Some Question Answer of 3G

1. If a UE is on a data call (CELL-DCH state) and there is in no activity

for awhile what would you expect to see occur?

UE should go from CELL-DCH to CELL-FACH then if still no activity to

either CELL-PCH or URA-PCH (via CELL-FACH). If they talk about

inactivity timers and mention that the state goes from CELL-DCH

straight to CELL-PCH or URA-PCH that is also possible. Bonus they say

they would see RADIO BEARER RECONFIGURATION messages when the

states are changing.

2. Explain the concept of a Monte Carlo Simulation for UMTS Design.

This is a simulator that randomly distributes terminals/users

geographically onto the network and then checks the link budget for

each terminal/connection to see if they can successfully connect or not.

The simulator modifies parameters such has UE Tx Power, BTS Tx

Power, requested bearer (in the case that multiple bearers could support

the same service) when checking if a connection can be made. In every

snapshot the simulator runs through the list of terminals/connections and

attempts to make them all connect successfully, it starts a new snapshot

when the number of successful connections converges. The process then

starts on a new snapshot.

3. In what cases is Open Loop Power Control used?

This is a simulator that randomly distributes terminals/users

geographically onto the network and then checks the link budget for

each terminal/connection to see if they can successfully connect or not.

The simulator modifies parameters such has UE Tx Power, BTS Tx

Power, requested bearer (in the case that multiple bearers could support

the same service) when checking if a connection can be made. In every

snapshot the simulator runs through the list of terminals/connections and

attempts to make them all connect successfully, it starts a new snapshot

when the number of successful connections converges. The process then

starts on a new snapshot.

4. Explain Inner and Outer loop power control and who controls them.

If they start talking about Open and Closed Loop PC, tell them you want

Inner/Outer Closed Loop PC. Inner loop power control is performed by

the NodeB to set the transmit power of the UE and BTS to compensate

for signal variations due to fading or path loss to maintain the set SIR

(occurs up to 1500 times per sec). Outer loop power control is performed

by the RNC to set the target SIR based on the required BER/BLER for the

requested services (occurs up to 100 times per sec).

5. In HSDPA, how does the network manage the throughput on the Radio

Interface for a user/ connection?

Modulation (16QAM, QPSK etc), Coding (convolution coding, fire codes

etc), number of codes allocated and scheduling (it's a shared resource)

6. Depending on the RF conditions, what can the network do to manage

call quality?

AMR - for good conditions use codec will low redundancy/overhead; for

poor conditions use codec with lower bit rate requirement but higher

overhead, stronger coding and more redundancy.

7. What is the typical/most common bit rate that a voice call uses?

They should say 12.2kbps but may be different if they start talking about

AMR and the different rates then the know more. Prod them to see if they

know the Spreading Factor (SF) used for the radio bearer, should be 128

8. In Release '99, how does the network manage the throughput on the

Radio Interface for a user/connection?

This question is a little harder to ask, so you may need to work it

differently a few times. Perhaps leading questions could be: What

parameter/configuration does the network change on the air interface

What you are trying to hear from the candidate is that the network

assigns a radio bearer with a channelization code with a spreading factor

that matches the requested service maximum bit rate.

9. Name the 4 RRC Connected Modes (states) and describe the

characteristics of each.

Cell-DCH: UE has been allocated a dedicated physical channel in uplink

and downlink.

Cell-FACH: UE listens to RACH channel (DL) and is allocated a FACH

channel (UL). Small amounts of UL/DL data can be transfers in this state.

The RNC tracks the UE down to the cell level and cell reselections are

possible with the CELL UPDATE message.

Cell-PCH: UE monitors (using discontinuous reception) a PCH channel

(PCH) indicated by the PICH channel. The RNC tracks the UE down to

the cell level and cell reselections are possible with the CELL UPDATE

message. No data can be transferred in the UL in this state.

URA-PCH: UE monitors (using discontinuous reception) a PCH channel

(PCH) indicated by the PICH channel. The RNC tracks the UE down to

the URA level.

10. What is compressed mode, what is it's function, and what impact

does it have on the network?

Compressed mode is when the mobile goes into a slotted transmit mode

whereby it opens up an idle period (transmission gap) where it can

monitor another carrier or technology (GSM). The impact is that to

maintain the same bit rate, it halves the SF, and therefore increases

power level causing higher interference to the network. If the SF cannot

be halved then the bit rate of the bearer decreases. If they seem

knowledgably, ask them if they know what messages and events trigger

and configure compressed mode on/off. 2D event for on, 2F for off.

Messages would for configuration would be RADIO BEARER

RECONFIGURATION, TRANSPORT CHANNEL RECONFIGFURATION or

PHYSICAL CHANNEL RECONFIGURATION.

11. What are the general triggers for an iRAT handover?

Ec/Io of best cell below a certain threshold (usually around -16 to -18 dB)

or RSCP of best cell below a certain threshold (usually around -100 dBm).

12. What would you define as a pilot polluter?

Many definitions: A cell that has high signal strength at a location but is

not part of the active set. A cell that meets the criteria for addition into

the Active Set but can not enter because the active set is full.

