GSM - Paging Success Rate (PSR) Analysis

Paging success rate can be degraded due to increase in MOC/MTC attempts, low TCH availability of source and neighboring cells, high LAC/BSC utilization. Following steps should be taken to analyze the root cause:

• Check for MOC/MTC trend - increase may cause high PPS (Paging Per Second) causing paging discards, indicating paging capacity limitation. LAC splitting could be done to resolve the issue
• Examine TCH Availbility of sites in the affected BSC
• Inspect cell level paging stats from the core network to zero in on the problematic LACs
• Parameter discrepancy between BSC and MSC can cause degradation - check parameters such as Location Update Timer, paging retransmissions by MSC, paging times etc.
• Interference, low coverage due to large distance or dense areas, path balance issues contribute to PSR degradation

GSM - TCH Availablility

TCH Availability is the ratio available TCH (Full Rate, Half Rate, dynamic radio channels) to the total available and unavailable TCH (Full Rate, Half Rate, dynamic radio channels). Generally, TCHA follows the pattern of available TCH/F and dynamic radio channel.

TCHA is one of the first KPI that should be checked in environments where power outages are common. It degrades blocking, DCR, HOSR and other KPIs to a large extent. More often than not, low TCHA of neighboring cell causing severe KPI degradation in a particular cell. It is always safe to revive TCHA of the network before implementation of parametric or design changes for KPI improvement.

Major alarms causing low TCHA are STP link association broken (transmission), and GENSET on (power outage).

UMTS Low CS/PS IRAT Handover Success Rate Analysis

Analysis of low CS/PS IRAT HOSR can be done using the below mentioned procedure:

1. Neighbor Audit - removal of illogical neighbors is equally important as adding Tier 1 cells
2. External neighbors configuration check - Often times, BCCH/NCC/BCC is modified and should be updated in external cells on the UMTS end
3. Parameter check
4. GSM State mode - can be modified in case of severe handover failures on one relation 

Apart from these steps, it should be checked whether the IRAT HOSR is low for both CS and PS or just one of these. If it is low for only one of the services, compressed mode parameters can be tuned to trigger early handover when the signal strength is high. Analysis should be followed from cluster to cell to relation level to pinpoint the issue.

Base Station Solution

Base station solution can be classified into two types:

Macro: where BBU and RFU are part of the cabinet of the base station
Distributed: where the cabinet comprises of BBU and RRU. RRU can be deployed close to the antenna which reduces antenna loss and can contribute to enhanced coverage.

RRU can be singe mode or multimode (GSM, UMTS, and/or LTE). A typical RRU contains sub systems such as Transciever (TRX), Power Amplifier (PA)/LNA, and Duplexer (DFL).

UMTS Reselection Parameters

General parameters with typical network values of the three types of reselection in UMTS is presented below:

3G to 3G

• qrxlevmin = -109 dBm     (min acceptable threshold of cell RSCP)
• qqualmin = -18 dB           (min acceptable threshold of cell quality EcNo)

• sintersearch =  10 dB   (f1,f2 case)
• sintrasearch =  14 dB    (same frequency)

3G to 2G

• ssearchrat = 2dB            (indicates when to trigger measurements on GSM cells, increasing this value quickens while decreasing it delays the process)
• treselect = 2s
• qoffset1sn = 7dB for gsmrelation; 0 dB for utranrelation                                                       (Source and target signal offset when reselection strategy is RSCP)
• qoffset2sn = 0 dB                 (Source and target signal offset when reselection strategy is EcNo)

2G to 3G

• FDDqRSCPmin = -102 dBm
• FDDqmin = -12 dB

Equation

The equations on which the decision is made using these parameters are:

For EcNo based reselection:

Squal <= sintrasearch     OR        sintersearch           OR            ssearchrat

where Squal = qmeasured  -    qqualmin

Thus the equation becomes:

qmeas - qqualmin <= sintrasearch

qmeas  <= qqualmin + sintrasearch

Similarly, EcNo values can be replaced with RSCP for the RSCP based reselection.

GSM TCH Channel Allocation Priorities - Zero Traffic Analysis

Zero traffic means the hardware is not carrying traffic even if there is blocking on the cell. Below are the steps to check for this issue:

1- Filter out TRXs having zero attempts/traffic
2- Check those cells TCH availability - if there is low TCH, then resolve low TCH first
3- Examine TCH blocking on these cells - if there is zero blocking, then the traffic is not high enough
4- Inspect the below TCH channel allocation priorities to finalize the zero traffic TRXs and recommend for health check

Note: TRXs can have zero or low assignments with normal ranges of availability if there is no blocking.

GSM - Low Paging Sucess Rate Issue

Paging discards can be a source low PSR or users complaining about getting "user not responding" messages as they call party B in GSM. The steps that can be taken to reduce or eliminate discards are as follows:

1- LAC Splitting
2- Cell paging capacity
      - can be increased of worst offenders
      - Shifting of SD/PD timeslots to non-BCCH TRXs
3- Audit of Periodic Location Update BSC Timer Value (t3212)
      - Can be set to 6 hours
      - This timer value at the MSC should be greater than the BSC value
4- Activation of Precise Paging Feature
      - Paging is done only on cells where there is high probability of finding subscribers according to the location record saved in the BSC

BS PA MFRMS - PSR Alcatel-Lucent

Improvement in PSR (Paging Success Rate) can be achieved by tuning of BS_PA_MFRMS and reducing Paging discards. Paging discards are mostly due to congestion in the BSC, on the Abis link and/or on the PCH (Paging Channel) ie Air interface.

Parameter:

• BS_PA_MFRMS

Target Cells: Cells with higher number of paging discards

Theory: 

• This parameter defines the the number of 51 multiframes between two consecutive occurrences of Paging group (a number of pagings)
• Higher value means less number of pagings will be transmitted in a given interval thereby conserving lesser power of the MS
• Reducing this parameter increases number of pagings per paging group (for a given value of MSC paging timer T3113) and therefore probability of no paging response decreases

Mobile Originating Call Flow

Here is the detailed Mobile Originating Call Flow divided into 4 functional modules depicting the network nodes for easy comprehension:

Concentric Cell (Co-BCCH) Technology

Concentric cell (also known as Common BCCH) are those that operate with both GSM and DCS TCH frequencies but with a single BCCH (of GSM), thereby having below advantages compared with dual-BCCH:

• DCS BCCH resource is saved per cell
• Reduced BA List
• Reduced DCS BCCH interference
• Reduced number of neighbours
• Potential for Reduction in number of Handovers

An important physical parameter consideration for Concentric Cells is the tilt difference between the DCS and GSM antennas. Generally, in an interfered GSM network, the DCS antenna has lower downtilt than the GSM antenna to equalize the coverage area.

Repeated SACCH/FACCH - CDR Alcatel-Lucent

Call Drops can be reduced to a great extent in the equipment of Alcatel-Lucent (B11, B10) by using the feature of repeated SACCH/FACCH frames.

Parameter:

• EN_REP_DL_FACCH
• EN_REP_SACCH

Target Cells: Radio Drops, HO Drops

Theory: 

• Each DLFACCH frame is sent twice by enabling this parameter thereby improving handover reliability in AMR Calls reducing call drop rate
• If a SACCH is not decoded correctly by MS, the BTS will resend it that enhances coverage and decreases drops (SACCH is used for transmission of radio measurement data such as TA, Power Control Info, SMS during call etc)