Feedback Reduction for Beyond 5G Cellular Systems

Cell-less system is a promising technology of the next generation wireless communications where a group of basestations intelligently recognizes user's communication environments and cooperatively serves the user. In order to maximize the gain obtained by the basestation cooperation, acquisition of accurate downlink channel state information (CSI) at the basestation is crucial. While this task is relatively easy for the time division duplexing (TDD) systems due to the channel reciprocity, it is not easy for the frequency division duplexing (FDD) systems due to the CSI feedback overhead. This issue is even more pronounced in the cell-less systems since the user needs to estimate and feed back the downlink CSIs of multiple basestations. In this paper, we propose a multi-path selection based feedback reduction technique for the FDD-based cell-less systems. Key ingredient of the proposed technique is that the spatial domain channel can be represented by a small number of multi-path components (angle of departure (AoD) and path gain). By exploiting the property that the AoDs are quite similar in the uplink and downlink channels, we only feed back the path gain information (PGI) to the basestations. Furthermore, by choosing a few paths maximizing the sum-rate and only feed back the PGI of the chosen paths, we can further reduce the feedback overhead substantially. From the simulations on realistic scenarios, we demonstrate that the proposed selective PGI feedback scheme is very effective in achieving the feedback overhead reduction compare to the conventional scheme relying on the CSI feedback.