Sunday, May 27, 2012

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.