tion of half-and full- duplex terminals in future multihop FDO-based cellular networks. Proceedings of the 4th International Conference on Wireless Communications (WiCom 2008). October 2008. © 2009 by Taylor & Francis Group, LLC Chapter 9 User Plane Protocol Design for LTE System with Decode-Forward Type of Relay Jijun Luo Contents 9.1 Introduction RLC Layer Overview Impact of Broadband RAN Discussion of RLC Configurations 9.5 Cellular Network with Relay 9.6 Simulation Model and Results 9.7 Conclusion and Discussion References 9.1 Introduction In the 3rd Generation Partnership Project (3GPP) WCDMA (wideband code divi- sion multiple access)/HSPA (high-speed packet access) standards, the radio link con- trol sublayer [1] is defined on top of the medium access control (MAC) sublayer and C 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology under other higher sublayers such as RRC (radio resource control) and PDCP (packet data convergence protocol) [2]. Its main function is to guarantee reliable data trans- mission by means of segmentation, ARQ (automatic retransmission request), in- sequence delivery, etc. The radio link control (RLC) layer has three functional modes: the TM (transparent mode), UM (unacknowledge mode), and AM (acknowledge mode), enabling services with different speed and reliability for the upper layer. In this chapter, we mainly study the AM mode of RLC for the unicast traffic on the downlink user plane. The AM mode incorporates the ARQ functionality, SO it is by far the most important operation mode for reliable data transfer. 9.2 RLC Layer Overview In the traditional WCDMA context, the responsibility of providing ARQ lies solely on the RLC layer because the MAC layer assumes no retransmission functional- ity. Moreover, the RLC layer and MAC layer are separated by the lub interface [2] between the radio network controller (RNC) and the base station (BS). Such sepa- ration determines that the RLC layer transmission is much slower than the MAC layer, which