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4

Long Term Evolution (LTE) and LTE‐Advanced Pro

4.1 Introduction and Overview

Despite constant evolution, the Universal Mobile Telecommunications System (UMTS), as
described in Chapter 3, has reached a number of inherent design limitations in a manner
similar to GSM and GPRS at the end of the 1990s. The Third Generation Partnership
Project (3GPP), the organization of mobile device manufacturers, infrastructure develop-
ers and mobile network operators responsible for the GSM and UMTS specification, hence
decided to once again redesign both the radio network and the core network. The result is
commonly referred to as ‘Long Term Evolution’ or LTE for short and has been included in
3GPP Release 8. The main improvements over UMTS are in the areas described below.
When UMTS was designed, it was a bold approach to specify an air interface with a
carrier bandwidth of 5 MHz. Wideband Code Division Multiple Access (WCDMA), the
air interface chosen at that time, performed very well within this limit. Unfortunately, it
does not scale very well. If the bandwidth of the carrier is increased to attain higher
transmission speeds, the time between two transmission steps has to decrease. The
shorter a transmission step, the greater the impact of multipath fading on the received
signal. Multipath fading can be observed when radio waves bounce off objects on the
way from transmitter to receiver, and hence the receiver does not see one signal but
several copies arriving at different times. As a result, parts of the signal of a previous
transmission step that has bounced off objects and thus taken longer to travel to the
receiver overlap with the radio signal of the current transmission step that was received
via a more direct path. The shorter a transmission step, the more the overlap that can
be observed and the more difficult it gets for the receiver to correctly interpret the
received signal. With LTE, a completely different air interface has been specified to
overcome the effects of multipath fading. Instead of spreading one signal over the com-
plete carrier bandwidth (e.g. 5 MHz), LTE uses Orthogonal Frequency Division
Multiplexing (OFDM), which transmits the data over many narrowband carriers of 180
kHz each. Instead of a single fast transmission, a data stream is split into many slower
data streams that are transmitted simultaneously. As a consequence, the attainable
datarate compared to UMTS is similar in the same bandwidth but the multipath effect
is greatly reduced because of the longer transmission steps.
To increase the overall transmission speed, the transmission channel is enlarged by
increasing the number of narrowband carriers without changing the parameters for the
narrowband channels themselves. If less than 5 MHz bandwidth is available, LTE can

From GSM to LTE-Advanced Pro and 5G: An Introduction to Mobile Networks and Mobile Broadband,
Third Edition. Martin Sauter.
© 2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.