WCDMA Overview

What is 3G?
3rd Generation of mobile communication based on IMT-2000 global standardization

3G characteristic:
Designed for multimedia communication from the beginning
Provides more efficient means for image and video transfer
Variable bit rates up to 2 Mbps (Rel’99)
Multiple access using Wideband CDMA (WCDMA)
All cells are using same frequency band, no need Frequency planning
Using codes for separate user and channel
Effective Signal Bandwidth 3.84MHz (wider bandwidth compared to GSM)
Short time delay (compared to GSM), 10ms frame length with 15 time slots
Multiple carriers can be used to increase capacity
Inter frequency functionality to support mobility between frequencies carrier
Compatibility with GSM technology
Inter System functionality to support mobility between GSM and UMTS
Soft Handover, add margin which improved performance

Network Element

User Equipment (UE)
Mobile equipment (ME): Radio communication over Uu interface
UMTS Subscriber Identity Module (USIM): Subscriber identity information, authentication algorithms, encryption keys, etc
UMTS Terrestrial Radio Access Network (UTRAN)
Node B (Base Station): Handles/manages the traffic between Uu and Iub interfaces. Basic tasks like coding, interleaving, rate adaptation, modulation, spreading, closed loop power control
Radio Network Controller (RNC): Control radio resources in its operation area. Provide services for Core Network (CN). Load and congestion control, admissions control, code allocation, radio resource management tasks.
Mobile Services Switching Centre (MSC) / Visitor Location Centre (VLR)
Handles switching in Circuit Switched (CS) connections and hold visiting users service profiles.
Serving GPRS Support Node (SGSN)
Similar functionality as in MSC/VLR but used for Packet Switched (PS) services
Other CN elements
Gateway MSC (GMSC): Handles incoming and outgoing CS connections
Gateway GPRS Support Node (GGSN): Like GMSC but in PS domain
Home Location Register (HLR): Master copy of users service profiles
Authentication Center (AuC): Handles authentication based on SIM data
Equipment Identity Register (EIR): Handles blacklist of Mobile Phone based on IMEI.

 

Iklan

GSM Timer

GSM Timers

Call Failure for mobile calls can be caused by many reasons. Some can be an expired range of Timers. For example we can see that timer T3216 (below) in essence relates to the failure of a Immediate Assignment Request, but the “root cause” of the failure is due to SDCCH congestion or poor radio link, such as: interference, coverage restriction or radio path imbalance. Understanding the “Causes for the cessation or loss of mobile communication” requires more than knowing the Cause Code or Timer but all the “root cause” behind them.

The Timer table below provides a useful but not exhaustive list. It important to keep monitors the GSM and 3GPP standards. Finally, it is important to recognize that Timers have different durations dependent upon when the timer is applicable. For instance, for radio resources management the durations are often denoted in seconds and some timers are in milliseconds.

However, other timer durations (expiration) are used for internal operation for devices such as mobile telephone or SIM and can be in minutes and in some instances hours. An example of the latter can be the elementary file EFHPLMN (7F206F31) – see GSM11.11. The Timer is set in decimal-digit increments e.g. 01, 02, 03 and so on. Each increment represents a value of n-minutes which the standard GSM0211 refers to as 6 minutes, but commonly rapid updates can cause drain on the mobile telephone’s battery it is understood that n-minutes can be 30-minutes. The maximum the timer can be set for is 8-hours. The timer value is network operator dependent, which means either timer method may be used.

Timers and counters for radio resource management

Timers on the mobile station side

T3122: This timer is used during random access, after the receipt of an IMMEDIATE ASSIGN REJECT message. Its value is given by the network in the IMMEDIATE ASSIGN REJECT message.

T3124: This timer is used in the seizure procedure during a hand-over, when the two cells are not synchronized. Its purpose is to detect the lack of answer from the network to the special signal. Its value is set to 675 ms if the channel type of the channel allocated in the HANDOVER COMMAND is an SDCCH (+ SACCH); otherwise its value is set to 320ms.

T3126:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the immediate assignment procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station’s RACH. S and T, the maximum value of this timer is 5 seconds.

T3128: This timer is started when the mobile station starts the uplink investigation procedure and the uplink is busy. It is stopped at receipt of the first UPLINK FREE message. At its expiry, the uplink investigation procedure is aborted. The value of this timer is set to 1 second.

