Transmit phase control to increase the minimum eigenvalue of the channel correlation matrix in the ETD system

For conditions under which the channel conditions between the transmitting and receiving antennas are known on the transmitter side when TDD (Time Division Duplex) is used in the MIMO (Multi-Input Multi-Output) method, much attention has been devoted to E-SDM (Eigenbeam Space Division Multiplexing), in which the influence of CCI (Co-Channel Interference) is eliminated by making the different transmission signal sequences orthogonal in space by means of the eigenvectors of the channel matrix used as the transmission and reception weights. However, in such a system as E-SDM, in which eigenvectors of the channel matrix are used, more than two antennas are needed on the receiving side in order to perform eigenvalue decomposition of the channel matrix. In this paper, in order to realize a system using the eigenvectors as the transmission and reception weights even in a system with two transmitting antennas and one receiving antenna under TDD, the ETD (Eigenbeam Transmit Diversity) system is proposed. In this system, the concept of frequency diversity is applied, in which identical signals are transmitted on two subcarriers on one transmitting antenna, so that a MIMO system is virtually realized with subcarriers of the OFDM (Orthogonal Frequency Division Multiplexing) in place of receiving antennas in the row direction of the channel matrix. In the ETD system, the minimum eigenvalue of the channel matrix can be made larger by adaptively interchanging the signals among antennas and by transmission phase control. Hence, the BER characteristics of the signal sequence by which the minimum eigenvalue is multiplied can he improved. The characteristics are evaluated by computer simulations. It is shown that the proposed system is superior in terms of BER characteristics to the transmission diversity system in which transmission phase control is carried out among the antennas.