Soft Handover vs Hard Handover
Now
before we move to other things like measurements in handover, handover
decision, handover procedure, handover interruption time, and mobility
robustness optimization, let us take a look into difference between soft
handover and hard handover.
Soft Handover |
Hard Handover |
The
handover in which radio links are added and removed in a way that MS always
keeps at least one radio link to the UTRAN is known as soft handover.
|
The
handover in which all the old radio links in MS are removed before the new
radio links are established is known as hard handover.
|
This
can also be simplified as make before break
|
This
can also be simplified as break before make
|
As
call drop rate is lower this handover is used to lower the rate of call drop.
|
In
this case higher rates of call drops are found
|
Why soft handover is not included in
LTE?
To
answer this, there are many specific reasons that why soft handover was skipped
in LTE from system design. As we all know that connect-before-break (soft
handover) is a category of handover procedure where radio links are removed or
added in such a manner that there is always one radio link connected to the UE.
The entire radio links connected to UE at a particular instant of time are part
of active set (This number can vary from 1 to 8). This means at least one and
maximum 8 radio links are connected to the UE.
In soft handover the mobile
station is in the overlapping cell coverage area of two sectors that belongs to
different base stations. In the uplink, for the particular UE, the scrambled
signals from different base station are compared frame by frame at RNC and the
best candidate is selected after ach interleaving period (i.e. every 10, 20,
40, or 80 ms).
In downlink direction
signals that are received from different base stations are combined at the rake
receiver of the UE. Also soft handovers are only possible when UE is in connected
state with the DCH (dedicated channel). In case of HSDPA communication there is
only hard handover (break before connect).
Now, Basic reasons why LTE does not have soft handover are:
In
there is no central node controller like the BSC or RNC. Thus there is no need
to sum up multiple active signals like it’s done CDMA. Also, there is no need
for power control in LTE because of the presence of orthogonal modulation
scheme; there is no self interference like CDMA. So no worries about Rx
diversity gain and so soft handover can be skipped.
Cell
edge reception which was the heart of the soft handover design in WCDMA is not
applicable in LTE networks due to orthogonality in both downlink and uplink.
Thus soft handover is dropped from LTE system.
So here the UE has to actually resync to a different set of frequency subcarriers when it hands over between cells thus removing the possibility for soft handover.
In fact, when a handover begins, an LTE UE goes into a compressed mode where it hears to its current cell for part of the time.
Due to advancement in technology this retuning happens fast enough to make the inter frequency retuning much more seamless than it was in older technology like GSM, thus eliminating the need of soft handover.
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