receiver MAC or the RLC. If it is placed in the MAC, the MAC PDU header has to carry an additional sequence number to support ordering. In addition, not all RLC PDUs need in-sequence delivery; reordering at the MAC delays the delivery of all PDUs, regardless of the ordering requirement. Thus, the reordering function should be placed in the RLC; only if the reordering fails at the expiry of a timer would the missing PDU be reported to trigger a status report. © 2009 by Taylor & Francis Group, LLC User Plane Protocol Design for LTE System Sender Retransmissions, NACK Maximum (M-1)*RTT Receiver Forward PDU #N + 1 to RLC Forward PDU #N to RLC Figure 9.1 Worst-case HARQ reordering delay. Figure 9.1 shows how to set the reordering timer. Because the RLC sender always send PDUs to the MAC in their original order, the worst-case delay situation happens when PDUs N and N + 1 are sent in the same TTI. PDU N successfully arrives at the receiver, but PDU N + 1 takes the maximum number of transmis- sions (M) to get to the receiver. The times in which the receiver RLC gets those two PDUs differ by (M - 1) X RTT, where RTT is the HARQ round-trip time, if we assume HARQ retransmissions are always scheduled instantly. Thus, the RLC reordering timer should be set to (M - 1) X RTT because a smaller value may trig- ger a status report prematurely, ignoring the unfinished HARQ retransmissions. Tosummarize, given the sender side ARQ/HARQ interaction in the new Beyond 3G system architecture, we suggest the following principles for RLC layer configuration: The receiver side should configure the "missing PDU indicator" to report errors as quickly as possible. To prevent immature status reports due to PDU disordering by HARQ operation, a reordering timer should be con- figured with (M - 1) X RTT and started whenever a "hole" in the received PDU sequence is observed. The sender side should enable polling for "the last PDU in the buffer" and "the last PDU in the retransmission buffer," as well as for the "poll timer," to prevent the loss of