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The authors investigate hybrid automatic repeat request (H-ARQ) schemes for spatially multiplexed multiple-input multiple-output (MIMO) systems with channel state information available only at the receiver. In particular, the authors compare the multiple H-ARQ scheme and the single H-ARQ scheme with repetition. The authors first propose a system model for symbol detection for the multiple H-ARQ processes, and then discuss joint and separate detection algorithms for both multiple H-ARQ and single H-ARQ. Simulation results show that with linear detection the single H-ARQ outperforms multiple H-ARQ in the high signal-to-noise ratio region. With the vertical Bell Labs space-time (V-BLAST) architecture, multiple H-ARQ always outperforms single H-ARQ. Additionally, joint detection always outperforms separate detection.