T3130: This timer is started after sending the first UPLINK ACCESS message during a VGCS uplink access procedure. It is stopped at receipt of a VGCS ACCESS GRANT message. At its expiry, the uplink access procedure is aborted. The value of this timer is set to 5 seconds.

T3110: This timer is used to delay the channel deactivation after the receipt of a (full) CHANNEL RELEASE. Its purpose is to let some time for disconnection of the main signaling link. Its value is set to such that the DISC frame is sent twice in case of no answer from the network. (It should be chosen to obtain a good probability of normal termination (i.e. no time out of T3109) of the channel release procedure.)

T3134: This timer is used in the seizure procedure during an RR network commanded cell change order procedure. Its purpose is to detect the lack of answer from the network or the lack of availability of the target cell. Its value is set to 5 seconds.

T3142: The timer is used during packet access on CCCH, after the receipt of an IMMEDIATE ASSIGNMENT REJECT message. Its value is given by the network in the IMMEDIATE ASSIGNMENT REJECT message.

T3146:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during a packet access procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message during a packet access procedure, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the packet access procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station’s RACH. S and T are defined in section 3.3.1.2. The maximum value of this timer is 5 seconds.

T3164: This timer is used during packet access using CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message. It is stopped at the transmission of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expire, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

T3190: The timer is used during packet downlink assignment on CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message or of an PDCH ASSIGNMENT COMMAND message when in dedicated mode. It is stopped at the receipt of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expiry, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

Timers on the network side

T3101: This timer is started when a channel is allocated with an IMMEDIATE ASSIGNMENT message. It is stopped when the MS has correctly seized the channels. Its value is network dependent. NOTE: It could be higher than the maximum time for a L2 establishment attempt.

T3103: This timer is started by the sending of a HANDOVER message and is normally stopped when the MS has correctly seized the new channel. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the HANDOVER COMMAND, plus the value of T3124, plus the maximum duration of an attempt to establish a data link in multiframe mode.)

T3105: This timer is used for the repetition of the PHYSICAL INFORMATION message during the hand-over procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.

T3107: This timer is started by the sending of an ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly seized the new channels. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the ASSIGNMENT COMMAND message plus twice the maximum duration of an attempt to establish a data link multiframe mode.

T3109: This timer is started when a lower layer failure is detected by the network, when it is not engaged in a RF procedure. It is also used in the channel release procedure. Its purpose is to release the channels in case of loss of communication. Its value is network dependent. NOTE: Its value should be large enough to ensure that the MS detects a radio link failure.

T3111: This timer is used to delay the channel deactivation after disconnection of the main signaling link. Its purpose is to let some time for possible repetition of the disconnection. Its value is equal to the value of T3110.

T3113: This timer is started when the network has sent a PAGING REQUEST message and is stopped when the network has received the PAGING RESPONSE message. Its value is network dependent. NOTE: The value could allow for repetitions of the Channel Request message and the requirements associated with T3101.

T3115: This timer is used for the repetition of the VGCS UPLINK GRANT message during the uplink access procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.

T3117: This timer is started by the sending of a PDCH ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly accessed the target TBF. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the PDCH ASSIGNMENT COMMAND message plus T3132 plus the maximum duration of an attempt to establish a data link in multiframe mode.

T3119: This timer is started by the sending of a RR-CELL CHANGE ORDER message and is normally stopped when the MS has correctly accessed the new cell. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the RR_CELL CHANGE ORDER, plus T3134, plus the maximum duration of an attempt to establish a data link in multiframe mode.

T3141: This timer is started when a temporary block flow is allocated with an IMMEDIATE ASSIGNMENT message during a packet access procedure. It is stopped when the mobile station has correctly seized the temporary block flow. Its value is network dependent.

 

Handover Problem Troubleshooting

Handover Analysis
Missing Neighbor is a very critical problem in optimizing GSM neighbors. Missing Neighbor as such doesn’t means that there is no neighbor. We define neighbors to a cell based on the geography, i.e. which base stations are nearby.
In certain cases we may also use the Planning Tools to predict neighbors. It is such not desirable to use excessive neighbors also, since this reduces the samples collected per neighbor.
Since Mobile has to look for each neighbor in one TDMA frame, and we have 104 frames for measurement. This means if we define 32 neighbors , the number of samples per neighbor will be only 3-4, which means the authenticity of Handovers will not be there. So we need to define appropriate neighbors only.

