RF INTERVIEW

1. 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 & externalhand over?
Ans
(1)INTERNAL HANDOVER
a) ( INTER BTS) ---- Transfer betweentwo 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 betweenBTS 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 betweencell 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.

2. 7. Explain the major diff betweenBBH & 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.

3. 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 reusedthat leads to co-channel interference.
 When TCH carrier have call activity.
 This is measuredby 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 performedon demodulated speechframes 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
L BC BS RXLEV
L TC TS TX
C/I RQ FE
TA DSC CHM
RH CiMd
MCC MNC LAC
RA CI

4.  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

5. 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 generatedif
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 practicalmaximum 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.
1.1 Hopping Sequence Number
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
reservedfor 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.

6. 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 presentedin Error!
Reference source not found.. All the time slots in RF hopping cell follow the HSN1 as presentedin Error!
Reference source not found..
1.2 Mobile Allocation Index Offset
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 comparedto 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 speechquality 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 betweenRXQUAL& 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 presentedin 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.

7. Typically, the share of the RXQUAL classes 6 and 7 may increase after FH is switchedon, even if
no other changes have been made. This may seem to be surprising since it is expectedthat 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 presentedsome trial results of a DL RXQUAL
distribution with different frequency allocation reuse patterns. As can be seenfrom 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
successfulerror correction leads to less erasedframes 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 measuredidle 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 measuredidle channel
interferences betweendifferent 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 measuredidle
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.

8. The most important component of DTX is, of course, Voice Activity Detection. It must distinguish
betweenvoice 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 createdat 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