Handover

UMTS supports two main categories of handovers—soft handovers and
hard handovers.A soft handover is make-before-break, whereby communication
exists between the UE and more than one cell for a period of time.A
hard handover is break-before-make, whereby communication with the
first cell is terminated before establishing communication with the second
cell.
A soft handover has two variants—soft handover and softer handover.
These two situations are depicted in Figure 6-26. A soft handover occurs
between two cells or sectors that are supported by different base stations.
The UE is transmitting to and receiving from both base stations at the
same time. The user information sent to the UE is sent from each base station
simultaneously and is combined within the UE. In the uplink, the
information sent from the UE is relayed from each base station to the RNC
where the combining takes place. In the case of a soft handover, each base
station is sending power control commands to the UE.
A softer handover occurs between two cells that are supported by the
same base station. In this case, only one power control loop is active and is
controlled by the base station that serves both cells. Depending on RF coverage,
both a soft handover and a softer handover may occur at the same
time for a given UE.
A hard handover can occur in several situations, such as from one cell to
another where the two cells are using different carrier frequencies, or from
one cell to another where the base stations are connected to different RNCs
and no Iur interface exists between the RNCs. UMTS also supports a hard
handover to and from GSM. This is a reasonable requirement as it takes
time to roll out a UMTS network nationwide, and one would like UMTS
subscribers to receive service from GSM in areas where holes occur in the
UMTS coverage.
Regardless of the type of handover to take place, the decision when and
how to invoke a handover is made at the serving RNC. This decision is
based upon measurements reported by the UE. The set of cells for which
measurement reports are to be generated is broadcast from the network on
the BCH or FACH. If a neighboring cell uses a different frequency and the
RNC requires reports related to that cell, then the UE needs time periodically
to tune to the frequency in question. This means that the UE and
UTRAN must operate in compressed mode. This mode means that in a
given radio frame, not all 15 slots are used. The unused slots correspond to durations where the UE can tune to another frequency to make the necessary
measurements. 276