Create Neighbors as efficiently as possible

neighbour

Too Many Unnecessary Neighbors

Handover Margin

Handover Margin is value set for all cells, which will allow mobiles to handover to the cell, only when this cells power budget balance exceeds the source cells power budget balance by this value.
Handover Margins main objective is to avoid ping-pong effects of handovers on cell periphery, when the power levels of two cells are near to equal , so due to multipath and mobility, this will result into frequent handovers between these two cells.
Setting this value low, will result into fast handover to the target cell, which may result in improvement in quality.
Setting this to high value will delay the handover to the target cell, and ensure that when handover takes place, the probability of the mobile going back to the source cell is very low for some significant period

Optimizing – Handover Margin

Adjacent Channel Interference Reduction
> Tight Re-use patterns can permit the use of Adjacent ARFCN in adjacent cell.
> This should not pose an problem if Handover margin is set appropriately low.

HO Margin

Multipath solutions
> Antenna Tilts
> Using Directional Antenna’s
> Shifting the BTS site

Handover Parameters

HO Para

 

 

 

Drop Call Troubleshooting

Call Drop

Call drops are identified through SACCH messages.  A Radio Link Failure Counter value is broadcast on the BCH. The counter value  may vary from network to network. At the establishment of a dedicated channel, the counter is set to the broadcast value (which will be the maximum allowable for the connection). The mobile decrements the counter by 1 for every FER (unrecoverable block of data) detected on the SACCH and increases the counter by 2 for every data block that is correctly received (up to the initial maximum value). If this counter reaches zero, a radio link failure is declared by the mobile and it returns back to the idle mode.

If the counter reaches  zero when the mobile is on a SDCCH then it is an SDCCH Drop.  If it happens on a TCH, it is a TCH drop.

Sometimes an attempted handover, which may in itself have been an attempt to prevent a drop, can result in a dropped call.

When the quality drops, a mobile is usually commanded to perform a handover.  Sometimes however, when it attempts to handover, it finds that the target cell is not suitable.  When this happens it jumps back to the old cell and sends a Handover Failure message to the old cell. At this stage, if the handover was attempted at the survival threshold, the call may get dropped anyway.  If on  the other hand the thresholds were somewhat higher, the network can attempt another handover.

Drop Calls Analysis

Call Drop

SDCCH Drops – Causes
Coverage
Interference & Multipath
BTS performance

TCH Drops – Causes
Coverage
Interference & Multipath
BTS performance
Pre-emption

Handover Failure – Causes
Threshold parameters
Missing neighbors

Solutions to Dropped Calls
Optimize Coverage
Interference Management
Optimize neighbors
Optimize handover parameters
Effective Frequency Hopping
Use of DTX & Power control

CallDrop Flow

 

 

Troubleshooting & Optimizing 2G

Blocked Call & Troubleshooting

Blocked Calls can occur due to :
Access Failures
SDCCH Congestion
SDCCH Drop
TCH Congestion

Trouble shooting cause :
Use Layer 3 messages to analyze the cause
Decode System Information Type 3 messages.
Note the parameter, “MSMAXRETRAN ”; “BSPAMFRAMS ” and “BSAGBLKSRES

The best way of analyzing blocked calls, to identify the cause, is from a Layer III protocol log.
* Paging Failure
A paging message always originates from the MSC and is sent to all the BSCs in the Location Area of the MS to be paged. The BSC will then calculate the Paging group of the MS and send a Paging Command to the BTSs controlling the Location Area of the MS. On the air interface there are two cases of Paging Failure, either the Mobile receives no Paging message or it receives a Paging message, but is not able to respond (not able to send a RACH) which could be due errors in the Paging message.
* Access Failure
Irrespective of the purpose, for any communication required with the network, a mobile sends a Channel Request (for SDCCH) on a RACH and waits for some time for a response which should come from the BTS on an AGCH. A mobile will do several retransmission of RACHs  (pre-defined) and if it still does not get a response, it goes back to idle mode and preferably does a cell reselection.  At this stage we call it an Access Failure.
* SDCCH Blocked
Once a mobile has sent a Channel Request on a RACH , it expects a response from the BTS on the AGCH. This should be an Immediate Assignment Command to an  SDCCH. If an Immediate Assignment Reject comes instead , then this is SDCCH blocking.
* TCH Blocked
After the completion of call set-up signaling, a mobile expects an Assignment Command to a TCH so that speech can commence.  If no Assignment occurs for a specific period and the Mobile has to return to idle mode, then it is due to TCH congestion.

