We propose a jointly designing transceiver for time-varying multiple-input multiple-output (MIMO) systems. Based on geometric mean decomposition (GMD), the scheme decomposes a MIMO channel into parallel subchannels with identical capacities and each subchannel meets certain BER constraints. Thus the power loading and modulation/demodulation process are particularly simplified because of the identical signal-to-interference-and-noise ratios (SINR) across the subchannels. We compare the average spectral efficiency (ASE) of our scheme with singular value decomposition-based continuous rate (SVD-CR) scheme, singular value decomposition-based discrete rate (SVD-DR) scheme and the scheme of GMD with subchannel selection (GMD-sel). Performance is investigated by simulation results.