not comparable (e.g., in terms of performance and reliability) to cellular systems, on which we focus in this chapter. All cellular systems require a centralized control of resources to avoid interference due to uncoordinated transmissions. With its frame-based transmission in TDD mode, WiMAX (IEEE 802.16) can also support a multihop operation (with RN) due to its centralized control [18]. This mode is preferred over the 802.16 mesh concept, which is not compatible. Mesh networks are not further covered in this chapter. 8.2.5 Theory of Relay-Enhanced Operation "Relaying" means that any traffic destined from the fixed network toward the UT goes through the BS and one or more RNs [27,29]. We assume one relay (two hops) now, but the principles are inductive for more relays in a row [20,33]. The traffic is packetized in PDUs transmitted by the BS to the RN using dedicated downlink resource blocks (a fraction of the time and bandwidth). In the BS, the RN is treated like a UT. For the next hop, the PDUs require resources (sending opportunities) between the RN and UT in the downlink. These resources are assigned by the BS, which is typically relay aware in modern multihop designs. Thus, the BS has reserved DL resources that it does not use itself but, rather, lets the RN operate on it. The RN has full freedom of use for these resources. It decides like a BS which resource to use for which of its UTs. The size of the resources for the second hop may differ from that of the first hop, if the SINR situation is different, and different PhyModes are used. This fact has to be taken into account when reserving resources for hop two. UTs receive PDUs from an RN just like they would from a BS. They do not see a difference and do not need to be relay aware for that purpose. For the uplink, a UT announces any available traffic to its master station, which is the RN. The RN assigns UL resources to UT for the PDU transmission while it also announces new uplink traffic toward the BS. PDUs received in the uplink of hop two are stored 