 Francis Group, LLC Long Term Evolution: GPP LTE Radio and Cellular Technology ISO/OSI model Application Application Presentation Presentation Session Logical link Session Transport Transport Network Network Relay Network Data Link Data Link Data Link DataLink Physical Physical Physical Physical Figure 8.4 The ISO-OSI view on relays. On the user plane, they behave like a bridge by using the lowermost two layers. multiplexing (OFDM) symbol creation, and decoding. This layer is common in all station types-BS, RN, or UT. They may only differ in points like transmit power level, RF (radio frequency) filter quality, and antenna gains. Thus, for a link between communication peers, the link between BS and RN is just like that from BS to UT, as is the link between RN and UT. It is the higher layer that gives the link a mean- ing. Whether it is a broadcast or unicast transmission or a data or control channel, uplink or downlink is something decided by the medium access control (MAC) functionality of the data link layer (DLL), or layer 2. This is where the radio system becomes a character and distinguishes LTE from WiMAX [4,9] (worldwide interop- erability for microwave access), WiFi, or other OFDM-based systems. A wireless relay is simply a device that receives incoming bits up to layer 2 and forwards them down again on layer 2 of the sending side, SO this decode-and-for- ward relay acts like a repeater, bridge, or router known in the classical wired world. Figure 8.4 shows this layer model. In these systems, received protocol data units (PDUs) are fully error corrected, automatically repeated (ARQ), stored and sched- uled for transmission, and, if necessary, even segmented and reassembled prior to their next-hop transmission. Conventional cellular systems use one-hop bidirectional (duplex) communica- tion between BS and UTs. UTs are immediately associated (linked) to the BS, which coordinates resources needed for the downlink (DL) and uplink (UL) trans- mission according to the traffic demand in both directions. Simply s