Rate-allocation strategies for closed-loop MIMO-OFDM

A closed-loop MIMO-OFDM transmitter can exploit channel knowledge using eigenbeamforming to create MN parallel channels, where M is the number of antennas at both ends and N is the number of OFDM tones. We find that the penalty due to a flat-frequency constraint, which forces each tone to convey the same amount of information, becomes negligible as M grows. We propose low-complexity bit-allocation strategies by combining the flat-frequency constraint with previously-reported spatial bit-allocation strategies (Sung, J.H. and Barry, J.R., Proc. IEEE VTC 2003 Spring, p.813-17, 2003). A fixed (nonadaptive) allocation across space and frequency performs remarkably well with M as small as 4.