MIMO precoder design in the relay-assisted cellular network

In this thesis, we consider a practical relay-assisted cellular downlink system. The system transmits information to users close to the base station (“near users”) and users further away (“far users”). Cellular systems face a typical challenge where the base station cannot easily maintain a reliable link with the far users. A possible solution to this problem is to use relays. In this paper, a fixed relay is used to enhance the signal transmitted to the far users, while the base station directly serves the near users. We assume M multiple antennas are installed and full channel state information is available at both the base station and the relay. We jointly optimize the precoders at the base station and the relay, and compare different design schemes when the relay adopts either linear processing or a decode-and-forward technique. We show that the spatial multiplexing gain is an inappropriate performance measure in a relay system since relays typically operate in the moderate signal-to-noise-ratio (SNR) regime where, unlike in the asymptotic SNR regime, the base station and the relay do not pose significant interference to each others’ signals. We define a path-loss adjusted multiplexing gain and derive this metric for the relay system under study