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In this paper, we investigate several possibilities to increase the efficiency of hybrid automatic retransmission request (HARQ) for multi-input multi-output (MIMO) systems. At the receiver side, two HARQ combining schemes, namely pre-combining and post-combining, are discussed and analyzed. We show that pre-combining is superior to post-combining. In addition, we propose a transmission technique, termed the basis hopping, which improves the HARQ diversity gain especially in slow fading channels. We demonstrate that the use of pre-combining in conjunction with the proposed basis hopping technique provides dramatic gain over a simplistic application of HARQ scheme for MIMO.