Monday, June 4, 2012

What happened before you made a Call

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.