Limited Feedback in Multiuser MIMO OFDM Systems Based on Rate Approximation

We propose a new limited feedback scheme for the downlink of multiuser MIMO OFDM systems based on fixed linear beamforming, i.e. the linear beamforming vectors are chosen from a fixed transmit codebook. The proposed feedback method allows the base station to uniformly approximate all multiuser rates for any selection of users and any combination of beamforming vectors defined by the transmit codebook; thus providing all degrees of freedom for the user selection. The approximation of the multiuser rates is enabled by using an additional codebook for the feedback. This has several advantages: the transmit codebook can be designed independent of the feedback codebook, the accuracy of the approximated rates can be scaled by changing the size of the feedback codebook and the feedback codebook can be adapted to the environment. We show how feedback codebooks can be designed for arbitrary environments using the LBG algorithm. Moreover, we show how the computational complexity of the proposed feedback method can be reduced without a significant performance loss. In the simulations we demonstrate that the proposed method outperforms other methods within the LTE context.