13. What is typically the requirements (criteria) for a cell to be

added/removed/replaced to/from/in the active set?

For addition (Event 1a), candidate cell needs to have an Ec/Io value that

is within a T_ADD threshold of the primary/reference (usually the best)

cell for a specify time hysteresis. For removal (event 1b), cell needs to

have Ec/Io lower than T_DROP margin for a specific time hysteresis. For

replacement (event 1c), cell needs to have an Ec/Io better than the worst

cell in the active set by the T_REPLACE and for a specific time hysteresis.

14. What is the typical maximum active set size and what needs to be

considering when setting this?

3 to 4 cells, the larger the active set size the more likely it is that Iub link

efficiency is reduced (more than one resource for a single connection due

to SHO).

15. In the Link Budget, what is a Shadow Fade Margin for and what

factors does it depend on?

The shadow fade margin is dependent on the target percentage area

coverage, the propagation model, and the standard deviation of the

lognormal shadowing (usually the same as the model's standard deviation

if the fast fading effects are removed). The Shadow Fade Margin is a

added margin placed in the link budget such that a guaranteed level of

service can be offered "in the worst case".

16. What would the call flow be for a Mobile Originated Call (major RRC

messages)?

RRC Connect Request -> RRC Connection Setup -> RRC Setup Complete

-> (SETUP, authentication encryption, TMSI reallocation etc) -> CALL

PROCEEDING-> Radio Bearer Setup -> Radio Bearer Setup Complete ->

ALERT -> CONNECT -> CONNECT ACK ->DISCONNECT -> RELEASE.

17. How would you find such cells from a planning tool and from a drive

test tool?

Ignoring low signal conditions, if the best cell RSCP is greater than say -

85dBm and there are cells not in the active set but are strong enough to

be in the active set then they are candidate for pilot polluters. Looking at

cells that have a high noise rise, high amount of traffic compared to

surrounding cells, may also indicate a pilot polluter. Areas with high

Signal strength for the (Active Set Size + 1) best pilot (like the 4th best

pilot if AS size is 3). In DTT, areas with poor Ec/Io but good RSCP, in the

monitored set contains a cell with a good Ec/Io but cannot enter the AS

because it is full. Areas where scanner shows a strong signal for a far

away cell.

18. What is the major difference in link budgets between UMTS and

GSM/TDMA?

In UMTS you generally have a link budget for each service (voice, data,

video etc), in GSM you usually only use 1 for voice. Each service has a

different Eb/No target. In UMTS you have to consider the target traffic

load you will have and add a noise-rise margin, in GSM you may have a

slight interference margin but not normally related to traffic. In UMTS

some services (like voice) will show up as uplink limited but other

services (like HSDPA, 384kbps service) will show as downlink limited. In

UMTS you usually have to consider that all users use the same power

from the BTS therefore the more number of users the lower the

maximum power available per user (maximum power per connection)

which is a starting point in the link budget.

19. What is an active set, monitor set and detected set?

Active Set: the set of cells with which the UE is currently

connected/communicating with; Drive test usually show them as SC or

Pilots but they are actually cells; Monitored Set: Cells that the UE has

detected and is monitoring and are known to the network, they either

don't meet the criteria or the active set is full; Detected Set - Cells that

the UE has detected but are not known to the network as yet (missing

neighbor likely).

20. Explain the different Handover types in UMTS.

Soft(er) Handover: connected to more than one cell on the same

frequency, softer occurs when 2 cells in the active set belong to same

Node-B; Intra-frequency Hard Handover: Occurs when UE moves from

one cell in one RNC to a cell in another RNC and the RNCs do not have

an Iur link between each other; Inter-Frequency Hard Handover: when

UE changes from one frequency to another frequency (usually due to

traffic layer management or Quality reasons);Inter-technology (iRAT)

Hard Handover: Handover from UMTS to GSM (v.v.) usually at the edge

of UMTS service area but also due to quality reasons.

21. Explain the concept of Cell Breathing. How is the accounted for in the

link Budget?

Io or No (the interference part of Ec/Io and Eb/No) increase as the traffic

on the network increases since everyone is using the same frequency.

Therefore as Io or No increases the UE or BTS needs to use more power

to maintain the same Eb/No or Ec/Io. When the power required is more

than the maximum power allowed, the connection cannot be made. Users

at the cell edge are usually the first to lose service; hence the service

area of a cell shrinks. As traffic decreases the reverse happens and the

service area increases. They should say that it is accounted for in the

Noise Rise Margin found in the Link Budget.

22. What does the scrambling code do and function?

Scrambling Code makes it possible for the UE to distinguish the

transmissions from different cells/NodeBs. Bonus if he knows there are

512 primary scrambling codes and that the are broken up to 64 groups of

8 codes each.

23. What does channelization codes do and function?

Channelization codes are used for spreading and dispreading of the

signals, they also create the "channels" making it possible to distinguish

between users/connections/channels. Bonus if they know that they have

an associated Spreading Factor and are allocated depending on the

bandwidth required by the service.

