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Multiple-input-multiple-output (MIMO) systems generally fall into two categories, depending on the kind of gain they provide: spatial multiplexing (SM) methods yield capacity gain and diversity methods yield link quality gain [measured here by the post-processing signal-to-noise ratio (SNR)]. We consider a set of systems from each category, quantify their gains analytically or via simulations, and show how these gains vary with the receiver input SNR and the numbers of antennas. The contribution of this work resides in both the closed-form analytical results and the numerical comparisons. We both highlight the benefits of using additional transmit antennas and provide comparisons among diversity-based and SM-based MIMO schemes. The analytical results are for a diversity-based scheme that combines selection diversity at the transmit side with maximum ratio combining (MRC) at the receive side, which we show to upper bound the SNR performance of other diversity-based schemes. We find that, for practical system parameters, the relevant SNR metric is 6-12 dB higher for the diversity-based schemes than for SM-based schemes. At the same time, SM-based schemes yield capacity metrics which range from 30% higher to double that of diversity-based schemes.
Date of Publication: June 2003