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This correspondence studies linear receivers for multiple-input-multiple-output (MIMO) channels under frequency-nonselective (flat) quasi-static Rayleigh fading. The outage probability and diversity gain of minimum mean square error (MMSE) and zero forcing (ZF) receivers are investigated. Assuming M. transmit and JV receive antennas, the ZF receiver always has diversity N - M + 1, unlike the MMSE receiver which may exhibit a rate-dependent behavior. Under separate spatial encoding, where the parallel data streams are not jointly encoded, MMSE is no better than ZF in terms of diversity. Under joint spatial encoding, the MMSE receiver achieves diversity MN at low spectral efficiencies but has diversity only M - N + 1 at high spectral efficiencies. These results are established via simulations and an outline for the corresponding analysis is presented.