Page 269 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 255

1 frame (10 milliseconds) = 10 subframes (1 millisecond) = 20 slots (0.5 millisecond)

SSS
Timing
Normal cyclic prefix difference

Extended cyclic prefix PSS

1 Slot = 0.5 millisecond
Figure 4.17 PSS and SSS in an LTE FDD frame.

The signals transmitted from the different cells on the same channel interfere with
each other. As a channel is used only by one operator except at national borders, the
mobile device would attempt to start communication only with the cell with the strong-
est synchronization signals and ignore other cells on the same frequency. If the mobile
device has found the cell it used before it was switched off, it may go directly to this cell
and stop searching for other cells on different channels in the current frequency band,
even if the cell is not the strongest on the current channel. After a successful attach
procedure as described below, the cell reselection mechanism or a handover will ensure
that the mobile device is served by the strongest cell it receives.
The next step in the cell search procedure is to read the MIB from the PBCH, which
is broadcast every 40 milliseconds in the inner 1.25 MHz of the channel. The MIB con-
tains the most important information about the configuration of the channel, which is
essential for the mobile before it can proceed. Very conservative modulation and strong
error detection and correction information is added to allow successful decoding of this
information even under very unfavorable reception conditions. The first information
that the mobile device gets from the MIB is the total bandwidth used for the channel
since all decoding attempts so far were only performed in the inner 1.25 MHz of the
channel. Further, the MIB contains the structure of the HARQ indicator channel
(PHICH, see Section 4.3.10 ‘HARQ Operation in the MAC Layer’) and the System
Frame Number (SFN), which is required, for example, for ciphering and calculation of
paging opportunities, as described later on.
With the information from the MIB, the mobile device can then begin to search for
the SIB 1. As it is broadcast on the downlink shared channel every 80 milliseconds, the
mobile device needs to decode the ‘common’ search space in the control region of a
subframe to find a downlink control channel (PDCCH) message that announces the
presence and location of the SIB 1 in the subframe. Once found, the SIB 1 message
provides the following information:
The MCC and MNC of the cell. These parameters tell the mobile device if the cell
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belongs to the home network or not.
The NAS cell identifier, which is similar to the cell‐ID in GSM and UMTS.
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