DCR Analysis

Pakistan Mobile Communications Limited 1

Contents

■ QOS Parameters

■ Interference

■ Quality

■ Dual & Common BCCH concept

■ Cell Site & Azimuth

■ DCR Analysis

■ Power Control


QOS Parameters

Network Accessibility:Network should have sufficient radio conditions (Typically signal Level and Quality) so that the Mobile Station (MS) can camp onto the Network without going into emergency mode.

Service Accessibility:Service Accessibility is the successful connection of a call (typically affected by open and closed loop no audios, error in connection, network busy).

Network Retainability: Quality of service that the subscriber experiences while using the Network (depending upon drops after call is established).


Interference

Types:

Co-channel interference (C/I)

Adjacent Channel interference(C/A)

External interference


Co-channel Interference

■ When a neighboring cell has been designated the same frequency. The later will interfere with the former.


Adjacent Channel Interference

■ Adjacent frequencies (A), that is frequencies shifted 200 kHz from the carrier frequency (C), must be avoided in the same cell and preferably in neighboring cells also.


External Interference & GSM Specification

External Interference:

External interference is caused by a GSM jammer or by any other wireless transmitter which is not operating within its assigned frequency range. External interference degrades the KPIs of nearby cells, so it is important to identify the source of interference and get it removed.

GSM Specification for C/I and C/A GSM specifications recommend that the carrier-to interference (C/I) ratio is

greater than 9 decibels (dB). GSM specification states that the carrier-to-adjacent ratio (C/A) be greater

than -9dB.


Quality

■ Quality reflects RF environment in call mode. It is measured as Received Quality (Rx-Qual), and is heavily dependent on interference.

■ Rx-Qual is measured in terms of Bit Error Rate (BER). There are eight ranges of BER that reflect upon Rx-Qual; higher the BER, worse will be Rx-Qual. Rx-Qual mapping with respect to BER is shown below:


Dual BCCH & Common BCCH


Dual BCCH Cell:

Better utilization of 1800 band

Traffic Sharing and reduction in blocking.

Initial SDCCH channel for a call set-up is allocated in the respective frequency band where the cell’s BCCH lies.

Increased intercell handovers.

Decreased efficiency of HO procedure, as there is a greater number of neighboring cells to monitor for an MS.

More LACs need to be defined in network resulting in increased location updates

Increased trunk load due to separate signaling links for both layers


Common BCCH Cell

Number of neighboring cells is reduced, since each sector is represented by a single CI.

MS has fewer cells to monitor i.e. more reliable measurements.

Only one layer BCCH frequency plan.

Higher traffic efficiency of the second frequency band not limited by BCCH/SDCCH allocations

Combined signaling channels . Combined traffic channels, therefore improved trunking gain.

Quality improvement due to decreased number of handovers.


Cell Site & Azimuth

■ First sector is “A” as it is at least degrees from North moving clockwise. Similarly, “B” is the second sector, and “C” is third one.


DCR

■ DCR is measure of dropped calls over total number of successfully set-up calls. A call is affirmed to be dropped if TCH is dropped after assignment.

■ DCR is dependent on Radio Link Timeout (RLT), and Handover (HO) drops from RF point of view. RLT is a counter for loss of SACCH frames It decrements by value of 1 when one SACCH is lost and increments by value of 2 if one SACCH is received.

■ RLT value is affected due to poor RF environment (Interference, low coverage). Probability of increase in HO Drops is directly proportional to increase in HO failures.


Types of TCH drops:DCR

RF Drops

HO Drops

Abis Drops

Transcoder Drops

BTS Drops

LAPD Drops

User-Action Drops

A-interface Drops


Radio DropsPoor signal level (Rx-Level) in dedicated (call) mode.

Rx level on DCS layer

Aggressive power reduction can lead to poor Rx-level

Poor signal level from neighboring cells if its logical server has outage

Poor C/I causing degraded quality.

Pro-interference frequency plan leads to very low C/I.

High VSWR between BTS cabinet and antenna (feeder cable problem, combiner issues etc.) could also lead to poor quality.

Overshooting cell

HO & Adjacency Parameter setting


HO Drops

Bad Neighbor Plan.

Poor RF Environment

Database Discrepancies

Non-standardized values of Adjacency and HO control parameters

Wrong/Incomplete External LAC Plan


Bad Neighbor Plan

■ Ensure that neighbor plan contains all logical relations (geographically first tier cells at least). An example of a precise neighbor plan is:


Database Discrepancies

■ Since database changes for handover parameters are quite frequent, database needs to be periodically checked for discrepancies. There might be some discrepancies between adjacency database and actual target cell information. Such discrepancies could include:

■ BCCH

■ BSIC (NCC/BCC)

■ LAC

■ Cell ID


Non-Standardized values of Adjacency and HO Control Parameters

Adjacency parameters majorly include:

i.) BCCH, BSIC, LAC, Cell ID information for target cell

ii.) Rx-Level margins for making handover attempts on the basis of relative signal strength (Power Budget Handover Margins)

iii.) Thresholds for minimum acceptable Rx-level of target cell to qualify for making handovers.

Handover Control (HOC) parameters majorly include:

i.) Thresholds and averaging mechanisms for making Quality & Level based handovers (Both Uplink and Downlink)

ii.) Non-BCCH Layer Access and Exit Thresholds & averaging mechanisms.


Wrong/Incomplete External LAC Plan

■ The External LACs are defined in MSCs so that inter-MSC handovers can be done. LACs have to unique for MSCs in the Network.

■ If the External LAC definitions are not correct or are incomplete in an MSC, then handovers will fail between MSCs; thus increasing the probability of HO Drops.

■ External LAC plans are provided by RF engineer, and are implemented in MSCs by NSS Department.


TCH Drops - Continued

•ABIS failures/drops are due to issues on ABIS interface. Such cases need to be raised to O&M.

Abis Drops

•BTS drops appear due to problem on the BTS, and have to be rectified by O&M.

BTS Drops

•These drops also appear when there is cell/site outage i.e. all calls, when outage appears, are dropped and pegged into LAPD failure. Such issues need to be raised to BSS O&M

LAPD Drops

•User action drops appear when an engineer (user) locks the TRX/cell/site for some configuration changes.

User Action Drops

•A-interface drops appear due to issues on the interface between BSC and MSC. These issues also need to be raised.

A-interface Drops

•Transcoder drops appear due to problems on E1 terminating at transcoder. These issues need to be raised to BSS O&M.

Transcoder Drops


Power Control

■ Power Control is used to reduce interference by decreasing power of BTS as well as MS. The power reduction adjustment is based on Rx-Level and Rx-Qual thresholds

Documents

DCR Analysis