2. UMTS
Within UMTS it is possible to define a number of
different types of UMTS handover or handoff.
With the advent of generic CDMA technology, new
possibilities for effecting more reliable forms of
handover became possible, and as a result one of a
variety of different forms of handover are
available depending upon the different
circumstances.
3. TYPES
Hard handover: This form of handover is essentially the
same as that used for 2G networks where one link is broken
and another established.
Soft handover: This form of handover is a more gradual
and the UE communicates simultaneously with more than
one Node B or base station during the handover process.
Softer handover: Not a full form of UMTS handover, but
the UE communicates with more than one sector managed
by the same NodeB.
UMTS inter RAT handover: This form of handover occurs
when mobiles have to change between Radio Access
Technologies, e.g. UMTS to / from GSM or UMTS to / from
4G LTE, etc..
4. UMTS HARD HANDOVER
The name hard handover indicates that there is a
"hard" change during the handover process. For hard
handover the radio links are broken and then re-
established. Although hard handover should appear
seamless to the user, there is always the possibility that
a short break in the connection may be noticed by the
user.
5. UMTS SOFT HANDOVER
Soft handover is a form of handover that was
enabled by the introduction of CDMA. Soft
handover occurs when a UE is in the
overlapping coverage area of two cells. Links
to the two base stations can be established
simultaneously and in this way the UE can
communicate with two base stations. By
having more than one link active during the
handover process, this provides a more
reliable and seamless way in which to
perform handover.
6. IRAT HANDOVER
In many instances it is necessary for the UMTS radio
access network to handover to the 2G GSM network.
These handovers are given a variety of names
including Inter-RAT handover as they are handing
over between different forms of Radio Access
Technology, Intersystem Handover, and UMTS / GSM
Handover. These handovers may be required for one
of a variety of reasons including:
Limited UMTS coverage
UMTS network busy whereas spare capacity is
available on GSM network.
7. UMTS TO GSM HANDOVER
Compressed mode handover: Using compressed mode
handover the UE uses the gaps in transmission that occur to
analyze the reception of local GSM base stations. The UE uses
the neighbour list provided by the UMTS network to monitor
and select a suitable candidate base station. Having selected a
suitable base station the handover takes place, but without any
time synchronisation having occurred.
Blind handover: This form of handover occurs when the base
station hands off the UE by passing it the details of the new cell
to the UE without linking to it and setting the timing, etc of the
mobile for the new cell. In this mode, the network selects what it
believes to be the optimum GSM based station. The UE first
locates the broadcast channel of the new cell, gains timing
synchronisation and then carries out
8. HANDOVER FROM GSM TO UMTS
Handover from GSM to UMTS : This form of
handover is supported within GSM and a "neighbour
list" was established to enable this occur Seasily. As the
GSM / 2G network is normally more extensive than
the 3G network, this type of handover does not
normally occur when the UE leaves a coverage area and
must quickly find a new base station to maintain
contact. 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 neighbour list will inform the UE when this may
happen.
