15 research outputs found
Fundamental Asymptotic Behavior of (Two-User) Distributed Massive MIMO
This paper considers the uplink of a distributed Massive MIMO network where
base stations (BSs), each equipped with antennas, receive data from
users. We study the asymptotic spectral efficiency (as )
with spatial correlated channels, pilot contamination, and different degrees of
channel state information (CSI) and statistical knowledge at the BSs. By
considering a two-user setup, we can simply derive fundamental asymptotic
behaviors and provide novel insights into the structure of the optimal
combining schemes. In line with [1], when global CSI is available at all BSs,
the optimal minimum-mean squared error combining has an unbounded capacity as
, if the global channel covariance matrices of the users are
asymptotically linearly independent. This result is instrumental to derive a
suboptimal combining scheme that provides unbounded capacity as
using only local CSI and global channel statistics. The latter scheme is shown
to outperform a generalized matched filter scheme, which also achieves
asymptotic unbounded capacity by using only local CSI and global channel
statistics, but is derived following [2] on the basis of a more conservative
capacity bound.Comment: 6 pages, 2 figures, to be presented at GLOBECOM 2018, Abu Dhab
Self-Learning Detector for the Cell-Free Massive MIMO Uplink: The Line-of-Sight Case
The precoding in cell-free massive multiple-input multiple-output (MIMO)
technology relies on accurate knowledge of channel responses between users
(UEs) and access points (APs). Obtaining high-quality channel estimates in turn
requires the path losses between pairs of UEs and APs to be known. These path
losses may change rapidly especially in line-of-sight environments with moving
blocking objects. A difficulty in the estimation of path losses is pilot
contamination, that is, simultaneously transmitted pilots from different UEs
that may add up destructively or constructively by chance, seriously affecting
the estimation quality (and hence the eventual performance). A method for
estimation of path losses, along with an accompanying pilot transmission
scheme, is proposed that works for both Rayleigh fading and line-of-sight
channels and that significantly improves performance over baseline
state-of-the-art. The salient feature of the pilot transmission scheme is that
pilots are structurally phase-rotated over different coherence blocks
(according to a pre-determined function known to all parties), in order to
create an effective statistical distribution of the received pilot signal that
can be efficiently exploited by the proposed estimation algorithm.Comment: Paper accepted for presentation in IEEE SPAWC 2020 - 21st IEEE
International Workshop on Signal Processing Advances in Wireless
Communications. {\copyright} 2020 IEEE. Personal use of this material is
permitted. Permission from IEEE must be obtained for all other use
Decentralized shaping for pilot generation and detection in opportunistic communications
© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.The uncoordinated design of pulse shaping filters for opportunistic communications is addressed. We show that under degrees-of-freedom sensing uncertainties the waveform design problem can be cast as a minimum-norm optimization, admitting hence a closed-form expression. Because designed waveforms are adapted to scenario working conditions, proposed design scheme may be considered in pilot reference signals design to achieve orthogonality, regardless the traditionally considered pilot symbols orthogonality. Hence, the effect of interferences such as pilot contamination is diminished. However, a crucial aspect relies on their detectability. Since each node uses only local observations from the wireless network, the sensed degrees-of-freedom may slightly differ from one node to others. In this paper we prove that, thanks to the existence of some invariances, designed waveforms can be detected by neighboring nodes. Even though degrees-of-freedom sensing uncertainties may incur in a performance loss, we propose a least-squares constrained basis pursuit algorithm to reduce the effect of uncertainties by considering only the degrees-of-freedom subspace intersection.Peer ReviewedPostprint (published version
Massive MIMO has Unlimited Capacity
The capacity of cellular networks can be improved by the unprecedented array
gain and spatial multiplexing offered by Massive MIMO. Since its inception, the
coherent interference caused by pilot contamination has been believed to create
a finite capacity limit, as the number of antennas goes to infinity. In this
paper, we prove that this is incorrect and an artifact from using simplistic
channel models and suboptimal precoding/combining schemes. We show that with
multicell MMSE precoding/combining and a tiny amount of spatial channel
correlation or large-scale fading variations over the array, the capacity
increases without bound as the number of antennas increases, even under pilot
contamination. More precisely, the result holds when the channel covariance
matrices of the contaminating users are asymptotically linearly independent,
which is generally the case. If also the diagonals of the covariance matrices
are linearly independent, it is sufficient to know these diagonals (and not the
full covariance matrices) to achieve an unlimited asymptotic capacity.Comment: To appear in IEEE Transactions on Wireless Communications, 17 pages,
7 figure