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.

GSM Handover/Handoff

Both Handover and Handoff is used to describe the same process.  There is a process called Cell reselection and I will talk about it later. A particular mobile service provider is given a set of frequencies. Form that few set of frequencies, by doing Frequency Reuse they have to provide the coverage. The total coverage is divided into large number of cells. When the customer moves from one cell to another while taking a call Handover takes place to retain the connection.

This must be carefully considered when you are planning a network. Because this is a measure of the Quality of Service. If we fail to handover properly the call will drop. And when the number of call drops goes high customer may tend to go for another service provider.

There are few ways of categorizing Handover in GSM systems. For GSM only systems there are four categories.

     1. Intra-BTS handover

This occurs within the same BTS when there are some interference takes place. In this case mobile will be locked to the same BTS but the channel allocated to that mobile/time slot will change.

    2. Inter-BTS Intra BSC handover

This type of handover occurs when the mobile moves out of the coverage of one BTS into another BTS and both BTSs are controlled by the same BSC. BSC will take care of the handover by allocating a channel for the user in the second BTS.

  3. Inter-BSC handover

This is a special case of previous one and this time handover occurs between two BSCs. Therefore it has to be controlled by MSC.

  4. Inter-MSC handover

      In this occasion handover occurs between two MSCs.

 

As I mentioned above there are few categories but as far as the mobile is concerned they will look like the same. For GSM systems we use TDMA. Transmitter only transmits in one out of eight time slots and similarly receiver receives in one slot. As a result of this RF module of the mobile may be idling for the remaining six time slots. But it is not the case. During those slots mobile will scans for beacon frequencies which may be more suitable/stronger. When the mobile deals with the BTS it will send the list of radio channels of the beacon frequencies of neighboring BTSs via Broadcast Channel known as BCCH. In addition to this mobile will report back the quality of the existing link with BTS. It is not only the telecommunication network but also the mobile is helping in doing handover. This form of handover is also known as Mobile Assisted Handover (MAHO).

Now with the help of the mobile network has the details regarding the current link quality and the available links or availability of channels nearby cells. Depending on the configurations set or the parameters available, network will decide when to handover and to which cell it should be handed. If network decide to go with the handover it will assign a new time slot to the mobile and also inform the relevant BTSs about the change. Mobile will return during the idling period and will synchronize with the new available parameters and continue the conversation. This can be considered as the perfect scenario. But in practice we may come across few issues.

They are:

·         Old and new BTSs synchronized: As I mentioned above mobile is provided with all the required details. For fine adjustment of synchronization mobile may optionally send four access bursts even though the synchronization is already good. They are shorter than the standard bursts. Because of that they will not overlap with other bursts.

·         Time offset between synchronized old and new BTS: if there exist a time offset between the old and new BTSs, mobile will be informed about the offset. So mobile station can make the adjustment and then the handover takes place as a standard synchronized handover.

·         Non-synchronized handover: For this to happen mobile will transmit 64 access bursts on the new channel. That will help the BTS to determine and adjust the timing of the mobile. After it has done mobile can access the new BTS and it will enable the mobile to re-establish the connection through new BTS with correct timing. 

As time passes new technologies arises. Earlier we talked about how the handover takes place within GSM/2G network. But later we see 3G, HSPA and LTE. So we may encounter a situation where we want to handover from GSM to any other or vice-versa. It is known as Inter-system/inter-RAT handover.

·         UMTS / WCDMA to GSM handover

We can divide this further into two.

o   Blind handover: This form of handover occurs when the BTS hands off the mobile by just passing it the details of the new cell to the mobile without linking to it and setting the timing, other parameters of the mobile for the new cell. In this mode, the network selects what it believes to be the optimum GSM based station. The mobile first locates the BCCH of the new cell, gains timing synchronization and then carries out non-synchronized inter-cell handover.

o   Compressed mode handover:   The mobile uses the gaps of transmission that occur to analyze the reception of local GSM base stations using the neighbor list to select suitable candidate base stations. Having selected a suitable base station the handover takes place, again without any time synchronization having occurred.

·         Handover from GSM to UMTS / WCDMA:   This form of handover is supported within GSM and a "neighbor list" was established to enable this occur easily. As we know the GSM/2G network is normally more extensive than the 3G network, this type of handover does not normally occur. If a mobile go away from a coverage area, then it will have to quickly find a new base station to stay in touch. The handover from GSM to UMTS occurs to provide an improvement in performance and can normally take place only when the conditions are right. The neighbor list will inform the mobile when this may happen.