Antenna grouping algorithms are hybrids of beam-forming and spatial multiplexing. With antenna grouping, we can achieve diversity gain through beamforming, and high spectral efficiency through spatial multiplexing simultaneously. In an IID channel, at a fixed transmission rate, we can get better BER performance by antenna grouping compared with beamforming. But if the channel is ill-conditioned or there is some correlation between transmit or receive antennas, the BER performance of antenna grouping is significantly degraded. In that case, beamforming is better than antenna grouping algorithm. We introduced existing antenna grouping algorithms and proposed our new simple antenna grouping algorithms in our previous paper. In this paper, we investigate mode selection algorithms which select antenna grouping or beamforming mode. By selecting a suitable mode for instantaneous channel, we can prevent fluctuation of BER performance caused by channel variation and antenna correlation. We introduce some existing mode selection criteria and also propose a new criterion which is derived from the simple antenna grouping algorithm in the previous paper. This proposed mode selection criterion requires little additional calculations. By comparing with existing mode selection criteria, we verify the performance of our criterion.
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