In this paper, we propose a greedy user selection with swap (GUSS) algorithm based on zero-forcing beamforming for multiuser multiple-input-multiple-output (MIMO) downlink channels. Existing user selection algorithms such as zero forcing with selection (ZFS) have the flaws of "redundant users" and "local optimum," which compromise the achieved sum rate. GUSS improves the performance by adding the "delete" and "swap" operations to the user selection procedure of ZFS to eliminate "redundant users" and escape from "local optimum," respectively. In addition, an effective-channel-vector-based effective-channelgain-updating scheme is proposed to reduce the complexity of GUSS. With the help of this updating scheme, GUSS has the same order of complexity as ZFS with only a linear increment. Simulation results indicate that over the range of transmit signal-tonoise ratios (SNRs) considered, on average, the sum rate of GUSS reaches 99.3% of the upper bound that is achieved by exhaustive search, with only 1.51 to 2.29 times the complexity of ZFS.Index Terms-Broadcast channel, multiuser multiple-inputmultiple-output (MIMO), user selection, zero-forcing beamforming (ZFBF).

We have proposed that ancient and divergent classes of plant actin genes have been preserved throughout vascular plant evolution, because they have distinct patterns of gene regulation. The hypothesis was explored for ACT1 and ACT3, which represent one of the six ancient subclasses in the Arabidopsis actin gene family. Comparison of ACT1 and ACT3 cDNA and genomic sequences revealed highly divergent flaking and intron sequences, whereas they encoded nearly identical proteins. Quantification of their level of divergence suggests that they have not shared a common ancestor for 30 to 60 million years. Gene-specific RNA gel blot hybridization and reverse transcriptase-polymerase chain reaction analyses demonstrated that the distribution of ACT1 and ACT3 mRNAs was very similar: both preferentially accumulated at high levels in mature pollen and at very low levels in the other major organs. The 5' flanking regions of both genes, including the promoter, leader exon and intron, and the first 19 condons, were fused to the beta-glucuronidase (GUS) reporter gene. The expression of these reporter fusions was examined in a large number of transgenic Arabidopsis plants. Histochemical assays demonstrated that both ACT1-GUS and ACT3-GUS constructs were expressed preferentially in pollen, pollen tubes, and in all organ primordia, including those in roots shoots, and the inflorescence. Comparison of the 5' flanking regions of ACT1 and ACT3 revealed a number of short conserved sequences, which may direct their common transcriptional and post-transcriptional regulation. The expression patterns observed were distinct from those of any other other Arabidopsis actin subclass. The conservation of their expression pattern and amino acid sequences suggests that this actin subclass plays a distinct and required role in the plant cytoskeleton.

Abstract-This paper proposes a decremental user selection algorithm based on zero-forcing beamforming when the number of users K in the network is smaller than the number of antennas M at a base station. The algorithm is specifically designed for large-scale multi-user multiple-input multiple-output (MIMO) downlink channels. While previous user selection algorithms are based on incremental search that starts from an empty user set, our proposed delete the minimum lambda (DML) algorithm starts by selecting all users and then deleting one user per iteration. DML substantially reduces the computational complexity as the cardinality of the final user set is close to K. Simulation results indicate that on average DML achieves an equal or higher sum rate performance than previous algorithms with greatly reduced complexity of O(MK 2 ).Index Terms-Large-scale MIMO, decremental user selection, multi-user MIMO, zero-forcing beamforming.

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