Since the capabilities of MIMO systems were discovered, much research effort has been invested in this field. However, in most applications, the channel state information (CSI) is assumed to be known to the receiver only. To further improve the performance, the transmission rate will adapt to the levels of CSI fed back from the receiver. The capacity and performance of linear dispersion code (LDC) are studied. An analytical expression of the ergodic capacity and a tight upper bound of the pairwise error probability of full-rate LDC are derived. The tight upper bound demonstrates the relationship between the pairwise error probability, the constellation size and the space-time (ST) symbol rate, which will be a guideline for the adaptation. The probability density function of the signal-to-interference-noise ratio (SINR) of a MIMO transceiver using LDC and linear minimum-mean-square-error (MMSE) receiver is then derived, over a Rayleigh fading channel. With these theoretical results as a guideline, we study the design of adaptive systems with discrete selection modes. An adaptive algorithm for the selection-mode adaptation is proposed. Based on the proposed algorithm, two adaptation techniques are presented, using constellation size and ST symbol rate, respectively. To improve the average transmission rate, a new adaptation design is developed, which is based on joint constellation size and ST symbol rate adaptation. Next, we propose a novel scheme called "beam-nulling" for MIMO adaptation. In the beam-nulling scheme, the eigenvector of the weakest subchannel is fed back and then signals are sent over a generated subspace orthogonal to the weakest subchannel. Theoretical analysis and numerical results show that the capacity of beam-nulling is close to the optimal water-filling scheme at medium SNR. Additionally, the SINR of an MMSE receiver is derived for beam-nulling, followed by a presentation of the associated numerical average bit-error rate (BER) of beam-nulling. Finally, to further improve the performance, beam-nulling is concatenated with LDC. Simulation results show that the concatenated beam-nulling schemes outperform the beamforming scheme at higher rate. Additionally, the existing beamforming and new proposed beam-nulling schemes can be extended if more than one eigenvector is available at the transmitter. Theoretical analysis and simulation results are also provided to evaluate the new extended schemes.
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