Page 175 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 161

EcNo. The received energy per chip (Ec) of the pilot channel divided by the total noise
●
power density (No). In other words, the EcNo is the RSCP divided by the RSSI. The
better this value the better the signal can be distinguished from the noise. The EcNo
is usually expressed in decibels as it is a relative value. The value is negative as a loga-
rithmic scale is used and the RSCP is smaller than the total received power. The EcNo
can be used to compare the relative signal quality of different cells on the same fre-
quency. Their relative difference to each other, independent of their absolute signal
strengths, can then be used, for example, to decide which of them should be the serv-
ing cell.
In UMTS a number of different handover variants have been defined.

Hard Handover
This is as shown in Figure 3.24. This kind of handover is very similar to GSM hando-
ver. By receiving measurement results from the mobile device of the active connec-
tion and measurement results of the signal strength of the broadcast channel of the
neighboring cells, the RNC is able to recognize if a neighboring cell is more suitable
for the connection. To redirect the call into the new cell, a number of preparatory
measures have to be performed in the network before the handover is executed. This
includes, for example, the reservation of resources on the Iub interface and, if neces-
sary, also on the Iur interface. The procedure is similar to the resource reservation of
a new connection.
Once the new connection is in place, the mobile device receives a command over the
current connection to change into the new cell. The handover command contains,
among other parameters, the frequency of the new cell and the new channelization and
scrambling code to be used. The mobile device then suspends the current connection
and attempts to establish a connection in the new cell. The interruption of the data
stream during this operation is usually quite short and takes about 100 milliseconds on
average, as the network is already prepared for the new connection. Once the mobile
device is connected to the new cell the user data traffic can resume immediately. This
kind of handover is called UMTS hard handover, as the connection is briefly inter-
rupted during the process.

User moves to the
coverage area of a
new cell. The network
performs a hard
handover

Iub

Iu(cs), Iu(ps)
RNC

Figure 3.24 UMTS hard handover.