Trellis-Extended Codebooks and Successive Phase Adjustment: A Path From LTE-Advanced to FDD Massive MIMO Systems It is of great interest to develop efficient ways to acquire accurate channel state information (CSI) for massive multiple-input-multiple-output (MIMO) systems using frequency division duplexing (FDD). It is theoretically well known that the codebook size (in bits) for CSI quantization should be increased as the number of transmit antennas becomes larger, and 3GPP Long Term Evolution (LTE) and LTE-Advanced codebooks have sizes that scale according to this rule. It is hard to apply the conventional approach of using unstructured and predefined vector quantization codebooks for CSI quantization in massive MIMO because of the codeword search complexity. In this paper, we propose a trellis-extended codebook (TEC) that can be easily harmonized with current wireless standards, such as LTEor LTE-Advanced, because it can allow standardized codebooks designed for two, four, or eight antennas to be extended to larger arrays by using a trellis structure. TEC exploits a Viterbi decoder for CSI quantization and a convolutional encoder for CSI reconstruction. By quantizing multiple channel entries simultaneously using standardized codebooks in a state transition of a trellis search, TEC can achieve a fractional number of bits per channel entry quantization and 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 for TEC. This is similar to the dual codebook concept of LTE-Advanced. TE-SPA can reduce CSI quantization error with lower feedback overhead in temporally and spatially correlated channels. Numerical results verify the effectiveness of the proposed schemes in FDD massive MIMO systems.