Channel quantization for limited feedback-based block diagonalization in MIMO broadcast channels

We consider block diagonalization (BD) based on limited feedback for multiple-antenna broadcast channels. With limited feedback, achievable sum rate is reduced due to multi-user interference. When the chordal distance (ChD) is used as a quantization measure, the number of quantization bits should scale with the signal to noise ratio (SNR) to guarantee full multiplexing gain of MIMO channel. Although the ChD can be used to prove such bit-scaling law, it is not an optimal quantization criterion for limited-feedback-based BD. Moreover, the required number of quantization bits for full multiplexing gain is too large to be realized in practical wireless communications. In this paper, we propose a more-appropriate quantization criterion for channel direction information that can achieve much higher sum capacity with moderate feedback rate than can ChD. We also consider a sufficient condition for quantization bits to maintain achievable multiplexing gain of the proposed quantization criterion.