2G Optimization

Low Signal Strength Analysislets starts todays topic that is Low signal strength analysis

What could be the probable cause of low signal while you drive

or optimize.

First see the following flow chat and try to understand the things

Remember that

Low Signal strength is one of the reason of drop call. It

can be indicated by many calls disconnected at low signal

strength by subscriber, drop calls due to excessive TA,

poor handover performance and poor call setup

performance.

What could be the probable reasons

Probable ReasonPoor BSC

Exchange Property setting

High LOWSSDL & LOWSSUL will give more

drop reason due to SS and this might not show

the actual drop. It is because drop due to SS is

more priority than Quality.

No dominant

cell

Cell might be isolated or standalone.

Antenna tilt &

orientation

Too much downtilt sometimes might not

cover a larger area and the subscriber might lose

the SS.Output Power

Low output power might cause smaller border cell.

Just try to observed what could be the right cause :-

The following procedure should be performed for low signal strength

analysis:

1:

Identify the baseline requirement of design and BSC exchange property (setting for LOWSSUL/LOWSSDL).

2:

Check the value for LOWSSDL & LOWSSUL. If it is higher than ACCMIN, change the parameter to a reasonable value since the drop reason will be more priority to SS compared to Quality.

3:

Check the site position, antenna direction, position etc. This is to ensure the possible location is open to interference (open water environment) or isolated. Good map is needed for this.

4:

Check if the site is sectorized or Omni. If it is Omni, set the cell into sectorized cell.

5:

Check if the signal strength is uplink or downlink limited. Mostly, It is designed to be downlink limited.

6:

Check the coverage cover expected area from the planet. If it is not, check the antenna tilt and orientation. Change the direction or tilt if it is too much downtilt or pointing to a wrong direction.

7:

Sometime, low output power might cause low SS. Check output power and if it is low, increase the output power.

8:

Check cell whether it has hotspots from drivetests. If found, adding new site is recommend.

9:

In order to check power distribution, run Cell Traffic Recording (CTR) to that particular cell.

10:

Check if the cell has indoor coverage problem. If yes, add micro site instead.

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TCH Assignment Analysis

Successful assignments show the number of successful TCH allocations at call setup.At unsuccessful assignment, the Assignment Complete message, sent by the MS, was never received by the BTS.

The formula is defined as:

% TCH ASSIGNMENT

SUCCESS RATE

= TFCASSALL X 100 %

TASSATTProbable Reason

No dominant serving cell

The serving cell cannot cope with the TCH traffic.

Severe congestion on

TCH

Failing TCH allocation for assignment or handover due to

congestionLow signal

strength for call access

The signal strength might be higher on the BCCH than on the

TCH.Interference Disturbance on SDCCH or target

TCHFaulty

transceiverFaulty equipment

The following procedure should be performed for TCH Assignment analysis:

1: For TCH assignment success rate, the first thing, check the TCH Time Congestion.

2: If there is congestion on TCH, it is recommend doing the dimensioning and adding TRU based on carried TCH traffic demand.

3: If there is no congestion on TCH, check the output power of the BTS. If the output power is low, increase the output power.

4: If the output power is ok, check the faulty BTS by extracting BTS error log.

5: If hardware fault found, swap or repair HW.

Perform drivetests to check the coverage and received RxLEV.

6: If no dominant cell or similar signal strengths of a few cells found during drivetests, it is recommended to add BTS.

7: If there is no problem on the dominant cell, check the interference whether co-channel or adjacent channel.

8: Check the disturbance whether it is on SDCCH or target TCH. If disturbance found, improve the frequency plan.

Mostly, the problems of low TCH assignment are TCH availability and interference.

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Interference Analysis

The following procedure should be performed for interference analysis:

Check the drop call performance for that particular cell. When the interference problem occurs in the cell, the drop call will be higher than usual. This might depends on the severity of the interference whether it is co-channel or adjacent channel.

Check the handover performance of the cell. The HO performance will also look bad especially when you look into the neighbor relation that has interference. For e.g. adjacent channel. (CNA consistency checking can detect this)

Check the antenna direction, position etc. This is to see whether the direction covers the right area, open space area (this can be seen by having good and updated map)

Check if the site is a dragon site. If the site is a dragon site, it might be possible to get interfered by co-channel from far away.

Check the co-channel sites, if found, change frequency and see the result. Mostly, changing the frequency will solve the interference problem.

Check the Intracell handover (normally for Intracell handover 25% Uplink and 75% Downlink) and if the variation is different from this. Intracell handover usually indicates bad quality and high signal strength. Too high number of intracell handover show a bad quality cell and if possible, you can reduce the number of intracell handover of MAXIHO to a smaller value based on the channel group.

Locate the interference from statistics based on MS reversion to old channel of total attempt. High number of reversion will show that the target frequency might be interfered.

Check the statistics from Outgoing Handover decision due to bad quality Uplink or Downlink from handover decision. High decision of handover due to quality will show the direction of interference.

Check if the interference is uplink interference (this might be an interference from other MSs) by analyze the ICM band for other band (not include band 1). If found on ICM > 3, change frequency.

