The anti-intereference ability can be strengthened and the BER can be decreased when space-time block code (STBC) is applied to MIMO systems. The existing quasi-orthogonal code TBH can effectively improve the system capacity, but the effect is not good at high signal-to-noise ratio (SNR). In this paper, we introduce an improved quasi-orthogonal code, derive system capacity formula not with space-time coding but with TBH and improved quasi-orthogonal STBC, and simulate the relationship curve of the SNR and capacity in these systems based on MATLAB. The simulation shows that the system capacity with the improved code is bad in low SNR, but much better than that of the existed code with the increasing SNR, and the advantage on increasing capacity is outstanding, especially when the SNR is high.

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