Page 182 - From GMS to LTE

168 From GSM to LTE-Advanced Pro and 5G

frequencies and thus cannot be received simultaneously with the UMTS cells of the
Active Set. The same problem occurs when signal quality measurements need to be
made for UMTS cells that operate on a different frequency to increase the capacity of
the radio network. The only way for the mobile device to perform measurements for
such cells, therefore, is to stop transmitting and receiving frames in a predefined pattern
to perform measurements on other frequencies. This mode of operation is referred to
as compressed mode and is activated by the RNC, if necessary, in the mobile device and
all cells of the Active Set of a connection. The standard defines three possibilities for
implementing compressed mode. While network vendors can choose which of the
options described below they want to implement, support for all options is required in
the mobile device:
Reduction of the spreading factor. For this option, the spreading factor is reduced
●
for some frames. Thus, more data can be transmitted during these periods, which
increases the speed of the connection. This allows the insertion of short transmission
gaps for interfrequency measurement purposes without reducing the overall speed of
the connection. As the spreading factor changes, the transmission power has to be
increased to ensure an acceptable error rate.
Puncturing. After the channel coder has added error correction and error detection
●
bits to the original data stream, some of them are removed again to allow time for
interfrequency measurements. To keep the error rate of the radio bearer within
acceptable limits, the transmission power has to be increased.
Reduction of the number of user data bits per frame. As fewer bits are sent per
●
frame, the transmission power does not have to be increased. The disadvantage is the
reduced user datarate while operating in compressed mode.
The goal of the measurements in compressed mode is to allow successful decoding of
the Frequency Correction Channel (FCCH) and the Synch Channel (SCH) of the sur-
rounding GSM cells. For further information on these channels see Section 1.7.3.
Figure 3.32 shows how an intersystem handover from UMTS to GSM is performed.
The procedure starts on the UTRAN side just like a normal inter‐MSC handover by the
RNC sending an SRNS relocation request. As the SRNS relocation is not known in
GSM, the 3G MSC uses a standard 2G Prepare Handover message to initiate the com-
munication with the 2G MSC. Thus, for the 2G MSC, the handover looks like a normal
GSM to GSM handover and is treated accordingly.

3.7.2 Mobility Management in Idle State
While in idle state, the mobile device is passive, that is, no data is sent or received.
Nevertheless, there are a number of tasks that have to be performed periodically by the
mobile device.
To be able to respond to incoming voice calls, short messages, MMS messages, etc.,
the PCH is monitored. If a Paging message is received that contains the subscriber’s
International Mobile Subscriber Identity (IMSI) or Temporary Mobile Subscriber
Identity (TMSI), the mobile device reacts and establishes a connection with the net-
work. As the monitoring of the paging channel consumes some power, subscribers are
split into a number of groups based on their IMSI (paging group). Paging messages for
a subscriber of each group are then broadcast at certain intervals. Thus, a mobile device