Page 232 - From GMS to LTE

218 From GSM to LTE-Advanced Pro and 5G

eNode‐Bs consist of three major elements:
the antennas, which are the most visible parts of a mobile network;
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radio modules that modulate and demodulate all signals transmitted or received on
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the air interface;
digital modules that process all signals transmitted and received on the air interface
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and that act as an interface to the core network over a high‐speed backhaul
connection.
Many vendors use an optical connection between the radio module and the digital
module. This way, the radio module can be installed close to the antennas, which
reduces the length of costly coaxial copper cables to the antennas. This concept is also
referred to as Remote Radio Head (RRH), and significant savings can be achieved, espe-
cially if the antennas and the base station cabinet cannot be installed close to each other.
At this point it is important to introduce the concept of ‘bearer’ as the term will be
frequently used in this chapter. A bearer is a logical connection between network enti-
ties and describes quality of service (QoS) attributes such as latency, maximum through-
put, etc. for the data that flows over it. All transmissions between a mobile device and a
radio base station are managed via a Radio Access Bearer (RAB). The RAB assigned to
a mobile device during connection establishment includes a Signaling Radio Bearer
(SRB) for exchanging session management, mobility management and radio resource
configuration (RRC) messages and at least one Data Radio Bearer (DRB) over which IP
user data packets are transferred.
Unlike in UMTS where the base station at the beginning was little more than an intel-
ligent modem, LTE base stations are autonomous units. Here, it was decided to inte-
grate most of the functionality that was previously part of the radio network controller
(RNC) into the base station itself. Hence, the eNode‐B is not only responsible for the air
interface but also for:
user management in general and scheduling air interface resources;
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ensuring QoS such as ensuring latency and minimum bandwidth requirements for
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real‐time bearers and maximum throughput for background applications depending
on the user profile;
load balancing between the different simultaneous radio bearers to different users;
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mobility management;
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interference management, that is, reducing the impact of its downlink transmissions
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on neighboring base stations in cell‐edge scenarios. Further details are given below.
For example, the eNode‐B decides on its own to hand over ongoing data transfers to
a neighboring eNode‐B, a novelty in 3GPP systems. It also executes the handover
autonomously from higher‐layer nodes of the network, which are only informed of the
procedure once it has taken place. The air interface is referred to as the LTE Uu inter-
face and is the only interface in wireless networks that is always wireless. The theoreti-
cal peak datarates that can be achieved over the air depends on the amount of spectrum
used by the cell. LTE is very flexible in this regard and allows bandwidth allocations
between 1.25 and 20 MHz. In a 20 MHz and 2 × 2 MIMO configuration, which is typical
for current LTE networks and mobile devices, peak speeds of up to 150 Mbit/s can be
reached. Speeds that can be achieved in practice depend on many factors such as the
distance of a mobile device from the base station, transmission power used by the base