From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

4.1 Introduction and Overview

Despite constant evolution, UMTS, as described in Chapter 3, is approaching a number of inherent design limitations in a manner similar to what GSM and GPRS did a decade ago. The Third Generation Partnership Project (3GPP) 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. The main improvements over UMTS are in the following areas:

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 took 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 ...