Hybrid Active-Passive RIS Transmitter Enabled Energy-Efficient Multi-User Communications
IEEE Transactions on Wireless Communications(2024)
摘要
A novel hybrid active-passive reconfigurable intelligent surface (RIS)
transmitter enabled downlink multi-user communication system is investigated.
Specifically, RISs are exploited to serve as transmitter antennas, where each
element can flexibly switch between active and passive modes to deliver
information to multiple users. The system energy efficiency (EE) maximization
problem is formulated by jointly optimizing the RIS element scheduling and
beamforming coefficients, as well as the power allocation coefficients, subject
to the user's individual rate requirement and the maximum RIS amplification
power constraint. Using the Dinkelbach relaxation, the original mixed-integer
nonlinear programming problem is transformed into a nonfractional optimization
problem with a two-layer structure, which is solved by the alternating
optimization approach. In particular, an exhaustive search method is proposed
to determine the optimal operating mode for each RIS element. Then, the RIS
beamforming and power allocation coefficients are properly designed in an
alternating manner. To overcome the potentially high complexity caused by
exhaustive searching, we further develop a joint RIS element mode and
beamforming optimization scheme by exploiting the Big-M formulation technique.
Numerical results validate that: 1) The proposed hybrid RIS scheme yields
higher EE than the baseline multi-antenna schemes employing fully
active/passive RIS or conventional radio frequency chains; 2) Both proposed
algorithms are effective in improving the system performance, especially the
latter can achieve precise design of RIS elements with low complexity; and 3)
For a fixed-size hybrid RIS, maximum EE can be reaped by setting only a
minority of elements to operate in the active mode.
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关键词
Multi-user communication,energy-efficient design,hybrid reconfigurable intelligent surface,RIS element scheduling,mixed-integer nonlinear programming
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