Check the MS power regulation setting. If any poor setting found, correct the parameter. Improper setting of MS power regulation might cause interference. The feature used to reduce the MS power when the MS is near to the BTS and hoping that it might not interfere the uplink.

Check if the frequency hopping on or off. If more than 1 TRU, turn on the frequency hopping. Turning on the frequency will help to reduce interference by interference averaging.

Check if DTXU feature is on or off. If off, turn the DTX feature on. This will save the battery in the MS and reduce the interference.

If the interference is downlink (causing by other BTS interference), Check BTS power regulation. If any poor setting found, correct parameter setting.

Check if DTXD feature is on or off. If off, turn the DTX feature on. This is used to reduce interference and decrease BTS power consumption

If changing frequency or parameter cannot solve the interference for both uplink & downlink, it might be external interference.

Check antenna installation, ensure that the antenna is correctly installed

Check if another mobile network like AMPS is near to the location.

Check from the statistics if there is any pattern of bad quality reason. For example, for surveillance purpose, the CCTV or wireless alarm system might be turn on during nighttime only.

If external interference problem occurs, do drive test and report the usage of the frequency to authorities.

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Congestion Analysis

Hi all

This is second topic for today that is "Congestion Analysis" a well

known word for Telecom professionals .

Traffic congestion is one of the major network problems in a

mobile system. A high congestion deteriorates the overall

performance of the network and should be minimized.

· 1: Short term growth

If the high traffic related to an occasional event, like sports event, fairs, conference, a temporary solution might be considered.

· 2: Long term growth

If there is a long-term growth the network capacity has to grow according to the demand.

Type of Congestion

The congestion analysis begins by identifying if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may mean that the only way to get rid of the congestion is to add more physical capacity in terms of transceivers or sites.

Consider how many channels that are allocated in the cell. If possible, expand the capacity with new transceivers, otherwise a new site must be implemented. Frequency planning schemes such as MRP and FLP could be used to relieve congestion. Microcells could be used to take traffic in severe congested areas.

SDCCH Congestion

In R8, the time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does.

The flowchart below, explains a general approach to investigate SDCCH Congestion. The next section describes the action points in this flowchart. The reference to each action point is indicated on the flow chart as well.

TCH Drop Analysis

1. Radio Link Time-Out

Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACCH

periods before the timeslot is released and a new call can be established on the channel.

2. Layer 2 Time-Out

If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.

3. Release Indication

When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received.

4. MSC Time-OutNormal Release:

If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH.

Reject (only SDCCH):

If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release).

5. Assignment to TCH

Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:

a. There must be a TCH channel available, i.e. no congestionb. The locating algorithm must have received at least one valid measurement report.

If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC.

TCH Drop reason (1)The classification of TCH Drop Reasons are arranged in the order of priority:1.ExcessiveTiming Advance2.Low Signal Strength3.Bad Quality4.Sudden Loss of Connection5.Other Reasons

Excessive Timing Advance

The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is commonly apparent to isolated or island sites with a wide coverage area.

Action:Check if the cell parameter TALIM is < "63" Solution:Set TALIM to a value close to 63.Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.

TCH Drop Reasons (2)Low Signal Strength on Down or Uplink or Both Links

The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and LOWSSUL are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason.

Action:Check coverage plots.Check output power.

Check power balance and link budget.Check if Omni site.Check antenna configuration & type.Check antenna installation.Perform drive tests & site survey.Check TRX/TS with high CONERRCNT.

Solution:Add a repeater to increase coverage in for example a tunnel.Change to a better antenna (with higher gain) for the base station.Add a new base station if there are large coverage holes.Block/Deblock TRX

TCH Drop Reasons (3)Poor Quality on Down or Uplink or Both Links

The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and BADQUL (expressed in DTQU) are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD Quality BL will pegged.

Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH. Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality.

Action:Check C/I and C/A plots.Check Frequency Plan (Co-BCCH or Co-BSIC Problem).Check MAIO, HOP, HSN parameters.Check FHOP if correctly configured (BB or SY).Check for External Interference.Perform drive tests.

Solution:Change BCCH frequency.Change BSIC.

Change MAIO, HOP, HSN.Change FHOP.Record RIR or on-site Frequency Scanning to identify source of interference.Use available radio features.

TCH Drop Reasons (4)Sudden Loss of Connection

Drops due to Sudden Loss are drops that have not been registered as low signal strength, excessive timing advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement results from the MS.

There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very sudden and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission faults), and

MS Faults.

Action:Check BTS Error Logs, Alarms and Fault Codes.Check CONERRCNT per TRX and TS.Check Transmission Link (A-bis).Check for DIP Slips.Check LAPD Congestion.Correlate Handover Lost to Drops due to Sudden Loss

Solution:Fix Hardware Faults and Alarms.Reset TRX with high CONERRCNT.Ensure that Synchronization and A-bis Link are stable.Change RBLT with high DIP Slips.Change CONFACT or increase Transmission CapacityInvestigate HO Lost Problem

TCH Drop Reasons (5)TCH Drops due to Other ReasonsTCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad Quality and

Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated with hardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost.

