ded for that data sink [39,45,56,57]. Interference coordination and filtering. If certain knowledge about the occur- rence of interference is available, then appropriate countermeasures can be taken (e.g., interference filtering by signal processing or interference avoidance by proper resource allocation; see Figure 7.5). Coordination strategies can be centralized or decentralized. For example, a central control unit can assign orthogonal resources such that interference is avoided. But the nodes can also act independently. In this © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes Relay Source Destination Figure 7.4 A basic relay channel; the relay assists the direct link. case, they need to observe the interference from neighboring nodes. Strategies based on observation are often categorized as "cognitive radio." One can further distin- guish strategies according to the degree of channel knowledge. Sometimes, efficient coordination is possible based on medium-term statistical channel knowledge. Game theory can be used for analyzing such networks. The users can be regarded as players competing for the available resources. Coherent base station cooperation. Information can be distributed to several network nodes, from which it is simultaneously transmitted to the destination. Conversely, a received signal can be reconstructed by combining the outputs of multiple distributed receivers. If this is done coherently, then the network terminals form a distributed "antenna array." This is sometimes referred to as "joint transmis- sion" or "network MIMO" (see Figure 7.6). Cooperation can be demanding in terms of infrastructure and synchronization, but it is nevertheless interesting because it provides a benchmark for achievable per- formance. The cooperation (combining) gain can be significant (see, for example, Section 7.2.2.4). Partial cooperation with reasonable implementation and overhead Figure 7.5 Interference can be avoided or reduced by interference coordination, either by properly allocating 