First of all I would like to ask you a question. How many
different modes are there in a mobile phone? Most probably your answer will be
two (on/off). But when we look at it technically there are three modes and they
are switched off, idle and dedicated mode. When we take a call we can say that
our phone is in dedicated mode. If our phone is switched on and not taking any
calls we could say it is in idle mode. When we switch on the phone it will
immediately goes in to idle mode. Depending on the signal strengths present MS
(Mobile Station) will decide to which cell it should camp. There are two
important things should happen before setup a call.
1.
Cell Selection
Immediately after MS switched on Cell Reselection occurs. It refers to the initial registration
that a MS will make with a network. C1 is the parameter used for
selection. The most favorable cell is indicated by the so called C1 parameter
for a MS. MS will search all RF channels and decide which cell to camp using C1
values. A basic equation is given below,
C1 = Rx level – Rx level Access min
Rx level: Received signal power for the MS or in other words what signal
strength the MS sees the tower at.
Rx level Access min: Minimum access level of the tower. This is a
parameter of the antenna of the site.
Normally MS will receive signals from many cells. For those cells
MS will calculate the C1 and find out the maximum value. Then it will camp into
that cell. That’s not the end and it will keep gathering information of nearby
cells. Normally it will keep tow lists named as Active set and Merge set. In active
set we have a primary cell (the cell mobile has already camped at the moment)
and another four cells which are having highest signal strengths next to
primary cell. And there are several cells in the merge set. If anyone in the
merge set gets better than the active set, it will replace the active set cell.
This will keep happening.
2.
Cell Reselection
Cell reselection is performed as MS traverses through a network in
idle mode. MS will keep details of strongest BCCH cells as I mentioned above. As
in previous case we have another parameter known as C2 for Reselection. It can
be also called as an improved version of C1.
C2= C1 + CRO – TO(t)
[CRO (Cell Reselection Offset), TO (Temporary Offset)]
The mobile shall regularly search for a better cell according to
the cell reselection criteria. If a better cell is found, that cell is
selected. But before we select a one we should check for the neighbors. That means
for each and every cell we define neighbors. Neighbors are defined using their
locations, directions and distances to a particular cell. If it is a neighbor
cell we can camp on to it. But if it isn’t MS will not camp into that.
If we want to reduce the number of customers per a cell we can
adjust CRO parameter. Or we can say we will give higher priority to a cell by
giving a high value to that cell than its neighboring cells. TO is used to
avoid unnecessary re-selections. For example consider a person moves along a
highway. As he moves along there may be many cells serving him at different
points. But there may be one or two major cells which can serve him for a
longer distance. In such occasions we prefer him to stay camp in to that major
serving cell. To do that we can give a proper timing value for TO. That means MS
will not camp into that cell until it stays in that cell for a particular time
period. Normally in high ways we go fast and it will avoid unnecessary re-selections and keep camp on to a major cell.