Action:Check BTS Error Logs.Check Alarms and Fault Codes.Check CONERRCNT per TRX and TS.Check Transmission Link (A-bis).Check for DIP Slips.Correlate Handover Lost to Drops due to Other Reasons

Solution:Fix Hardware Faults and Alarms.Reset TRX with high CONERRCNT.Ensure that Synchronization and A-bis Link are stable.Change RBLT with high DIP Slips.Investigate HO Lost Problem

Problem reason of drop in SDCCH

Low Signal Strength on Down or UplinkThe reason for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure.Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error logSolution: Add new sites. Increase output power. Repair faulty equipment.

Poor Quality on Down or UplinkAction: Check C/I and C/A plots. Check frequency plan. Perform drive tests.Solution: Change frequency. Use available radio features.

Too High Timing AdvanceAction: Check if the cell parameter TALIM is < style="font-weight: bold;">Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for cochannel cells.

Mobile Error

Some old mobiles may cause dropped calls if certain radio network features are used. Another reason is that the MS is damaged and not working properly.Action: Check MS fleet.Solution: Inform operator.

Subscriber BehaviorPoorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing illadvised locations to attempt calls, etc.Action: Check customer complaints and their MS.

Battery FlawWhen a subscriber runs out of battery during a conversation, the call will be registered as dropped call due to low signal strength or others.Action: Check if MS power regulation is used. Check if DTX uplink is used.

Congestion on TCHThe SDCCH is dropped when congestion on TCH.Action: Check TCH congestionSolution: Increase capacity on TCH or using features like Assignment to another cell, Cell Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc

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SDCCH drop due to "Other Reason"

How to Analyse SDCCH drop due to "Other Reason"

SDCCH drop

SOLUTION:

CONDITIONS:

PROCEDURE:

Our experience says that the "Other Reasons" is the area which is

Really difficult to point out as there could be various possibilities of the same.

There is list of some possibilities that has to be taken care while

analysing the SDCCH drop:

1.Hardware Fault at BTS

For BTS, HW fault should be the prime and preliminary point to be

concentrated. This can be seen by collecting The OMT Logs of CF,TRX,

ECU etc. This type of fault could also be intermittent in nature.

2.Definition of Parameter

Wrong definition or parameter in the BSC could lead to this type of fault.

Few important ones are as follows

The MS power control and BTS power control : As the MS and BTS powers are regulated in order to decrease battery consumption of the MS and not to saturate the multicoupler of the BTS as well as decreasing the

Interference in the system.So the fine tuning of the control parameters are very important in accordance

with R&D recommendation.

Activation of Adaptive configuration : This could also be the reason for the SDCCH drop. So proper R&D consultation is needed before activating the same, if required . It is advisable to use of this only in those cells which are really needing the same not the entire BSC area.

3.DIP Status

DIP with poor quality may lead to drop call problem in different cells.

Poor quality may implicate High BER or Frame Loss in the DIP connected to a site. DIP Quality Supervision tools should be initiated for the suspected connecting Digital Path. Mismatch of DIP could be another reason behind drop call occurrence in some of the cell.

4.frequency Interference Problem

Interference is generally related with Quality Drop, but sometimes it may provoke Sudden Drop as per situation.

5.C7 link

The C7 link between BSC to MSC should be properly dimensioned and check The command C7ERP for SCCP information.

Stability of C7 link

6.Proper definition of MSC parameter

The main functions of SDCCH are as follows :

-Call Setup

-Paging

-Location Updating

-Authentication

-Ciphering

-SMS

-IMEI Check

As we could see above, the MSC is also very well involved in above functions so it is equally required to check MSC part also while analyzing the SDCCH drop.

Few important parameters definition are given below :

1.The Exchange properties IMEICONTROLSMS and IMEICONTROLSS to "0" (No IMEI related checks ON if

you don't have a EIR),

2 PHASE2-1 in the BSC parameters in MSC (MGBSP).

Same as above, we may also check the parameter related to Location

updating, SMS, ciphering , authentication etc

------------------------------------------------------------------------------------------------

HOSR AnalysisProbable Reasons of Bad Handover Performance

---Neighboring Cell RelationAction:Add neighbor cell relation.

---Missed measurement frequencies in BA-listAction:Check measurement frequencies list.

---Permitted Network Color Code problemAction:Check NCC Permitted

---HW faults.Action: Check BTS error log.

---Blocking on Target CellAction:Remove Blocking on Tager Cell

---CongestionA high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers.Action: Check TCH congestion.

---Timer Expire After MS is LostThe MS never answers the base station.Action: Check coverage. Check interference.

---Link Connection or HW FailureAction: Check BTS error log. Perform site visit. Perform link performance measurements.

---Bad Antenna InstallationAction: Perform site survey and check antenna installation. Check antenna cabling.

---Many Neighbors DefinedMany defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few

samples per frequency and problem for mobiles to decode the BSIC.

Action: Check number of definitions.

---Delayed Handover Decision

A delayed handover decision can be due to congestion in the

target cell.

