Precoding and Relaying Algorithms for Multiuser MIMO Downlink Channels

In the last years, research has focused on multiple-input multiple-output (MIMO) wireless technology due to the capacity and performance improvement it provides, offering a higher spectral efficiency. In addition, when multiple users take part in the network, the scenario becomes much more complex, since resources like bandwidth, time or transmission power must be shared. Furthermore, the performance of the system is degraded as a consequence of the noise and multiuser interference (MUI). When the transmission is conducted from a base station (BS) to multiple users, a pre-equalization stage called precoding is applied. By means of this, each user will be able to interpret the signal independently, without the knowledge of the channel. Precoding techniques are classified into linear and non-linear. In fact, the non-linear Tomlinson-Harashima precoding (THP) and vector precoding (VP) techniques have been shown to achieve very good results in multiuser broadcast channels, outperforming the performance of linear systems considerably. Nevertheless, the computational complexity also increases as a consequence of successive interference cancellation in THP or the search for the closest point in an infinite lattice for the case of VP. In order to improve the coverage and range of the network in fading environments, as well as for MUI mitigation, a relaying terminal can be introduced between the BS and the users, being the signal modified either regeneratively or not to reach the destination. Hence, apart form incorporating a precoding stage at the BS for the separation of user streams, multiuser MIMO relaying systems can include a processing stage at the relay. By means of this, the signal will be provided by the characteristic necessary to reach the destination in a more efficient way. Throughout this thesis, the design of transceivers for non-regenerative multiuser MIMO relay systems is addressed, where the main challenge resides on the joint design of the precoder at the base station and the relaying matrix. We propose the use of linear and non-linear precoding techniques such as THP and VP at the downlink relayed transmission scenario. Apart from the joint design of the linear and non-linear transceiver design, block diagonalization (BD) is proposed in combination with the mentioned precoding techniques. BD, which maps each user signal at the null space created by the interfering users, is applied at the relay for interference cancellation whilst linear processing and VP are set at the BS for users signal separation and overall error minimization. The main contributions of this research work are the novel optimal proposals based on non-linear precoding techniques and the consideration of BD for the joint design. Furthermore, simple and computationally efficient suboptimal approaches based on linear and non-linear precoding techniques are first here proposed. In addition, block diagonal geometric mean decomposition is presented in combination with VP for the optimal and suboptimal design of the transmitter. To conclude, a complete complexity analysis is given in order to show the effectiveness of the proposed schemes.