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).

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The 5th-generation mobile communication system (5G) has higher security requirements than previous systems. Accordingly, international standard organizations, operators, and equipment manufacturers are focusing extensively on 5G security technology. This paper analyzes the security requirements of 5G business applications, network architecture, the air interface, and user privacy. The development trends of 5G security architecture are summarized, with a focus on endogenous defense architecture, which represents a new trend in 5G security development. Several incremental 5G security technologies are reviewed, including physical layer security, lightweight encryption, network slice security, user privacy protection, and block chain technology applied to 5G.

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