where k is an integer that may indicate the subcarrier number (or frequency bin), Y(k) is an MR???1 vector where MR is the number of receive antennas, Hl(k) is the MR???1 channel vector for stream l, xl(k) is the frequency-domain data symbol for stream l on subcarrier k (for single-carrier, xl(k) is the Fast Fourier Transform (???FFT???) of N time-domain symbols), and N(k) is additive noise that m