Blocked Call Analysis – L3 messages

bl

Access Failures
– CCCH Overload at the Base Station
– Uplink Interference at the Base Station
– Low Rxlev at the Base Station
– Base Station TRX decoder malfunctioning
– Downlink Low Rxlev ( Coverage Hole )
– Downlink Interference
– Excess Cell Range

accacc2

Access Failure – Uplink Problem

Causes:
1.  AGCH Overload at Base Station
2.  RACH Collisions
3.  MS out of Range
4.  Poor Uplink quality
5.  BTS Receiver Problem
agch

AGCH Overloading – Root Cause Analysis

If Multiple Immediate Assignment Extended Messages are seen, problem could be AGCH overloading OR  RACH Collisions/Non-detection

If MSMAXRETRAN and TX-Integer are set to a lower value, problem could be more towards RACH Collisions/ Non-detection

If set high, then possibility of overloading is high!!

Check for CCCH Configuration (combined or un-combined) and BSAGBLKSRES.
If less blocks are reserved, problem is overloading.

Analyze OSS data for the same period for the following stats:
– No of Deletions
– No of Successful RACHs
– RACH Busy counts
– No of RACH’s with invalid establishment cause.

RACH Collisions/ Max Range – Root Cause Analysis

MSMAXRETRAN and TX-Integer set low –  RACH Collisions is possible

Check for Distance from Base Station
– Calculate distance between “Channel Request” and BTS
– Compare this distance with the MAXTADROPCALLTHRESHOLD set for this cell (Table SET GCELLCCAD)
– If the MS distance is more than MAXTADROPCALLTHRESHOLD then problem with Maximum Range

If both the above conditions don’t meet, then problem is Non-Detection

RACH Non-Detection – Root Cause Analysis

Downlink is fine !!!  Parameters are well set for RACH control !!!

Problem could be Uplink Quality /  Base Station

Analyze the following OSS Data
– No of Invalid RACHs
– Interference on Idle Channel
– SDCCH RF Loss / TCH RF Loss

If Interference and RF Losses are above normal, problem is Uplink Interference.

If RF Losses are high, but interference is low, problem is Uplink level
Uplink level poor indicates Link Imbalance.

If all above conditions are satisfactory and still “No of Invalid RACH’s” high, problem could BTS Receiver Problem.

rach

SDCCH Congestion
Causes:
Location Updates;
To be analyses with OSS statistics first. If  high, determine cause of location update (border LAC, user mobility, etc.).
Drive around the suspected area in the Idle Mode

Interference;
Analyze OMC statistics on “Idle Channel Interference”

Heavy  Traffic;
Verify from OSS statistics SDCCH Congestion with number of SDCCH on cell.
Modify channel configuration or dynamic SDCCH Channel

TCH Blocked
Causes:
Interference ;
– Verify Idle Channel Interference reports from OSS
– If suspected, carry out uplink interference check

Heavy Traffic;
– Verify TCH utilization from OSS measurements
– Modify Idle mode parameter to share traffic to another cells
– Request upgrade TRX.

Solutions To Blocked Calls:
1. Optimize coverage
2. Optimize Cell Traffic
3. Interference management
4. Channel configurations
5. Optimize neighbors

 

 

 

 

 

Optimization Process (2G) Part 2

Performance Message Flow (Call Flow)

Overview

It’s a sequential order of specific message flows at Air Interface (Um) between MS and BTS and BSC.

In this section we’ll discuss some important event:
– Mobile Originating Call (MOC)
– Mobile Terminating Call (MTC)
– Handover
– Location Update

Mobile Originating Call (MOC)

Picture1

Mobile Terminating Call (MTC)

Picture2

Location Update

Picture3

KPIs (Key Performance Indicators)

Accessibility
This KPI measures the ability to set up a call. This range from the arrival of random access burst to the event users get TCH assignment. (Random Access Success Rate, SDSR, SDCCH Drop Rate)

Retainability
This KPI measures the ability to maintain the existing connection. This range from the service has been accessed to the event user disconnected the service. (TCH Drop Rate, Erlang per Minute Drop/EMD,)

Mobility
This KPI measures the ability to maintain user connection while moving in network.
(HOSR)

Integrity
This KPI measures the degree of service after the service has been accessed by user.
(Throughput, FER, SQI, RxQual)

 

a b c d

Idle Mode Operation (2G)

Cell selection and reselection

Cell Selection
It is a process when MS doing synchronization and registration to a network for the first time until MS can fully using service (MS Power ON or MS roams from one network to another)
Using BCCH to synchronize frequency, time and others.
Cell Selection using C1 Criteria.

