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We propose the incorporation of an RF-adaptive array antenna (RF-AAA) into each receive branch on multiple-input multiple output (MIMO) systems. The RF-AAA consists of multiple antenna elements where a variable-gained low-noise amplifier (VG-LNA) and a 360°- continuously adjustable phase shifter are inserted in each antenna element and an adder for the array output is connected into the RF front-end of each receive branch. The phase shifters are adaptively controlled to increase the total channel capacity of MIMO systems. Computer simulations showed that the capacity of a 2-element-receive-RF-AAA incorporated 2 × 2 MIMO system is higher than that of a conventional 3 × 3 MIMO system (without RF-AAA) under average transmit power constraints.