 we make only a maximum of three transmissions on the HARQ. In Figures 9.5 and 9.6, the throughput and CDF of RLC SDU (IP packet) delays are shown for both the two-hop RLC and per-hop RLC cases, with different RLC transmission window sizes and unlimited RLC receiving window sizes. If the SDU is not successfully delivered to the UT, its delay is the simulation time minus the packet generation time. A big problem with single RLC over two hops is that the tuning of RLC transmission window size is quite difficult. In single-hop transmission, if, without the constraint of the receiver RLC window, the RLC transmission window is adapted to the link capacity, overflow or underflow at the MAC layer can be avoided. In the REC, where the two hops could have distinctive link capacity, the two-hop RLC scheme could only adapt the transmission window size to one of the two links. The RLC standards with a universal mobile telecommunications system (UMTS) [1] allow the receiver to adjust the window size of the transmitter. However, in the two-hop RLC transmission case, the UT is ignorant about whether the RN buffer undergoes overflow or underflow due to inappropriate sender window size; hence, it cannot tell the sender the suitable window size. Because the first hop normally transmits more quickly than the second hop, setting a big transmission window size © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology CDF of RLC SDU Delay Per-hop RLC Two-hop RLC, W = 100 Two-hop RLC, W =300 Two-hop RLC, W =500 Two-hop RLC, W =700 Seconds Figure 9.6 Delay of RLC operation with REC. would make data queue up at the RN; setting a small window size might under- utilize the capacity provided by the first hop and might result in a window stall problem. Also, the threat of overflow on the RN buffer-having too much data queuing at the RN-slows down the retransmissions. Although the BS always gives RLC retransmission higher priority, the retrans- mitted PDU nevertheless must queue behind the already trans