mitted data that are waiting on the RN buffer. This makes retransmission rather slow and, even worse, the data queuing before the retransmitted PDU might contain the polling, which triggers a status report asking again for the retransmission of the same PDU. Duplicate retransmissions would then be issued by the BS because the first retrans- mission is still on the way. 9.7 Conclusion and Discussion In this chapter, we examined the legacy UMTS RLC protocols and discussed their evolution for the future broadband network. We investigated different operating schemes of RLC for the multihop REC. The conclusion is that the two-hop RLC scheme performs poorly compared to the per-hop RLC scheme, due to the asym- metric capacity on the two hops. We also concluded that, even with the more © 2009 by Taylor & Francis Group, LLC User Plane Protocol Design for LTE System efficient per-hop RLC scheme, a flow control mechanism on the BS-RN hop has to be developed. However, the topic of flow control, as well as the impact of the HARQ/ARQ cross-layer interaction, is left for our future study. References 1. 3GPP. 2006. TS 25.322 V7.2.0. Radio link control (RLC) protocol specification (2006-09). 2. Holma, H., and A. Toskala. WCDMA for UMTS: Radio access for third generation mobile communications, 3rd ed. New York: John Wiley & Sons, 2000. 3. Teyeb, O., M. Boussif, T. Sorensen, J. Wigard, and P. Mogensen. 2005. The impact of RLC delivery sequence on FTP performance in UMTS. WPMC 05, Aalborg, Denmark, September 2005. 4. Zhang, Q., and H.-J. Su. 2002. Performance of UMTS radio link control. IEEE International Conference on Communications (ICC 2002). 5. Li, J., D. Y. Montuno, J. Wang, and Y. Q. Zhao. 2004. Performance evaluation of the RLC protocol in 3G UMTS. Wireless Networks and Emerging Technologies (WNET). 6. 3GPP. 2006. TR 25.913 V7.3.0 (2006-03). Requirements for evolved UTRA (E-UTRA) and evolved UTRAN (E-UTRAN) (release 7), 16. 7. 3GPP. 2007. TS 36.300 V8.1.0 (2007-06). Evolved universal terrestrial radio access (E-UTRA) and evol