Page 178 - From GMS to LTE

164 From GSM to LTE-Advanced Pro and 5G

ensure that there are no areas of the network in which more than three cells need to
be used for the soft handover state.
A soft handover gets even more complicated if cells need to be involved that are not
controlled by the S‐RNC. In this case, a soft handover is only possible if the S‐RNC is
connected to the RNC that controls the cell in question. RNCs in that role are called Drift
RNCs (D‐RNC). Figure 3.28 shows a scenario that includes an S‐RNC and a D‐RNC. If a
foreign cell needs to be included in the Active Set, the S‐RNC has to establish a link to the
D‐RNC via the Iur interface. The D‐RNC then reserves the necessary resources to its cell
on the Iub interface and acknowledges the request. The S‐RNC in turn informs the mobile
device to include the new cell in its Active Set via an Update Active Set message. From this
point onward, all data arriving at the S‐RNC from the core network will be forwarded via
the Iub interface to the cells that are directly connected to the S‐RNC and also via the Iur
interface to all D‐RNCs that control a cell in the Active Set. These in turn forward the data
packets to the cells under their control. In the reverse direction, the S‐RNC is the point of
concentration for all uplink packets as the D‐RNCs forward all incoming data packets for
the connection to the S‐RNC. It is then the task of the S‐RNC to decide which of the
packets to use on the basis of the signal quality indications embedded in each frame.
A variation of the soft handover is the so‐called softer handover, which is used when
two or more cells of the same Node‐B are part of the Active Set. For the network, the
softer handover has the advantage that no additional resources are necessary on the Iub
interface as the Node‐B decides which of the frames received from the mobile device via
the different cells are to be forwarded to the RNC. In the downlink direction, the point
of distribution for the data frames is also the Node‐B, that is, it duplicates the frames it
receives from the RNC for all cells that are part of the Active Set of a connection.
One of the most important parameters of the GSM air interface is the timing advance.
Mobile devices that are farther away from the base station have to start sending their frames
earlier compared to mobile devices closer to the base station, owing to the time it takes the
signal to reach the base station. This is called timing advance control. In UMTS controlling

SGSN
This link is not used as all data
Iu(ps)
from and to the SGSN is sent
via the Serving RNC (S-RNC)
S-RNC D-RNC
Iur

Iub Iub

Node-B 1 Node-B 2 Node-B 3

Soft handover with three cells and two RNCs

Figure 3.28 Soft handover with S‐RNC and D‐RNC.