Separability of closely spaced users in massive MIMO systems

Abstract

Massive multiple-input multiple-output (MIMO) evolved to a key enabling physical layer (PHY) technology for the fifth generation (5G) of mobile communication systems and beyond. While the envisioned use cases of such communications systems are diverse, so are the challenges to meet their respective requirements. As a large scale evolution of already well established MIMO communications technologies, massive MIMO promises benefits with respect to all possible use cases. Theoretical works on massive MIMO, however, typically assume i.i.d. Rayleigh fading channels without spatial channel correlation. The application of this model is justified through the assumption of rich scattering environments, which is claimed to hold, for example, in indoor environments. Spatial correlation of wireless channels leads to inter-stream interference in single-user MIMO communications systems and to inter-user interference in multi-user MIMO systems with linear precoding. Channel correlation is therefore crucial for the performance of such MIMO techniques for mobile communications systems. In my work, I show the importance of spatial correlation of wireless channels for massive MIMO systems analytically, through measurements and through simulation. Since the correct modeling of this correlation is necessary for accurate performance evaluation, I propose a spatially consistent wireless channel model for massive MIMO. I investigate the antenna array property of user separability and perform array design to reduce the spatial channel correlation.