On Some Techniques for Reducing the Feedback Requirement in Precoded MIMO-OFDM

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is a spectrally efficient modulation scheme which combines the advantages of having multiple antennas at both transmitter and receiver and the ease of equalization afforded by OFDM. When channel state information (CSI) is available at the transmitter a simple technique called linear precoding can be used to improve the error rate performance in both beamforming and spatial multiplexing systems. However, in frequency division duplex (FDD) systems there is typically a lack of channel reciprocity and precoder matrices have to be designed at the receiver and sent back through a limited feedback channel with the help of codebooks. By constraining the precoders to lie on the Grassmann manifold optimal codebook design for the single carrier MIMO channel has been well studied. If these codebooks are used in a MIMO-OFDM system and precoder information for all subcarriers is sent back, the amount of feedback can get prohibitively large. In this paper we use simple geometrical ideas on the Grassmann manifold to further reduce this feedback requirement. The performance of these algorithms are shown to provide improvement over existing schemes.