LMMSE Receivers in Uplink Massive MIMO Systems With Correlated Rician Fading

Abstract

We carry out a theoretical analysis of the uplink (UL) of a massive MIMO system with per-user channel correlation and Rician fading, using two processing approaches. Firstly, we examine the linearminimum-mean-square-error receiver under training-based imperfect channel estimates. Secondly, we propose a statistical combining technique that is more suitable in environments with strong Line-of-Sight (LoS) components. We derive closed-form asymptotic approximations of the UL spectral efficiency (SE) attained by each combining scheme in single and multi-cell settings, as a function of the system parameters. These expressions are insightful in how different factors such as LoS propagation conditions and pilot contamination impact the overall system performance. Furthermore, they are exploited to determine the optimal number of training symbols which is shown to be of significant interest at low Rician factors. The study and numerical results substantiate that stronger LoS signals lead to better performances, and under such conditions, the statistical combining entails higher SE gains than the conventional receiver. Index TermsMassive MIMO, correlated Rician fading, imperfect channel estimation, optimal training, LMMSE combining.

Multi-Cell MMSE Combining Over Correlated Rician Channels in Massive MIMO Systems

“…asymptotic spatial orthogonality is unlikely to occur in practice [10] and is a much stronger condition than the linear independence (assumption 2) considered in this work. As for the classical single-cell MMSE, the work [4] demonstrates that although the interference due to pilot contamination decreases with stronger LoS signals, it is only canceled as the Rician factor grows extremely large.…”

“…This work, however, considers Rayleigh fading which does not encompass Lineof-Sight (LoS) components. On the other hand, in [4], the authors investigate single-cell combining schemes and show that interference generated by pilot contamination decreases as the LoS signals become stronger. Therefore, it is paramount to investigate M-MMSE when the system involves both LoS and NLoS signals which calls for Rician fading.…”

“…Examining the performance of massive MIMO over Rician fading channels has received a lot of interest (see [4]- [9] and references therein). Among these, only a few study the multi-cell setting with pilot contamination [4], [7], [8]; however with either uncorrelated channels, single-cell detection techniques, or only intra-LoS channels. Accordingly, in this work we consider a more general system model wherein every channel (including the inter-cell links) accounts for a distinct Rician factor and correlation matrix, in addition to pilot contamination and imperfect channel state information (CSI).…”

Hybrid Precoding for Massive MIMO With Low Rank Channels: A Two-Stage User Scheduling Approach

“…Statistical beamforming for multiuser massive MIMO systems have been extensively investigated based on the assumption of correlated Rayleigh fading channels (see e.g., [21], [22]) and cannot be easily generalized to scenarios where we have a dominant line-of-sight (LOS) component. In the context of Rician fading channels, the authors in [4], [23] exploited the knowledge of channel mean information at the transmitter to introduce a sub-optimal precoder for the traditional fully digital structure. Therefore, contrary to prior works, we consider a two-stage hybrid precoding in generalized fading conditions, namely rank deficient correlated Rician fading channels.…”

“…The Karhunen-Loève correlated channel model representation is extended here for the case where there is a strong LOS component 4. The rank of the channel covariance matrix depends on the angular spread, inter-antenna spacing, and wavelength[14],[24].…”