Efficient Feedback via Subspace Based Channel Quantization for Downlink Transmission in Distributed Cooperative Antenna Systems

It is one of the biggest challenges of multi-user distributed cooperative antenna (COOPA) systems to provide base stations (BSs) with downlink channel information for transmit filtering (precoding). The analog pilot retransmission method is efficient in terms of resources but is vulnerable to noise enhancement effects. The precoder codebook based feedback schemes are not applicable since they are developed for single user cases. In this paper we propose a new feedback scheme via a subspace based channel quantization method. The proposed scheme adopts the chordal distance as a channel quantizer criterion so as to capture channel characteristics represented by subspaces spanned by the channel matrix. We then develop a channel quantizer codebook construction method based on the Linde, Buzo and Gray (LBG) vector quantization algorithm. Our new codebooks have better minimum distance properties than currently available Grassmannian codebooks. Simulation results show that the proposed subspace based channel quantization method outperforms the analog pilot retransmission method and the Euclidean distance based channel quantization method.