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.

	L        BC            BS                 RXLEV  L        TC            TS                    TX            C/I            RQ                   FE             TA             DSC               CHM                                 RH                  CiMd             MCC          MNC               LAC                                  RA                  CI

 

                          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.

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 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..

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 MAIO_offset is a cell specific parameter defining the MAIO_TRX for the first hopping TRX in a cell. The MAIOs for the other hopping TRXs are automatically allocated according to the MAIO_step -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 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 MHz. A dual-band phone operates at two frequencies, such as 850/1900 MHz or 900/1800