porated in link-level result graphs, which are also shown in this chapter. Because this is a one-way, single-link mechanism, there is no relay-specific issue with that. 8.4.5 Automatic Repeat Request (ARQ) ARQ is the most important function to ensure error-free reception above layer 2. All PDUs that still have bit errors after FEC are detected by an additional CRC check- sum. ARQ then sends negative acknowledgments (NACKs) to the sender asking for the transmission of this PDU again. In a multihop setup, this can be done either hop by hop or end to end. From simulative studies and real-world experiments, it is known that a hop-by-hop solution is robust and performs best. End-to-end ARQ suffers from the problem that there is not only one link in between, but also two (or three) links, as well as buffers in the relay in between. Any buffer overflow looks like a link failure and triggers a retransmission, which leads to more traffic and even more buffer loss. This situation is both severe and common because the links of hop one and hop two are typically not the same in terms of capacity; thus, any unmatched link capacity leads to transient buffer overflows from time to time. 8.4.6 Hybrid ARQ (HARQ) HARQ is a technique to combine FEC with ARQ already in the physical layer. In case of a transmission error, the old received bits are not discarded but, instead, combined with a new, retransmitted PDU, and the result is more reliable than a normal retransmission would be. Because this is a link-local mechanism, there is no effect on relaying. 8.4.7 Radio Resource Management (RRM) The resource schedulers decide upon the resources given to them. In a long-term view, these resources vary as new UTs come into the cell and some are handed off. Each time, the coarse resources must be redistributed. The RRM unit knows the number of UTs associated with the BS and each RN and its traffic contract, SO it can make a proper decision [36]. This unit must be relay aware because it must send this partitioning decision to the RNs. 8.4.8 S