n for those UTs connected to it. Figure 9.3 shows how the downlink transmission is organized in the OFDMA- based REC for every two TTIs. In the first TTI, the BS transmits to the RN and the UTs directly connected to it. In the second TTI, the resources for RN transmis- sion and BS transmission are separated, in order to eliminate interference between the two. We assume a fixed allocation of resources for the RN to UT transmission; the detailed parameter settings are listed in Table 9.1. Our focus is to investigate the role of the RLC layer on the RN. Basically, two RLC deployment modes can be chosen by the REC, as shown in Figure 9.4. In the upper case, the RLC layer is also implemented on the RN, SO each hop can Figure 9.2 Relay-enhanced cell deployment. © 2009 by Taylor & Francis Group, LLC User Plane Protocol Design for LTE System RN/UT 1st TTI 2nd TTI Figure 9.3 Downlink transmission with relay. Table 9.1 Physical Layer Parameters Inter-BS distance BS-RN distance 543 m Carrier frequency (DL) 2.5 GHz Bandwidth 2 X 20 MHz BS antenna gain 14 dBi BS-RN antenna array gain 6 dBi RN antenna configuration Single antenna, omnidirectional Receiver noise figure BS max. transmit power per sector 46 dBm RN max. transmit power 37 dBm TTI length Chunk size 12 OFDM symbols No. of chunks for RN downlink No. of chunks for BS-RN downlink User speed 30 m/s RN-UT channel Non-line of sight RLC PDU size HARQ round-trip time 4 TTI RLC status report feedback delay Fixed to 5 ms/hop have a separate RLC connection that handles hop-wise ARQ; this is referred to as per-hop RLC. In the lower case, no RLC layer is implemented on the RN; a single RLC connection handles ARQ over the two hops, which is referred to as two-hop RLC. Simulation Model and Results We are interested only in the protocol behavior; thus, in the simulation, only UTs connecting to the RN are modeled. The OFDMA-based system takes the param- eters listed in Table 9.1 and operates in the FDD mode. Also, we assume that the © 2009 by Taylor & Francis Group, LLC Long Term Evo