  It is of great interest to develop efficient ways to acquire accurate channel
state information (CSI) for frequency division duplexing (FDD) massive
multiple-input multiple-output (MIMO) systems for backward compatibility. It is
theoretically well known that the codebook size for CSI quantization should be
increased as the number of transmit antennas becomes larger, and 3GPP long term
evolution (LTE) and LTE-Advanced codebooks also follow this trend. Thus, in
massive MIMO, it is hard to apply the conventional approach of using
pre-defined vector-quantized codebooks for CSI quantization mainly because of
codeword search complexity. In this paper, we propose a trellis-extended
codebook (TEC) that can be easily harmonized with current wireless standards
such as LTE or LTE-Advanced by extending standardized codebooks designed for 2,
4, or 8 antennas with trellis structures. TEC exploits a Viterbi decoder and
convolutional encoder in channel coding as the CSI quantizer and the CSI
reconstructer, respectively. By quantizing multiple channel entries
simultaneously using standardized codebooks in a state transition of trellis
search, TEC can achieve fractional bits per channel entry quantization to have
a practical feedback overhead. Thus, TEC can solve both the complexity and the
feedback overhead issues of CSI quantization in massive MIMO systems. We also
develop trellis-extended successive phase adjustment (TE-SPA) which works as a
differential codebook of TEC. This is similar to the dual codebook concept of
LTE-Advanced. TE-SPA can reduce CSI quantization error even with lower feedback
overhead in temporally correlated channels. Numerical results verify the
effectiveness of the proposed schemes in FDD massive MIMO systems.
