Multi-antenna base stations (BSs) in cellular systems require channel state information (CSI) of the downlink channel for proper transmit beamforming. In frequency division duplexing (FDD) systems the downlink channel is usually estimated at the user equipment (UE) and then fed back to the BS with a limited number of bits. We instead exploit the partial correlation of the uplink and downlink channels and propose a new CSI reporting procedure that reduces the signaling overhead by splitting it between uplink and downlink. In our procedure the codebook used for the quantization of the downlink channel at the UE is adapted to the uplink channel estimated at the BS: therefore the BS estimates and quantize the uplink channel, then sends to the UE the quantization index, which is used in turn to select the quantization codebook for the downlink channel. The BS reconstructs the downlink channel by either using only the feedback from the UE or merging it with its own knowledge of the "unquantized" uplink channel. Numerical results obtained on the COST 2100 model confirm the correlation among uplink and downlink channels and support the validity of the proposed techniques in balancing the uplink and downlink signaling overhead.
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