Page 263 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 249

This compares to a theoretical maximum uplink speed of 75 Mbit/s of FDD‐LTE, where
a channel with up to 20 MHz bandwidth is used for downlink transmissions and an
additional 20 MHz channel for uplink transmissions.

4.5 Scheduling

Data transmissions in LTE in both the uplink and the downlink directions are controlled
by the network. This is similar to other technologies such as GSM and UMTS. In these
systems, some or all of the scheduling control is located in centralized network compo-
nents such as the PCU (see Chapter 2) or the RNC (see Chapter 3). In LTE, the schedul-
ing is fully controlled by the eNode‐Bs as higher‐layer radio network control instances
were removed from the overall network design. Some of the advantages of network‐
based scheduling are as follows:
The network can react to changing radio conditions of each user and optimize the
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overall throughput.
The network can ensure the QoS for each user.
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Overload situations can be dealt with.
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Other technologies such as Wi‐Fi do not use centralized control and leave it to the
devices communicating in the network to cooperatively use the air interface. Here, cen-
tral control is not necessary as the number of devices simultaneously communicating
over a Wi‐Fi access point is much lower and the coverage area is much smaller. Details
are discussed in Chapter 5.

4.5.1 Downlink Scheduling
In the downlink direction, the eNode‐B’s scheduler is responsible for forwarding the
data that it receives from the network for all users it serves over the air interface. In
practice, a single logical default bearer is usually assigned to a mobile device, over which
the data that flows from and to the Internet are transported. To ensure QoS for applica-
tions such as VoLTE (cf. Chapter 5), it is also possible to assign more than one logical
bearer to a mobile device. The VoLTE data stream then uses a dedicated bearer for
which the requested bandwidth and a low time variation between two packets (jitter) is
ensured by the network.

Dynamic Scheduling
Scheduling is a simple task if there is only one user and if there is less data waiting in
the transmission buffer than can be sent over the air interface. When the eNode‐B
serves several users, or several bearers to be precise, and the amount of data in the
downlink buffer exceeds that which can be sent in a subframe, then the scheduler has
to decide which users and bearers are given an assignment grant for the next sub-
frame and how much capacity is allocated to each. This decision is influenced by
several factors.
If a certain bandwidth, delay and jitter have been granted for a bearer to a particular
user then the scheduler has to ensure that this is met. The data of this bearer is then
given preference over the data arriving from the network for other bearers for the same