Cell Re-Selection
It is a process when MS choosing and camped to a better new cell at Idle Mode.
Cell Re-Selection using C2 Criteria.

C1 Criteria
C1 is the path-loss parameter that is used to determine the strongest cell for selection.
The C1 uses the following parameters for calculation:
RXLEV = average receive level sense at MS
RXMIN = Minimum received signal level of the MS (Table GCELLBASICPARA)
FMSMAXOPCC = Maximum transmit power level of MSs when accessing network (Table GCELLCCCH)
Max RF Output MS = Max power MS for transmitting signal.

The formula for calculating C1 is given as:

C1 = (A) – Max(B,0)

Where:

  • C1 > 0
  • A = (RXLEV – RXMIN)
  • B = MS Transmit Power Max CCH (FMSMAXOPCC) – Max RF Output of MS

A -> This value is merely a difference between what RSSI is required to select that cell and what RXLEV the MS sees the tower at. If the RLAM is -110dB and the MS sees the tower at -90dB then the value of A is 20dB.

B ->This value is merely a difference between Power Transmit that BTS allowed MS to transmit (MS Transmit Power Max CCH) and Maximum Power Transmit that MS able to do (Max RF Output of MS).

C2 Criteria

C2 is the parameter used for cell reselection. After camped in, MS will continuously monitor neighbor cells by monitoring BCCH signal strength.

The C2 is calculated using the following parameters:

  • C1 = path-loss parameter
  • CRO = cell reselection offset, a value added to C1 (Table GCELLIDLEBASIC), each step equals 2dB
  • PT = cell reselect penalty time, is used to ensure the safety and validity of cell reselection because it helps to avoid frequent cell reselection (Table GCELLIDLEAD), each step equal 20s
  • TO = cell reselect temporary offset, (Table GCELLIDLEAD), each step equal 10dB
  • PI = Cell Reselect Parameters Indication, indicates whether CRO and TO is used or not, PI = Yes/No value (Table GCELLIDLEBASIC)

The formula for calculating C2 is:

C2 = C1 + CRO – (Temp_Offset * H),

Where:

  • H = 1 if the MS has been monitoring a particular cell for less than the penalty time.
  • H = 0 if the MS has been monitoring the particular cell for longer than the penalty time (expired).
  • H = 0 if the MS has been monitoring the original serving cell that MS camped.

And, C2 = C1 – CRO, if PENALTY TIME = 31.

Cell Bar Access

  • Work together with CBQ to make priority of a cell.
  • CBA = YES/NO value, if CBA TRUE then no MS can select this cell in idle mode (cell selection or cell re-selection) but existing calls are continued and handovers to this cell are still possible but as soon as the busy MSs return to idle mode they perform a cell reselection, too.
  • (Table GCELLIDLEBASIC)

Cell Bar Qualify

  • CBQ is used together with Cell Bar Access to decide the priority status of a cell. This parameter does not affect cell reselection but cell selection only.
  • (Table GCELLIDLEBASIC).

Cell Reselect Hysteresis
This is one of the parameters used for deciding whether to reselect cells in different location areas.
Parameter = CRH, value = 0dB, 2dB, 4dB, 6dB, 8dB, 10dB, 12dB, 14dB

So, in conclusion, When there is no abnormal situation happened, MS will trigger normal cell reselection as described below:
Cell reselection for cells in the same location area
C2(n)> C2(s) for 5 Seconds

Cell reselection for cells in different location areas
C2(n) > C2(s) + CRH for 5 seconds.

Cell Selection/Reselection Parameter

Conclusion:

Parameter for Cell Selection / Re-Selection are:

  • RXLEV-ACCESS-MIN
  • MS_TXPWR_MAX_CCH
  • CBQ (Cell_Bar_Qualify)
  • CBA (Cell_Bar_Access)
  • CRO (Cell Reselection Offset)
  • TO (Temporary Offset)
  • PT (Penalty Time)
  • PI (Cell Reselect Parameters Indication)
  • CRH ( Cell Reselection Hysteresis)