GSM systems require a procedure known as a Handover to maintain the continuity of the call. This is because a single cell does not cover the whole service area e.g. a whole city or country. However a single cell has a maximum service area of approximately 23 miles (35 km) for each antenna (Tripathi, et al. 1998). The smaller the size of the cell and the faster the movement of the MS through the cells (Up to 155 mph (250 kph) for GSM), the more handovers of ongoing calls are required, but a handover should not cause the a call drop. Basically there are two main reasons for handovers, however the GSM Specification identifies 40 reasons.
The MS moves out of coverage of the serving BTS thus the signal level becomes lower continuously until it falls beneath the minimal requirements for communications. Or the error rate may grow due to interference, the distance to the BTS may be do high. All these effects may diminish the quality of the radio link and make transmission impossible in the near future.
The wired infrastructure i.e. the MSC, BSC may decide that the traffic in one cell is too high thus introducing congestion and hence decides to shift some MSs to other cells with a lower level of traffic, if that is possible. Thus, handovers can be used as a method of controlling traffic through load balancing to relieve localised congestion.
Figure 11 shows four possible handover scenarios with in the GSM system.
- Intra Cell Handover : This happens when within a cell, when narrowband interference could make transmission at a certain frequency impossible. The BSC could then decide to change the carrier frequency. (1)
- Inter Cell, intra BSC handover : This type of handover is a typical handover within the GSM system and occurs when the MS moves from one BTS to another but stays within the control of same BSC. The BSC performs the handover and assigns a new radio channel in the new BTS, then releases the old BTS. (2)
- Inter BSC, Intra MSC handover : Since a BSC controls a limited number of BTSs, the GSM system has to perform handovers between BSCs. This form of handover is controlled by the MSC. (3)
- Inter MSC handover : A handover could also be required between two BTSs that belong to two different MSCs, now both MSCs perform the handover together.(4)
In order to provide all the information necessary for a handover due to a weak link, the MS and the BSC both perform periodic measurements of the downlink and the uplink quality respectively. The link quality measurement is made up from receive signal level (RxLev) and the bit error rate (BER) and form part of the layer 3 messaging function. Measurement reports are sent by the MS about every 0.5 seconds and these contain the quality of the current link used for transmission as well as the quality of certain channels in neighbouring cells (the BCCHs).