Page 266 - From GMS to LTE

252 From GSM to LTE-Advanced Pro and 5G

1. Read the PCFICH
to get size of control region

2. Decode PDCCH to find
DL scheduling grants

3. Get data from PDSCH

Figure 4.16 Downlink data reception overview.

its ID, it then computes the search spaces in which it has to look for assignment messages.
It then decodes these areas to see if it can find messages for which it can successfully
calculate the CRC checksum, which implicitly contains the device’s ID. If a Downlink
Assignment message is found, the resource allocation type then determines how the
information is given (bitmap or a starting point with length). With this information, the
mobile device can now go ahead and attempt to decode all areas of the subframe in
which its data is transmitted.
Depending on the current activity and the amount of data that arrives for a user from
the network, it might not be necessary to schedule data for a user in every subframe. In
such cases, it is also not necessary for the mobile device to search for potential schedul-
ing grants in every subframe. To reduce the power consumption of a mobile device, the
network can thus configure discontinuous reception (DRX) periods during which the
control channel does not have to be observed. Further details are discussed in Section 4.7.

Semi‐Persistent Scheduling
While dynamic scheduling is ideal for bursty, infrequent and bandwidth‐consuming
data transmissions such as web surfing, video streaming and e‐mails, it is less suited for
real‐time streaming applications such as voice calls. Here, data is sent in short bursts at
regular intervals. If the datarate of the stream is very low, as is the case for voice calls,
the overhead of the dynamic scheduling messages is very high as only a small amount of
data is sent for each scheduling message. The solution for this is semi‐persistent scheduling.
Instead of scheduling each uplink or downlink transmission, a transmission pattern is
defined instead of single transmission opportunities. This significantly reduces the
scheduling assignment overhead.
During silence periods, wireless voice codecs stop transmitting voice data and only
send silence description information with much longer time intervals in between.
During these silence times persistent scheduling can be switched off. Hence, grants are
semi‐persistent. In the uplink direction, the semi‐persistent grant scheme is implicitly
canceled if no data is sent for a network‐configured number of empty uplink transmis-
sion opportunities. In the downlink direction, semi‐persistent scheduling is canceled
with an RRC message. Details can be found in section 5.10 of the LTE MAC protocol
specification [17].
In practice, it is left to the network vendor to decide when to use which scheduling
type. The challenge with semi‐persistent scheduling for the network is to detect when
it can be used. For network operator‐based VoIP services such as Voice over LTE (see
Section 4.13), the voice service can request the LTE network to establish a logical