We propose an innovative and practically attainable downlink multi-cell MIMO system with distributed transmit beamforming design. The proposed system is referred to as the MIMO-MAP system which is aimed to mitigate the rank deficiency problem of those MIMO wireless channels that can not support high-order multiplexing gains. In the MIMO-MAP system, each mobile station is allowed to receive several independent data streams from multiple access points at the same time and the same frequency. To do this, a set of noise-subspace-based receive beamformers are employed to suppress the interference among the data streams from different access points. On the other hand, if we consider each receive beamformer as part of its associated wireless channel, we virtually reduce the antenna array at each receive mobile station to a single antenna. With this arrangement, we may have the transmit signal dimension high enough to pre-cancel the inter-stream-interferences at each transmit access point. As a result, the MIMO-MAP channel can be decomposed into a large number of independent subchannels which significantly increase the channel capacity.

In multiple-input multiple-output (MIMO) wireless relay networks, simultaneously using multiple relay nodes can improve the capacity of source-to-destination communications. Recent information theories have shown that passing the same message across multiple relay nodes can improve the capacity of source-to-destination communications. We have previously proposed three relay schemes that use jointly QR decomposition and the phase control matrix; computer simulations have confirmed the superiority of these schemes over conventional ones such as amplify-and-forward and zero-forcing schemes. In this paper, we analyze the capacity and achievable gains (distributed array gain, intra-node array gain and spatial multiplexing gain) of the previously proposed relay schemes (QR-P-QR, QR-P-ZF, and ZF-P-QR) and thus provide an insight into what contributes to their superiority over conventional schemes. The analyses show that the location of the relay nodes used has a significant impact on capacity. On the basis of this observation, we further propose a method that enables each relay node to individually select its relay scheme according to its channel conditions so as to maximize the capacity. A computer simulation confirms the capacity improvement achieved by the proposed selection method.