Page 249 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 235

Downlink channels
Logical
channels PCCH BCCH DTCH DCCH
(what)

Transport PCH BCH DL-SCH
channels
(how)

Physical PBCH PDSCH PCFICH PDCCH PHICH
channels
Figure 4.10 LTE downlink channel structure.

network side. The PCCH is first mapped to the transport layer Paging Channel (PCH),
which is then mapped to the PDSCH.
The only exception to the general mapping of all higher‐layer information to the
shared channel is the transmission of a small number of system parameters that are
required by mobile devices to synchronize to the cell. They are transmitted on the
Physical Broadcast Channel (PBCH), which occupies three symbols on 72 subcarriers
(=6 RBs) in the middle of a channel every fourth frame. Hence, it is broadcast every 40
milliseconds and follows the PSSs and SSSs.

4.3.6 Downlink Management Channels
As discussed in the previous section, most channels are mapped to a single PDSCH,
which occupies all RBs in the downlink direction except for those symbols in each RB
which are statically assigned for other purposes. Consequently, a mechanism is required
to indicate to each mobile device when, where and what kind of data is scheduled for
them on the shared channel and which RBs they are allowed to use in the uplink direc-
tion. This is done via Physical Downlink Control Channel (PDCCH) messages.
The downlink control information occupies the first one to four symbols over the
whole channel bandwidth in each subframe. The number of symbols that are used for
this purpose is broadcast via the Physical Control Format Indicator Channel (PCFICH),
which occupies 16 symbols. This flexibility was introduced so that the system can react
to changing signaling bandwidth requirements, that is, the number of users that are to
be scheduled in a subframe. This topic is discussed in more detail in Section 4.5.
Downlink control data is organized in Control Channel Elements (CCEs). One or
more CCEs contain one signaling message that is either addressed to one device, or, in
the case of broadcast information, to all mobile devices in the cell. To reduce the
processing requirements and power consumption of mobile devices for decoding the
control information, the control region is split into search spaces. A mobile device
therefore does not have to decode all CCEs to find a message addressed to itself but only
those in the search spaces assigned to it.