Submitted to IEEE Transactions on Signal Processing, November 2006
We propose a low-complexity multi-mode transmission technique for downlink multiuser MIMO systems with block diagonalization (BD). The proposed technique adaptively adjusts the number of data streams for each user, switching between single-stream beamforming and multi-stream spatial multiplexing, as a means to exploit the multi-mode switching diversity. Multi-mode transmission is studied in the multiuser context, where there are more mobile users than the base station can serve simultaneously, therefore a subset of users need to be selected. Joint user and antenna selection has been proposed as a multiuser multi-mode switching technique, where theoptimal subset of receive antennas and users are chosen to maximize the sum throughput. The brute-force search, however, is prohibitively complicated and impractical due to the computational demand. In this paper, we propose two low-complexity near-optimal user/antenna selection algorithms. The first algorithm aims at maximizing a capacity lower bound, derived in terms of the effective channel energy, while the second algorithm greedily maximizes the sum capacity. We analytically evaluate the complexity of our proposed algorithms and show that it is orders of magnitude lower than the exhaustive search. Simulation results demonstrate that the proposed algorithms achieve up to 98% of the sum throughput of the exhaustive search, for most system configurations, while the complexity is substantially reduced.
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