Page 177 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 163

Figure 3.26 Soft handover reduces the
energy consumption of the mobile due
to lower transmission power.
Node-B 2

Used data stream
Discarded data stream
Node-B 1

information included in both frames, that the frame received from cell 2 is to be
forwarded into the core network. This decision is made for each frame, that is, the RNC
has to make a decision for every connection in handover state every 10, 20, 40 or 80
milliseconds, depending on the size of the radio frame.
In the downlink direction, the mobile device receives identical frames from cell 1 and
cell 2. As the cells use different channelization and scrambling codes the mobile device
is able to separate the two data streams on the physical layer (see Figure 3.27). This
means that the mobile device has to decode the data stream twice, which of course
slightly increases power consumption as more processing power is required.
From the network point of view, the soft handover procedure has an advantage
because the mobile device uses less transmission power compared to a single cell sce-
nario in order to reach at least one of the cells in the Active Set, and so interference is
reduced in the uplink direction. This increases the capacity of the overall system, which
in turn increases the number of subscribers that can be handled by a cell.
On the other hand, there are some disadvantages for the network, as in the down-
link direction data has to be duplicated so that it can be sent over two or even more
cells. In the reverse direction, the RNC receives a copy of each frame from all cells of
the Active Set. Thus, the capacity that has to be reserved for the subscriber on the
different interfaces of the radio network is much higher than that for a subscriber who
only communicates with a single cell. Therefore, good network planning tries to

UL: identical
scrambling code

DL: scrambling code 2 DL: scrambling code 1

Figure 3.27 Use of scrambling codes while a mobile device is in soft handover state.