When LTE system was developed one of the big technical feature from UMTS Soft/Softer handover was missing from the architecture. There are many specific reasons why LTE dropped altogether soft handover (connect-before-break) from the system design.
Before discussing about why LTE skipped soft handover and only implemented hard handover, let’s check the basic concept of soft/softer handover.
Soft handover concept
Soft handover or connect-before-break handover is a category of handover procedure where radio links are added or removed in such a fashion that there is always one radio link connected to the UE.
All the radio links connected to the UE at a particular time are part of the ACTIVE SET and the number can vary from 1 to 8. This means at least one and maximum of 8 radio links are connected to the UE.
Soft handover is a part of WCDMA system which is basically same in concept as in CDMA systems.
In the case of soft handover the mobile station is in the overlapping cell coverage area of two sectors belonging to different base stations. In downlink direction signals received from different base stations are combined at the rake receiver of the UE. In the uplink the scrambled signals from different base stations for a particular UE are compared frame-by-frame basis at the RNC and the best candidate is selected after each interleaving period; i.e. every 10, 20, 40 or 80ms.
UMTS Soft Handover Architecture
Soft and softer handovers are only possible when UE is in connected state with dedicated channel (DCH).
Even with HSDSCH (High-Speed Downlink Shared Channel) there is no soft handover in UMTS. In cased HSDPA communication there is only hard handover which is break before connect. Means UE is never connected to more than one radio link at a time and when UE goes out of coverage of a particular cell that radio link is teared down before adding the better radio link.
Requirements for soft handover and what is missing in LTE
Let’s discuss why LTE does not have soft handover as in WCDMA and CDMA systems. Though there are various reasons, I documented here three causes why soft handover was omitted from LTE.
Adjacent Cell Frequency Reuse
Soft handover is possible in CDMA because adjacent cells can operate on the same frequencies as long as they use different scrambling codes. So, a UE can listen to two different cells by decoding the received signals twice, using a scrambling code from each cell on each for each decoded signal. This allows a UE to communicate with both cells during handover, making it a soft handover.
LTE is based on OFDMA, which is fundamentally a frequency division method. This means that a UE has to actually resync to a different set of frequency subcarriers when it hands over between cells, removing the possibility for soft handover. In fact, when a handover is beginning, an LTE UE has to go into a ‘compressed mode’ where it listens to its current cell for part of the time and searches for a new cell the rest of the time. With modern radio technology, this retuning can happen fast enough to make the inter-frequency retuning much more seamless than it was in older technologies like GSM, obviating the need for soft handover.
LTE has flat architecture which means there’s no central node controller like the BSC or RNC. Therefore there’s no need to sum up multiple active signals like you would say for in cdma. Another reason being, there’s no need for power control in LTE because of its orthogonal modulation scheme, there’s no self interference (like in cdma) so no worries about Rx diversity gain and therefore SHO can be dropped.
Orthogonality in LTE
Cell-edge reception (which was at the core of Soft HO design in WCDMA) issue is not quite applicable in LTE networks because of orthogonality in both uplink and downlink. Therefore soft handover can be dropped from the LTE system.
Though LTE does not have soft or softer handover it still maintains seamless mobility using hard handover. In LTE there are three different types of handover can be possible
- Intra-LTE Handover: In this case source and target cells are part of the same LTE network.
- Inter-LTE Handover: Handover happens towards other LTE nodes. (Inter-MME and Inter-SGW)
- Inter-RAT Handover: Handover between different radio technologies. For example handover from LTE to WCDMA.
More details on Intra-LTE handover
Do you have some questions or more suggestions why LTE removed soft handover from the system design please? Please add your views in comment area below.