The Eigenmode Transmission Coupled with Space-Time Codes in Adaptive Power Allocation MIMO Systems

For the wireless communication systems coupled with multiple transmit receive antennas and orthogonal frequency-division multiplexing (OFDM), data sub-streams can be directed along orthogonal modes of channels, when spatial channel information is known at transmitters. These eigenmodes are generally obtained by the singular value decomposition (SVD) of spatial channel fading matrix at every carrier, but they could not be directly used to steer data symbols encoded by space-time codes, as they ignore the effects of space-time diversity gains on data symbols. For the MIMO OFDM systems with universal space-time codes and correspondent least-squared decoders, we present a novel eigenmode transmission according to the SVD of equivalent channel matrix. With these novel eigenmodes, data symbols encoded by space-time codes can be steered to these eigenmodes similar to MIMO wireless communication systems with single-carrier transmission. Moreover, the relations among different code-rate space-time codes, system capacity and number of eigenmodes are respectively analyzed and evaluated by numerical simulation. Furthermore, an improved water-filling scheme is also proposed for determining the optimal transmit powers for orthogonal eigenmodes. The classical water-filling strategy is firstly adopted to determine the optimal power allocation and correspondent bit numbers for every Eigenmode, followed by a residual power reallocation to further determine the additional bit numbers carried by every eigenmode. Compared with classical water-filling schemes, it can also obtain larger throughputs via residual power allocation. Numerical simulation results indicate that the improved water-filling scheme can obtain good tradeoff with respect to throughputs and complexity, with comparison to classical water-filling schemes and greedy algorithms respectively.