Soft Handover vs Hard Handover

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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.

In CDMA soft handover is possible as adjacent cells operate on same frequency using different scrambling codes. On the other side we know that LTE is based on OFDMA, which is fundamentally a frequency division method. 

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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