Action: Check handover parameters.

---Wrong Locating Parameter Setting

Action: Check locating parameters.

---Bad Radio Coverage

Action: Check coverage plots.

---High Interference, Co-Channel or Adjacent

The potential handover candidate is disturbed by interference.

Outgoing handover due to bad uplink quality may indicate

interference from co-channel another MS. On the border, the

quality may be rather bad and the signal strength low. Bad

downlink quality may indicate interference from another co-

channel base station.

Action: Check interference. Check if many handovers are

performed due to downlink or uplink bad quality.

---Receiver Antenna Problem or RBS HW problems (in

candidate cell)

Action: Check antenna installation. Check RBS HW and Error

log of the target cell

---Poor Inter-MSC/BSC Handover Performance

For outer or external cell, wrong definitions in either MSC or

BSC may be reason for the problem.

Action: Check inter-MSC/BSC handover performance.

---Incorrect Down Tilt

Action: Perform site survey and check antenna installation.

Solution: Correct antenna tilting.

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FREQUENCY HOPPINGWhat is Frequency Hopping?

Frequency Hopping is an old technique introduced firstly in military

transmission system to ensure the secrecy of communications and combat

jamming. Frequency Hopping is mechanism in which the system changes the

frequency (uplink and downlink) during transmission at regular intervals. It

allows the RF channel used for signaling channel (SDCCH) timeslot or traffic

channel (TCH) timeslots, to change frequency every TDMA frame (4.615 ms).

The frequency is changed on a per burst basis, which means that all the bits in a

burst are transmitted in the same frequency.

Advantages of Frequency Hopping

1. Frequency Diversity

In cellular urban environment, multipath propagation exists in most cases. Due

to Rayleigh fading, short-term variations in received level are frequently

observed. This mainly affects stationary or quasi-stationary mobiles. For a fast

moving mobile, the fading situation can be avoided from one burst to another

because it also depends on the position of the mobile so the problem is not so

serious. Frequency Hopping is able to take the advantage due to frequency

selective nature of fading to decrease the number of errors and at the same

time they are temporally spread. As a result, the decoding and de-interleaving

processes can more effectively remove bit errors caused by bursts received

whilst on fading frequencies (errors will be randomly distributed instead of

having long bursts of errors). This increase in effectiveness leads to a

transmission quality improvement of the same proportion.

· Frame Erasure Rate reduces due to 6 dB to 8 dB gain.

· Number of reports with rxqual 6 and 7 reduce.

· Reported values of rxlev are more concentrated around mean.

2. Interference Averaging

Interference Averaging means spreading raw bit errors (BER caused by the

interference) in order to have random distribution of errors instead of having

burst of errors, and therefore, enhance the effectiveness of decoding and de-

interleaving process to cope with the BER and lead to better value of FER.

With hopping, the set of interfering calls will be continually changing and the

effect is that all the calls experience average quality rather than extreme

situations of either good or bad quality. All the calls suffer from controlled

interference but only for short and distant periods of time, not for all the

duration of the call.

· For the same capacity, Frequency Hopping improves quality and for a given

average quality Frequency Hopping makes possible increase in capacity.

· When more than 3 % of the reports have rxqual of 6 or 7 then voice quality

disturbances start to appear.

· Gains (reduction in the C/I value needed to satisfy the quality requirements

involved in the criterion) from hopping relative to fixed frequency operation can

be achieved.

1/3 interference: 1 dB gain

i.e. if 1 out of 3 frequencies are experiencing a continuous interference a gain of

1 dB in C/I requirement is obtained.

Similarly,

1/4 interference: 4 dB gain

1/5 interference: 6 dB gain

2/4 interference: 0 dB gain

2/5 interference: 4 dB gain

The effective gain obtained with Frequency Hopping is due to the fact that the

interference effect is minimized and it is easier to keep it under control.

Types of Frequency Hopping

There are two ways of implementing Frequency Hopping in a Base Station

System, one referred as Base Band Frequency Hopping (BBH) and another as

Synthesizer Frequency Hopping (SFH). Their operation differs in the way they

establish the Base to Mobile Station link (downlink), however there is not

difference at all between Mobile Station to Base Station link in both types of

hopping. Motorola does not allow BBH and SFH to be used together on the

same site

1. Base Band Frequency Hopping

This is accomplished by routing the traffic channel data through fixed frequency

DRCUs via the TDM highway on a timeslot basis. In this case, the DRCU would

have fixed tuned transmitters combined either in low loss tuned combiners or

hybrid combiners.

· DRCU always transmits fixed frequency.

· The information for every call is moved among the available DRCUs on a per burst

basis. (Burst of 577 µs)

· Call hops between same timeslots of all DRCUs.

· Processing (coding and interleaving) is done by digital part associated with DRCU

on which call was initially assigned.

· For uplink – call is always processed by DRCU on which the call was initially

assigned.

· Number of DRCUs needed is equal to the number of frequencies in the hopping

sequence.

· BCCH frequency can be included in the hopping sequence.

· Power control does not apply to BCCH or bursts transmitting BCCH frequency.

