Page 152 - From GMS to LTE

138 From GSM to LTE-Advanced Pro and 5G

Unsynchronized start of the
network search procedure 256 chips
1 Slot = 2560 chips
1. P-SCH
64 possible
combinations
2. S-SCH
Beginning of the frame found,
UE und UTRAN are synchronized 8 possible
codes
3. CPICH
Primary scrambling code found
P-CCPCH can now be decoded
4. P-CCPCH
15 slots = 1 frame = 10 ms
2560–256 chips = 2304 chips,
SF 256 = 9 bits on the I and Q path.
Together: 18 bits per slot on the
P-CCPCH. Resulting datarate: 27 kbit/s.

Figure 3.13 Network search after the mobile device is switched on.
this channel each slot has a different chip pattern. As the patterns and the order of
the patterns are known, the mobile device is able to determine the slot that contains the
beginning of a frame.
If an operator only has a license for a single channel, all cells of the network operator
send on the same frequency. The only way to distinguish them from each other is by
using a different scrambling code for each cell. The scrambling code is used to encode
all downlink channels of a cell including the P‐CCPCH, which contains the system
broadcast information. The next step of the process is therefore to determine the pri-
mary scrambling code of the selected cell. The first part of this process was already
started with the correct identification of the S‐SCH and the chip pattern. Altogether, 64
different S‐SCH chip patterns are specified in the standard. This means that in theory
the mobile device could distinguish up to 64 individual cells at its current location. In an
operational network, however, it is very unlikely that the mobile device would receive
more than a few cells at a time. To determine the primary scrambling code, the mobile
device then decodes the Common Pilot Channel (CPICH), which broadcasts another
known chip pattern. Eight possible primary scrambling codes are assigned to each of
the 64 chip patterns that are found on the S‐SCH. To find out which code is used by the
cell out of the eight scrambling codes for all other channels, the mobile device now
applies each of the eight possible codes on the scrambled chip sequence and compares
the result to the chip pattern that is expected to be broadcast on the CPICH. As only
one of the scrambling codes will yield the correct chip pattern the mobile device can
stop the procedure as soon as it has found the correct one.
Once the primary scrambling code has been found by using the CPICH, the mobile
device can now read the system information of the cell, which is broadcast via the