Page 267 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 253

dedicated bearer for the data flow. All IP packets that meet certain criteria such as
source and destination IP addresses or certain UDP port numbers are then always sent
over the air interface in this logical dedicated bearer for which a certain bandwidth,
delay and priority are ensured by the network. On the air interface, packets flowing
through this dedicated bearer can then be scheduled using semi‐persistent grants. All
other data to and from the device would continue to use the default bearer for which the
eNode‐B uses dynamic scheduling. Internet‐based voice services such as Skype, how-
ever, have no means to interact with the LTE network to request a certain QoS. For such
services, the IP transport channel is transparent and VoIP packets use the same default
bearer together with IP packets to and from all other connected services on the mobile
device. A potential solution for the future to optimize the use of scheduling grants
might thus be that the network analyzes the traffic flow and then adapts its use of the
different types of scheduling method accordingly.

4.5.2 Uplink Scheduling
To get resources assigned on the PUSCH, the mobile device has to send an assignment
request to the eNode‐B. If no physical connection currently exists with the eNode‐B,
the mobile first needs to reestablish the link. This is done as already described above by
sending an RRC Connection Request message on the RACH. The network then estab-
lishes a channel and assigns resources on the PUSCH so that the mobile device can
transmit signaling and user data in the uplink direction. The assignment of uplink
resources is performed via PDCCH messages in the control section of each subframe.
While a mobile device is in active communication with the network and has resources
assigned on the uplink shared channel, it includes buffer status reports in the header of
each packet. This information is then used by the eNode‐B to assign uplink resources in
the following subframes. If the connection is active but the mobile device currently has
no resources on the uplink shared channel, it has to send its bandwidth requests via the
PUCCH as described earlier.
Uplink scheduling grants are sent as PDCCH messages in the same way as described
earlier for downlink scheduling grants. For this purpose, DCI messages of type 0 are
used as shown above in Table 4.6. In fact, while scanning the search spaces in the con-
trol section of each subframe, both uplink and downlink scheduling grants that can be
given as full‐duplex FDD devices can transmit and receive data at the same time.
To make the best use of the resources of the uplink physical shared channel, the eNode‐B
must be aware of how much transmission power a mobile device has left compared to its
current power output. It can then select an appropriate modulation and coding scheme
and the number of RBs on the frequency axis. This is done via power headroom reports,
which are periodically sent by the mobile device in the uplink direction.

4.6 Basic Procedures

Now that the various reference signals and channels of the air interface have been introduced,
the following sections give an overview of the different procedures required for com-
munication with the network with reference to the previous section for details on the
channels.