Subspace-Based Pilot Decontamination in User-Centric Scalable Cell-Free Wireless Networks

Abstract

We consider a cell-free wireless system operated in Time Division Duplex (TDD) mode with localized user-centric clusters of remote radio units (RUs). Since the uplink pilot dimensions per channel coherence slot is limited, co-pilot users might incur mutual pilot contamination. In the current literature, it is assumed that the long-term statistical knowledge of all user channels is available. This enables MMSE channel estimation or simplified dominant subspace projection, which achieves significant pilot decontamination under certain assumptions on the channel covariance matrices. However, estimating the channel covariance matrix or even just its dominant subspace at all RUs forming a user cluster is not an easy task. In fact, if not properly designed, a piloting scheme for such long-term statistics estimation will also be subject to the contamination problem. In this paper, we propose a new channel subspace estimation scheme explicitly designed for cell-free wireless networks. Our scheme is based on 1) a sounding reference signal (SRS) using latin squares wideband frequency hopping, and 2) a subspace estimation method based on robust Principal Component Analysis (R-PCA). The SRS hopping scheme ensures that for any user and any RU participating in its cluster, only a few pilot measurements will contain strong co-pilot interference. These few heavily contaminated measurements are (implicitly) eliminated by R-PCA, which is designed to regularize the estimation and discount the "outlier" measurements. Our simulation results show that the proposed scheme achieves almost perfect subspace knowledge, which in turns yields system performance very close to that with ideal channel state information, thus essentially solving the problem of pilot contamination in cell-free user-centric TDD wireless networks.

UL-DL duality for cell-free massive MIMO with per-AP power and information constraints

“…When feasible, Problem (18) and Problem (19) are known to have unique and positive solutions meeting the SINR constraints with equality, and to attain the same optimum.…”

“…The main focus is the evolution of known coordinated multi-point (CoMP) concepts towards practically attractive access solutions that combine the benefits of access point (AP) cooperation and ultra-dense deployments. To this end, considerable research effort has been devoted to the development of scalable and possibly user-centric system architectures and algorithms covering, for instance, power control, pilot-based channel estimation, joint processing such as precoding and combining, fronthaul overhead, network topology, and initial access [1]- [18].…”

Joint optimal beamforming and power control in cell-free massive MIMO

“…Remark 1. Alternative rate regions based on TIN with various forms of CSI at the decoder are often considered (see [9], [17], [18] and references therein). These regions may provide less pessimistic capacity approximations, but optimization problems become challenging because of the expectation outside the logarithm in the resulting rate expressions.…”