Long Term Evolution (Advanced) - LTE(-A)
Relays

Along with femtocells, so-called relay stations are defined for the 4G networks. In general, the relays are simplified (and thus low cost) eNodeBs that relay data from the eNodeB to the user. The data exchange between UE and the network are routed via one or several relays.

Each part of communication (e.g., from UE to relay, from relay to eNodeB, etc.) represents one hop in communication. Therefore, communication using relays is also known as multihop communication.

The relays can be used either to extend coverage of an eNodeB or to increase throughput in specific area as shown in figure 27. For increasing the throughput, a relay is deployed within the coverage area of the eNodeB. The increase in throughput is reached as the distance between communicating nodes is decreased and signal level observed by target node or UE is thus increased. For enlarging the coverage, relay is deployed close to the edge of eNodeB cell. The relay can be deployed even outside the cell border experienced by UEs. This is due to the fact that the relay can be deployed in a place with direct visibility to the eNodeB and thus the communication channel between eNodeB and relay is of a higher quality comparing to the channel between eNodeB and UEs without direct visibility to the eNodeB.

In opposite to femtocells, relays are expected to be fully controlled by operators and their connection to the network is via wireless link shared with data connection of users served by the eNodeB.

Relays deployment and purpose

The relays can be classified according to the relays mobility to:

- Fixed relay - this relay is permanently installed at the same place without enabled mobility of the relay. The fixed relays are supported since Release 10 of LTE-A.
- Mobile relay - assumed to be deployed on moving vehicles such as bus or trains. In this case, the relay is supposed to perform often handover among eNodeBs. During the movement of the relay, handover among eNodeBs must be ensured even if the mobile relay serves UEs. The mobile relays are not supported in Release 10 but should be considered in Release 11.