For efficient heterogeneous network planning, 3GPP LTE-Advanced has introduced concept of Relay Nodes (RNs). The Relay Nodes are low power eNodeBs that provide enhanced coverage and capacity at cell edges. One of the main benefits of relaying is to provide extended LTE coverage in targeted areas at low cost.
The Relay Node is connected to the Donor eNB (DeNB) via radio interface, Un, a modified version of E-UTRAN air interface Uu. Donor eNB also srves its own UE as usual, in addition to sharing its radio resources for Relay Nodes.
Source: http://lteworld.org/ltefaq/what-relay-node-and-how-does-relaying-works-lte-advanced
