Why Handover Required in GSM and its Procedure
Handover is the mechanism that transfers an ongoing call from one cell to another as a user moves through the coverage area of a cellular system. The number of cell boundary crossings increases because smaller cells are deployed in order to meet the demands for increased capacity.
If we minimize the expected number of handovers the switching load minimizes as well, because each handover requires network resources to reroute the call to the new base station. In GSM, measurement reports, which are transmitted periodically from MS to BS on the SACCH assigned to each communication, are available for each connection. The repetition duration of the SACCH produces a fixed time grid of 480 ms in which the measurement reports occur.
In above Figure the measured RXLEVs from the serving BTS and from a neighbor one (NC1), according to the measurement reports submitted during a call, are shown. The horizontal axis represents the number of measurement reports.
Obviously the handover procedure consider a set of parameters in such a way to avoid shortcomings. On the other hand, it is not possible to have a safe handover execution in cases like the one shown in Figure above, since the location and direction of user as well as the area characteristics are not known. In many cases the execution takes place and after a couple of measurement reports the handover procedure is triggered again.
There are several different reasons for a handover. Each mobile terminal attempts to use the radio channel that will provide the best connection quality, i.e., the best C/I (carrier-to interference ratio). Co-channel interference is unavoidable because of multiple use of the same time and frequency channels due to existing cell layouts, and consequently quality can be poor (i.e., bit-error ratio high) despite a high signal level.
The connection of a mobile terminal to the base stations can be the cause of interference to other mobile stations, even if it is a high-quality one. The interference can be minimized if the interfered station changes to a different radio channel. It is also possible for mobile users to have the same good receive quality from more than one cell. The service quality of the network can then be optimized if mobile users are equally distributed over the available cells.
The following pie-chart summarizes the handover causes, showing the percentage of the different reasons for handover:
In order to measure the handover performance in a cellular network several counters are used. As far as the procedure is concerned, each counter is triggered when a Handover Required message, containing the respective cause, is routed from the BSC to the MSC (inter-BSC handover). When the handover is internal (intra-cell or inter-cell, intra-BSC handover), the procedure (decision and execution, respective cause counted) is undertaken by the responsible BSC and the MSC is informed by a Handover Performed message.
An in-depth statistical evaluation shows that, there are several shortcomings in the normal handover procedure. The major ones are the following:
- High failure of handovers, due to an insufficient planning in certain areas.
- “Far-away-cell”, where subscribers are served from a BTS that is far away from the cell where the user is located
- “Ping-pong” effect, the repeated handover between two base stations caused by rapid fluctuations in the received signal strengths from both base stations.
- Unnecessary handover often leads to increased signaling traffic, which can result in traffic congestion in the call-setup procedure of other subscribers intending to set up calls.