Page 151 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 137

While the separation of channels in GSM into logical and physical channels is quite
easy to understand, the UMTS concept of logical, transport and physical channels and
the mappings between them is somewhat difficult to understand at first. Therefore, the
following list summarizes the different kinds of channels and their main tasks:

Logical Channels. These channels describe different flows of information like user
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data and signaling data. Logical channels contain no information about the characteristics
of the transmission channel.
Transport Channels. These channels prepare data packets that are received from
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logical channels for transmission over the air interface. Furthermore, this layer defines
which channel coding schemes (e.g. error correction methods) are to be applied on
the physical layer.
Physical Channels. These channels describe how data from transport channels is
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sent over the air interface and apply channel coding and decoding to the incoming
data streams.
The next paragraph gives an idea of the way the channels are used for two different
procedures.

3.4.4 Example: Network Search
When a mobile device is switched on, one part of the startup procedure is the search for
available networks. Once a suitable network has been found, the mobile device performs
an attach procedure. Now the mobile device is known to the network and ready to accept
incoming calls, short messages and so on. When the user switches the mobile device off,
the current information about the network (e.g. frequency, scrambling code and cell ID
of the current cell) is saved. This enables the mobile device to skip most activities required
for the network search once it is powered on again, which substantially reduces the time
it takes to find and attach to the network again. In this example, it is assumed that the
mobile device has no or only invalid information about the last‐used cell when it is pow-
ered on. This can be the case if the SIM card is used for the first time or if the cell for
which information was stored on the SIM card is not found anymore.
As in all communication systems, it is also necessary in UMTS to synchronize the
mobile device with the network. Without correct synchronization it is not possible to
send an RRC Connection Request message at the correct time or to detect the begin-
ning of an incoming data frame. Therefore, the mobile device’s first task after it is
switched on is to synchronize to the cells of the networks around it. This is done by
searching all frequency bands assigned to UMTS for Primary Synchronization Channels
(P‐SCH). As can be seen in Figure 3.13, a UMTS data frame consists of 15 slots in which
2560 chips per slot are usually transported. On the P‐SCH only the first 256 chips per
slot are sent and all base stations use the same code. If several signals (originating from
several base stations) are detected by the mobile at different times owing to the different
distances between the mobile device and the various cells, the mobile device synchro-
nizes to the timing of the burst with the best signal quality.
Once a P‐SCH is found, the mobile device is synchronized to the beginning of a slot.
In the next step, the mobile device then has to synchronize itself with the beginning of
a frame. To do this, the mobile device will search for the Secondary Synchronization
Channel (S‐SCH). Again only 256 chips per slot are sent on this channel. However, on