Page 248 - From GMS to LTE

234 From GSM to LTE-Advanced Pro and 5G

1 Subframe =
2 resource blocks =
2 slots (0.5 ms) =
2 × (12 subcarriers × 7 symbols)

= reference signal

Figure 4.9 Symbols in a resource block used for the reference signal.

For initial synchronization, two additional signal types are used. These are referred to
as the primary and secondary synchronization signals (PSSs and SSSs) and they are
transmitted in every first and sixth subframe on the inner 72 subcarriers of the channel.
On each of those subcarriers, one symbol is used for each synchronization signal.
Hence, synchronization signals are transmitted every 5 milliseconds. Further details
can be found in Section 4.6.1 where the initial cell search procedure is described.

4.3.5 The LTE Channel Model in the Downlink Direction
All higher‐layer signaling and user data traffic are organized in channels. As in UMTS,
logical channels, transport channels and physical channels have been defined as shown
in Figure 4.10. Their aim is to offer different pipes for different kinds of data on the
logical layer and to separate the logical data flows from the properties of the physical
channel below.
On the logical layer, data for each user is transmitted in a logical Dedicated Traffic
Channel (DTCH). Each user has an individual DTCH. On the air interface, however, all
dedicated channels are mapped to a single shared channel that occupies all RBs. As
described above, some symbols in each RB are assigned for other purposes and hence
cannot be used for user data. Which RBs are assigned to which user is decided by the
scheduler in the eNode‐B for each subframe, that is, once per millisecond.
Mapping DTCHs to a single shared channel is done in two steps. First, the logical
DTCHs of all users are mapped to a transport layer Downlink Shared Channel (DL‐
SCH). In the second step, this data stream is then mapped to the Physical Downlink
Shared Channel (PDSCH).
Transport channels are able to multiplex not only data streams from several users but
also several logical channels of a single user before they are finally mapped to a physical
channel. An example is as follows. A UE that has been assigned a DTCH also requires a
control channel for the management of the connection. Here, the messages that are
required, for example, for handover control, neighbor cell measurements and channel
reconfigurations are sent. The DTCH and the DCCH are multiplexed on the DL‐SCH
before they are mapped to the PDSCH, that is, to individual RBs. In addition, most of
the cell‐specific information that is sent on the logical broadcast control channel
(BCCH) is also multiplexed on the transport downlink shared channel, as shown in
Figure 4.10.
In LTE, all higher‐layer data flows are eventually mapped to the physical shared chan-
nel, including the Paging Control Channel (PCCH), which is used for contacting mobile
devices that are in a dormant state to inform them of new IP packets arriving from the