An achievable rate region of broadcast relay channel with state feedback

In this paper, we investigate the fast fading broadcast relay channel (BRC) with one source (macrocell BS), one relay (smallcell BS), and two destinations (mobile users). We assume that the instantaneous channel state information (CSI) is known only at the receivers´ side, whereas the relay can obtain a noiseless CSI feedback from the destinations about the source-destination channels (with delay). An inner bound on the capacity region of this channel is derived based on a partial-decode-compress-forward (PDCF) scheme. The main idea is to let the relay decode a significant portion of the source messages and then generate and forward to the receivers some useful side information using both the decoded messages and the CSI feedback. Numerical results show that, thanks to the feedback, the proposed scheme provides a non-negligible gain over conventional decode-forward/compress-forward schemes in terms of sum-rate performance, especially in the high SNR regime. Quite remarkably, we show that overall transmit power of the macrocell network can be significantly reduced owing to the feedback from users to the smallcell BS.