· BCCH, timeslot 0 will never hop.

· Any timeslot with CCCH will never hop.

· Timeslot carrying all SDCCHs can hop.

If a network running with fixed frequency plan is switched over to BBH (BCCH

included in MA list) without any frequency changes, significant quality

improvement can be observed in the network. As a result drop call rate reduces

in the network. Alternatively, for the existing network quality additional

capacity can be provided. FHI can be used effectively in BBH. Further details

regarding FHI planning are discussed later in the document.

2. Synthesizer Frequency Hopping

This is accomplished by high speed switching of the transmit and receive

frequency synthesizers of the individual DRCUs. As a result of dynamic nature

of the transmit frequency, broadband (hybrid) combining of the transmitters is

necessary.

· DRCU changes transmitting frequency every burst.

· Call stays on the same DRCU where it started.

· Remote tune combiners (RTC) are not allowed.

· Number of DRCUs is not related to number of frequencies in hopping sequence.

· BCCH can be included in the hopping sequence:

1. If BCCH is included in the hopping sequence, timeslots 1 to 7 can not be

used to carry traffic. They transmit dummy burst when BCCH frequency

is not in the burst. Whenever BCCH frequency is being transmitted in a

burst by DRCU, it will be transmitted at full power.

2. BCCH DRCU will never hop. It either carries traffic in timeslots 1 to 7 or

it transmits dummy bursts.

· Transmission and reception is done on the same timeslot and same DRCU.

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 pseudorandom

hopping sequences.

Mobile Allocation Index Offset (MAIO): Determines inside the hopping

sequence, which frequency the mobile starts to transmit on. The value of MAIO

ranges between 0 to (N-1) where N is the number of frequencies defined in the

MA list. MAIO is set on per carrier basis.

Motorola has defined an additional parameter, FHI.

Frequency 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.

GSM algorithm

GSM has defined an algorithm for deciding hopping sequence. The algorithm is

used to generate Mobile Allocation Index (MAI) for a given set of parameters.

ARFCN: absolute radio frequency channel number

MA: mobile allocation frequencies.

MAIO: Mobile allocation offset (0 to N-1), where N is the number of frequencies

defined in MA.

HSN: Hopping sequence number (0-63)

T1: Super frame number (0-2047)

T2: TCH multiframe number (0-25)

T3: Signaling multiframe number (0-50)

This algorithm generates a pseudorandom sequence of MAIs. MAI along with

MAIO and MA will decide the actual ARFCN to be used for the burst.

Planning for Frequency Hopping

1. Frequency Plan:

Frequency Hopping plan differs from the conventional fixed frequency plan. The

plan depends upon the type of Frequency Hopping system used. In case of SFH

including BCCH frequency in hopping sequence is not a practical option, as it

results in loss of traffic channels on BCCH carrier. A separate frequency plan is

prepared for the BCCH carriers. This planning is very much similar to the

conventional fixed frequency plan with lesser number of frequencies. This plan

needs to be done very carefully as the system monitors cells based on the BCCH

frequency only. Since BCCH carrier radiates continuously without downlink

power control, frequencies used for BCCH on one cell should not be used as

hopping frequencies on other cell. The reason is to avoid continuous

interference from BCCH carriers. The benefits of hopping increase if more

frequencies are available for hopping. Generally the frequency band is divided

into two parts, one used for BCCH frequency plan and other for hopping

frequencies. The division of frequency band for allocation of BCCH and hopping

carriers should be done to maintain reasonable C/I for BCCH carriers as well as

to have enough frequencies for hopping.

e.g. consider a network with 31 frequencies, using 12 frequencies for BCCH and

using 18 for hopping with 1 frequency as guard, is the ideal option. But it may

not be practically possible to plan BCCHs with 12 frequencies (4/12 reuse).

Using 15 for BCCH plan and 15 for hopping frequencies is more practical.

There always exists a trade-off between BCCH and hopping plans. Using very

less frequencies for BCCH plan might result in poor quality on BCCH carrier

and the advantages of having quality improvement on hopping carriers may be

lost.

In case of BBH, generally BCCH carrier is included in the hopping sequence.

The benefits of BBH can be obtained only when most of the sites in the network

are having more than one NBCCH carriers. Benefits of BBH comparable to SFH

can only be obtained by equipping additional hardware in order to include more

frequencies in hopping sequence. However BBH without additional hardware

will result in quality improvements and provide scope of additional capacity as

compared to fixed frequency plan though the benefits may not be as significant

as seen in SFH.

2. Planning of HSN:

HSN allocation to the cells is done in random fashion. Various scenarios are

explained below:

a. MA list is same for all the cells of the site – In this case HSN is kept same for all

the cells of the site. MAIO is used on per carrier basis to provide offset for

starting frequency in hopping sequence and avoid hits among carriers of the

site. Practically it is possible to achieve 0% hit rate within the site, as all the

cells of the same site are synchronized.

b. MA list is same for the cells of different sites – In this case HSN should be

different for all such cells. MAIO can be same or different in this case as HSN is

different.

c. MA list is different for the cells – In this case HSN planning is not important, as

there can not be any hits between these cells.

d. HSN is set to 0 – This is the case of cyclic hopping. The sequence for hopping

remains same and is repeated continuously. This is not recommended in the

urban environment where frequency reuse is more. This is because the network

is not synchronized so if there is any one hit it will result in continuous

sequence of hits. Cyclic hopping is preferred in rural environment as it provides

the maximum benefits of frequency diversity.

