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In this paper, a two-stage detector is proposed for signals transmitted using multiple antenna configurations over frequency-selective channels. For the first stage, a frequency domain equalizer applying the minimum mean-squared error criterion is used to mitigate the intersymbol interference introduced by the channel's non-flat frequency response. Following, the output of this frequency domain equalizer, assumed to be intersymbol interference free, is used as an input for a sphere detector. The sphere detector then processes its input, returning log-likelihood ratios for all transmitted bits from all transmitting antennas. These log-likelihood ratios can be used, for instance, as inputs for decoders following the sphere detector in the receiving chain. The proposed method is shown to be well suited for the uplink scenario of the Long Term Evolution (LTE) Advanced standard, where high-modulation order signals can be transmitted by multiple antennas using a single-carrier frequency division multiplexing access (SC-FDMA) scheme that is prone to suffer from the effects of intersymbol interference.