Evolved Packet System (EPS): The LTE and SAE Evolution of 3G UMTS

2.8 Major Disruptions with 3G UTRAN-FDD Networks

The choice of OFDM technology on the radio interface and the various options chosen for the E-UTRAN radio interface have some significant and disruptive impacts on the way the overall network behaves. This section aims at describing the main impacts of some E-UTRAN technical choices made in the standard definition on network architectures and supported features, as regards to 3G/UTRAN FDD networks.

2.8.1 About Soft Handover

One of the main consequences of using a CDMA-based radio interface is about the need for soft handover, also known as macro-diversity. In CDMA, each transmission channel behaves as an interferer for the other channels. The consequence is that transmission power tuning is a key point to preserve CDMA system capacity. This becomes critical at cell edge or in poor coverage areas, where maintaining the radio link transmission quality is often a synonym to increased transmission power. This is where soft handover helps, allowing the information to be transmitted on different links – called the active set – and adding transmission diversity gain. As illustrated in Figure 2.29, soft handover is a mechanism by which a terminal maintains simultaneously several radio links in different cells for one single session or data flow. Information is then recombined from the received radio links, either on the network or on the terminal side, for the sake of transmitted power and associated interference. In 3G/UTRAN, soft handover ...