  We propose a low-complexity transmission strategy in multi-user
multiple-input multiple-output downlink systems. The adaptive strategy adjusts
the precoding methods, denoted as the transmission mode, to improve the system
sum rates while maintaining the number of simultaneously served users. Three
linear precoding transmission modes are discussed, i.e., the block
diagonalization zero-forcing, the cooperative zero-forcing (CZF), and the
cooperative matched-filter (CMF). Considering both the number of data streams
and the multiple-antenna configuration of users, we modify the common CZF and
CMF modes by allocating data streams. Then, the transmission mode is selected
between the modified ones according to the asymptotic sum rate analyses. As
instantaneous channel state information is not needed for the mode selection,
the computational complexity is significantly reduced. Numerical simulations
confirm our analyses and demonstrate that the proposed scheme achieves
substantial performance gains with very low computational complexity.
