the
link quality in terms
of BER. The RXQUAL doesnt indicate how the bit errors were
distributed in a speech
frame. The bit error distribution affects the ability of the
channel decoding to correct
the errors.
The following table gives an idea of the correlation between
RXQUAL and FER
and between subjective speech quality and different FER
classes.
31. What is the relation link between RXQUAL& FER?
Table 1. RXQUAL vs. FER comparison according to the laboratory
tests.
The relation of downlink FER and RXQUAL was measured during a FH
trial. The
relation is clearly different in the hopping case compared to
the non-hopping case. The
distributions of FER in each RXQUAL class are presented in
Error! Reference source not
found. and Error! Reference source not found.. One clear
observation can be made; in
the non-hopping case there are significant amount of samples
indicating deteriorated
quality (FER>10%) in RXQUAL class 5 while in the hopping case
the significant quality
deterioration (FER>10%) happens in RXQUAL class 6. Thus, it
may be concluded that in
the frequency hopping networks significant quality deterioration
starts at RXQUAL
class 6 while in non-hopping network this happens at RXQUAL
class 5.
This improvement of FER means that the higher RXQUAL values may
be
allowed in a frequency hopping network. RXQUAL thresholds are
used in the handover
and power control decisions. Because of the improvement in the
relative reception
performance on the RXQUAL classes 4-6, the RXQUAL thresholds
affecting handover
and power control decisions should be set higher in a network
using frequency hopping
network. In a frequency hopping network RXQUAL classes 0-5 are
indicating good
quality.
Typically, the share of the RXQUAL classes 6 and 7 may increase
after FH is
switched on, even if no other changes have been made. This may
seem to be surprising
since it is expected that frequency hopping improves the network
quality. However, in
most cases the quality is actually improved, but the improvement
is more visible in
the call success ratio. The improved tolerance against
interference and low field
strength in FH network means that it is less likely that the
decoding of SACCH frames
fails causing increment in the radio link timeout counter. Thus,
it is less likely that a
call is dropped because of the radio link timeout. Instead, the
calls generating high
RXQUAL samples tend to stay on. This may lead to increase in the
share of RXQUAL 6-
7. However, at the same time the call success rate is
significantly improved.
In the Error! Reference source not found., there are presented
some trial results
of a DL RXQUAL distribution with different frequency allocation
reuse patterns. As can
be seen from the figures, the tighter the reuse becomes, the
less samples fall in quality
class 0 and more samples fall in quality classes 1-6. Theres
bigger difference in
downlink than in uplink direction.
This difference is a consequence of interference and frequency
diversities that
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affect the frequency hopping network. Because of these effects,
the interference or low
signal strength tend to occur randomly, while in a non-hopping
network it is probable
that interference or low field strength will affect several
consecutive bursts making it
harder for the error correction to actually correct errors. The
successful error
correction leads to less erased frames and thus improves the
FER.
32. What do you understand by idle channel measurement?
When a new call is established or a handover is performed,
the
BSC selects the TRX and the time slot for the traffic channel
based on the idle
channel interference measurements. The frequency hopping has a
significant
effect on the idle channel interference measurement results.
When the frequency hopping is used, the frequency of a hopping
logical channel
is changed about 217 times in a second. The frequency of the
idle time slots changes
according to the same sequence.
In a case of the random hopping, this means that the measured
idle channel
interference is likely to be the same for all the TRXs that use
the same MA-list. If the
interference is averaged over more than one SACCH frame, the
averaging effect is even
stronger. However, normally the interferers are mobiles located
in interfering cells. In
this case, there are probably differences in the measured idle
channel interferences
between different time slots in the cell. This happens, because
the interfering mobiles
are only transmitting during the time slot that has been
allocated to them. This is
illustrated in Figure Error! No text of specified style in
document.-1.
If the cyclic hopping sequence is used, there might occur
differences on the
measured idle channel interference levels between the TRXs on
the same time slot as
explained in the following section.
Figure Error! No text of specified style in document.-1. Idle
channel interference
in a case of the random RF hopping
33 .what are types of handover?
There are four different types of handover in the GSM system,
which involve
transferring a call between:
Channels (time slots) in the same cell
Cells (Base Transceiver Stations) under the control of the
same
Base Station Controller (BSC),
Cells under the control of different BSCs, but belonging to
the
same Mobile services Switching Center (MSC), and
Cells under the control of different MSCs.
34. what are important parameter of power saving in GSM
Discontinuous transmission
Minimizing co-channel interference is a goal in any cellular
system, since it
allows better service for a given cell size, or the use of
smaller cells, thus increasing the
overall capacity of the system. Discontinuous transmission (DTX)
is a method that
takes advantage of the fact that a person speaks less that 40
percent of the time in
normal conversation [22], by turning the transmitter off during
silence periods. An
added benefit of DTX is that power is conserved at the mobile
unit.
The most important component of DTX is, of course, Voice
Activity Detection. It
must distinguish between voice and noise inputs, a task that is
not as trivial as it
appears, considering background noise. If a voice signal is
misinterpreted as noise, the
transmitter is turned off and a very annoying effect called
clipping is heard at the
receiving end. If, on the other hand, noise is misinterpreted as
a voice signal too often,
the efficiency of DTX is dramatically decreased. Another factor
to consider is that when
the transmitter is turned off, there is total silence heard at
the receiving end, due to
the digital nature of GSM. To assure the receiver that the
connection is not dead,
comfort noise is created at the receiving end by trying to match
the characteristics of
the transmitting end's background noise.
Discontinuous reception
Another method used to conserve power at the mobile station is
discontinuous
reception. The paging channel, used by the base station to
signal an incoming call, is
structured into sub-channels. Each mobile station needs to
listen only to its own sub-
channel. In the time between successive paging sub-channels, the
mobile can go into
sleep mode, when almost no power is used.
All of this increases battery life considerably when compared to
analog
: What is Tri-band and Dual-band?
A: A tri-band phone operates at three supported frequencies,
such as 900/1800/1900 MHz or 850/1800/1900
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Posted by Dharmendra singh at Friday , January 18, 2013
MHz. A dual-band phone operates at two frequencies, such as
850/1900 MHz or 900/1800
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3 comments:
swetank aginhotri said...
these are very helpful.
thanks
May 9, 2014 at 1:37 PM
swetank aginhotri said...
these are very helpful.
thanks
May 9, 2014 at 1:38 PM
swetank aginhotri said...
these are very helpful.
thanks
May 9, 2014 at 1:39 PM
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