  In this paper, we propose a greedy user selection with swap (GUSS) algorithm
based on zero-forcing beamforming (ZFBF) for the multi-user multiple-input
multiple-output (MIMO) downlink channels. Since existing user selection
algorithms, such as the zero-forcing with selection (ZFS), have `redundant
user' and `local optimum' flaws that compromise the achieved sum rate, GUSS
adds `delete' and `swap' operations to the user selection procedure of ZFS to
improve the performance by eliminating `redundant user' and escaping from
`local optimum', respectively. In addition, an effective channel vector based
effective-channel-gain updating scheme is presented to reduce the complexity of
GUSS. With the help of this updating scheme, GUSS has the same order of
complexity of ZFS with only a linear increment. Simulation results indicate
that on average GUSS achieves 99.3 percent of the sum rate upper bound that is
achieved by exhaustive search, over the range of transmit signal-to-noise
ratios considered with only three to six times the complexity of ZFS.
