User Selection for Multi-user MIMO Downlink with Zero-Forcing   Beamforming
  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. 
Shengchun Huang, Hao Yin, Jiangxing Wu, Victor C. M. Leung
cs.IT math.IT