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.
( 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
· 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
· FE –frame erasure rate.
· TA -- Timing advance
· CHM --C hannel Mode
· RH -- cell reselction Hystresis
· CiMd—Ciphering mode
· RAC – Routing area code.
· LAC—Location 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
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. 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 doesn’t 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. There’s bigger difference in downlink than in uplink
direction.
This difference is a
consequence of interference and frequency diversities that 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
sameMobile 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 MHz. A dual-band phone operates at two frequencies, such as 850/1900 MHz or 900/1800