AMC provides the coarse data rate selection, and HARQ provides for fine data rate adjust- ment based on channel conditions. Adaptive MIMO. In realistic wireless transmission environments, channel char- acteristics of MIMO systems behave in time, frequency, and space dimensions. In order to guarantee system performance in various channel conditions, adaptive MIMO, which is able to choose appropriate MIMO schemes adaptively based on different channel environments, should be supported in the FuTURE TDD sys- tem. Technology of transmit beam-forming (TxBF) is an advisable choice in high spatial correlated channels. In low correlation cases, technology of spatial division multiplexing (SDM) is more suitable. Therefore, adaptive MIMO should be supported to maintain high performance under various channel environments. The switching criterion according to differ- ent channel conditions for choosing appropriate MIMO schemes such as TxBF or SDM should be further discussed. For simple demonstration, the primary diagram of adaptive MIMO is shown in Figure 2.8. © 2009 by Taylor & Francis Group, LLC Evolution from TD-SCDMA to FuTURE Switching criterion Mode Recovery Module Mode Selection Module Feedback channel Figure 2.8 Primary diagram of adaptive MIMO. Additionally, the adaptive power allocation can be combined with the V-BLAST technique to bring good performance. Because the overall bit error rate (BER) per- formance of spatial multiplexing systems is limited by early detection stages, allo- cating a low data rate for early detection stages may reduce the probability of error propagation and thereby improve the averaged BER performance for all detection stages. Thus, different transmit antennas of the V-BLAST system should be allo- cated different transmit data rates if better BER performance is expected. One important advantage of the method is that the optimal ordering does not need to be determined because the allocation of the data rate assumes the receiver detects the streams in a predefined order. When the data rate o