Page 153 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 139

P‐CCPCH. The P‐CCPCH is always encoded with spreading code C256, 1 with a
spreading factor of 256, which is easy for the mobile device to find even under difficult
radio conditions. Only after deciphering the information broadcast on this channel is
the mobile aware which network the cell belongs to. The following list shows some
parameters that are broadcast on the P‐CCPCH:
The identity of the network the cell belongs to (MCC/MNC), location area (LAC) and cell ID.
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Cell access restrictions. This suggests which groups of subscribers are allowed to
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communicate with the cell. Usually all subscribers are allowed to communicate with
a cell. Only under certain conditions will the network operator choose to temporarily
restrict access to parts of the network for some subscribers. This can help during
catastrophic events to allow important users of the network like the police and doc-
tors to communicate with facilities like hospitals. Without access restrictions, cells
quickly overload during such events as the number of call attempts by normal users
increases dramatically and can thus delay important calls.
Primary scrambling codes and frequencies of neighboring cells. As described above, the
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frequencies of the other cells in the area are usually the same as the frequency of the cur-
rent cell. Only in areas of very high usage might operators deploy cells in other frequency
bands to increase the overall available bandwidth. Both scrambling codes and frequencies
of neighboring cells are needed by the mobile device to be able to easily find and measure
the reception quality of other cells while they are in idle mode for cell reselection purposes.
Frequency information of neighboring GSM cells. This information is used by the
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mobile to be able to reselect a GSM cell in case the signal quality of the current cell
deteriorates and no suitable neighboring UMTS cell can be received.
Parameters that influence the Cell Reselection Algorithm. This way the network is
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able to instruct the mobile device to prefer some cells over others.
Maximum transmission power the mobile is allowed to use when sending a message
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on the RACH.
Information about the configuration of the PRACH and S‐CCPCH, which transport
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the RACH and FACH respectively. This is necessary because some parameters, like
the spreading factor, are variable to allow the operator to control the bandwidth of
these channels. This is quite important as they transport not only signaling informa-
tion but also user data, as described below.
If the cell belongs to the network the mobile device wants to attach to, the next step
in the process is to connect to the network by performing a circuit‐switched location
update and a packet‐switched attach procedure. These procedures use the higher pro-
tocol layers of the MM and PMM, respectively, which are also used in GSM and GPRS.
For UMTS, both protocol stacks were only slightly adapted. Further information on
these procedures is described in Sections 1.8.1 and 2.8.1.

3.4.5 Example: Initial Network Access Procedure
If the mobile device is in idle state and wants to establish a connection with the net-
work, it has to perform an initial network access procedure. This may be done for the
following reasons:
to perform a location update;
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for a mobile‐originated call;
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