Q:how to Plan MAIO and HSN

Ans:

 For telecom engineers, it is very help full to know about MAIO. Basically there is a set of frequencies where the channels are hop to avoid interference that set of frequencies is called MA-LIST. suppose MALIST contained frequencies [A,B,C,D,E,F,G,H,I,J,K,L].
Then MAIO corresponds the starting point of hopping sequence. suppose there are four TRx in one sector. then MAIO will be, for example, 0,2,4.
for first TRx for traffic, the MAIO is 0. means that hopping sequence starts from first frequency in the MALIST, that is frequency A. and for MAIO 2 it starts from frequency C and for MAIO 4 it starts from frequency E.

Also HSN (hopping sequence number), ranges from 0 to 63 . if we use HSN as, for example 4 it means that for MAIO 0, the sequence starts from frequency A and repeats frequencies in following manner.

For MAIO 0 and HSN 4
A,F,K,D,I,B,G,L,E,J,C,H,A,F

For MAIO 2 and HSN 4
C,H,A,F,K,D,I,B,G,L,E,J,C,H

For MAIO 4 and HSN 4
E,J,C,H,A,F,K,D,I,B,G,L,E,J

Q:Why TSC must be equal to BCC????????

Ans:

Training Sequence Code, this optional parameter specifies the Training Sequence Code of the radio channel. The TSC is part of the 'Normal Bursts' which are used for all channel types except RACH,SCH and FCCH. The TSC for the BCCH must correspond to the BCC (part of the BSIC sent on the SCH, so that the MS can derive the TSC of the BCCH from the SCH. This is necessary for the correct selection and decoding of the BCCH bursts, especially if within a limited geographical area a frequency is used several times. If no value is entered for the parameter TSC the BCC is automatically selected.

Questions and answers

Hi
   Lets start to discuss your frequent questions and try to get their answers.....

Q:What are the reason of TCH Drop due to bad quality Uplink

Ans:

 Probable reasons for poor Uplink Quality
1. Poor Frequency plan (TCH)
2. Interference in UL
3. Overshoot of neighboring cells.
4. Poor Level on the UL.
5. Fault in TCH TRX
6. Codec issues.
7. Transmission related issues.
8. Boosters/Repeater related issues.
9. External interferences – Restricted area/Electronics equipment Factory/Jammers.


Uplink Quality Checkpoints
1. Poor Frequency Plan
a. Always resolve DL quality issues before attempting to correct the UL.
2. Interference in UL
a. Avoid using Co & adjacent TCH frequencies in the same cell or site.
b. Avoid using co TCH on neighboring cells.
c. Wrong values of the HSN or MAIO/MAIO Offset.
i. Do not reuse HSN in nearby sites.
ii. Review MAIO & MAIO Offsets as per the TCH plan(1x1, 1x3, adhoc etc).
d. In cases where intra-cell handovers are allowed, interference will cause a lot of intra-cell handovers thus degrading the UL & DL quality
3. Overshooting of neighboring cells.
a. TCH reuse becomes tight is there’s a lot of overshooting of nearby cells. Allow only 1 or up to 3 dominant servers only.
b. Reduce the overshooting of neighboring cells.
4. Poor level on the uplink
a. Possible coverage problem.
b. Rx Level Versus Rx Quality distribution per TRX on a cell.
c. TMA’s can be used judiciously used to enhance UL level.
5. Fault in TCH TRX
a. Check the alarm printouts.
b. (BTS UL Quality history at 24 Hour/10 day resolution). Check if the BTS is within defined interference boundary limits & for how long it has been out of limits of acceptable interference.
c. BTS Analyzer. Check if the BTS is within defined interference boundary limits.
d. Path balance for TCH TRXs

6. Codec Issues
a. AMR related quality issues.
b. AMR related parameter settings. Check if the Codec (AMR/Non-AMR) settings are OK.
c. Check if the BTS s/w version is OK & supports the codec.
d. If the problem is with many sites in a BSC then that BSC’s parameter file needs to be checked/updated.
7. Transmission related issues.
a. Check the BSC ET availability & Quality profile .
b. Check the TCSM ET availability and quality,.
c. Check the QOS
8. Boosters/Repeater related issues.
a. Check for repeaters and/or booster installations. Check for spillage of the repeater signals in areas not intended to be covered.
b. If repeaters are a problem( you will know once you switch off the repeater for a short duration), adjust the gain of repeater
c. Adjust the repeater antenna & orient it to closest serving cell. DO NOT latch a repeater with a DISTANT dominant cell.
9. External Interferences
a. If you suspect external interference on single TRX, lock the TRX & see if the problem persists. If the problem is resolved then the TRX or its implementation is at fault.
b. If you suspect external interference on the entire site, Lock the site & scan the area served by the site for potential interferers. Somebody may be transmitting the same frequencies in the serving area of affected site.
c. Workaround: If you indeed found an interferer over which you have no control, try allocating a different frequency (beyond 2 MHz). There’s a high probability that the frequency which is being severely interfered now will no longer be interfered after frequency change.

Q:how cross polarised antenna is better than space div. antenna

Ans: 

Polarisation diversity Vs Space Diversity

It also depends upon the kind of clutter or area where you want to use these. Cross Polar antenna which has its dipoles placed at +45 and -45 degrees provide us better diversity in a multipath environment where we have lot of reflections,diffractions as chances are mostly that polarisation of the signal transmitted would change by the time it reaches Rx antenna and +-45 degree configuration of dipoles provide maximum probability that the signal will be matched to these phase difference. Another point in favour of crosspolar antennae in city is slant gain which is around 1.5dB. This comes from the angle at which we hold our mobile handsets (mostly at around 45 degrees). 
Space diversity first of all needs space between antennae atlest 10times the wavelength. Secondly the tower structure has to be strong and two different units have to be used. These are the physical limitations. But, it is useful for open areas or highways where chances are most of the times signal transmitted with be in LOS and without phase change.

what are important parameter of power saving in GSM?

Q: what are important  parameter of power saving in  GSM?

Ans: 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

Q:What do you understand by idle channel measurement?


Ans:

• 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 7‑5.

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.

What is the relation link between RXQUAL& FER?

Q: What is the relation link between RXQUAL& FER?

Ans: 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 Figure 7‑1 and Figure 7‑2. 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 Figure 7‑3, 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.

.Explain in brief what is FER.

.Q: 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.

2          to 15% ,   slightly degraded

Greater than 15%, useless

Define the freq. hopping parameters?

Q:Define the freq. hopping parameters?


Ans: 

              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.

How in frequency hopping there is enhancement of network capacity?

Q: How in frequency hopping there is enhancement of network capacity?


Ans: ·        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 

Lets Talk about Cellular

Dear Readers
                     I'm going to start a new section " Cellular Talk" for all the telecom engineers.
It is just like a group discussion where we can discuss our problem and can get the solutions and guidance of experts.


I invite all of my telecom engineers friends to take participate and make this section useful to all.


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