Page 169 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 155

Idle State
In this state, a mobile device is attached to the network but does not have a physical or
logical connection with the radio network. This means that the user is involved neither
in a voice call nor in a data transfer. From the packet‐switched core network point of
view, the subscriber might still have an active PDP context (i.e. an IP address) even if no
radio resources are assigned at the moment. Owing to the user’s period of inactivity, the
radio network has decided to release the radio connection. This means that if the user
wants to send some data again (e.g. request a new web page), the mobile device needs to
request the establishment of a new radio bearer.

Cell‐DCH State
The Cell‐DCH RRC state is used similarly to the GSM dedicated mode for circuit‐
switched voice calls. While in this state, a physical connection is established between
the mobile device and the network. In the UTRAN this means that the mobile device
has been assigned its own spreading code in the downlink direction and its own spread-
ing and scrambling codes in the uplink direction.
The Cell‐DCH state is also used for packet‐switched connections. At first, the term
contradicts the packet‐switched approach. The advantage of packet‐switched connec-
tions is that resources are shared and used only while they are needed to transfer data.
In Release 99, however, air interface resources were not immediately freed once there
was no more data to be transferred. If a subscriber did not send or receive data for some
time, only control information was sent over the established channel. Other subscribers
benefited indirectly from this owing to the reduced overall interference level of the cell
during such periods, as shown in Figure 3.22. If new data arrived which had to be sent
over the air interface, no new resources had to be assigned as the dedicated channel was
still established. Once data was sent again the interference level increased again for
other subscribers in the cell.
In practice, the Release 99 Cell‐DCH state for transferring packet‐switched data has
been replaced by the HSDPA (High‐Speed Downlink Packet Access) Cell‐DCH state in
the downlink direction. While shared channels are used for user data transfer, a
dedicated connection per user still exists for the control channels. Consequently, the
following section on mobility management in dedicated mode applies for both setups.
Using signal measurements of the mobile device and the Node‐B, it is possible to
control the power level of each mobile device in a cell, which is a task that is shared
between the Node‐B and the RNC. Using the downlink of the PDCCH the network is
able to instruct the mobile device to adapt its transmission power to the current conditions,
that is, 1500 times a second. The rate at which power control is performed shows the

Figure 3.22 Discontinuous
Transmission (DTX) on a dedicated
Interference level The interference level is reduced by
channel reduces the interference for
other subscribers.

DTX of a subscriber.

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