ration. As the 3GPP sets its target for the next generation of broadband radio access network (RAN)-commonly referred to as long term evolution (LTE) [6]-the RLC layer also has to evolve. Two significant changes of the RLC layer are quite clear. First, due to architectural changes [6], the RLC layer now resides also on the BS. Second, due to the much faster air interface, the RLC layer should also strive to make error recovery much faster than before. As indicated in reference 7, this is partly enabled by closer interaction between the MACHARQ and the RLC ARQ. The draft standard mentions three optional ways of interaction: If the HARQ transmitter detects a failed delivery of a PDU-for exam- ple, due to a maximum retransmission limit being reached-the relevant transmitting ARQ entities are notified and potential retransmissions can be initiated. © 2009 by Taylor & Francis Group, LLC User Plane Protocol Design for LTE System If the HARQ receiver is able to detect a NACK (negative acknowledge) to ACK (acknowledge) error, it is FFS (for further study) if the relevant trans- mitting ARQ entities are notified via explicit signaling. If the HARQ receiver is able to detect transmission failure, the receiving ARQ entities could be notified. Among the three possible strategies, the first one is rather straightforward. Because the RLC and MAC reside in the BS, a transmission failure can be reported by the MAC to the RLC to trigger faster retransmission without waiting for the status report. The latter two strategies mostly try to solve the problem of NACK ACK error with the HARQ because this cannot be recovered by the first strategy. However, for the moment, let us leave the NACK ® ACK error out of the picture. In our simulation model (introduced in Section 9.6), we incorporate the first HARQ/ ARQ interaction listed before and we will neglect the NACK R ACK error. 9.4 Discussion of RLC Configurations The RLC operation in the future broadband RAN must be able to recover the HARQ transmission failures efficiently. Within the 