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160 From GSM to LTE-Advanced Pro and 5G

3.7 Radio Network Mobility Management

Depending on the MM state of the core network, the radio network can be in a number
of different RRC states. How mobility management is handled in the radio network
depends on the respective state. Table 3.5 gives an overview of the MM and PMM states
in the core network and the corresponding RRC states in the radio network.

3.7.1 Mobility Management in the Cell‐DCH State
For services like voice or video communication it is very important that little or no
interruption of the data stream occurs during a cell change. For these services, only the
Cell‐DCH state can be used. In this state, the network constantly controls the quality of
the connection and is able to redirect the connection to other cells if the subscriber is
moving. This procedure is called handover or handoff.
A handover is controlled by the RNC and triggered based on measurement values of
the quality of the uplink signal measured by the base station and measurement reports
on downlink quality sent by the mobile device. Measurement reports can be periodic or
event triggered. Different radio network vendors use different strategies for measurement
reporting. Unlike in GSM where only the signal strength, referred to as Received Signal
Strength Indication (RSSI), is used for the decision, UMTS needs additional criteria as
neighboring base stations transmit on the same frequency. A mobile device thus not
only receives the signal of the current serving base station but also the signals of neigh-
boring base stations, which, from its point of view, are considered to be noise. In UMTS,
the following values are used:
RSSI. To describe the total signal power received in milliwatts. The value is usually
●
expressed in dBm (logarithmic scale) and typical values are −100 dBm for a low signal
level to −60 dBm for a very strong signal level.
Received Signal Code Power (RSCP). The power the pilot channel of a base station
●
is received with. The RSCP can be used, for example, to detect UMTS cell‐edge
scenarios where no neighboring UMTS cell is available to maintain the connection. In
this case, the network takes action when the RSCP level falls below a network‐operator‐
defined threshold. If the network is aware of neighboring GSM cells, it can activate
the compressed mode so that the mobile device can search for and report neighboring
GSM cells to which the connection could be handed over.

Table 3.5 Core network and radio network states.

MM states and possible
RRC states MM idle MM connected PMM idle PMM connected
Idle × − × −
Cell‐DCH − × − ×
Cell‐FACH − − − ×
Cell‐PCH − − − ×
URA‐PCH − − − ×