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The capacity of MIMO systems with perfect transmitter and receiver channel state information (CSI) can be attained by decoupling the MIMO channel into a set of independent subchannels, and distributing the power among these subchannels in accordance with the water-fitting solution. Implementation of this scheme on a time-varying channel requires however continuous rate adaptation and is not feasible in any practical such system. In this paper, we show how to maximise the average spectral efficiency (ASE) of dual-branch MIMO systems (either two transmit or two receive antennas) with perfect transmitter and receiver CSI when using a fixed number of codes (discrete rate adaptation), and subsequently compare the resulting maximum ASE to the system's ergodic capacity. We assume that capacity-achieving codes for AWGN channels are available, and that the power available to the transmitter for transmitting the ith symbol frame is fixed and independent of the frame index i.