DL - 4G Americas

traffic/signaling forwarding between eNB and UE to improve the coverage of high data rates, cell edge
coverage and to extend coverage to heavily shadowed areas in the cell or areas beyond the cell range. It
provides throughput enhancement especially for the cell edge users and offers the potential to lower the
CAPEX and OPEX by keeping the cell sizes relatively large by limiting the number of macro sites.
The relay nodes are wirelessly connected to the radio access network via a donor cell. The RN is
connected to the donor eNB via the Un interface and the UEs are connected to the RN via the Uu
interface as shown in Figure 7.3. The Un connections can be either in-band or out-band. In an in-band
connection, the eNB-to-relay link shares the same band with the direct eNB-to-UE link within the donor
cell. In this case, Rel-8 UEs should have the ability to connect to the donor cell. For out-band connection,
on the other hand, the eNB-to-relay connection is in a different band than the direct eNB-to-UE link.
Figure 7.3. A Diagrammatic Representation of a Relay Network.
The types of relays that were studied in 3GPP during the LTE Rel-10 timeframe can be roughly separated
by the layers within the protocol stack architecture that are involved in the relay transmission:
� Layer 1 (L1) Relay. Also called Amplify-and-Forward Relay, Layer 1 (L1) Relay is simple and
easy to implement through RF amplification with relatively low latency. The noise and
interference, however, are also amplified along with the desired signal. Moreover, strict isolation
between radio reception and transmission at RN is necessary to avoid self-oscillation, which limits
its practical applications.
� Layer 2 (L2) Relay. Layer 2 (L2) Relay performs the decode-and-forward operation and has
more freedom to achieve performance optimization. Data packets are extracted from RF signals,
processed and regenerated and then delivered to the next hop. This kind of relay can eliminate
propagating the interference and noise to the next hop, so it can reinforce signal quality and
achieve much better link performance. (Difficulty for HARQ techniques)
� Layer 3 (L3) Relay. Also called Self-Backhauling, Layer 3 (L3) Relay has less impact to eNB
design and it may introduce more overhead compared with L2 Relay.
From the point of view of UE knowledge, the relays that were studied in 3GPP can be classified into two
types; transparent and non-transparent. In transparent relay, the UE is not aware that it is communicating
with the eNB via a relay. Transparent relay was proposed for the scenarios where it is intended to
achieve throughput enhancement of UEs located within the coverage of the eNB with less latency and
www.4gamericas.org February 2011 Page 56