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In this paper, we study the diversity-multiplexing tradeoff assuming groupwise successive interference cancelation (GSIC). GSIC with layered codes in multiple-input multiple-output (MIMO) can vary from the plain SIC to the maximum likelihood (ML) decoding according to the group sizes, thus is flexible in both complexity and performance. The maximum group size is a dominating factor for the computation complexity of GSIC when the processing complexity is polynomially dependent on the group size with a high order. In this paper, we show the tradeoff among the maximum group size, diversity gain, and multiplexing gain (GDMT) under the GSIC and under the GSIC combined with antenna selection (GSICAS). For a given group-size bound c, we show that the number of groups achieving the optimal DMT under the GSIC with α selected transmit antennas is equal to ⌈α/c⌉ and a larger group should have a higher priority in decoding order. The optimal grouping is obtained in a systematic way. Based on these results, we find a limited set of groupings that contains the grouping achieving the optimal GDMT under GSIC and under GSICAS. The optimal multiplexing gain allocation for the optimal grouping is then found systematically and efficiently within the limited set.