3-Overshooting;
When we get the signal from an away site that not close to the current area drive test, whether this site was serving
you or just appears as a neighbor with good Rx levels to you.
-In case this overshooting site was serving with good levels:
Usually we get bad [Rx Qual] due to down-link
Usually we get bad [Rx Qual] due to down-link
interference from neighboring cells with Co. or Adjacent channel BCCH and large value for TA [Time Advance] also it
may causes call drop by the time as you will not be able to perform handover because all the neighboring sites to
you will be missing neighbors for it [not in your serving site BA list], you will remain on serving site until call drops
due to bad radio conditions.
It may also cause [congestion] problem under this overshooting site as it is now serving away from its supposed
coverage region so the site near-by users may found no resources to use + levels under site may be bad causing
coverage problems specially indoor.
-In case this overshooting site appears as a neighbor with good levels:
It may cause a bad [Rx Qual] due to 2G DL
interference problem [Co. or Adjacent channel interference] as due to the frequency reuse pattern, this
overshooting site may be using an ARFCN which your serving site are using as BCCH or TCH, so it will impact it
causing bad [C/I] value.
NOTE: Any sites near water may overshoots due to the ducting phenomena in water that it could transfers signal
for a large distances.
It may cause a bad [Rx Qual] due to 2G DL
interference problem [Co. or Adjacent channel interference] as due to the frequency reuse pattern, this
overshooting site may be using an ARFCN which your serving site are using as BCCH or TCH, so it will impact it
causing bad [C/I] value.
for a large distances.
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4-Blocking/Reflections
When Rx levels from your serving site suddenly drops to the -90's/-100's dbm which is very bad [While it was very
good] in the -60's during DT, while you are near to site and its levels to you was supposed to be very good but it
suddenly turned bad for a while then while moving levels turned very good again [Temporary bad levels].
In reality field you find a body [building-metal-trees-mountain or hill-etc……] which existed in the bad levels region
between you and serving site, this obstacle:
-Whether blocks LOS (Line Of Sight) for you from site, so you can't see site from your position of bad level. [Blocking]
Back then blocking body height is = or higher than serving site.
-Or makes reflections to received signal so that it doesn't reaches you with good levels. [Reflections]
Back then blocking body height may not be = or higher than site height, its height may be lower than site height
but made from a metal/conductor material or trees causing reflection, you even may see the site but you still find
bad levels.
As shown above, levels turned bad [Red color] suddenly after it was good ,once approaching near a region levels
turned bad and suddenly it turned good again, a metal container exists in the area of about same height as serving
site causing blockings to the received signal from site for a while.
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5-Cell Barred
When served by a cell in dedicated mode and as soon as you end your call you find yourself in (No Service Mode) as
this cell is banned to be accessed in idle mode, so this cell only accepts handovers in dedicated mode with good
levels normally but you can't access or initiate a call on it as it is closed/barred in idle mode.
As shown in snaps before, when served by sector 2 BCCH: 59 in dedicated mode after making a handover from
sector 1 BCCH: 29, levels were good and coverage in front of sector 2 was good, but once call is ended while in front
of sector 2 you find (No Service Mode) on TEMS as cell is barred and your MS can't make cell selection or reselection
to it in idle mode as it is barred.
Once you end your call and be in idle mode, your MS will try to camp and perform cell reselection to the cell BCCH
59 considering that it was with the highest level in idle mode, but once MS try to read the system information on
BCCH 59 for this cell, it will know that this cell is barred and so that it should leave this cell immediately, so MS will
perform forced cell reselection to any other nearby cell even if it was lower in level than this barred cell BCCH 59 and
sometimes your MS may not find any neighbors to perform this forced cell reselection to any of them so it will enter
SOS emergency call mode and you will see No Service on TEMS.
NOTE: A cell may sometimes barred by optimization teams if this cell suffers:
1- High SDCCH congestion causing bad KPI's in CBR (Call Block Rate) due to high utilization on SD channels.
2- Temporary hardware problem
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Prepared by:
Eng. Abdelrahman Ashraf Mostafa
abdelrahman_ashraf@hotmail.com