Page 181 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 167

In UMTS there are a number of different possibilities for performing an intersystem
handover. The first intersystem handover method is the blind intersystem handover. In
this scenario, the RNC is aware of GSM neighboring cells for certain UMTS cells. In the
event of severe signal quality degradation, the RNC reports to the MSC or SGSN that a
handover into a 2G cell is necessary. The procedure is called a ‘blind handover’ because
no measurement reports of the GSM cell are available for the handover decision.
The advantage of this procedure is the simple implementation in the network and in
the mobile devices. However, there are a number of downsides as well:

The network does not know if the GSM cell can be found by the mobile device.
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The mobile device and the target GSM cell are not synchronized. This considerably
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increases the time it takes for the mobile device to contact the new cell once the
handover command has been issued by the network. For the user, this means that
during a voice call they might notice a short interruption of the voice path.
If a UMTS cell has several GSM neighboring cells, as shown in Figure 3.31, the RNC
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has no means to distinguish which would be the best one for the handover. Thus, such
a network layout should be avoided. In practice, however, this is often not possible.
To improve the success rate and quality of intersystem handovers, the UMTS standards
also contain a controlled intersystem handover procedure, which is usually used in
practice today. To perform a controlled handover, UMTS cells at the border of the coverage
area inform mobile devices about both UMTS and GSM neighboring cells. A mobile
device can thus measure the signal quality of neighboring cells of both systems during
an active connection. As described before, there are several ways to report the measurement
values to the RNC. The RNC, in turn, can then decide to request an intersystem hando-
ver from the core network on the basis of current signal conditions rather than simply
guessing that a certain GSM cell is suitable for the handover.
Performing neighboring cell signal strength measurements is quite easy for UMTS
cells as they usually use the same frequency as the current serving cell. The mobile
device thus merely applies the primary codes of neighboring cells on the received signal
to get signal strength indications for them. For the mobile device this means that it has
to perform some additional computing tasks during an ongoing session. For neighboring
GSM cells, the process is somewhat more complicated as they transmit on different

Figure 3.31 A UMTS cell with several GSM neighboring
cells presents a problem for blind intersystem handovers.

GSM cell A
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UMTS cell

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GSM cell B