A Two-Step Precoding Scheme for Multi-User Joint Transmission in Coordinated Multi-Point System

Joint transmission (JT) allows multiple base stations (BSs) to serve multiple users (UEs) in the same time and frequency resources, which will causes inter-user interference (IUI). Moreover, different signals from different BSs can´t be executed coherent combination directly in non-coherent JT. In this paper, a two-step precoding scheme (TSPS) is proposed to solve these problems. Firstly, an alternating minimization algorithm over the precoders at BSs and the interference subspace matrixes at UEs based on interference alignment (IA) is applied, which can force interference at each UE into the subspace and make each UE observe the interference-free desired signals. Secondly, we use the phase adjustment (PHA) based on QR decomposition to implement the coherent combination of desired signals from different BSs. This paper proves the convergence of alternating minimization algorithm in TSPS. Furthermore, numerical results show TSPS can improve the system performance by the IUI suppression and the signal coherent combination.