tated, for every certain rate to be supported, a proportional radio resource has to be allocated. Second-generation systems can only handle constant-rate traffic, while 3G systems have more flexibility in resource allocation. But they still suffer from assumptions © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks that came from the circuit-switched, voice-only view on the customer. Only the next-generation systems will make no difference between data and voice communi- cation. They even sacrifice the advantages of classical digital voice transmission for a more contemporary voice over Internet protocol (VoIP) convergence. Multihop systems are transparent for the UT. The RN acts like a BS toward the UT on the second (last) hop. Additional resources are required only for the first hop, where the RN acts like a UT toward the BS. This resource coordination was not foreseen in older systems-one reason why the deployment of smart relay concepts did not come at that time. 8.2.2 Generic Multimode Protocol Stack Once the protocol stack for a relay is clear, it is simply the exchange of the uplink or the downlink side to construct a heterogeneous relay. For example, LTE could be used on the first hop; WiMAX or some backward-compatible GSM or UMTS could be used on the second hop. Each system requires different layers, especially on layer 2. Within a layer, we could think of these systems as a "mode" and simply supply an implementation for each mode. This is most use- ful for UTs that can adapt to the best (e.g., least expensive or best performing) radio interface at the current location. See Berlemann et al. [11] for the ideas behind it. 8.2.3 Link-Level Models and Performance This chapter deals with performance results on the MAC layer. They are very much linked with the raw performance on the PHY layer. Therefore, link-level perfor- mance results are needed at first. Here a way is shown that is abstract enough not to require simulations anymore and is suitable for studies on higher layers. For