in turn selects © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer the appropriate rank (number of layers) of the transmission. This decision is based on the reported RI (rank indicator), as well as on other parameters as the reported CQI. As an example, a user under poor channel conditions (i.e., low CQI values) will be transmitted on rank 1 even if the channel matrix has rank 2. This proce- dure, referred to as rank adaptation, is illustrated in Figure 3.27. Closed-loop-based configurations, such as precoded spatial multiplexing and beam forming, exhibit good performance when the terminal speed is not very high. However, for high-mobility scenarios, the UE reports become obsolete sooner and hence adaptive precoding may be outdated. Therefore, two modes of operation can be identified when considering low- or high-mobility scenarios: Closed-loop operation implies adaptive precoding and rank adaptation (1, 2, 3, or 4) based on the UE reports. A rank 1 transmission corresponds to beam forming, whereas a rank 2, 3, or 4 is associated with spatial multi- plexing configuration. The UE reports must be reliable for a proper adap- tation process; therefore, this mode is only suitable for users moving at low or medium speed. Open-loop operation is employed for high-mobility scenarios and consists of applying either transmit diversity or fixed (nonadaptive) precoding. Switching between these alternatives is performed by means of rank adap- tation (i.e., transmit diversity for rank 1 and spatial multiplexing with fixed precoding for rank 2). This operation mode allows for increasing through- put of high-mobility users experiencing good channel conditions. As stated earlier, multiantenna configurations are adapted to channel variations and user-specific conditions (e.g., user speed). Adaptation or switching between dif- ferent configurations or modes of operation is performed at two levels: Dynamic switching (or physical layer switching) occurs in the context of rank adaptation. This does not mean that rank