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Relay nodes (RNs) in LTE-Advanced have several different classifications according to different criteria.

Amplify-and-Forward v.s. Decode-and-Forward RNs

In the Amplify-and-Forward (AF) relaying scheme, the transmitted data is received and amplified by the RNs. There are less computation and delay with this scheme. However, the interference and the thermal noise at the RNs are also amplified.

In the Decode-and-Forward (DF) relaying scheme, the RNs decode the received signal from the eNB and transmit the encoded data to the destination UEs in the downlink. Better channel conditions of the two-hop transmission can be obtained.

S1 UE UE Uu IP Networks PGW SGW/MME SGi S5/S8 eNB RN Ba_ck ha ul l_ink Dire ct lin k Acce ss lin k Un Uu

Figure 2.10: Simplified LTE-Advanced network architecture with relay node

Since the RN buffer is finite, all the data received from the eNB should be trans- mitted to the UEs to avoid resource waste.

L1, L2 and L3 RNs

From the perspective of the protocol architecture of relaying, L1, L2 and L3 RNs are specified in [34]. The L1 RN just amplifies the signal from eNB/UE and forwards it to the UE/eNB. The L2 RN performs the scheduling function. The resource alloca- tion between the UE and the L2 RN is performed in coordination with eNB and the other L2 RNs, taking inter-cell interference and load conditions into consideration. The L3 RN has partial or full functions of RRC resided in eNB. The latency due to the handover and fast data routing can be reduced. The L3 measurements may be utilised for handover decisions in the RNs.

In-band v.s. Out-band RNs

With respect to the usage of spectrum, the operation of RNs can be classified into in-band and out-band. The eNB-RN links of the out-band RNs are not allowed to operate on the same carrier frequency band as RN-UE links; the in-band RNs can operate the eNB-RN links and the RN-UE links on the same carrier frequency band, which can reduce the complexity of frequency band planning. In addition, for both in-band and out-band relaying, it is possible to operate the eNB-RN link on the same carrier frequency band as eNB-to-UE links.

Transparent v.s. Non-transparent RNs

In relation to the information for UEs, RNs can be classified into transparent and non-transparent, according to whether the UEs are aware of their associating RN or not. The transparent RNs will not extend the coverage, but is beneficial for cooperative communication. The non-transparent RNs have cell IDs and can provide some RRM functions.

Type 1 and Type 2 RNs

In the LTE-Advanced standards [14], Type 1 and Type 2 RNs are defined with the following characteristics:

• A basic Type 1 RN is an in-band relay node with the following characteristics: – It controls a cell, and appears to a UE as a separate cell different from

the donor cell;

– The cells controlled by the RNs shall have their own physical cell Iden- tifier (ID), defined in LTE Rel-8, and the RNs shall transmit their own synchronization channels, reference symbols and so on;

– In the context of single-cell operation, scheduling and HARQ functions can be conducted by the RN. The control channels also exist at the RNs; – It shall appear as a special UE to the eNB.

• Type 1a and Type 1b RNs are characterised by the same set of features as the basic Type 1 RN described above, except that the Type 1a RN operates out- band and the Type 1b RN operates in-band with adequate antenna isolation. A basic Type 1 RN is expected to have little or no impact on LTE Release-8 specifications.

• A Type 2 relay node is an in-band relay node with the following characteristics: – It does not have a separate Physical Cell ID and thus would not create

any new cells;

– It is transparent to Rel-8 UEs; a Rel-8 UE is not aware of the presence of a Type 2 relay node;

– It can transmit Physical Downlink Shared Channel (PDSCH) as the data channel;

– At least, it does not transmit Common Reference Signal (CRS) and Phys- ical Downlink Control Channel (PDCCH) for control functionalities.

Depending on different types of RNs, a RN may be part of the donor cell or controls a cell of its own. In the case that the RN is part of the donor cell, it does not have a cell identity of its own, but still has a relay ID. Most of RRM functions are executed by the eNB of the donor cell, and few parts of the RRM functions may be located in the RN. In the case that a RN is in control of a cell of its own, a unique physical layer cell ID is provided in its cell. The same RRM mechanisms as the eNB are available at the RNs. There is no significant difference between accessing cells controlled by a RN and connecting cells controlled by a normal eNB from the perspective of the RRM of a UE. The cells controlled by the RNs should support also LTE Rel-8 UEs.

Among the above types of relay nodes in LTE-Advanced, basic Type 1 decode- and-forward in-band non-transparent half-duplex fixed RNs are considered as the research objective in this thesis.