A study on CSI feedback schemes exploiting feedforward information in FDD cellular systems

In frequency-division duplexing systems, the downlink (DL) channel is usually estimated at the user terminal (UT) and then fed back to the base station (BS) with a limited number of bits. In this paper, we exploit the partial dependency of the uplink (UL) and DL channels to design a new channel-state information (CSI) reporting procedure that splits the signaling overhead between UL and DL. In particular, the DL channel quantization at the UT is adapted to the UL channel estimated at the BS. The BS estimates and quantizes the UL channel, and then sends to the UT the quantization index as feedforward signaling. This index is used in turn by the UT to select the quantization codebook for the DL channel. Finally, the BS reconstructs the DL channel by either using only the feedback from the UT or merging it with its unquantized knowledge of the UL channel. The number of feedforward and feedback bits is fixed, and the proposed approach balances the signaling overhead between UL and DL. For a given average number of total signaling bits, it turns out that a variable-length feedback signaling avoids the use of feedforward, while still providing the minimum mean-square-error DL channel estimate at the BS. Numerical results prove the validity of proposed fix-rate and variable-rate solutions in the scenario of the COST 2100 model.