3. Planning of MAIO:

The benefits of MAIO planning can be best achieved only in case when sectors

having same MA list are synchronized. For non-synchronized sectors MAIO can

be the same. In the present version (GSR2), Motorola does not provide manual

MAIO setting. It is set automatically by the system. However from GSR3

onwards it will be possible to set MAIO manually. It has to be changed on a case

to case basis. In cases where there are large numbers of hits, MAIO change can

be very effective as it adds the offset in the hopping sequence and hitrate can

be reduced.

4. Planning of FHI:

This parameter is not specified in GSM. FHI is the Motorola defined hopping

system. It actually means an independent hopping system consisting of MA and

HSN. Total of 4 such hopping systems can be set in a cell.

FHI can be defined on a timeslot basis.

e.g. consider a cell with 3 carriers i.e. 2 carriers are hopping. It is then possible

to define 4 different FHIs for 16 timeslots. That means timeslot 0 to 3 of 1

carrier can have one FHI and so on.

Benefits and Drawbacks of FHI

· Separate FHI can be defined even for each carrier with separate MA list.

· For a fully utilized cell, FHI can be used to control increase in hitrate during peak

hours. This can be done by defining different MA list associated with a FHI for

one of the carriers.

· Main benefits of FHI can be obtained in BBH. Consider a cell with 2 carriers using

BBH with BCCH included in the hopping sequence. Timeslot 0 of BCCH will not

hop. A separate FHI (with MA list without BCCH frequency) has to be defined

for timeslot 0 of NBCCH.

· Different FHIs in the same cell is not used extensively in Motorola networks with

SFH, where BCCH frequency is not included in hopping sequence.

· One drawback of using FHI on timeslot basis is that it adds more complexity to

the database.

5. Reuse pattern for hopping carriers:

Conventionally there are 3 main reuse patterns followed for hopping

frequencies.

1 X 1: It means all the cells in the network use the same frequencies for

hopping.

e.g. If 15 frequencies are to be used for hopping, then every cell will have all 15

frequencies in the MA list. This type of reuse is useful in urban areas, where

capacity requirement is large. However there is very less planning involved and

so less control over quality problems.

3 X 9: Three hopping groups are used in 3 sites, one per site. In this case all the

sites should be considered as omni sites for planning frequency reuse. The

advantage of this scheme is it provides better isolation between sites using

same hopping frequencies. The problem with this method is that, addition of

new site may require frequency replan for the area.

1 X 3: This scheme is very commonly used in Motorola networks. Hopping

frequencies are divided in 3 groups. Each cell on a site uses one group and it is

repeated on all sites. e.g. consider a network with standard orientation, all V1

sectors will use the same group and so on. It is very easy to add a site in the

network. This reuse scheme is suitable for homogeneous network with minimum

overlapping areas. The problem with this scheme is in peak hours there may be

more hits.

6. Effect of Frequency Hopping

Handovers: When SFH is implemented, BCCH plan is done using lesser

number of frequencies as compared to fixed frequency plan. This may result in

quality degradation. However quality of hopping carriers improves than before.

Also, quality threshold for handovers on hopping carrier should be increased as

compared to fixed frequency plan. In the present version (GSR2), same quality

threshold settings are set for both BCCH and NBCCH. This may result on more

drop calls on BCCH carriers. However GSR 3 provides separate settings for

BCCH and NBCCH carriers. By setting lower quality thresholds for BCCH as

compared to NBCCH, number of dropped calls can be controlled.

Call setup: In call setup, SDCCH hopping is also possible. There are no

separate settings required for SDCCH hopping. b Since GSR3 allows control

over SDCCH configuration (location of SDCCH on timeslot basis), SDCCH

hopping depends on the location of SDCCH. In case of SFH (with BCCH not

included in MA list), if SDCCHs are on BCCH carrier they will not hop whereas

SDCCHs on NBCCH carriers may hop. Generally it is preferred to keep SDCCHs

on hopping carriers as they have better C/I compared to BCCH carriers. Call

success rate will depend on the cleanliness of BCCH carriers.

Frame Erasure Rate (FER): FER indicates the number of TDMA frames that

could not be decoded by the mobile due to interference. This parameter gives

the indication of hitrate. FER improves (gain of 6 to 8 dB) after implementation

of frequency hopping.

7. Tools for simulation and drive test: Motorola uses a tool “Handsem”

which can simulate SFH plan (different reuse patters and HSN plan). Latest

versions of plaNET and Golf are supposed to support Frequency Hopping

simulation. Drive test tools that display decoded layer 3 information are used

for monitoring frequency hopping networks. TEMS is one of the drive test tools

that can be used for the purpose.